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Chapter 6: Problem 27

Two normal-looking fruit flies were crossed, and, in the progeny, there were 202 females and 98 males. a. What is unusual about this result? b. Provide a genetic explanation for this anomaly. c. Provide a test of your hypothesis.

Short Answer

Expert verified
The unusual result is the 2:1 sex ratio skew. A sex-linked lethal gene could explain male lethality. Conduct further crosses to test the hypothesis.

Step by step solution

01

Identify the Expected Ratio

Under normal circumstances, we expect an approximately 1:1 sex ratio in the progeny of fruit flies. This means that the number of male and female offspring should be roughly equal.
02

Analyze the Given Progeny Data

The given data shows 202 females and 98 males. This is an approximate ratio of 2:1 in favor of females, which is unusual for fruit flies.
03

Propose a Genetic Explanation

One possible genetic explanation is the presence of a sex-linked lethal gene. This could mean a genetic factor on one of the sex chromosomes (often on the X chromosome) is causing male lethality. If such a lethal gene is present on the X chromosome, males (who are XY and have only one X chromosome) would be more affected, leading to their higher mortality rate.
04

Design a Hypothetical Test

To test the hypothesis of the sex-linked lethal gene, one could conduct a cross involving another set of fruit flies with known genetic backgrounds. Monitoring the offspring's sex ratio in multiple subsequent generations could verify whether this skewed ratio is consistent, indicating a genetic issue on the sex chromosome.

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Key Concepts

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

Sex-linked Genes
Sex-linked genes are a fascinating aspect of genetics. They are located on sex chromosomes, which determine the sex of an organism. In humans and many other species, these are known as X and Y chromosomes.

In fruit flies, like in humans, males have one X and one Y chromosome (XY), while females have two X chromosomes (XX). When a gene is located on a sex chromosome, it can result in traits that manifest differently depending on whether the individual is male or female.

In the case of our exercise, the anomaly in the sex ratio implies that a sex-linked gene might be influencing the results. When a potentially harmful or lethal gene is on the X chromosome, males are more vulnerable because they have just one X chromosome. This can lead to a higher mortality rate in males if the gene has lethal effects.
Fruit Fly Genetics
Fruit fly genetics is a popular field for genetic study due to the short reproductive cycle of fruit flies and their relatively simple genetic structure. Scientists often use fruit flies, scientifically known as _Drosophila melanogaster_, as a model organism to study genetic inheritance.

One important reason is their short life cycle. Fruit flies can produce a new generation in just about two weeks, which makes them ideal for studying inheritance patterns across multiple generations.

  • Fruit fly genetics is governed by similar genetic principles as other species, like humans.
  • They have four pairs of chromosomes, but focus often lies on the X chromosome due to the legacy of sex-linked traits.
In our specific example, the altered sex ratio may be attributed to a mutation or sex-linked genetic anomaly affecting the viability or survival of one sex over the other.
Sex Ratio
The sex ratio in a population refers to the proportion of males to females. In many organisms, including fruit flies, the expected sex ratio is often 1:1, meaning you expect equal numbers of males and females in offspring. This expected outcome occurs because each parent typically contributes one sex chromosome randomly to the offspring.

A deviation from this expected ratio can indicate underlying genetic anomalies or environmental influences impacting one sex more than the other.

In the provided scenario, the unusual ratio of approximately 2:1 in favor of females suggests something unusual is influencing the survival or viability of the males.
  • A sex-linked lethal gene is one plausible explanation, particularly impacting males.
  • To verify such an anomaly, scientists can perform controlled crosses and meticulously observe the resulting sex ratios.
This careful analysis and experimentation allow researchers to determine if genetic factors are influencing the skewed sex ratio.

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Most popular questions from this chapter

Consider the production of flower color in the Japanese morning glory (Pharbitis nil). Dominant alleles of either of two separate genes \((A /-\cdot b / b \text { or } a / a \cdot B /-)\) produce purple petals. \(A /-\cdot B /-\) produces blue petals, and \(a / a \cdot b / b\) produces scarlet petals. Deduce the genotypes of parents and progeny in the following crosses: $$\begin{array}{cll} \text { Cross } & \text { Parents } & \text { Progeny } \\ \hline 1 & \text { blue } \times \text { scarlet } & \frac{1}{4} \text { blue }: \frac{1}{2} \text { purple : } \frac{1}{4} \text { scarlet } \\ 2 & \text { purple } \times \text { purple } & \frac{1}{4} \text { blue }: \frac{1}{2} \text { purple }: \frac{1}{4} \text { scarlet } \\ 3 & \text { blue } \times \text { blue } & \frac{3}{4} \text { blue }: \frac{1}{4} \text { purple } \\ 4 & \text { blue } \times \text { purple } & \frac{3}{8} \text { blue }: \frac{4}{8} \text { purple }: \frac{1}{8} \text { scarlet } \\ 5 & \text { purple } \times \text { scarlet } & \frac{1}{2} \text { purple }: \frac{1}{2} \text { scarlet } \\ \hline \end{array}$$

Most of the feathers of erminette fowl are light colored, with an occasional black one, giving a flecked appearance. A cross of two erminettes produced a total of 48 progeny, consisting of 22 erminettes, 14 blacks, and 12 pure whites. What genetic basis of the erminette pattern is suggested? How would you test your hypotheses?

Several mutants are isolated, all of which require compound G for growth. The compounds (A to E) in the biosynthetic pathway to G are known, but their order in the pathway is not known. Each compound is tested for its ability to support the growth of each mutant (1 to 5). In the following table, a plus sign indicates growth and a minus sign indicates no growth. $$\begin{array}{rcccccc} {5}{c} {\text {compound tested}} \\ \ { 2 - 6 }{1}{c} {} & &\mathrm{A} & \mathrm{B} & \mathrm{C} & \mathrm{D} & \mathrm{E} & \mathrm{G} \\ \hline \text { Mutant } & 1 & \- & \- & \- & \+ & \- & \+ \\ & 2 & \- & \+ & \- & \+ & \- & \+ \\ & 3 & \- & \- & \- & \- & \- & \+ \\ & 4 & \- & \+ & \+ & \+ & \- & \+ \\ & 5 & \+ & \+ & \+ & \+ & \- & \+ \\ \hline \end{array}$$ a. What is the order of compounds A to Ein the pathway? b. At which point in the pathway is each mutant blocked? c. Would a heterokaryon composed of double mutants 1,3 and 2,4 grow on a minimal medium? Would 1,3 and \(3,4 ?\) Would 1,2 and 2,4 and \(1,4 ?\)

Wild-type strains of the haploid fungus Neurospora can make their own tryptophan. An abnormal allele \(t d\) renders the fungus incapable of making its own tryptophan. An individual of genotype \(t d\) grows only when its medium supplies tryptophan. The allele \(s u\) assorts independently of \(t d ;\) its only known effect is to suppress the \(t d\) phenotype. Therefore, strains carrying both \(t d\) and \(s u\) do not require tryptophan for growth. a. If a \(t d ; s u\) strain is crossed with a genotypically wildtype strain, what genotypes are expected in the progeny and in what proportions? b. What will be the ratio of tryptophan-dependent to tryptophan-independent progeny in the cross of part \(a ?\)

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?
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