Question

Please answer the following two questions. Show all your work. Thanks in advance.

9. In Drosophila yellow body color is caused by a sex-linked recessive allele and brown

eye color is an autosomal recessive trait. A phenotypically normal female fly,

heterozygous for both genes, is crossed to a male with normal body color and brown

eyes. What proportion of their offspring should have yellow body color and brown

eyes?

A) 1/16

B) 1/8

C) 1/4

D) 3/16

E) 3/8

ANSWER: C

28. A testcross is performed on an individual to examine three linked genes. The most

frequent phenotypes of the progeny were Abc and aBC, and the least frequent

phenotypes were abc and ABC. What was the genotype of the heterozygous individual

that is test-crossed with the correct order of the three genes?

A) Abc aBC

B) BAC/bac

C) bcA/BCa

D) aBc/AbC

E) bAc/BaC

ANSWER: C

Answer 1

9.As the yellow body colour is said to be due to a sex linked recessive allele and the female is said to be heterozygous for the gene, clearly the gene is X linked and not Y linked (because if it were Y linked, only males will have yellow colour and no question if heterozygous females would have appeared). Now let us represent the allele for normal body colour with X_Y and that for yellow body colour with X_y. Therefore genotype of the female will be X_YX_y. The female is also heterozygous for eye colour, brown eye being the autosomal recessive trait. Let E and e represent the alleles for normal and brown eye colours respectively so the genotype of the female for eye colour would be Ee. Now the male has normal body colour and the gene for body colour being X linked, the male must have only one allele for the body colour gene and that too the dominant normal one. Hence the genotype of the male would be X_YY. Also the male has brown eyes means it is homozygous for the recessive eye colour allele. So it's genotype for eye colour would be ee. Therefore we have a cross between a female with X_YXyEe and a male with X_YYee. Now let us see what will be the genotypes of their progenies:

Progenies with yellow body colour and brown eyes must have a genotype either X_yX_yee (females) or X_yYee (males). We can clearly see that in this particular cross there is no possibility of obtaining X_yX_yee (as we can clearly see in the above diagram) and there are only two X_yYee out of total sixteen possible offspring genotypes giving a ratio of 1:8. So the answer should be B.

28. Linked genes are genes on a single chromosome that are so close to each other that they behave almost like a single unit and are likely to be inherited together as there is very little space between them for cross over between the homologous sets of the chromosome during meiosis. A test cross refers to cross between a heterozygote with dominant phenotype with a homozygous recessive one. Eg a cross between Aa and aa where A is dominant and a is recessive allele and Aa is a heterozygote with dominant phenotype (for A) and aa is homozygous recessive individual. Since the genes A, B and C are said to be linked, the test cross will not give normal Mendelian ratios for trihybrid cross but will behave like a monohybrid cross ie as if not three but only one gene is present. So the phenotype of most of the progenies will be same as it's parents. Since most frequent phenotype of the progeny is Abc and aBC clearly the genotype of the heterozygous parent is also Abc/aBC (gene order being A,B,C) and the very few progeny with phenotype abc and ABC are the ones obtained by recombination between A and B genes.