91Ó°ÊÓ

A dominant allele H reduces the number of body bristles that Drosophila flies have, giving rise to a "hairless" phenotype. In the homozygous condition, \(H\) is lethal. An independently assorting dominant allele Shas no effect on bristle number except in the presence of \(H,\) in which case a single dose of \(S\) suppresses the hairless phenotype, thus restoring the hairy phenotype. However, \(S\) also is lethal in the homozygous (S/S) condition. a. What ratio of hairy to hairless flies would you find in the live progeny of a cross between two hairy flies both carrying \(H\) in the suppressed condition? b. When the hairless progeny are backcrossed with a parental hairy fly, what phenotypic ratio would you expect to find among their live progeny?

Step by step solution

01

Determine the Genotypes of the Parental Flies

The hairy phenotype in the suppressed condition suggests that the flies have one dose of both the dominant alleles, i.e., H (dominant for hairless) and S (suppressor for hairless) while being heterozygous for both alleles to be viable. Therefore, the genotypes of the hairy parental flies are HhSs.

02

Set up the Punnett Square for the Cross

We perform a dihybrid cross of HhSs x HhSs. The possible gametes of each parent are HS, Hs, hS, and hs.

03

Use the Punnett Square to Determine Offspring Genotypes

From the Punnett square, the offspring genotypes are: 1 HHSS, 2 HHSs, 1 HHss, 2 HhSS, 4 HhSs, 2 Hhss, 1 hhSS, 2 hhSs, 1 hhss.

04

Identify Lethal Genotypes

HH and SS in homozygous forms are lethal. Therefore, genotypes HHSS, HHSs, HHss, HhSS, hhSS are non-viable.

05

Calculate Viable Offspring for Phenotypic Ratio

From viable offspring (HhSs, Hhss, hhSs, hhss), identify: HhSs = Hairy, Hhss = Hairless, hhSs = Hairy, hhss = Hairless. Thus, we get 4 hairy and 2 hairless.

06

Determine Phenotypic Ratio of Hairy to Hairless

The phenotypic ratio of viable offspring is 4 hairy: 2 hairless, simplifying to 2:1.

07

Cross Hairless With Parental Hairy

Hairless (Hhss) with parental hairy (HhSs) results in possible genotypes: HhSs, Hhss, hhSs, hhss for viable combinations.

08

Calculate Phenotypic Ratio for Backcross

For viable offspring, categorize HhSs and hhSs as hairy, Hhss and hhss as hairless, yielding 2 hairy: 2 hairless, simplifying to a ratio of 1:1.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Punnett Square

The Punnett Square is a useful diagram that helps visualize the possible genotypes of offspring from a genetic cross. Imagine it as a simple grid. Each parent's potential gametes are placed along the top and side of the grid. With Drosophila crosses such as HhSs x HhSs, each parent can produce four types of gametes: HS, Hs, hS, and hs. You place these gametes across the grid and systematically fill in the squares to see all possible offspring combinations. This tool makes it clear which genotypes are viable and which phenotypes these will manifest as based on dominance and recessiveness of alleles.

Drosophila Genetics

Drosophila melanogaster, commonly known as the fruit fly, is a vital model organism in genetics. Researchers often choose Drosophila due to its short life cycle and simple genetics. In our exercise, we consider dominant alleles, such as H for 'hairless' and S for 'suppressor'. This cross elucidates the importance of allele interaction within the species. For example, the presence of one S allele can suppress the hairless phenotype caused by H, showcasing how genetic interactions go beyond simple dominance-recessiveness and can lead to interesting phenotypic outcomes.

Allele Interaction

Alleles don't work in isolation. They interact in both expected and unexpected ways. Here, we have two dominant alleles: H, which induces a hairless phenotype, and S, which can suppress this phenotype, restoring hairiness. Such interactions suggest epistatic relationships, where the expression of one gene masks or modifies the expression of a second gene. Additionally, both H and S in a homozygous form (HH or SS) are lethal, illustrating a critical allele relationship where viability is concerned. This gives students insightful examples of how allele interaction complicates genetic predictions compared to simple Mendelian inheritance.

Phenotypic Ratio

Phenotypic ratios reveal the observable characteristics produced by certain genetic crosses. For instance, from a cross between two Drosophila flies with genotypes HhSs, some offspring are non-viable due to lethal allele combinations, impacting the expected ratios. In our example, removing the lethal and non-viable combinations, the phenotypic ratio of hairy to hairless flies is calculated as 2:1. In contrast, backcrossing hairless offspring with a hairy parent changes this ratio. After recalculating, we get an expected phenotypic ratio of 1:1. Understanding these ratios help students predict the distribution of traits within a population based on parental genotypes.