Inheritance of quantitative characters Concept of polygenes
•Colour, sex etc which shows distinct categories are known as qualitative characters.
•They are usually governed by one or major genes or oligogenes.
•Characters like length of ear in corn, yield of grain, yield of milk, stature etc do not fall into clear cut classes and shown more or less continuous variation and are governed by a large number of minor genes called multiple genes or polygenes.
•The characteristic feature of quantitative characters is 1) continuous variation and 2) a marked influence of the environment on their expression.
Multiple factor hypothesis (Nilsson – Ehle 1908)
•He effected crosses between different true breeding strains of wheat with red kernels and with white kernel and the result of F1 and F2 were obtained.
•The F1 was (medium) red In F2 15:1 ratio was obtained.
•Careful examination revealed that the red colour of F1 was not as intene as the red colour of the parent and that in the F2 some red grains were as dark as those of the parent and other only as dark as those of the F1.
•F2 plants revealed marked difference in the intensity of colour depending upon the ratio of dominant and recessive genes present in them.
•Thus he obtained 1:4:6:4:1 ratio for dark red, medium dark red medium red, light red and white.
•It is evident that red colour is due to two pairs of genes.
•Each gene is capable of producing red colour.
•Each is incompletely dominant over white and is cumulative in its effect.
•The intensity of the red color depends upon the number of colour producing genes present.
•Dark red is due the presence of four genes for red medium dark red to three genes, medium red, to two genes and light red to one gene.
•Nilson Ehle from his studies proposed the multiple gene hypothesis for the inheritance of quantitative characters.
•This assumes that there is a series of independent genes for a given quantitative traits.
•Dominance is usually incomplete and there is a strength of expression of the character, whereas its alleles do not posses any effect.
•The F1 essentially uniform but intermediate between the two parents.
•The F2 shows considerable variability, but is intermediate between the two parents.
•The F2 mean value being approximately equal to the parental mean and also the F1 mean.
•Studies on Nicotiana (East and Emerson 1916) worked on quantitative characters to explain the inheritance of corolla length in nicotiana longiflora.
•He crossed two inbred with contrasting corolla length of 40 and 93 mm.
•The F1 was intermediate with mean corolla length of 63 mm.
•In F2 a much larger variation for corolla length than the parents and F1 was observed.
•This variation was continuous as well and the F2 mean was close to that of F1 and intermediate between those of the parent.
•This is precisely what is expected in case of polygenic inheritance.
•The appearance in F2 individuals with higher or lower intensity of characters than the parents is called as transgressive segregation.
•It is produced when the parents have positive alleles of different genes affecting a quantitative traits and segregation of these genes produce two extreme homozygotes in F2, which transgress the parental limit for the character.
•The reappearance of ancestor is called atavism, throw back or reversion.
•The degree of phenotypic expression of a penetrant gene is called
•In other words, the ability of a gene to produce identical phenotypes in all the individuals carrying it in the appropriate genotype is known as incomplete expressivity.
•Many genes have incomplete expressivity, while the wild type (normal) alleles are buffered against such variations.
•The frequency with which a gene produces a phenotypic or visible effect in the individuals, which carry it, is known as penetrance.
•In other words penetrance refers to the proportion of individuals which exhibit phenotypic effect of a specific gene carried by them.
•In general genes express themselves in all the individuals in which they are present in the appropriate genotype is known as penetrance.
•It indicates the number of individuals that give the expected phenotype to any degree.
•Polydactyly is a condition with extra fingers and toe or toes in man is due to the presence of dominant gene P.
•The normal condition is produced by the genotype PP.
•The genotype and pp produce polydactyly. Some heterozygous individual are not polydactly. (Pp).
•Therefore the gene has penetrance of less than 100 per cent and said to be incompletely penetrant.
•A gene though penetrant, may be quite variable in its expression.
•The degree of expression produced by a penetrant genotype is termed expressivity.
•The polydactylous condition may be penetrant in the left hand and not in the right hand or may be penetrant in the feet and not in hands.