Ch. 4 - Extensions of Mendelian Genetics

Klug - Concepts of Genetics 12th Edition

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Klug 12th Edition

Ch. 4 - Extensions of Mendelian Genetics

Problem 2

Chapter 4, Problem 2

Write a short essay that explains why multiple and lethal alleles often result in a modification of the classic Mendelian monohybrid and dihybrid ratios.

Verified step by step guidance

Understand that Mendelian genetics is based on the principle that alleles segregate and assort independently, leading to predictable ratios in monohybrid (3:1) and dihybrid (9:3:3:1) crosses.

Recognize that multiple alleles refer to the presence of more than two alleles for a given gene within a population, which can lead to a greater variety of genotypes and phenotypes than predicted by simple Mendelian ratios.

Consider lethal alleles, which are alleles that cause the death of an organism when present in a certain genotype, often leading to deviations from expected Mendelian ratios because certain genotypes are not viable.

Explore how the presence of multiple alleles can result in modified phenotypic ratios, such as in the case of blood type inheritance in humans, where the ABO blood group is determined by three alleles (I^A, I^B, i) leading to more than the typical two phenotypes.

Examine how lethal alleles can alter expected ratios, such as in the case of a recessive lethal allele where a 2:1 phenotypic ratio might be observed instead of the expected 3:1, due to the death of homozygous recessive individuals.

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Key Concepts

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

Mendelian Genetics

Mendelian genetics is the study of how traits are inherited through generations, based on the principles established by Gregor Mendel. It includes concepts such as dominant and recessive alleles, which determine the phenotypic expression of traits. In monohybrid crosses, the classic 3:1 ratio of dominant to recessive phenotypes is observed, while dihybrid crosses typically yield a 9:3:3:1 ratio. These ratios assume simple inheritance patterns without complications from multiple alleles or lethal alleles.

Multiple Alleles

Multiple alleles refer to the existence of more than two allelic forms for a given gene within a population. This complexity can lead to a variety of phenotypes that do not conform to Mendelian ratios. For example, in the ABO blood group system, three alleles (IA, IB, and i) interact to produce four blood types, resulting in modified inheritance patterns that deviate from the expected ratios of simple Mendelian inheritance.

Lethal Alleles

Lethal alleles are alleles that can cause the death of an organism when present in a homozygous state. This phenomenon alters the expected Mendelian ratios by reducing the number of viable offspring. For instance, if a lethal allele is present in a dihybrid cross, the typical 9:3:3:1 ratio may shift to 2:1, as the homozygous lethal genotype does not survive to be counted among the offspring, demonstrating how lethal alleles can significantly modify inheritance patterns.

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Related Practice

Textbook Question

In the discussion, we focused on extensions and modifications of Mendelian principles and ratios. In the process, we encountered many opportunities to consider how this information was acquired. On the basis of these discussions, what answers would you propose to the following fundamental questions?

How did geneticists determine that inheritance of some phenotypic characteristics involves the interactions of two or more gene pairs? How were they able to determine how many gene pairs were involved?

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Textbook Question

The discussion centers on extensions and modifications of Mendelian principles and ratios. In the process, we encountered many opportunities to consider how this information was acquired. On the basis of these discussions, what answers would you propose to the following fundamental questions?

How do we know that specific genes are located on the sex-determining chromosomes rather than on autosomes?

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Textbook Question

For genes whose expression seems to be tied to the sex of individuals, how do we know whether a gene is X-linked in contrast to exhibiting sex-limited or sex-influenced inheritance?

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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

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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Textbook Question

In mice, a short-tailed mutant was discovered. When it was crossed to a normal long-tailed mouse, 4 offspring were short-tailed and 3 were long-tailed. Two short-tailed mice from the F1 generation were selected and crossed. They produced 6 short-tailed and 3 long-tailed mice. These genetic experiments were repeated three times with approximately the same results. What genetic ratios are illustrated? Hypothesize the mode of inheritance and diagram the crosses.

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