Polygenes in discontinuous traits

• The relationships between polygenes and the expression of discontinuous characters comes out through the establishment of ‘threshold’ i.e. polygenically determined genotypes that have the values below the threshold shows no expression of the characters.
• Expression only occurs when the genotype have values above the threshold.
• Thus, there are two scales involved:

a. the underlying polygenic distribution which is continuous

b. the superficial phenotypic distribution which is discontinuous

• The relationships between these two scales was first demonstrated by Wright in crosses between strains of guinea pigs that differ from each other in number of toes on hind legs.

i.e. Guinea pigs : Three toed x four toes = three toed

In F2 generation, 188T:45F

• Since, the ratio didn’t differ too greatly from 3:1 ratio, it seemed as though these crosses were segregating for only a single gene difference in F2 with T allele dominant to F allele.
• This view seemed to be supported by observation of back crosses.

i. First back crosses (BC₁) of F1 progeny to recessive stock i.e. four toed which produces three toed and four toed in equal proportion.

i.e. BC₁ = F1 x four toed (recessive)

ii. Second back crosses (BC₂) was made between heterozygote three toed of BC₁ with recessive stock ( Four toed) which result into 77% four toed and 28% three toed offspring

i.e. BC₂ = BC₁ x four toed (recessive)

• As a result, approximately four pairs of polygenes or eight alleles were involved in polydactyl.
• Individuals which had about 5 or more polydactyl alleles out of eight has exceeded the threshold and appeared as four toed.