GENETIC BASIS OF INHERITANCE

Transcription

1 GENETIC BASIS OF INHERITANCE. What is meant by like begets like? Ans. Every living organism reproduces to form offsprings which resemble to Themselves: 2. Thus offsprings are of their own kind. It is called like begets like. 3. Eg. Cat will always produce kittens and dogs will produces pups. 4. Pea seeds will germinate into pea Plant and mango seeds will grow Into mango plant. 2. Why Mendel is called Father of Genetics? Ans.. In 866, first scientific explanation of inheritance was given by Gregor John Mendel. 2. He performed experiments on garden pea in a very scientific manner and proposed postulates or principles which are now popularly known as Mendel s law s of inheritance. 3. As Mendel laid down the foundations of genetics and therefore is he known as Father of genetics. 3. What is the theory of blending inheritance? Ans.. Theory of blending inheritance says that there occurs mixing of characters of parents during inheritance in the offsprings. 2. Thus offsprings show intermediate characters. 3. This theory was believed till 9 th century as it cannot explain that in some cases offsprings resemble only one of the parents. E.g. Offsprings of a black and a white horse are not grey. 4. Theory also can not explain that sometimes offsprings resemble their grand parents or uncle or aunt and not their parents. 4. Why Mendel selected garden pea for his experiments? Ans. Mendel selected garden pea (Pisum sativum L) for his experiments because of following characters.. Garden pea is an annual plant and completes the life cycle within 3-4 months. Due to the short life span he could grow 3 generations in a year. 2. It is a small herbaceous plant that produces many seeds and so he could grow thousands of pea plants in a small plot. 3. It is naturally self pollinating and it is available as many varieties with contrasting characters. There were no intermediate characters. 4. Though flowers are naturally self pollinating but can undergo artificial cross pollinating and produce fertile offsprings. 5. Flowers are large enough for easy emasculation. 5. Enlist seven traits in pea selected by Mendel. Ans. In garden pea Mendel selected seven contrasting characters as below. Sr.N o. Character Dominant trait Recessive trait. Stem Tall (T) Dwarf (t) height 2. Seed colour Yellow (y) Green (y)

2 3. Seed chape Round (R) Wrinkled (r) 4. Pod Green (G) Yellow (g) Colour 5. Pod shape Inflated (I) Constricte d (i) 6. Flower position Axial (A) Terminal (a) 7. Flower colour Coloured (C) White (c) 6. Give graphic representation of Monohybrid cross. Ans. A cross between two pure parents in which inheritance pattern of only one pair of contrasting characters is studied, it is called monohybrid. Cross. Graphical representation of Monohybrid cross is as follows 7 Explain Law of dominance is not universally applicable. Ans.. According to law of dominance when a character is controlled by a pair of contrasting characters, then in F hybrid only one character is expressed (dominant) and other get suppressed or not expressed (recessive). 2. But there are many cases where the dominance is not complete or absent. 3. In case of incomplete dominance both

3 Alleles of a gene are expressed partially. 4. In case of co-dominance both alleles of a gene are expressed equally. Thus Law of dominance is not universally applicable. 8. Explain why law of segregation is also called Law of purity of gametes. Ans.. Law of segregation states that members of allelic pair in a hybrid remain together without mixing with each other and separate or segregate during gamete formation. 2. Thus gametes receive only one of the two factors and are pure for a given trait. 3. Therefore, this law also known as Law of Purity of gametes. 9. Law of segregation is universally applicable. Explain Ans.. Law of segregation states that gametes receive only one of the two contrasting factors (alleles/genes) and are pure for a given trait. 2. Thus each gamete is haploid (n) with one set of chromosomes. 3. It is true that all sexually producing higher organisms are diploid (2n) and they produce haploid gametes. Chromosomes. 3. But many genes are located on one chromosome i.e. they are linked. Therefore they pass to gametes in form of a linkage group. In this case only parental combinations of characters appear in progeny and not new combinations (recombination) 4. In case of linkage even though recombinations arise in progeny, the only reason is crossing over (not independent assortment). Test cross is a backcross but back cross is not necessarily a test cross. Ans.. Test cross is done to find out the genotype of Tall plant i.e. whether it is TT or Tt. 2. In test cross F hybrid (or F 2 ) is crossed with homozygous recessive parent. 3. Here offspring is crossed back with one of the parents it is also called back cross. 4. But back cross with homozygous dominant parent produces all tall plants and therefore cannot be used to test the genotype. Thus a test cross is a back cross but back cross is not necessarily a test cross. 0. Why law of independent assortment is not universally applicable? Ans.. Law of independent assortment says that appearance of new combinations of characters in dihybrid F 2 generation is due to independent assortment (Distribution) of alleles during gamete Formation. 2. This is possible only if traits (or their Alleles) are present on separate pairs of

4 2. Distinguish between - Homozygous. An Individual possessing similar alleles for a particular trait is called homozygous. 2.It is also called pure condition. 3.Homozygous breeds true to the trait. 4.They produce only one type of gamete. 5.Egs. Homozygous Tall is TT. Incomplete dominance. Here both alleles of a gene express themselves partially 2.F hybrids show intermediate expression of Phenotypes of parents. 3. What are the reasons for Mendel s success? Ans. Persons for Mendel s success could be. Right choice of experimental material garden pea plant, in which all seven pairs of contrasting characters show Complete dominance. 2. He selected seven pairs of contrasting characters which are present on separate pairs of chromosomes. Thus he could give law of independent assortment. 3. He considered one pair of contrasting characters at a time. 4. He done statistical analysis of his experiments. 5. He conducted ample crosses and reciprocal crosses to eliminate chance factor. 4. Explain /Define the terms- Genetic Terminology : Heterozygous. An individual possessing dissimilar alleles for a particular trait is called heterozygous. 2. It is also called a hybrid condition. 3. Heterozygous does not breed true to the trait. 4. They produce two types of gametes. 5. Egs Heterozygous Tall is Tt. Co-dominance. Here both alleles of a gene express themselves equally. 2. F hybrids show both the traits of parents. Character- It is a feature of the organism e.g. Height of stem. Trait An inherited character and its detectable variant e.g. Tall or dwarf. (Some traits can be environmentally determined or modified) Factor It is a unit of heredity, a particle present in the organism which is responsible for the inheritance and expression of a character. (Factor is passed from one generation to the next through gametes and not a character itself.) Gene. It is a particular segment of DNA which is responsible for the inheritance and expression of that character. (The term was coined by Johannsen) Alleles or Allelomorphs- The two (or more) alternative forms of a gene (factor) are called alleles of each other. They occupy identical loci (positions) on

5 homologous chromosomes. (These terms were coined by Bateson) Dominant It is allele that expresses its trait even in the presence of an alternative allele, i.e. in heterozygous condition. (Only dominants express in hybrids.) Recessive It is an allele which is not expressed in the presence of an alternative allele, i.e. in heterozygous condition. It expresses only in the presence of another identical allele. Phenotype- The external appearance of an individual for any trait is called phenotype for that trait. It is observable and is determined by different combinations of alleles. In pea, for height of the stem, Tall and dwarf are the two Phenotypes. (Tall is determined by TT or Tt.) Genotype- It is representation of the genetic constitution of an individual with respect to a single character or a set of characters, e.g. in pea, tall plants can have genotype TT or Tt and dwarf will have tt, Genotype for yellow and round seeds can be YYRR or YyRr and for green wrinkled seeds genotype is yyrr. (Entire genetic constitution of an organism is called genome.) Honozygous- An individual possessing similar alleles for a particular trait is called homozygous or pure for that trait. Homozygous breeds true to the trait and produces only one type of gamete. E.g. parental Tall with TT and dwarf plant with tt is homozygous. Heterozygous- An individual possessing dissimilar alleles for a particular trait is called heterozygous or hybrid for that trait. Heterozygous does not breed true for that trait and produce two types of gametes. E.g. F generation hybrids (Tt.) Pure line An individual or a group of individuals (population) that is homozygous or true breeding for one or more traits. Hybrid- It is heterozygous individual produced from any cross involving pure parents having one or more contrasting traits. Monohybrid- It is heterozygous for one trait and produced in a cross between two pure parents differing in single pair of contrasting characters. E.g. Hybrid tall produced in a cross between pure tall and dwarf parent. F generation- (F stands for filial meaning offsprings produced in sexual reproduction. Latin word filin means a son). The progeny (offsprings) produced from a cross is called first filial or F generation. It shows uniform expression. F 2 generation The second generation (progeny) produced by selfing (interbreeding) of F generation offsprings is called second filial or F2 generation. It shows two or more types of individuals in a particular ratio. Punnett Square/ checker board It is a diagram that is used to show possibilities of combinations in a particular cross or breeding experiment. It is named after. Reginald C. Punnett who devised this approach. It is used by breeders to know all the possible genotypes and phenotypes of offsprings produced in a cross. Homologous chromosomes The morphologically and structurally similar chromosomes present in a diploid cell are called homologous chromosomes or simply homologues.

6 They have identical gene loci (positions) bearing alleles. Long Answer Questions. Describe the steps of procedures of Mendel s experiments with suitable Ans. examples. Mendel selected Garden pea plant (Pisum Sativum) for his experiments. He selected seven pairs of contrasting characters of garden pea plant. He conducted experiments in 3 steps. Step :- Selection of parents and obtaining pure lines :- Mendel selected parents as male and female plants. Then is ensured that selected parents breed true to their selected traits by selfing them for 3 generations. Thus he obtaine pure lines of selected parents, Step 2 : Artificial cross of selected parents to raise F generation. Mendel first emasculated flowers of selected female plant. Then pollens from the flowers of selected male parent were dusted on the stigma of emasculated flowers i.e. artificial cross. He crossed many flowers in this way, then collected seeds and raised the plants which represented F generation hybrid plants. [Emasculation is removal of stamens from the floral bud before anthesis i.e. formation of pollen grains.] Step 3:- Selfing of F hybrids to raise F 2 generation :- Mendel allowed the natural self pollination in each F hybrid. He collected seeds separately and raised F 2 generation. F 3 generation was obtained by selfing of F 2 hybrids. 2. Explain monohybrid cross experiment and monohybrid Ratio. Ans. Monohybrid cross- A cross between two pure homozygous parents in which inheritance pattern of only one pair of contrasting monohybrid cross. Step :- Mendel first selected pure tall pea plant as female parent and pure dwarf pea plant as male parent. He conformed that these parents are true breeding by selfing them for 3 generations. Step 2 : He performed artificial cross, then collected seeds, sowed the seeds and waited for the results. He was expecting that offsprings F hybrids will either be of intermediate height or 50% tall and 50% dward. But he was surprised,but as F hybrids were all tall. To find out answer, he proceeded further. Step 3 : He allowed natural selfing of all the hybrids, collected seeds, sowed the seeds. He found dwarfness reappeared appeared in some plants and Tall and dwarf plants appeared. in 3 : ratio. Graphic representation of monohybrid cross is as follows. A cross between two pure parents in which inheritance pattern of only one pair of contrasting characters is studied, it is called monohybrid. Cross. Graphical representation of monohybrid cross is as follows

7 2.Eg. In garden pea plant for height of plant two contrasting characters are tall and dwarf. When pure Tall plant is crossed with pure dwarf plant, then F hybrid are all Tall. Thus Tall character is dominant and dwarf character which is not expressed or suppressed is called recessive character. It can be represented as follows. F generation : Tt F hybrid Phenotype All Tall 3. State and explain the Law of dominance with suitable example. Ans.. When a cross is made between two pure parents differing in any pair (or pairs) of contrasting characters, then the character that appears in F hybrid is called dominant and one which is suppressed is called recessive. This is called Law of dominance. 4. State and Explain Mendel s second law of inheritance. Ans..Mendel s second law of inheritance is Law of segregation. It states that in a hybrid, two alleles of a character remain together without mixing with each other and separate or segregate during gamete formation. 2. Thus gametes receive only one of the two alleles (factors) and thus gametes are pure for a given trait. One gamete represents one trait only. Therefore, this is also known as Law of Purity of Gametes. 3. Law of segregation also can be explained with the help of monohybrid cross experiment. (a) when pure Tall (TT) pea plant is crossed with pure dwarf (tt) pea plant, in F generation all plants are Tall.

8 (b) F hybrids (Tt) have two factors (T) and (t). These two factors do not mix with each other. As F hybrids undergo selfing, these factors separate during gamete formation and 2 types of gametes are produced (T) and (t) in equal proportion : (c) F 2 generation shows 3 different types of genotypes as (TT), 2 (Tt) and (tt) F 2 generation is (tt). Here these (tt) offsprings receive (t) from both the parents and are dwarf (recessive character) (d) Thus 25% or one out of 4 combinations 4. Significance of second Law is reappearance of recessive trait in F 2 generation. This proves the Law. 5. All sexually reproducing higher organisms are diploid. They produce haploid gametes by meiosis. Therefore Law of segregation is universally applicable. 5. What is dihybrid cross? Explain with suitable example and checker board method. Ans. A cross between two pure (homozygous) parents in which the inheritance pattern of two pairs of contrasting characters is considered simultaneously is called dihybrid cross.mendel followed the same 3 steps which is a classical experimental design. Steps involved in dihybrid cross experiment as follows. Step : Mendel selected 2 Pure pea plants as Female plant = forming yellow and Round seeds male plant = forming Green and wrinkled seeds. He obtained purelines by selfing them for 3 generations. Step 2 : He performed artificial cross pollination by dusting pollen grains from pure male plant on to the stigma of emasculated pure female plant. He collected pods and obtained seeds of F generation plants. He observed that all seeds of F hybrids are yellow and Round, thus they are dominant traits while green and wrinkled seeds are recessive trait. Step 3 :- He raised the F plants and allowed selfing in them. He opened the pods and collected the seeds of F2 generation. He was expecting 75% yellow round and 25% green wrinkled seeds. But he was surprised to find that seeds were of 4 types in 9 : 3 : 3 : proportion, as follows- Yellow Round (9), yellow wrinkled (3) Green Round (3), Green wrinkled () Out of the 4 different types, two were parental combinations (yellow rond, green wrinkled) and two were new or recombination (yellow wrinkled, green round) Dihybrid cross can be graphically represented as follows

9 6. What is law of Independent Assortment Explain with suitable example. Ans.. Law of independent assortment states that when the two homozygous parents differing in two pairs of contrasting characters are crossed, then the inheritance of one pair is independent of other. 2. Thus when a dihybrid (or polyhybrid) forms gametes, assortment (distribution) of alletes of different traits is independent of their original combinations in the parents. 3. In dihybrid cross appearance of new combinations in F 2 generation proves the law. 4. Explaination of Law with the help of Dihybrid cross as follows 5. In dihybrid cross pure dominant yellow round seed (YYRR) pea plant is crossed with pure recessive green wrinkled seed (yyrr) pea plant. As plants are homozygous they produce only one type of gametes i.e. (YR) and (yr) respectively. 6. All F dihybrid plants are heterozygous for both the traits and with yellow and round seeds. This is due to Law of dominance. 7. F hybrids are allowed to undergo selfing. During gamete formation by dihybrid, alleles in both the pair separate due to law of segregation. Thus each magete receives only one allele from each pair. 8. The assortment (distribution) of alleles of different traits is totally independent of their parental combinations. Thus each allele of any one pair is free to enter the gamete with any allele from remaining pair of alleles. 9. F 2 gametes are of & types as YR, Yr, yr, yr. It indicates that a gamete that receives Y for colour may receive R or r for shape. This results in the formation of YR and Yr types of gametes. Similarly a gamete that receives y for colour may received R or r for shape. This results in the formation of yr and yr types of gametes. Thus F dihybrid produces four different types of gametes in equal proportion (each 25%) 0. During selfing of F generation 4 types of male gametes undergo random fusion to form 6 combinations which has

10 9 types of genotypes and 4 types of phenotypes.. Yellow round (9), yellow wrinkled (3), green round (3), green wrinkled (), Out of 4 different type of phenotypes, two and two are new or recombination s i.e. green round and yellow wrinkled. 2. The appearance of recombination s is due to law of independent assortment 7. What is test cross? Explain the significance of test cross. Ans.. Test cross is a cross between plant with unknown genotype for a particular trait with a recessive plant for that trait. 2. It is a simple method devised by Mendel to verify/test the genotype of F (orf 2 /F 3 ) 3. If a plant has recessive trait for eg. dwarf, its genotype is definite i.e. tt. But if plant is Tall i.e. dominant trait, then its genotype may be either homozygous TT or heterozygous Tt. Therefore to find out the genotype of dominant trait is homozygous or heterozygous test cross in useful. 4. In test cross F hybrid (or F 2 ) in crossed with homozygous recessive parent (back cross) 5. Monohybrid test cross is as follows: Test cross:- F recessive dwarf parent Genotype:- Tt tt Gametes : - Progeny : - TT Tall tt Dwarf Thus Tall (dominant) : dwarf (recessive) : This is test cross, ratio. 6. When any dominant trait is crossed with recessive trait and ratio obtained is : for dominant : recessive, then the dominant trait is heterozygous 7. Another significance of test cross in that it is also used to introduce useful recessive trait in the hybrids for rapid crop improvement programmers. It is more popular with plant breeder for its simple : ratio. 8. Explain incomplete dominance. Ans.. When both alleles of a gene express themselves partially so that one allele can not suppress the expression of other completely, it is called incomplete dominance. 2. Thus allelic pair is not as one dominant and other recessive. In such cases there is intermediate expression in F hybrids. 3. Eg. In four O clock plant (Mirabilis jalapa) pure Red flower plant (RR) is crossed with pure white flower plant (rr), the F hybrids bear pink (Rr) flowers and not Red flowers. 4. On selfing F hybrids, the F generation 2 shows red (RR), pink (Rr) and white (rr) in : 2 : ratio. 5. Thus phenotypic and genotypic ratio is same. This means factors segregate and there is no mixing of the factors 6. The intermediate shade (pink) is produced due to incomplete dominance. 7. In complete inheritance does not favour blending theory of inheritance. Though in F all are pink, both the parental traits red

11 and white reappear each in 25% in F 2 generations. Incomplete dominance can be expressed as follows (eg. Snapdragon) one with red coat (skin with red colour hair) and other with white coat (with white hair) 4. When red cattle (RR) are crossed with white cattle (WW), F hybrids (RW) have roan colour. Roans have the mixture of red and white colour hair. Thus both the traits are expressed (co-dominance). 5. On interbreeding of roans, in F 2 generation red (RR), roans (RW) and white (WW) are produced in ratio : 2 :. Thus in co-dominance genotypic and phenotypic ratio are identical 6. Co-dominance can be represented as follows eg cattle coat colour P generation : Pure parents Phenotype : Red White Genotype : RR WW Gametes : F generation : RW Roan Coat Colour 9. Explain Co-dominance Ans.. When both alleles of a gene are expressed equally in F generation, it is called co-dominance. 2. Such alleles which are able to express themselves independently even when present together in hybrids are called codominant alleles. 3. Classical example of co-dominance in coat colour in cattle. There are 2 types F gametes 2 F generation 2 Gametes RR Red RW Roan RW Roan ww White Phenotypic ratio Red : Roan : White : 2 : Genotypic ratio RR : RW : WW : 2 : 0. Multiple alleles Explain. Ans.. Multiple alleles are more than 2 alterative forms of alleles of a gene in a population occupying same gene locus.

12 2. An individual carries any two alleles of a gene. 3. Multiple alleles arise by mutations of the wild type of gene. A gene can mutate to form several types of alternative forms. These alleles may show dominance recessive or incomplete dominance or codominance. 4. Example of multiple alleles is ABO blood groups in human beings. 5. Gene I contorts ABO blood groups. It A B has three alleles I, I and i. A B 6. The alleles I and I are responsible for A and B antigen while i does not produce any of these antigens. Blood group type can be determined by antigen types present in Blood. 7. Each person possesses any two of the A B three I gene alleles. I and I are codominants and are completely dominant over i. There are six different genotypes and only four different phenotypes i.e. blood groups as follows I I I i i A B A Genotype A I or B A I i B I or I i I B Phenotype (blood group) A B A B O 0. Explain Pleiotropy : - Ans.. When a single gene controls two (or more) different traits it is called pleiotropic gene and this phenomenon is called pleiotropy or pleiotropism. The phenotypic ratio is 2 :, instead of 3 :. 2. Mendels factor (gene) controls one trait but pleiotropic gene produces multiple phenotypic expressions because it inffuences a number of characters simultaneously. 3. For eg the disease sickle cell anaemia is S called by a gene Hb. Normal (healthy) A gene Hb is dominant. The heterozygous A S carriers Hb Hb show mild symptoms of anaemia in which their RBCs become sickle shaped when there is oxygen deficiency. 4. Such heterozygous carriers are said to have sickle cell trait and they are normal in normal conditions. 5. The individual with recessive S S homozygous Hb Hb condition die due to fatal anaemia. 6. Thus two different expressions i.e. sickle cell trait (heterozygous) and lethal (death) (homozygous), are produced by a S single gene ( Hb ) in dictates pleiotropy. 7. If marriage occurs between two carriers, children formed will be normal, carriers and sickle cell anemic in : 2 : ratio. Sickle cell anemic die leaving carriers and normal in the ratio 2:. Such marriage of carriers should be avoided. Carriers can be detected by microscopic examination of blood for sickle shaped RBCs. 8. Graphic representations of pleiotropy is as follows. Parents :- phenotype : sickle cell carrier x sickle cell genotype : A Hb S Hb carrier

13 . Explain Polygenic (Quantitative) inheritance : - Ans.. In human population, characters such as height, skin colour and intelligence show gradations (continuous variations) in expression and not the usual two contrasting expression. These characters are determined by two or more gene pairs and they have additive or cumulative effect. These genes are called cumulative genes or polygene s or multiple factors. 2. If there are two genes having additive effect (eg. Wheat kernel colour), the ratio is :4 : 6 : 4 : and in case of three such genes (poly genes), the ratio is : 6 : 5 : 20 : 5 : 6 : (eg. Skin colour in human) 3. Example : wheat kernel colour. When red kernel wheat is crossed with white kernel wheat, F hybrids have grain colour intermediate between red and white. In F 2 generation five different phenotypic expression appear in ratio : 4 : 6 : 4 :. 4.Kernel colour in wheat is controlled by 2 pairs of genes ; Aa and Bb. Gene A and B Determine the red colour of kernel ; and they are dominant over their recessive alleles a and b, which do not produce red colour pigment. They express white colour if dominant gene is not present. 5. Thus genotype of a parent with red kernel is AABB and that of the parent with white kernel is aabb. F hybrid AaBa is medium red and produces 4 types of gametes on selfing. 6. F 2 generation shows gradations (continuous variations) in expression as each dominant gene produces specific amount of Rd pigment and thus different shades of red colours depends upon the number of dominant genes present (quantitative effect) as shown in Punnett square below. 7. Graphical representation of polygenic inheritance Parents : Wheat varieties Phenotype : Red Kernel x White Kernel Genotype : AABB aabb F generation : AaBb Internediated Red kernel Gametes : F 2 generation : Gametes AB Ab ab ab AB Ab ab ab AABB RED darkest AABb Dark AaBB Dark AaBb medium AABb Dark AAbb mediu m AaBb Mediu Aabb Light AaBB Dark AaBb Medium aabb Medium aabb Light AaBb medium Aabb Light aabb Light aabb White 8. Thus F 2 generation has five different phenotypic expressions in the ratio Darkest : Dark : Medium : Light : white : 4 : 6 : 4 : 9. Continuous variations in the expression as follows.. The darkest red = /6 (with 4 dominant genes)

14 2. Medium red (Dark) = 4/6 The amount of melanin synthesised is (with 3 dominant genes) proportional to the number of dominant 3. Intermediate red (medium) = 6/6 (with 2 dominant genes) genes and thus effect of all genes in cumulative (quantitative effect). 4. Light red = 4/6 6. Negro parent has genotype AABBCC (with dominant genes) and that of the albino (white) aa bb cc. 5. White = /6 Genotype of mulattoes (F offsprings) is (without dominant genes) AaBbCc. Mulattoes produce 8 different 0. Explain polygenic inheritance with the help of Human skin colour. types of gametes an 64 combinations are possible in F 2 generation. 7. There are 7 different phenotypes due Ans..In the human population, characters such as height, skin colour and intelligence show gradiations (continuous variations) in expression and the not the only two to the cumulative effect of each dominant gene as follows:-. Pure black (Negro)- 6 dominant genes = /64 contrasting expressions. 2. There characters are determined by two or more gene pairs and they are called 2.Black (less dark than 5 dominant genes = 6/64 3. Less black or brown 4 dominant genes = cumulative genes or polygenes or multiple factors. 3.Example of polygenic inheritance is 5/64 4. Mulattoes (intermediate) - 3 dominant Human skin colour. Marriages between genes = negro and white show intermediate skin 20/64 colour and are called mulattoes. When 5. Fair - 2 dominant gene = 5/64 such mulattoes marry each other, all 6. very fair - dominant gene = 6/64 shades of colour (continuous variation) are observed in the population in the ratio 7.pure white (albino)- No dominate gene =/64 : 6 : 5 : 20 : 5 :6 : 4.Thus human skin colour is controlled by 3 pairs of g enes As, Bb, Cc 5. The amount of melanin in the skin determines the skin colour. Each dominant gene (A,B,C) are of melanin. 8.Graphical representation of a cross showing polygenic inheritance, as follows.

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