(5 marks) An experiment was done to determine the linkage relationship of three genes (a, b, and c are the recessive, mutant genes, a '+' indicates wild-type and dominant for each gene) in Drosophila melanogaster.
Pure-breeding females phenotypically a, b+, c were crossed with pure-breeding males phenotypically a+, b, c+. The F1 females were all wild type in appearance, and the F1 males were all a, b+, c in appearance.
The F1 females and males were each testcrossed with a fly of phenotype a, b, c. When the male F1 was testcrossed, all male offspring were a, b, c, and all female offspring were a, b+, c. When the F1 female was testcrossed, the following phenotypes were seen in all offspring, regardless of sex.
Phenotypes Frequency
a+ b+ c 252
a+ b c+ 248
a b c+ 28
a+ b+ c 32
a b c 167
a+ b+ c+ 173
a b+ c+ 49
a+ b c 51
Total 1,000
Calculate the recombination frequency between each pair of genes. Draw a genetic map showing the order of the genes and the distances between them. Re-write the genotypes of the parents and the F1 with the correct notation. Make sure to consider why the F1 showed different phenotypes for males and females, and why the results of the testcross were different for the F1 males and females. Calculate the interference.
In the following cross, the genes are inherited independently except for D and E, which are tightly linked and show zero recombination:
A/A b/b c/c D/D e/e x a/a B/B C/C d/d E/E.
Re-write the genotypes of the parents using the correct notation to show the arrangement of these genes. Also write the genotype of the F1 with the correct notation.
b. In the F2 (obtained by self-crossing the F1), what proportion of individuals will be:
pure breeding?
heterozygous for all loci?
homozygous recessive for all loci?
A/a B/B C/c D/D e/e?
(Don't worry about the genotype notation here, just calculate the probability of this combination of alleles.