The body color of Drosophila adult flies is regulated by multiple genes, includi

Question

The body color of Drosophila adult flies is regulated by multiple genes, including the autosomal ebony gene. Ebony body (e) is recessive to the wild type (yellowish) body (e+). A second gene, which is X-linked, controls bristle shape. Normal bristle shape (f+) is dominant to the mutant forked bristles (f). A female with forked bristles that is heterozygous for the body color gene is mated with an ebony body male (his bristles are normal).

(a) What are the genotypes of the parents?

(b) Using a Punnett square, determine the expected proportions of each possible progeny phenotype. Since there is a sex-linked gene here, be sure to include consider sex as part of the phenotype.

Explanation / Answer

Ebony body is due to the allele e & yellow body is due to the allele e+; e+ is dominant over e.

Normal bristles is  due to the X-linked allele f+ & forked bristle is due to the allele f; f+ is dominant over f.

In Drosophila, the chromosomal basis of sex determination is due to the ratio of X chromosomes to sets of autosomes ie. the X/A ratio.

Sex determination in drosophila- AAXX : female ( X/A ratio = 1); AAXY : male ( X/A ratio =0.5).

A female with forked bristles that is heterozygous for the body color gene is mated with an ebony body male (his bristles are normal).

Hence genotype of the female drosophila is e+eXfXf (it is heterozygous for autosomal gene that controls body color & has forked bristles, so two X-linked f alleles).

Genotype of the male drosophila is eeXf+Y.

(A) - Genotypes of the female & male parents are respectively e+eXfXf and eeXf+Y.

(B) - The cross can be shown as-

female.....................e+eXfXf...................................X................................eeXf+Y................male

gametes..........e+Xf, eXf.........................................................................eXf+, eY

F1 generation:

Punnett square is drawn:

Gametes

e+Xf

eXf

eXf+

e+eXfXf+   female, yellow body, normal bristle

eeXfXf+ female, ebony body, normal bristle

eY

e+eXfY male, yellow body, forked bristle

eeXfY   male, ebony body, forked bristle

Phenotypic ratio= male with yellow body & forked bristle: male with ebony body & forked bristle: female with yellow body & normal bristle: female with ebony body & normal bristle = 1:1:1:1

The expected proportion of each possible progeny phenotype to occur is 1/4 (equal chance in occurrence).

Gametes

e+Xf

eXf

eXf+

e+eXfXf+   female, yellow body, normal bristle

eeXfXf+ female, ebony body, normal bristle

eY

e+eXfY male, yellow body, forked bristle

eeXfY   male, ebony body, forked bristle

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