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UNIT 13:INHERITANCE AND MUTATIONS
Key unit competence: Explain the role of genes in inheritance, how genetic disorders occur and describe the types, causes and effects of mutation in organisms.
Introductory activity 13
Read the passage below and answer the questions that follow
We all are aware of the fact that we look similar to our parents (grand-parents) and siblings in our appearance such as eye color, hair texture, skin color etc. We are also aware of the fact that certain diseases run in the family such as albinism, hemophilia etc. which indicates that certain characters or traits are passed on from parents to their offspring.
a. Why do you resemble your parents or siblings?
b. Which structure do you think controls the transmission of information from parents to their offspring?
For thousands of years, humans have understood that characteristics such as eye colour or flower color are passed from one generation to the next. The passing of characteristics from parent to offspring is called heredity. Humans have long been interested in understanding heredity. Many hereditary mechanisms were developed by scholars but were not properly tested or quantified. The scientific study of genetics did not begin until the late 19th century. In experiments with garden peas, Austrian monk Gregor Mendel described the patterns of inheritance.
13.1 Concept of inheritance and Mendel’s laws
Activity 13.1
Analyze the photo below and answer the questions that follow:
The young cat look similar to her mother.
a. What characteristics does the young cat receive from its mother?
b. How are information transmitted from the mother cat to its offspring?
c. The diploid number of chromosome for the cat is 38 (2n = 38).
How many chromosomes does the young cat receive from its mother? Why?
An organism produced by sexual reproduction tends to have two parents and inherits certain traits from father and certain traits from mother. It leads to variation in organism. So heredity and variation is characteristics of sexually reproducing organism. The study of heredity and variations in biology is referred to as Genetics.
13.1.1 Definition of genetic terms
• Gene: Gene is the entity/unit which has the information for particular trait. For example: in garden pea, gene for stem height has information for height whether it would be long or small.
• Locus: The position of gene on chromosome constitutes its loci/locus.
• Allele: The alternate forms of genes are known as Alleles. A pair of alleles for each trait is present in the zygote of an organism. For example: in garden pea, true breeding tall parent plants have two similar alleles (TT).
- Dominant allele: In individual, out of two alleles for the particular trait, only one allele is expressed. The expressed allele is known as dominant. For example, allele (T) for tallness is expressed in F1 individuals (Tt), dominant allele. Dominant allele is generally referred by capital alphabet.
- Recessive allele: In individual, out of two alleles for the particular trait one allele is under expressed. The under-expressed allele is known as recessive. For example, allele (t) for shortness is not expressed in F1 individuals (Tt), recessive allele. Recessive allele is generally referred by small alphabet.
- Co-dominant: It’s a phenomenon when both alleles present in an individual, are equally expressed. For example, in humans, Blood cells express both the alleles M and N (alternate form of gene encoding Red blood cell membrane protein) when present together.
- Linkage: The genes are said to be linked when present on the same chromosome and inherited together as unit. • F1: F symbolized filial, which means “progeny” in Latin. F1 is the filial generation first, produced by cross between parent individuals.
- F2: F2 is the filial generation second, produced by cross between F1 individuals.
- Phenotype: The morphological appearance for particular trait constitutes its phenotype. For example: In the cross between tall and dwarf parent plants, F1 plants are tall. Tallness is their phenotype. In F2 plants, tall and dwarf plants are obtained in ration of 3: 1, it is phenotypic ratio.
- Genotype: The combination of allele for a particular trait in an individual constitutes its genotype. For example: In the cross between tall and dwarf parent plants, F1 plants are Tt. “Tt” constitutes their genotype for the trait stem height. Similarly, F2 plants are tall and dwarf. But genotype of all tall F2 plants is not same, one third are pure (TT) while two third are hybrid (Tt). So genotypically F2 ratio is 1: 2: 1.
- Homozygous: When in an individual, two alleles for a particular trait are alike, then individual is considered homozygous for the particular trait. For example, parent plants tall and dwarf plants are homozygous for stem height.
- Heterozygous: When in an individual, two alleles for a particular trait are different then individual is considered heterozygous for the particular trait. For example, F1 plants are genotypically “Tt”. They are heterozygous for stem height.
- Monohybrid cross: ‘It is a cross between two individuals of a species which is made to study the inheritance of a single pair of factors or genes of a trait.’ A ratio among the offspring of F2 generation of a monohybrid cross is called a ‘monohybrid ratio.’ It is usually 3 : 1 (phenotypic ratio) or 1 : 2 : 1 (genotypic ratio), in which 1/4 individuals carry the recessive trait, 1/4 pure dominant and 1/2 have impure dominant trait.
13.1.2 Mendel’s laws of inheritance
Mendel’s experiments
In 1856, Gregor Mendel conducted experiments in garden pea (Pisum sativum) in the limited space of a monastery garden. Garden pea plant has both male (pollen-producing part) and female parts (pollen-receiving part). Since both the male and female parts are on the same plant, it has tendency to undergo self-fertilization. Because of self-fertilization, the tall plants always give rise to tall plants and dwarf plants always produce dwarf plants. Such true breeding varieties are known as pure lines. Furthermore, he was lucky to get pure lines in garden pea.
He then carefully conducted hybridization experiments between two parent plants expressing contrasting form of single trait. He also made sure that selffertilization didn’t happen by removing male parts from one parent (say tall plants) before female part got matured. In his initial experiments, he carefully transferred pollen from male parent (say dwarf plant) to tall parent’s female part and analyzed transmission of one particular trait (stem height) in all progenies of the first generation (also known as F1 generation where F symbolizes the Latin word “filial” meaning progeny and 1 represents first). Furthermore, he followed the transmission of same trait (stem height) in second (F2) and third generation (F3) progenies as well which were naturally produced by self fertilizing power among first generation plants and second-generation plants. He maintained the quantitative records of all his experiments.
Since Mendel focused on one trait at a particular time, a cross between parents which differs in contrasting form of single trait is known as Monohybrid cross or inheritance.
13.1.2 Monohybrid inheritance
Mendel first worked with plants that differed in a single characteristic, such as flower color.
Hybridization is a cross between two individuals that have different traits. A hybridization in which only one characteristic is examined is called a monohybrid cross. The offspring of such a cross are called monohybrids. Mendel noted that hybridizing true-breeding (P-generation) plants gave rise to an F1 generation that showed only one trait of a characteristic. For example, a true breeding purple-flowering plant crossed with a true-breeding white-flowering plant always gave rise to purple-flowered hybrid plants. There were no white flowered hybrids!
Mendel wanted to know what happened to the white-flowered plants’ “heritable factors.” If indeed the white flower “heritable factor” had disappeared, all future offspring of the hybrids would be purple-flowered. To test this idea, Mendel let the F1 generation plants self-pollinate and then planted the resulting seeds.
Mendel’s results
The F2 generation plants that grew included white-flowered plants! Mendel noted the ratio of white flowered plants to purple-flowered plants was about 3:1. That is, for every three purple-flowered plants, there was one white flowered plant.
Mendel carried out identical studies over three generations, (P, F1, and F2), for the other six characteristics and found in each case that one trait “disappeared” in the F1 generation, only to reappear in the F2 generation. Mendel studied a large number of plants so he was confident that the ratios of different traits in the F2 generation were representative.
Mendel’s observation
Mendel carried out experiments to follow the pattern of inheritance of particular trait in several generations. On crossing tall plants (which provided the female part) verse dwarf plants (which provided pollen), he observed that (Figure 13.3).
• First generation progenies were always tall.
• Second generation progenies (also known as F2 generation) include tall plants as well as dwarf plants almost in ratio of 3 (tall plants): 1 (dwarf plants).
Mendel then performed the reciprocal cross (A similar cross where tall plants provided male parts whereas dwarf plants represented female plants). Mendel observed similar results.
On performing similar cross-fertilizing experiments with parent plants showing other contrasting set of traits such as seed colour, seed shape, seed coat colour, pod colour, pod shape and flower position/arrangement (figure 13.3), he observed similar observation and concluded that:
• First generation progenies were always showing one form of trait expressed in one of the parent plants.
• Second-generation progenies include the plants showing both contrasting forms of traits, almost in ratio of 3:1.
On self-fertilization of F2 plants for various contrasting traits (for example: for stem height), Mendel observed the following points:
• Dwarf F2 plants always yielded dwarf plants only.
• All F2 tall plants were not genetically same. The one-third tall plants produced tall plants only but two-third tall plants yielded both tall plants and dwarf plants in the ratio of 3: 1. It means phenotypic ratio is 3: 1 but genetically the ratio is 1:2:1.
The results of Mendel´s experiment were published in the monograph – “Experiments in Plant Hybridization” in 1866.
13.1.3 Principles of inheritance (Mendel’s postulates)
Based on consistency of his results in transmission of seven contrasting traits, he derived postulates which later became principles of inheritance.
• There are two factors (Unit factor in pairs) for each trait. In pure lines of plants, both the factors for particular trait (stem height) are alike. For example, if Factor “T” donates height, there are two factors for each trait. The tall plants have TT and dwarf plants have tt.
• At the time of gamete formation, the factors for particular trait randomly segregate with equal likelihood. Each gamete contain single factor, therefore the gamete is always pure for the trait. Later on, it becomes popular as “Mendel’s principle of segregation”. For example: all the gametes from tall plants have single factor “T” and dwarf plants have “t” (Figure 13.5).
• After fertilization, when gametes from parents randomly fuse, factors for a particular trait also unite together. For example, in a cross between tall and dwarf plants, gamete from tall plant with factor “T” fuses with gamete from dwarf plant with factor “t” to form “Tt” organism.
Test Cross: It is cross between hybrid forms (dominant phenotype) with other parent with recessive form of particular trait (homozygous recessive). It is generally used to identify the genotype of hybrid form. The progenies are observed. If all progeny demonstrates only dominant form of trait thereby indicating that unknown genotype must be homozygous for the particular trait. Or If F1 progeny shows both dominant and recessive form of trait in the ratio of 1: 1 indicating that unknown genotype must be heterozygous for the particular trait.
There can be two possible genotypes of an unknown dominant phenotype as illustrated below.
Possibility 1: If the unknown is homozygous yellow (YY), then crossing with green recessive (yy) gives all yellow offspring (i.e., all Yy) as shown below.
Possibility 2: If the unknown is heterozygous yellow (Yy), then crossing with green recessive results in 50% yellow (Yy) and 50% green (yy) progeny as shown below.
Mendel’s laws
• First law of Mendel: the law of segregation: there are two factors controlling a given characteristic and these factors separate during gamete formation.
• Second law of Mendel: the law of independent assortment: factors controlling different characteristics are inherited independently of each other.
Application activity 13.1
1. Complete the table below about the number of chromosomes (4 marks)
a. How many chromosomes does a gorilla receive from its father?
b. What is the number of sex chromosomes in an egg cell of a fruit fly?
c. What is the number of autosomes in a leaf cell of a cotton plant?
d. What is the number of autosomes in a sperm cell of a rat?
2. A plant with terminal flowers stems is crossed with a plant with axial flowers. All F1 plants produced had axial flowers. /6 marks
a. Which allele is dominant?
b. With a punnet square, show the genotypic and phenotypic ratio of the F1 and F2 generation.
c. If there are 360 plants with axial flowers in the F2 generation, what is the number of plants with terminal flowers?
13.2 Co-dominance, multiple alleles and lethal alleles
Activity 13.2
Analyze the photo below and answer the questions that follow:
The cow observed on the photo has a roan coat colour. The roan coat of this shorthorn cattle is made up of red and white hairs. Both the red and white hair alleles are codominant. Therefore cattle with a roan coat are heterozygous for coat color. It is an offspring of a cross between a red bull cattle and a white bull cattle. What is the difference between this mode of transmission with complete dominance?
Codominance occurs when both traits appear in a heterozygous offspring. Neither allele is completely dominant nor completely recessive. For example, roan shorthorn cattle have codominant genes for hair color. The coat has both red and white hairs. The letter R indicates red hair color and W white hair color.
In cases of codominance, the genotype of the organism can be determined from its phenotype.
The heifer below is RW heterozygous for coat color.
The roan coat of this shorthorn heifer is made up of red and white hairs. Both the red and white hair alleles are codominant. Therefore cattle with a roan coat are heterozygous for coat color (RW).
Incomplete dominance
Incomplete dominance occurs when the phenotype of the offspring is somewhere in between the phenotypes of both parents; a completely dominant allele does not occur.
Multiple alleles
When three or more alleles determine a trait, the trait is said to have multiple alleles. The human ABO blood group is controlled by a single gene with three alleles: IA, IB, and the recessive i allele. The gene encodes an enzyme that affects carbohydrates that are found on the surface of the red blood cell. A and B refer to two carbohydrates found on the surface of red blood cells. There is not an O carbohydrate. Type O red blood cells do not have either type A or B carbohydrates on their surface.
The alleles IA and IB are dominant over i. A person who is homozygous recessive ii has type O blood. Homozygous dominant IAI A or heterozygous dominant IAi have type A blood, and homozygous dominant IBI B or heterozygous dominant IBi have type B blood. IAI B people have type AB blood, because the A and B alleles are codominant. Type A and type B parents can have a type AB child. Type A and a type B parent can also have a child with Type O blood, if they are both heterozygous (I Bi, IAi). The table below shows how the different combinations of the blood group alleles can produce the four blood groups, A, AB, B, and O
Lethal alleles
Sometimes genes have serious effect on development, physiology of the organism in such a way that organism is unable to survive. Such genes are known as lethal genes. The particular allele responsible for death of the organism is known as lethal alleles. Lethal allele can be dominant or recessive.
For example: The dominant allele A in chicken has serious effect on development of the organism and results in following phenotype:
• Aberrant form “creepers” in Heterozygous individual (Aa)
• Completely “lethal” in homozygous dominant (AA).
When two heterozygous creeper individuals are mated, progeny are obtained in phenotypic ratio of 2 (Creeper): 1 (Normal) instead of 3: 1 monohybrid Mendelian ratio.
Application activity 13.2
1. When red-flowered petunia plants are crossed with white-flowered plants, all the resulting F1 plants have pink flowers.
a. Explain how this is possible using genetic diagrams.
b. The F1 plants are crossed to produce an F2. Draw a genetic cross to show the genotypes and phenotypes of the F2 plants.
2. A man with blood group B marries a woman with blood group AB. Indicate the type of blood group that their children will not have. Show your working.
13.3 Dihybrid inheritance
Activity 13.3
Make a research in different books and use the internet about the dihybrid inheritance.
Mendel then thought how the segregation of factors for a particular trait at the time of gamete formation (Principle of segregation) could be effected with the segregation of factors for the other traits. With this question in his mind, he carried out similar sets of cross hybridization experiments between parents differing in contrasting set of two traits, (for example, round or wrinkled seed shape and yellow or green seed colour). Such a cross between parents which differs in contrasting form of two traits is known as Dihybrid cross or inheritance. The F1 progeny generated is known as Dihybrid.
The cross was made between the double dominant plants (round seed shape with yellow seed colour) with double recessive parent (wrinkled seed shape with green seed colour) and the following points were observed:
• All round yellow seeds were observed in F1 generation indicating dominant factor for a gene was expressed in the same manner as in monohybrid cross.
• On self-fertilization of F1 plants, F2 seeds were obtained and segregated in the ratio of 9 : 3 : 3 : 1 based on their phenotype.
In addition to parental phenotype combination, two new phenotype combinations/ recombinants (wrinkled and yellow and round and green seeds) were observed. Mendel hypothesized that the factors for different traits separate and assort independently in the gametes (factor for seed shape can assort with any seed colour factor and vice versa) then F1 plants should produce four types of gametes.
So male and female F1 plant gametes can fuse randomly and combine in 16 possible ways which can be simply represented by a simple square popularly known as Punnett’s square.
Mendel observed similar results when he analyzed results of dihybrid cross for the other pair of traits as well.
• The dihybrid results did not contradict monohybrid results, the round seeds and wrinkled seeds as well as yellow and green seeds were in ratio of 3: 1. He hypothesized dihybrid cross event as two independent monohybrid cross events.
(Punnett’s square or checker-board: square-shaped presentation used to predict result of a particular cross or breeding experiment in which gametes from each parent are placed on the top and left side of the square. This diagram is used to predict the ratio of genotypes and phenotypes of the individual when gametes from parents randomly fuse. It is named after Reginald C. Punnett, who devised the approach.)
Figure 13.11: Dihybrid cross between plants with dominant round yellow seeds with plants with recessive traits wrinkled and green seeds through two generations.
The phenotypic ratio is: 9 Yellow - Round: 3 Yellow – wrinkled: 3 green - Round: 1 green: wrinkled.
Note: The genotypes can be seen in specific locations as seen below in this table where every genotype has a specific colour
Table 13.2: Ratio of genotypes: 1 YYRR: 2 YYRr: 1YYrr: 2 YyRR: 4 YyRr: 2 Yyrr: 1 yyRR: 2 yyRr: 1 yyrr
Law of independent assortment
From the result of dihybrid cross experiments, Mendel gave the following postulates:
• The dominant allele of a particular gene is expressed in the presence of alleles of other genes for different traits.
• On self-fertilization F1 plants, F2 plants were observed in the phenotypic ratio 9:3:3:1 (Dihybrid ratio). He concluded that factors for different traits assort segregate and assort independently in the gamete. This is popularly known as Law of Independent Assortment.
Significance of test crosses in dihybrid inheritance
Test cross can be used to differentiate genotype of dihybrid organisms (whether it is homozygous and heterozygous for the traits) if phenotypically same for a traits.
For example: plants with similar phenotype rounded seed shape and yellow seed color can have different genotype RRYY or RrYy. So the genotype of such plants can be identified by test cross. So the plant with unknown genotype is crossed with plant with recessive form of both the traits. There are two possibilities.
1. If progeny plants are observed in phenotypic dihybrid test ratio 1 (round and yellow):1 (round and green):1 (wrinkled and yellow):1 (wrinkled and green), then the parent plant must have heterozygous genotype for both the traits.
Expected ratio for dihybrid test cross
2. If after the cross all the plants are formed with dominant phenotype i.e., round seed shape and yellow seed colour, it indicates that given parent plant must have homozygous genotype for both the traits.
Application activity 13.3 1.
A homozygous purple-flowered short-stemmed plant was crossed with a homozygous red-flowered long-stemmed plant and the F1 phenotypes had purple flowers and short stems. When the F1 generation was test crossed with a double homozygous recessive plant, the following progeny were produced. - 52 purple flower, short stem - 47 purple flower, long stem - 49 red flower, short stem - 45 red flower, long stem
Explain these results fully.
2. In tomatoes, the allele for red fruit, R, is dominant to that for yellow fruit, r. The allele for tall plant, T, is dominant to that for short plant, t. The two genes concerned are on different chromosomes.
a. A tomato plant is homozygous for allele R. Giving a reason for your answer in each case, how many copies of this allele would be found in:
i. A male gamete produced by this plant.
ii. A leaf cell from this plant.
b. A cross was made between two tomato plants.
i. The possible genotypes of the gametes of the plant chosen as the male parent were RT, Rt, rT and rt. What was the genotype of this plant?
ii. The possible genotypes of the gametes of the plant chosen as the female parent were rt and rT. What was the genotype of this plant?
iii. What proportion of the offspring of this cross would you expect to have red fruit? Use a genetic diagram to explain your answer.
13.4 Linkage and crossing over
Activity 13.4
Genes may be located on the same chromosome or on different chromosomes. Make a research on the transmission of genes located on the same chromosome and genes located on different chromosomes.
According to chromosomes theory of Inheritance, it is the chromosomes which segregate and assort independently in the gametes. So the question arises as to then what happens to genes located on same chromosome? Do they always remain together or linked (exception to law of independent assortment)? Or, do they segregate and assort independently, if yes what could be the mechanism?
Linkage
There are cases when genes (present on the same chromosome) for different traits do not show independent assortment, inherit together and behave as if genes are linked; the phenomenon is known as linkage. For example: two genes for trait flower colour and pollen grain texture in sweet pea (Lathyrus odoratus) where blue flower colour (B) allele is dominant over red flower colour (b) and long pollen (L) is dominant over round pollen (l). A test cross was carried out between heterozygous plant with double homozygous recessive plant (bbII), the observed phenotype had higher frequency of parental phenotype (87.4%) and lower frequency of recombinants phenotype (12.6%) in contrast to expected dihybrid test ratio. It indicated that genes do not assort independently and appear as if they are linked.
However, occasionally they may separate therefore resulting in lower frequency of recombinants.
Such genes are identified as linked when present on the same chromosome and do not assort independently and tends to form parental phenotype but occasionally they may separate resulting in low recombinants frequency. This phenomenon is known as linkage.
Linkage with crossing-over
Now the question arises what could be the possible mechanism for the separation of the genes located on the same chromosomes. The answer is crossing-over or recombination. Crossing-over is the physical exchange of chromosome parts between non-sister chromatids of the homologous chromosomes during meiosis division. The chiasma formation (observed by Janssens in 1909) clearly provides the site at which non-sister chromatids of paired homologous chromosomes cross over. The cross-over event between two gene loci in non-sister chromatids is responsible for formation of recombinant chromatids and their separation.
Here two paired homologous chromosomes (each with two sister chromatids) with centromeres and gene loci are shown.
Two alleles of a gene A (A and a) and two alleles of gene B (B and b) occupy same position in homologous chromosomes. The crossing over between two non-sister chromatids involves breakage of non-sister chromatids and reunion of broken parts. The chromatids which participate in crossing over generate recombinants chromatids. In the recombinants, the alleles on the same chromatid get separated and combine with alleles of non-sister chromatid.
Significance of Recombination/crossing-over
• The major significance is generation of variations. Due to crossing over, genes even on the same chromosome can be assorted differently. It leads to variations in the progeny. The variations are very useful in nature as it provides raw material on which natural selection can act.
• The frequency of crossing over becomes higher with increase in physical distance between gene loci. So recombinant frequency between two genes can be used to determine distance between genes, hence it helps to create chromosome map.
The recombination frequency or crossover frequency or crossover value (COV) is calculated using the formula:
Application activity 13.4
1. Pure-breeding Drosophila with straight wings and grey bodies were crossed with pure-breeding curled-wing, ebony bodied flies. All of the offspring were straight-winged and grey-bodied. Female offspring were then test-crossed with curled-wing, ebony-bodied males, giving the following results.
Straight-wing, grey body 113
Straight-wing, ebony body 30
Curled-wing, grey body 29
Curled-wing, ebony body 115
a. State the ratio of phenotypes expected in a dihybrid test cross such as this.
b. Explain the difference between the expected result and the results given.
c. Calculate the crossover value.
13.5 Sex determination and sex linkage
Activity 13.5
In 1910 by Morgan while working with white-eye (mutant) Drosophila. He carried several breeding analysis with white-eyed male drosophila and red-eye female drosophila. The F1 flies (male and female) are all redeyed. On mating F1 male and female, he found F2 flies with red-eye and white eye in the ratio of 3 :1 in accordance with Mendelian monohybrid ratio thereby concluding that white-eye colour is recessive character. In Mendel’s cross, expression of recessive trait in F2 is not associated with sex of the individual. Can you explain why?
Mostly, the organisms that produce their progeny using sexual reproduction have two sexes, male and female. Occasionally, there are hermaphrodites which have characteristics of both sexes. Sex determination is the biological system which initially determines sex of the organism while development.
13.5.1 System for sex determination
Based on whether genes play an important role in sex determination, there are two types of systems:
a. Genetic sex determination in which chromosomes (especially sex chromosomes) play an important role in determining sex of the individual. For example: mammals
b. Non-genetic sex determination in which other environmental factors such as diet, temperature etc., play an important role in sex determination.
For example: Certain reptiles
13.5.2 Sex determination in humans
In humans and other placental mammals, male and female differ in their chromosome complement. Generally, there are two types of chromosomes, autosomes and sex chromosomes. Generally in one sex (mostly female), both the sex-chromosomes are alike/homomorphic (XX) and in other sex (male), there are two different/heteromorphic sex chromosomes (XY).
As the females are homomorphic (44 autosomes and XX, they produce single type of ovum, containing 22 autosomes and one X chromosome while males are heteromorphic (44 autosomes and XY) and therefore, they produce two types of sperm, one containing 22 autosomes and an X chromosome while other with 22 autosomes and a Y chromosome.
It is the Y chromosome which determines the sex of an individual. Y chromosome has Testis determining factor (TDF) gene which produces testis determining factor which causes primordial gonadal tissue in developing foetus to differentiate into testis. In the absence of TDF, tissue differentiates into ovaries. So, the
• Individuals with Y chromosome are genetically male.
• Individuals without Y chromosome are genetically female.
Thus, the sex in human is determined at the moment of conception or fertilization of male (sperms) and female gamete (ovum). If ovum gets fertilized by sperm containing an X-chromosome, then resulting zygote will have two XX chromosomes and will develop into female.
But if ovum gets fertilized by sperm containing a Y-chromosome, then resulting zygote will have two XY chromosomes and will develop into male. So biologically, father is responsible for sex of the child.
13.5.3 Sex linkage
Have you ever wondered that some variations are associated with particular sex of the individual? For example, the diseases like colour blindness, Haemophila etc., are more common in male as compared to female. Is mutation sex associated?
There are certain genetic traits, the expression of which depends upon sex of the individual or inheritance of sex chromosomes. The transmission of such traits (or alleles responsible for traits) is tied up or linked with the sex chromosomes; inheritance pattern of such genes is known as sex-linked inheritance. The phenomenon is called as sex linkage.
Sex linkage was first demonstrated in 1910 by Morgan while working with white-eye (mutant) Drosophila. He carried several breeding analysis with white-eyed male drosophila and red-eye female drosophila. The F1 flies (male and female) are all red-eyed. On mating F1 male and female, he found F2 flies with red-eye and white eye in the ratio of 3: 1 in accordance with Mendelian monohybrid ratio thereby concluding that white-eye colour is recessive character. In Mendel’s cross, expression of recessive trait in F2 is not associated with sex of the individual.
13.5.4 Types of sex linkage
There are two types of sex-linked inheritance:
1. Genes located on X chromosomes demonstrate X-linked inheritance.
It is of two types’ X-linked recessive inheritance and X-linked dominant inheritance.
• X-linked recessive inheritance, gene causing a mutant phenotype (variant phenotype) is recessive. It is more common in male. As male has single X chromosome only, they are pure for X-linked genes (hemizygous). While for female to express X-linked recessive trait, both the X chromosome should carry recessive allele. Here, criss-cross inheritance pattern is seen when recessive trait from male are transmitted through their daughter to their grandson.
• For example: Hemophilia A in human, here individuals lack a clotting factor; thus, a minor cut may cause excessive bleeding. It follows X-linked recessive inheritance.
• X-linked dominant inheritance: Here, the gene causing for a mutant phenotype (variant phenotype) is dominant. It is less common than X-linked recessive trait. Only a few X-linked dominant traits have been identified.
For example: X-linked hypophosphatemia is X-linked dominant trait that can cause bone deformity in human.
2. Genes located on Y-chromosomes demonstrate Y-linked inheritance
Here, genes are transmitted according to inheritance of Y chromosomes. All males receive Y chromosome from their father, so here Y-linked genes (hence their information) are directly passed from father to son. This type of inheritance is also known as Holandric (“wholly male”) inheritance.
Application activity 13.5
1. The diagram below shows the pedigree of a family with red-green color blindness, a sex linked condition.
a. Define the term sex linkage.
b. Deduce, with a reason, whether the allele producing the condition is dominant or recessive.
c. Determine all the possible genotypes of the individual (2nd generation-1) using appropriate symbols.
d. Determine all the possible genotypes of the individual (3rd generation-4) using appropriate symbols.
13.6 Mutations and genetic disorders
Activity 13.6
Read the sentence below and answer to the questions that follow:
The big fat cat ate the rat.
a. Suppose the sentence is message hold on chromosome and code for the development of body parts, what will happen on the body when the word ATE is deleted?
b. What would happen if T in the word CAT is replaced by R?
c. What would happen if word CAT is doubled?
d. Which one of the following can have a great effect on the meaning of the sentence: removing one letter or adding one letter to the sentence? Compare the effect to that of the development of body parts.
In humans, how can we study inheritance pattern of different genetic disorders? So here, we have to study the history of families of person suffering from particular genetic disease by making a tree or chart.
Also, we can predict the chance of transmission of disease to future generation. Genetic disorders are the diseases which are caused by abnormalities in genetic information of the organisms. Genetic diseases are quite rare in population and their frequency varies from 1 1000 to 100,000.
13.6.1 Types of genetic disease
Single gene disorder: caused by abnormalities in single gene so that its product becomes either non-functional or abnormal. For example: haemophilia.
There are two types:
1. Autosomal-linked disorder: in this case, the affected gene is located on the autosomes and it can be dominant and recessive. In autosomal dominant, the affected gene allele is dominant in its expression. Only one allele is sufficient to cause the disease in affected person. Affected person will have 50% chance to pass it to offspring if he or she marries a normal person and it inherits in every generation in affected person’s family. For example: Huntington’s disease is a neurodegenerative genetic disease that affects muscle coordination.
In autosomal recessive, affected gene allele is recessive. Both copies of allele must be recessive for a person to be affected by the disease.An affected person usually has unaffected parents who each carry a single copy of the mutated gene. For example: Albinism disease which is characterized by the complete or partial absence of the pigment in the skin, hairs and eyes.
2. Sex-chromosome linked disorder: here the affected gene is located on the sex chromosome. Inheritance of this genetic disorder depends upon sex of the affected person
Pedigree Analysis: Studies of Inheritance of Genetic Diseases in Humans
The inheritance pattern of different genetic diseases can be studied by pedigree analysis. It involves collection of information about the family’s history for a particular genetic trait. Then the expression of trait is represented into a family tree (also known as Pedigree tree).
In a pedigree, squares symbolize males, and circles represent females. A horizontal line joining a male and female indicates the couple. Vertical lines indicate offspring which are listed left to right, in order of birth. Shading of the circle or square indicates an individual who has the trait being traced.
Mutations
Mutations are changes in the genetic material of a cell (or a virus). If a point mutation occurs in a gamete, or in a cell that gives rise to gametes, it may be transmitted to offspring and to a succession of future generations. If the mutation has an adverse effect on the phenotype of a human or other animal, the mutant condition is referred to as a genetic disorder, or hereditary disease.
Application activity 13.6 1.
In humans, Huntington’s disease is caused by a dominant, mutant gene. Draw a genetic diagram to show the possible genotypes and phenotypes of the offspring produced by a man with one allele for the disease and a woman who does not suffer from the disease.
2. The diagram shows a family tree for a condition known as polydactyly
a. State whether polydactyly is controlled by a dominant or a recessive allele.
b. Explain which evidence from the family tree confirms your answer to (a).
c. Explain what the chances are for a third child of parents 2 and 3 having polydactyly.
You may use a genetic diagram to help your explanation.
13.7 Types of mutations
Activity 13.7
The mutations can occur in a gene or on a chromosome. Mutations can also occur in somatic cells and in germinal cells. What is the meaning of gene, chromosome, somatic cell and germinal cell.
Mutations can broadly be categorized into two types: gene mutations and chromosomal mutations.
13.7.1 Gene mutations (point mutations)
Mutations within a gene can be divided into three general categories: base-pair substitutions and base-pair insertions and deletions.
a. Substitution
A base-pair substitution is the replacement of one nucleotide and its partner in the complementary DNA strand with another pair of nucleotides. One purine replaced by another purine or pyrimidine replaced by another pyrimidine is called transition. However, pyrimidine replacing purine or purine replacing pyrimidine is called transversion. Some substitutions are called silent mutations because, due to the redundancy of the genetic code, they have no effect on the encoded protein. In other words, a change in a base pair may transform one codon into another that is translated into the same amino acid. For example, if CCG mutated to CCA, the mRNA codon that used to be GGC would be GGU, and a glycine would still be inserted at the proper location in the protein.
Substitution mutations are usually missense mutation; That is the altered codon still codes for an amino acid and thus makes a sense, although not necessarily the right sense. But if a point mutation changes a codon for an amino acid into a stop codon, translation will be terminated prematurely, and the resulting polypeptide will be shorter than the polypeptide encoded by the normal gene. Alterations that change an amino acid codon to a stop codon are called nonsense mutations, and nearly all nonsense mutations lead to nonfunctional proteins.
Table 17.4: Point mutations
b. Insertions and deletions
Insertions and deletions are additions or losses of one or more nucleotide pairs in a gene. These mutations have a disastrous effect on the resulting protein more often than substitutions do. Because mRNA is read as a series of nucleotide triplets during translation, the insertion or deletion of nucleotides may alter the reading frame (triplet grouping) of the genetic message.
Such a mutation called a frameshift mutation will occur whenever the number of nucleotides inserted or deleted is not a multiple of 3. All the nucleotides that are downstream of the deletion or insertion will be improperly grouped into codons, and the result will be the extensive missense ending sooner or later in nonsense premature termination.
13.7.2. Large-scale mutations in chromosomal structure (chromosomal mutations
Physical and chemical disturbances, as well as errors during meiosis, can damage chromosomes in major ways or alter their number in a cell. Here, we survey these chromosomal alterations and see how this information applies to some important disorders in humans.
a. Alterations of chromosome number: aneuploidy and polyploidy
Ideally, the meiotic spindle distributes chromosomes to daughter cells without error. But there is an occasional mishap, called a nondisjunction, in which the members of a pair of homologous chromosomes do not move apart properly during meiosis I, or in which sister chromatids fail to separate during meiosis II. In these cases, one gamete receives two of the same type of chromosomes and another gamete receives no copy. The other chromosomes are distributed normally. If either of the aberrant gametes unites with a normal one at fertilization, the offspring will have an abnormal chromosome number, known as aneuploidy. If the chromosome is present in triplicate in the fertilized egg (so the cell has a total of 2n+1 chromosomes), the aneuploid cell is said to be trisomic for that chromosome. If a chromosome is missing (so the cell has 2n-1 chromosomes), the aneuploid cell is monosomic for that chromosome. Mitosis will subsequently transmit the anomally to all embryonic cells.
If the organism survives, it usually has a set of symptoms caused by the abnormal dose of genes located on the extra or the missing chromosome.
Some disorders caused by the nondisjunction of chromosomes
• Trisomy 21: Down syndrome
One of the most common chromosome abnormalities is Down syndrome, due to nondisjunction of chromosome 21 resulting in an extra complete chromosome 21, or part of chromosome 21. Down syndrome is the only autosomal trisomy where an affected individual may survive to adulthood. Individuals with Down syndrome often have some degree of mental retardation, some impairment of physical growth, and a specific facial appearance. With proper assistance, individuals with Down syndrome can become successful, contributing members of society. The incidence of Down syndrome increases with maternal age.
• Abnormal numbers of sex chromosomes
Sex-chromosome abnormalities may be caused by nondisjunction of one or more sex chromosomes. Many conditions are known in which there are an abnormal number of sex chromosomes.
An X chromosome may be missing (XO), or there may be an extra one (XXX or XXY). There may also be an extra Y chromosome (XYY). Any combination of X and Y chromosomes, as long as there is a Y chromosome, will produce a male (up to XXXXY). These individuals can lead relatively normal lives, but they cannot have children. They may also have some degree of mental retardation. These syndromes include Klinefelter’s syndrome, Turner syndrome and trisomy X.
• Klinefelter’s syndrome is caused by the presence of one or more extra copies of the X chromosome in a male’s cells. Extra genetic material from the X chromosome interferes with male sexual development, preventing the testicles from functioning normally and reducing the levels of testosterone.
• Triple X syndrome (trisomy X) results from an extra copy of the X chromosome in each of a female’s cells. Females with trisomy X have a lower IQ than their siblings.
• Turner syndrome results when each of a female’s cells has one normal X chromosome and the other sex chromosome is missing or altered. The missing genetic material affects development and causes the characteristic features of the condition, including short stature and infertility.
Some organisms have more than two complete chromosome sets. The general term for this chromosomal alteration is polyploidy, with the specific terms triploidy (3n) and tetraploidy (4n) indicating three or four chromosomal sets respectively.
b. Alterations of chromosome structure
Breakage of a chromosome can lead to four types of changes:
• A deletion removes a chromosomal segment.
• A duplication repeats a segment.
• An inversion reverses a segment within a chromosome
• A translocation moves a segment from one chromosome to another nonhomologous one. The most common type of translocation is reciprocal, in which nonhomologous chromosomes exchange fragments. Nonreciprocal translocations, in which a chromosome transfers a fragment without receiving a fragment in return, also occur.
Somatic mutations occur in the body of an organism. Such mutations are passed on only to cells that come from the original mutant cell. They are never passed on to offspring. Germ mutations occur in the reproductive cells of an organism. Such mutations can be passed on to offspring.
While mutations can occur spontaneously, some can be caused by exposure to physical or chemical agents in the environment called mutagens. Common environmental mutagens include ultraviolet rays from the sun and various chemicals, such as asbestos, cigarette smoke, and nitrous acid. High-energy radiation, such as medical X rays, can cause DNA strands to break, leading to the deletion of potentially important genetic information.
Application activity 13.7
1. Suggest why:
a. A mutation in which one nucleotide of a triplet code is altered often makes no difference to the protein molecule coded by the DNA.
b. The addition or deletion of three nucleotides in the DNA sequence of a gene often has less effect on the encoded protein than the addition or deletion of a single nucleotide.
13.8 Causes, effects and significance of mutations
Activity 13.8
Nowadays there are different cases of antibiotic resistance, insecticide resistance and herbicide resistance. Antibiotics are not killing the bacteria that they used to kill. Insecticides are not killing the insects that they used to kill. Can you identify a reason for this case of resistance? Is this resistance good or bad?
13.8.1 Causes of mutations
Have you ever wondered for the causes of variation? Sometimes we say its spontaneous or sometimes we say don’t stand in sunlight for so long, or Nuclear weapons or World War II has prolonged mutagenic effect on the victims or don’t take particular medicine, it might be mutagenic. So what could be the causes of mutation? Discuss with your friends.
i. Random mutations can occur spontaneously due to chance as:
a. DNA replication errors
• Normally each base exists in its more stable keto form and is responsible for the normal Watson-Crick base pairing of T with A and C with G. However, under certain physiological conditions, rare imino and enol forms (tautomers) of the bases are present, leading to altered base pairing affinities.
• If by chance, there is looping out of DNA from the template strand, it may be missed by DNA polymerase, resulting in deletion mutation. Similarly, if additional untemplated base is synthesised by DNA polymerase, addition mutation results.
b. Spontaneous chemical changes include depurination and deamination
• When bond breaks between the base and the deoxyribose sugar, purine is removed from the DNA, resulting in an apurinic site. Thousands of purines are lost in each mammalian cell cycle. If these apurinic sites are not repaired, DNA polymerase will not be able to add a complementary base and will dissociate from the DNA. Induced mutation happens due to mutagens (agents that induce mutations). It can be physical mutagens or chemical mutagens.
13.8.2 Effects of mutations on phenotypes
Spontaneous or induced mutagens cause changes in genotype which influences the phenotype. The phenotype can be physiological, morphological, biochemical, anatomical etc. So let’s think of effect of mutation on phenotype.
A gene represents the smallest unit that can code for protein. Gene is made up of DNA consisting of four nucleotides present in a particular sequence, which, when read in triplet codons, code for a particular amino acid sequence of a protein. Proteins play a number of important roles in the body, such as enzymes, hormones, structural etc. Whenever nucleotide sequence in DNA changes, it can lead to alteration in amino acid sequence affecting the function of the protein. For example: Albinism is caused by an autosomal recessive mutation. Tyrosine is converted to DOPA by the enzyme tyrosinase and DOPA is converted to melanin, the pigment which gives color to the skin. Melanin absorbs light in the ultraviolet (UV) range and protects the skin against UV radiation from the sun. If a mutation occurs in the gene responsible for production of tyrosinase, tyrosine cannot be converted to DOPA and melanin cannot be produced. Therefore, people with such a mutation have white skin, white hair and red eyes and are very sensitive to light.
13.8.3 Significance of mutations
Mutations can be harmful, beneficial, or have no effect. If a mutation does not change the amino acid sequence in a protein, the mutation will have no effect. In fact, the overwhelming majority of mutations have no significant effect, since DNA repair mechanisms are able to mend most of the changes before they become permanent. Furthermore, many organisms have mechanisms for eliminating otherwise permanently mutated somatic cells.
• Harmful mutations
Mutations can cause result in errors in protein sequence, creating partially or completely non-functional proteins. These can obviously result in harm to the cell and organism. As discussed in the previous lesson, to function correctly and maintain homeostasis, each cell depends on thousands of proteins to all work together to perform the functions of the cell. When a mutation alters a protein that plays a critical role in the cell, the tissue, organ, or organ system may not function properly, resulting in a medical condition. A condition caused by mutations in one or more genes is called a genetic disorder, which will be discussed in the next chapter.
However, only a small percentage of mutations cause genetic disorders; most have no impact on health. If a mutation does not change the protein sequence or structure, resulting in the same function, it will have no effect on the cell. Often, these mutations are repaired by the DNA repair system of the cell. Each cell has a number of pathways through which enzymes recognize and repair mistakes in DNA. Because DNA can be damaged or mutated in many ways, the process of DNA repair is an important way in which the cell protects itself to maintain proper function.
A mutation present in a germ cell can be passed to the next generation. If the zygote contains the mutation, every cell in the resulting organism will have that mutation. If the mutation results in a disease phenotype, the mutation causes what is called a hereditary disease. On the other hand, a mutation that is present in a somatic cell of an organism will be present in all descendants of that cell. If the mutation is present in a gene that is not used in that cell type, the mutation may have no effect. On the other hand, the mutation may lead to a serious medical condition such as cancer.
Beneficial mutations
A very small percentage of all mutations actually have a positive effect. These mutations lead to new versions of proteins that help an organism and its future generations better adapt to changes in their environment. The genetic diversity that results from mutations is essential for evolution to occur. Without genetic diversity, each individual of a species would be the same, and no one particular individual would have an advantage over another. Adaptation and evolution would not be possible. Beneficial mutations lead to the survival of the individual best fit to the current environment, which results in evolution.
Application activity 13.8
End unit assessment 13 I.
Choose whether the given statements are True (T) or False (F)
1. Mutations can broadly be categorized as somatic and germ-line, depending on whether mutation occurs in a somatic cell or gamete.
2. When breaks occur in chromosomes, their structures do not change.
3. Induced mutation happens due to mutagens (agents that induce mutations).
4. Removal of amino group from a base is called deamination.
5. Albinism is caused by an autosomal recessive mutation.
6. Haemophilia A and Haemophilia B are a result of mutations in different genes.
7. There is no interaction between genotype and environment that determines the phenotype shown by any individual.
8. Sickle cell anaemia is due to a dominant sex-linked allele.
9. Mutagens are DNA sequences which get changed due to radiations and chemicals.
10. Mutation has important role in bacterial resistance to antibiotics.
II. Multiple choice questions
1. A point mutation that changes a codon specifying an amino acid into a stop codon is called
a. missense mutation b. nonsense mutation
c. Frame shift mutation d. silent mutation
2. Sickle cell anaemia results because of
a. deletion mutation b. insertion mutation
b. Substitution mutation d. chromosomal mutation
3. Which of the following is not ionising radiation
a. X rays c. UV rays
b. cosmic rays d. alpha rays
4. Which of the following chemicals can affect non-replicating DNA?
a. nitrous acid b. Acridine dyes
c. Bromouracil d. None of the above
5. Phenotype of individual depends upon
III. Long answer type questions
1. Mutations can broadly be categorized as somatic and germ-line, depending on whether mutation occurs in a somatic cell or gamete.
2. When breaks occur in chromosomes, their structures do not change.
3. Induced mutation happens due to mutagens (agents that induce
4. Describe the types of mutation and causes of mutations.
5. Explain the significance of mutations.
6. Explain that gene mutation occurs by substitution, deletion, inversion and insertion of base pairs in DNA. Outline how such mutations may affect the phenotype.
7. Answer the following question on genetics
a. Define the words below
i. Allele
ii. Locus
iii. Autosome
iv. Homologous chromosome
b. State and explain the laws of Mendel.
c. Some coat colours in cats are sex linked. Black coat colour is codominant to ginger. A cat that has one allele for black and one for ginger is tortoiseshell. The gene for this coat colour is carried on the X chromosome. Describe the genotype and phenotype of the offspring of a cross between a pure breeding black female cat and a ginger male cat.
8. In an experiment, a homozygous tomato plant with a purple hairy stem was crossed with a homozygous tomato with a green, hairless stem. Both purple and hairy are dominant. The F1 plants were allowed to self pollinate to produce an F2. The F2 seeds were planted and the resulting phenotypes are shown below:
Purple, hairy stem 150
Purple, hairless stem 48
Green, hairy stem 15
Green, hairless stem 15
a. What is the ratio of phenotypes in the F2?
b. What was the expected ratio of phenotypes? Why?
c. Why do you think there is a difference between the observed and expected results?
d. What could be the results if the two genes were linked?
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