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Chapter 6: Problem 25

On a fox ranch in Wisconsin, a mutation arose that gave a "platinum" coat color. The platinum color proved very popular with buyers of fox coats, but the breeders could not develop a pure-breeding platinum strain. Every time two platinums were crossed, some normal foxes appeared in the progeny. For example, the repeated matings of the same pair of platinums produced 82 platinum and 38 normal progeny. All other such matings gave similar progeny ratios. State a concise genetic hypothesis that accounts for these results.

Short Answer

Expert verified
The platinum coat color results from a lethal allele hypothesis, where platinum foxes are heterozygous (Pp), and the PP genotype is lethal.

Step by step solution

01

Analyze the Phenotypic Ratio

The observed phenotypic ratio of platinum to normal foxes is 82:38. Simplified, this is approximately 2:1 when divided by 38 (since 82/38 鈮� 2.16). This suggests a segregation pattern consistent with a 2:1 ratio, commonly observed in lethal allele scenarios.
02

Formulate the Hypothesis

Given the 2:1 phenotypic ratio, we hypothesize that the platinum coat color is influenced by a single gene with a lethal allele. This suggests that platinum foxes are heterozygous for the coat color gene (e.g., Pp), where P is the platinum-producing allele and p is the normal allele. Homozygosity for the platinum allele (PP) is lethal, resulting in only the Pp (platinum) and pp (normal) genotypes being observed.
03

Predict the Genotypes

Under this hypothesis, when two Pp foxes are bred, the expected genotypic ratio is 1PP : 2Pp : 1pp. Since PP is lethal, only the 2Pp and 1pp survive, resulting in a 2 (platinum) to 1 (normal) phenotypic ratio.

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Key Concepts

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

Lethal Alleles
Lethal alleles are fascinating aspects of genetics. They help explain why a certain gene never seems to have a homozygous form in a population. Simply put, lethal alleles cause death when present in a certain combination or genotype.
Consider the foxes with the platinum coat. When two such foxes are bred, not all of their offspring have a platinum coat. The reason is that some combinations of genes don't allow the individual to survive.
For example, if we denote the allele for platinum as "P" and the normal allele as "p", then the combination "PP" is lethal. This means any fox with the combination does not survive to birth or maturity.
This phenomenon is common with certain coat colors, as we see in our fox example. Breeders find it challenging to have a pure-breeding population because the necessary allele combination for pure-breeding is lethal.
Phenotypic Ratio
The phenotypic ratio in genetics refers to the observable characteristics resulting from a specific genetic cross. This ratio helps us understand how certain traits are likely to appear in offspring.
In our fox example, the phenotypic ratio is 2:1, meaning that in every 3 offspring, 2 have the platinum coat and 1 has a normal coat.
  • A 2:1 ratio is a classic indicator in genetics that a lethal allele is at play.
  • Here, the phenotype ratio is not a simple 3:1, like one might expect with complete dominance, because one part of the genetic ratio (PP) results in a non-viable offspring.
Understanding phenotypic ratios helps breeders predict what results come from particular genetic combinations, despite not always reaching a 3:1 or other common ratios due to lethality.
Genotypic Ratio
The genotypic ratio refers to the genetic makeup of offspring that results from a genetic cross. Unlike the phenotypic ratio, which is about appearance, the genotypic ratio tells us about the genetic code.
In our case with the platinum foxes, the genotypic ratio predicted from crossing two platinum foxes is 1PP : 2Pp : 1pp.
  • This means for every 4 offspring, there should be 1 homozygous platinum (PP), 2 heterozygous platinum (Pp), and 1 normal (pp).
  • "PP" is lethal, so these offspring do not survive, altering our observed phenotypes.
By understanding both the genotypic and phenotypic ratios, breeders can better predict and manage the traits within a breeding population, ensuring they know what combinations might not survive.

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Most popular questions from this chapter

In Drosophila, an autosomal gene determines the shape of the hair, with \(B\) giving straight and \(b\) giving bent hairs. On another autosome, there is a gene of which a dominant allele \(I\) inhibits hair formation so that the fly is hairless ( \(i\) has no known phenotypic effect). a. If a straight-haired fly from a pure line is crossed with a fly from a pure-breeding hairless line known to be an inhibited bent genotype, what will the genotypes and phenotypes of the \(\mathrm{F}_{1}\) and the \(\mathrm{F}_{2}\) be? b. What cross would give the ratio 4 hairless: 3 straight: 1 bent?

The production of pigment in the outer layer of seeds of corn requires each of the three independently assorting genes \(A, C,\) and \(R\) to be represented by at least one dominant allele, as specified in Problem \(64 .\) The dominant allele \(P r\) of a fourth independently assorting gene is required to convert the biochemical precursor into a purple pigment, and its recessive allele \(p r\) makes the pigment red. Plants that do not produce pigment have yellow seeds. Consider a cross of a strain of genotype \(A / A ; C / C ; R / R ; p r / p r\) with a strain of genotype \(a / a ; c / c ; r / r ; \operatorname{Pr} / \operatorname{Pr}\) a. What are the phenotypes of the parents? b. What will be the phenotype of the \(\mathrm{F}_{1}\) ? c. What phenotypes, and in what proportions, will appear in the progeny of a selfed \(\mathrm{F}_{1}\) ? d. What progeny proportions do you predict from the testcross of an \(\mathrm{F}_{1}\) ?

In a maternity ward, four babies become accidentally mixed up. The \(A B O\) types of the four babies are known to be \(\mathrm{O}, \mathrm{A}, \mathrm{B},\) and \(\mathrm{AB}\). The \(\mathrm{ABO}\) types of the four sets of parents are determined. Indicate which baby belongs to each set of parents: (a) \(A B \times O,(b) A \times O,(c) A \times A B\) (d) \(\mathrm{O} \times \mathrm{O}\)

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?

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?
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