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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 9b
Chapter 4, Problem 9b

Given the inheritance pattern of coat color in rats described in Problem 17, predict the genotype and phenotype of the parents who produced the following offspring:
9/16 gray: 3/16 yellow: 4/16 albino

Verified step by step guidance
1
Identify the inheritance pattern described in Problem 17, which likely involves two genes with epistatic interactions affecting coat color in rats. Typically, such ratios suggest a dihybrid cross with epistasis.
Assign symbols to the genes involved. For example, let gene A control pigment production (A = pigment, a = no pigment) and gene B control pigment color (B = gray, b = yellow). Albino rats result from homozygous recessive at the A locus (aa), which masks the effect of the B locus.
Write down the expected phenotypes based on genotypes: gray rats have at least one dominant A and one dominant B allele (A_B_), yellow rats have at least one dominant A allele but are homozygous recessive at B (A_bb), and albino rats are homozygous recessive at A (aa regardless of B genotype).
Use the offspring phenotypic ratios (9/16 gray : 3/16 yellow : 4/16 albino) to deduce the genotypes of the parents. These ratios correspond to a dihybrid cross where both parents are heterozygous at both loci (A/a B/b).
Conclude that the parents' genotypes are both A/a B/b, and their phenotypes are gray, since the presence of dominant alleles at both loci produces gray coat color.

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Key Concepts

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

Mendelian Inheritance and Dihybrid Ratios

Mendelian inheritance explains how traits are passed from parents to offspring through dominant and recessive alleles. The given offspring ratios (9:3:4) suggest a dihybrid cross involving two genes, where the phenotypic ratio deviates from the classic 9:3:3:1, indicating gene interaction such as epistasis.
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Punnet Square

Epistasis in Coat Color Genetics

Epistasis occurs when one gene masks or modifies the expression of another gene. In rat coat color, certain alleles can suppress pigment production, leading to phenotypes like albino regardless of other color genes, explaining the altered phenotypic ratios observed in the offspring.
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Epistatic Genes

Genotype-Phenotype Relationship and Predicting Parental Genotypes

Understanding how specific genotypes produce phenotypes allows prediction of parental genotypes from offspring ratios. By analyzing the proportions of gray, yellow, and albino offspring, one can infer the parents' alleles and their combinations responsible for the observed phenotypic distribution.
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Related Practice
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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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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Textbook Question

Given the inheritance pattern of coat color in rats described in Problem 17, predict the genotype and phenotype of the parents who produced the following offspring:

9/16 gray: 3/16 yellow: 3/16 black: 1/16 cream

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Textbook Question

Given the inheritance pattern of coat color in rats described in Problem 17, predict the genotype and phenotype of the parents who produced the following offspring: 27/64 gray:

16/64 albino: 9/64 yellow: 9/64 black: 3/64 cream

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Textbook Question

A husband and wife have normal vision, although both of their fathers are red–green color-blind, an inherited X-linked recessive condition. What is the probability that their first child will be (a) a normal son, (b) a normal daughter, (c) a color-blind son, (d) a color-blind daughter?

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Textbook Question

In humans, the ABO blood type is under the control of autosomal multiple alleles. Color blindness is a recessive X-linked trait. If two parents who are both type A and have normal vision produce a son who is color-blind and is type O, what is the probability that their next child will be a female who has normal vision and is type O?

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