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Ch. 4 - Modification of Mendelian Ratios
Klug - Essentials of Genetics 10th Edition
Klug10th EditionEssentials of GeneticsISBN: 9780135588789Not the one you use?Change textbook
All textbooksKlug 10th EditionCh. 4 - Modification of Mendelian RatiosProblem 5
Chapter 4, Problem 5

In foxes, two alleles of a single gene, P and p, may result in lethality (PP), platinum coat (Pp), or silver coat (pp). What ratio is obtained when platinum foxes are interbred? Is the P allele behaving dominantly or recessively in causing (a) lethality; (b) platinum coat color?

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1
Identify the genotypes and their associated phenotypes: PP results in lethality, Pp results in platinum coat, and pp results in silver coat.
Set up a Punnett square for the cross between two platinum foxes, both with genotype Pp.
Fill in the Punnett square to find the genotypic ratios of the offspring: PP, Pp, and pp.
Translate the genotypic ratios into phenotypic ratios, considering that PP individuals are lethal and will not survive.
Analyze the phenotypic outcomes to determine if the P allele is dominant or recessive for (a) lethality and (b) platinum coat color by observing which phenotypes appear in heterozygotes.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Mendelian Inheritance and Allele Interactions

Mendelian inheritance describes how alleles segregate and assort during reproduction, producing predictable genotype and phenotype ratios. Understanding dominant, recessive, and lethal alleles helps explain how different allele combinations affect traits and survival, such as when homozygous genotypes cause lethality.
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Interacting Genes Overview

Lethal Alleles and Their Effects

Lethal alleles cause death when present in certain genotypes, often homozygous. They reduce or eliminate specific genotypes from offspring ratios, altering expected Mendelian ratios. Recognizing lethality is crucial to interpreting deviations in phenotypic ratios from typical dominant-recessive patterns.
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Dominance Relationships in Phenotypic Expression

Dominance refers to how one allele masks the effect of another in heterozygotes. An allele is dominant if it determines the phenotype when present, and recessive if its effect appears only in homozygotes. Determining dominance for traits like coat color or lethality involves analyzing offspring phenotypes from specific crosses.
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Related Practice
Textbook Question

In shorthorn cattle, coat color may be red, white, or roan. Roan is an intermediate phenotype expressed as a mixture of red and white hairs. The following data were obtained from various crosses:

How is coat color inherited? What are the genotypes of parents and offspring for each cross?

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Textbook Question

With regard to the ABO blood types in humans, determine the genotype of the male parent and female parent shown here:

Male parent: Blood type B; mother type O

Female parent: Blood type A; father type B

Predict the blood types of the offspring that this couple may have and the expected proportion of each.

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Textbook Question

Three gene pairs located on separate autosomes determine flower color and shape as well as plant height. The first pair exhibits incomplete dominance, where the color can be red, pink (the heterozygote), or white. The second pair leads to personate (dominant) or peloric (recessive) flower shape, while the third gene pair produces either the dominant tall trait or the recessive dwarf trait. Homozygous plants that are red, personate, and tall are crossed to those that are white, peloric, and dwarf. Determine the F₁ genotype(s) and phenotype(s). If the F₁ plants are interbred, what proportion of the offspring will exhibit the same phenotype as the F₁ plants?

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Textbook Question

Flower color may be red, white, or pink, and flower shape may be personate or peloric. For the following crosses, determine the P₁ and F₁ genotypes:

549
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Textbook Question

In rats, the following genotypes of two independently assorting autosomal genes determine coat color:

A third gene pair on a separate autosome determines whether or not any color will be produced. The CC and Cc genotypes allow color according to the expression of the A and B alleles. However, the cc genotype results in albino rats regardless of the A and B alleles present. Determine the F₁ phenotypic ratio of the following crosses:

AAbbCC×aaBBcc

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