Ch. 3 - Mendelian Genetics

Klug - Concepts of Genetics 12th Edition

Klug12th EditionConcepts of Genetics ISBN: 9780135564776Not the one you use?Change textbook

Klug 12th Edition

Ch. 3 - Mendelian Genetics

Problem 12b

Chapter 3, Problem 12b

In Drosophila, gray body color is dominant to ebony body color, while long wings are dominant to vestigial wings. Assuming that the P₁ individuals are homozygous, work the following crosses through the F₂ generation, and determine the genotypic and phenotypic ratios for each generation.
gray, vestigial x ebony, long

Verified step by step guidance

Identify the traits and their dominance relationships: gray body color (G) is dominant to ebony (g), and long wings (L) are dominant to vestigial wings (l). Since the P₁ individuals are homozygous, assign genotypes accordingly. For example, gray body with long wings would be GGLL, and ebony body with vestigial wings would be ggll.

Set up the P₁ cross using the homozygous parents: cross GGLL (gray, long) with ggll (ebony, vestigial). This will produce the F₁ generation, which will be heterozygous for both traits (GgLl).

Determine the phenotype and genotype of the F₁ generation: since both traits are dominant, all F₁ individuals will show gray body and long wings, and their genotype will be GgLl.

Set up the F₁ cross to produce the F₂ generation: cross two F₁ individuals (GgLl x GgLl). Use a dihybrid Punnett square to determine the possible genotypes and phenotypes of the F₂ offspring.

Calculate the genotypic and phenotypic ratios for the F₂ generation by analyzing the Punnett square results. The phenotypic ratio for two independently assorting dominant traits typically follows a 9:3:3:1 pattern (9 gray long, 3 gray vestigial, 3 ebony long, 1 ebony vestigial), and the genotypic ratio can be derived by counting all genotype combinations.

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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. A dominant allele masks the expression of a recessive allele in heterozygotes, determining the phenotype. In this question, gray body and long wings are dominant traits, while ebony body and vestigial wings are recessive.

Genotypic and Phenotypic Ratios

Genotypic ratios describe the frequency of different allele combinations in offspring, while phenotypic ratios describe the frequency of observable traits. These ratios are predicted using Punnett squares, especially when crossing homozygous parents and analyzing subsequent generations like F1 and F2.

Dihybrid Cross and Independent Assortment

A dihybrid cross involves two traits, each with two alleles, to study how they assort independently according to Mendel’s second law. This allows prediction of combined phenotypes and genotypes in offspring, such as body color and wing type in Drosophila, by analyzing the F1 and F2 generations.

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Related Practice

Textbook Question

Mendel crossed peas having green seeds with peas having yellow seeds. The F₁ generation produced only yellow seeds. In the F₂, the progeny consisted of 6022 plants with yellow seeds and 2001 plants with green seeds. Of the F₂ yellow-seeded plants, 519 were self-fertilized with the following results: 166 bred true for yellow and 353 produced an F₃ ratio of 3/4 yellow: 1/4 green. Explain these results by diagramming the crosses.

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Textbook Question

In Drosophila, gray body color is dominant to ebony body color, while long wings are dominant to vestigial wings. Assuming that the P₁ individuals are homozygous, work the following crosses through the F₂ generation, and determine the genotypic and phenotypic ratios for each generation.

gray, long x ebony, vestigial

Textbook Question

gray, long x gray, vestigial.

Textbook Question

What is the basis for homology among chromosomes?

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Textbook Question

How many different types of gametes can be formed by individuals of the following genotypes:

(a) AaBb

(b) AaBB

(d) AaBBcc

(e) AaBbcc

(f) AaBbCcDdEe

What are the gametes in each case?

Textbook Question

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. Two different matings were made between black members of the F₂ generation, with the following results. Diagram each of the crosses.

Cross Offspring

Cross 1 All black

Cross 2 3/4 black, 1/4 white