57. The allele l in Drosophila is recessive, sex- linked and lethal’ when homozygous or hemizygous. If a female of the genotype Ll is crossed with a male, what is the ratio of females : males in the progeny?
(1) 3 female: 1 male      (2) 2 female: 1 male
(3) 1 female: 1 male      (4) 1 female: 2 male

The ratio of females to males in the progeny when a female Drosophila with genotype Ll (heterozygous for a recessive, sex-linked lethal allele “l”) is crossed with a normal male is 2 females : 1 male. This occurs because males that inherit the lethal allele “l” (hemizygous for “l”) die, while all other genotypes survive.

Explanation of Cross and Genotypes

• The female is Ll (heterozygous, carrier of the lethal allele “l”).

• The male is assumed to be normal (L), having the normal allele on his single X chromosome.

Gametes:

• Female produces two types of X chromosomes: one with “L” and one with “l”.

• Male produces one type of X chromosome with “L”.

Offspring genotypes before lethality:

• Females: LL (normal), Ll (carrier)

• Males: L (normal), l (lethal allele)

Since “l” is recessive and lethal, males with “l” (hemizygous) will not survive. Females can survive if heterozygous (Ll) because they have one normal allele.

Resulting progeny:

• Female LL: survive

• Female Ll: survive (carrier)

• Male L: survive

• Male l: die (lethal)

Thus, viable progeny are 2 females (LL + Ll) and 1 male (L). The ratio is 2 females : 1 male.

Explanation of Options

(1) 3 females : 1 male — Incorrect; does not account for male lethality
(2) 2 females : 1 male — Correct; matches survival after lethal effect
(3) 1 female : 1 male — Incorrect; ignores death of lethal males
(4) 1 female : 2 males — Incorrect; lethal males do not survive

Introduction

In Drosophila genetics, recessive sex-linked lethal alleles cause the death of individuals carrying the lethal allele in homozygous females or hemizygous males. When a heterozygous female (Ll) for such an allele is crossed with a normal male, the progeny sex ratio is altered due to male lethality. This article explains the genetics behind this altered sex ratio and the expected outcome of such a cross.

Progeny Genotypes and Phenotypes

The female parent has two X chromosomes: one normal (L) and one with the lethal allele (l). The male has a normal X chromosome (L). The offspring genotypes include normal females (LL), carrier females (Ll), normal males (L), and males with the lethal allele (l). Males with the lethal allele do not survive.

Expected Progeny Ratio

Taking into account the male lethality caused by the recessive lethal allele, the surviving progeny comprise two types of females (LL and Ll) and one type of male (L). Therefore, the ratio of females to males in the progeny is 2:1.

Understanding the Options

This explains why options suggesting equal males and females or more males than females are incorrect. The lethal effect removes males carrying the lethal allele, skewing the ratio towards females.

This genetic principle is fundamental in Drosophila genetics experiments involving sex-linked lethal alleles.