Question

In chickens, silver and gold feathering is determined by a single pair of alleles as is slow and rapid feather development. True-breeding males that were silver, slow feathered were crossed with true-breeding females that were gold, fast feathered. All F1 were silver, slow feathered. The F2 phenotypes were as follows: 300 male progeny were silver, slow feathered; 120 females were silver, slow feathered; 123 females were gold, rapid feathered 35 females were silver, rapid feathered; and 32 females were gold, slow feathered. 1) Explain the different results obtained for females and males. 2) Are the two genes linked or independently segregating? Explain. 3) On which chromosomes, autosomes or sex chromosomes, are the traits located? Be specific.

Answer 1

In F1 generation, gender is not specified and only appearance was specified. That is, all of them are silver and slow feathered.

In F2 generation 300 male birds and 310 female birds are produced. It explains that 1 : 1 ratio of make and female birds are produced in F2 generation.

Out of which, all 300 male birds are silver and slow feathered.

120 - female are - silver slow feathered

123 - female are - gold rapid feathered

35 - female are - silver rapid feathered

32 - female are - gold slow feathered

Total silver slow feathered - 420

Total gold rapid feathered - 123

Total silver rapid feathered - 35

Total gold slow feathered - 32

Total - - 610

Silver slow : gold rapid : silver rapid : gold slow

420 : 123 : 35 : 32

0.68 : 0.20 : 0.06 : 0.05

It is in deviation with Mendelian dihybrid cross ratio of 9:3:3:1.

It shows that silver and slow alleles are linked and gold and rapid alleles are linked.

Otherwise the resulting offspring would have formed in Mendelian dihybrid cross ratio, but it didn't form. Instead, 7 : 2 : 0.5 : 0.5 ratio was obtained.

The alleles are located on autosomes, because the homozygous combination also is present in female birds for the traits rapid and gold also, if they would have to present on sex chromosomes of the female birds, the homozygous combination would not be possible because female birds have "ZW" sex chromosomes.