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Ch. 3 - Mendelian Genetics
Klug - Essentials of Genetics 10th Edition
Klug10th EditionEssentials of GeneticsISBN: 9780135588789Not the one you use?Change textbook
Chapter 3, Problem 3

In a cross between a black and a white guinea pig, all members of the F₁ generation are black. The F₂ generation is made up of approximately 3/4 black and 1/4 white guinea pigs. Diagram this cross, showing the genotypes and phenotypes.

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1
Identify the type of inheritance pattern based on the phenotypic ratios in the F₂ generation. Since the F₁ generation is all black and the F₂ generation shows a 3:1 ratio of black to white, this suggests simple Mendelian inheritance with black being dominant over white.
Assign symbols to the alleles: let \( B \) represent the dominant black allele and \( b \) represent the recessive white allele.
Determine the genotypes of the parental (P) generation. Since crossing black and white guinea pigs produces all black F₁ offspring, the black parent is likely homozygous dominant (\( BB \)) and the white parent is homozygous recessive (\( bb \)).
Write the cross for the P generation: \( BB \times bb \). The F₁ offspring will all be heterozygous (\( Bb \)) and phenotypically black.
Set up the F₁ cross: \( Bb \times Bb \). Use a Punnett square to determine the F₂ genotypes: \( BB \), \( Bb \), and \( bb \). The expected genotypic ratio is 1:2:1, which corresponds to a phenotypic ratio of 3 black (\( BB \) and \( Bb \)) to 1 white (\( bb \)).

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Key Concepts

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

Mendelian Inheritance and Dominance

Mendelian inheritance explains how traits are passed from parents to offspring through dominant and recessive alleles. In this case, black fur is dominant over white, so crossing a black and white guinea pig results in all black F₁ offspring. Understanding dominance helps predict phenotypic ratios in subsequent generations.
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Variations on Dominance

Punnett Square and Genotypic Ratios

A Punnett square is a diagram used to predict the genotypes and phenotypes of offspring from a genetic cross. It helps visualize allele combinations and calculate expected ratios, such as the 3:1 black to white ratio seen in the F₂ generation, indicating heterozygous parents in the F₁.
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Chi Square Analysis

Homozygous and Heterozygous Genotypes

Homozygous individuals have two identical alleles for a trait, while heterozygous individuals have two different alleles. The black F₁ guinea pigs are heterozygous (Bb), carrying one dominant black allele and one recessive white allele, which explains the 3:1 phenotypic ratio in the F₂ generation.
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