There are qualities that flow - CONTINUOUS variation. Different phenotypic classes are small so that classes aren't sharply noticeable or instantly obvious. For instance, in the population not everybody is same height; there appears to be somebody in every possible position from shortest to tallest. Weight and skin color are other characters that also show continuous variation. Differences between different classes of such genetically determined traits is therefore, described
Evidence from the number of experiments on quantitative inheritance illustrate that more than one gene is involved in phenotype which is produced. One can hence talk of the trait controlled by multiple genes or polygenes. Latter term is utilized more widely. Roles of alleles at different loci in production of phenotype is such that, on the simplified basis, one can distinguish two kinds of alleles at each locus - CONTRIBUTING and NONCONTRIBUTING alleles. These are only terms adopted for ease of description as it is extremely unlikely that allele has no effect. Therefore genotype with only non-controlling alleles still has basis phenotype. To facilitate study of quantitative inheritance some simplifying suppositions have to be made. Some assumptions are:
1) The effect of each contributing alleles on phenotype is equal to effect of any other contributing allele. Furthermore, effect is additionally to base or minimum value. Therefore we may say that each allele contributing to the height adds 2.5 centimeters to minimum height of one hundred and fifty centimeters.
2) Effects of contributing alleles are additive. Hence, if we continued with example in No. 1, we could consider the genotype having six contributing alleles. In this case total effect of contributing alleles would be 15 centimeters (i.e. 6 x 2.5) that would be added to minimum height. Therefore total height would be 165 centimeters (i.e. 150 + 15).
3) At any locus one alleles doesn't obscure effect of other alleles and effect of homozygosity for two contributing alleles is greater than that of heterozygosity for one contributing and one non-contributing allele. However when there is dominance, homozygotes and heterozygotes have identical phenotypes. Assumption of contributing and non-contributing alleles is also different from intermediate and codominance as in both of these cases and in dominance other alleles has a distinctive phenotype. Non-contributing allele isn't ascribed specific phenotype.
One significant result of first three assumptions is that different genotypes will have same phenotype is total number of contributing alleles is same in all genotypes.
4. There is no epistasis. It is essential to suppose this condition in which genotype at one locus doesn't mask effect of genotype at another locus as if that were not case it would be difficult to allot the value for effect of each allele. In absence of epistasis there is direct correspondence between genotype and phenotype.
5. There is no linkage among loci controlling the trait. It is one of the factors that modify genotypic and phenotypic ratios and altered ratios are hard to calculate.
6. Environment has no effect on genotype. Or we can say phenotype is completely attributable to genotype. Certainly this supposition is great simplification of true situation.
First important breakthrough on problem of quantitative inheritance was by Nilsson-Ehle in 1909. He worked on color of wheat kernel. F1 from the cross between pure-breeding dark red-kerneled and white-kneneled parents were all of the intermediate red color. This kind of F1 from such cross is constantly with incomplete dominance. F2 generation, though, forces rejection of that hypothesis. In F2 generation, one sixteenth (1/16) of progeny has same dark red color of Pgeneration parents. Same was true for white progeny in F2. Fact that fractions of different classes of progeny are in sixteenths, mean that trait is handled by two loci. Nilsson- Ehle attained five phenotype classes among F2, instead of four. Presence of 5 phenotypic classes was also different from what is attained when other kinds of genetic interactions are operative.
As parents were pure breeding dark red parent would be A1A1B1B1 and white would be A2A2B2B2. Rationale for characterizing dark red parent as being homzygos for only contributing alleles is that, color indicates presence of maximum amount of red pigment whereas white is because of total absence of red pigment. Put differently maximum number of contributing alleles any offspring can have is four if there are only 2 loci involved and four of such alleles would generate maximum amount of red pigment.
As contributing alleles have equal effects that are also additive, F1 with only two contributing alleles would have half as much red pigment as parent. Therefore its phenotype would be intermediate red. F1 is heterozygous at both loci (doubly heterozygous) and independent assortment in gamete production would yield gaments with 2, 1 or 0 contributing alleles Random Fertilization would in turn give F2 progeny with 4,3,2, 1 or 0 contributing alleles. Phenotypes corresponding, would be different. Thus, there would be five classes of F2 as Nilsson-Ehle observed.
It is evaluated that skin color in man is determined by the minimum of three and maximum of six additive loci. If four loci are involved with A1, B1, C1 and D1 alleles contributing to pigment production while A2, B2, C2 and D2 are non-contributing, marriage between pure black and pure white would generate mulatto children with intermediate skin color. Their genotype in this particular cross would be A1A2B1B2 C1C2D1D2. Marriage between two mulattoes would be capable of producing whole spectrum of skin colors. This is due to both parents can produce gametes that have only contributing (i.e. A1B1C1D1) or non-contributing (i.e. A2B2C2D2) alleles additionally to other gametic genotypes. Though discussion of polygenic inheritance so far has illustrated it as condition in which there is continuum of phenotypes that is not always case. There are polygenic features for which only two phenotypes are generated. For example, in the guinea pig normal hind foot has only three toes, but there are some strains having four toes. Crosses between pure breeding three-toed and four-toed strains generated F1 that are almost all three-toed. F2 from this type of cross comprises of three-toed and four-toed individuals in ratio of approximately 3 three-toed: 1 four-toed.
Crosess with F2 though don't support one locus hypothesis. Instead it is more rational to suppose that there are about four additive loci involved, but that is not all. To have fur toes there should be at least about 5 contributing alleles present. If there are less than 5 contributing alleles then only 3 toes develop, but if there are 5 or more, 4 toes develop. There is threshold or level that should be reached before there would be growth of four toes. Any number of contributing alleles below the threshold produces only three toes. The significant point to remember is that alleles are additive. Effects of different alleles should be added to reach threshold at which effect, in this case development of four toes, becomes visible. Equally significant is fact that once threshold is reached effect (phenotype) is same regardless of number of contributing alleles present.
According to hypothesis to account or three-and four-toed phenotypes if A1, B1, C1 and D1 are contributing and A2, B2, C2 and D2 are not, then parents who are A1A1B1B1C1C1D2D2 would be pure-breeding for four-toes. Same would be true of A1A1B1B1C1C1D1D2 parents but of A1A1B1B1C1C2D1D2 parents.
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