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Ch. 2 - Transmission Genetics
Sanders - Genetic Analysis: An Integrated Approach 3rd Edition
Sanders3rd EditionGenetic Analysis: An Integrated ApproachISBN: 9780135564172Not the one you use?Change textbook
All textbooksSanders 3rd EditionCh. 2 - Transmission GeneticsProblem 47c
Chapter 2, Problem 47c

The accompanying pedigree shows a family in which one child (II-1) has an autosomal recessive condition. On the basis of this fact alone, provide the following information.
Pedigree chart showing a family with one child affected by an autosomal recessive condition.
What are the probabilities for each of the possible genotypes for II-2, II-3, and II-4?

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1
Step 1: Understand the inheritance pattern. Autosomal recessive conditions require two copies of the recessive allele (aa) for an individual to express the condition. Individuals with one dominant and one recessive allele (Aa) are carriers but do not express the condition, while individuals with two dominant alleles (AA) are unaffected and not carriers.
Step 2: Analyze the pedigree. II-1 has the autosomal recessive condition, meaning their genotype is aa. This implies that both parents (I-1 and I-2) must be carriers (Aa) since they passed on one recessive allele each to II-1.
Step 3: Determine the probabilities for II-2. Since both parents are carriers (Aa), use a Punnett square to calculate the probabilities for II-2's genotype. The possible genotypes are AA, Aa, and aa, with probabilities of 25%, 50%, and 25%, respectively.
Step 4: Determine the probabilities for II-3 and II-4. These siblings also have the same parents (I-1 and I-2), so their genotype probabilities are identical to those of II-2. Use the same Punnett square analysis to conclude that the probabilities for AA, Aa, and aa are 25%, 50%, and 25%, respectively.
Step 5: Summarize the results. For each of II-2, II-3, and II-4, the probabilities of being AA (not a carrier), Aa (a carrier), and aa (affected) are 25%, 50%, and 25%, respectively, based on the autosomal recessive inheritance pattern and the genotypes of their parents.

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Key Concepts

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

Autosomal Recessive Inheritance

Autosomal recessive inheritance occurs when a trait or condition is expressed only when an individual has two copies of the recessive allele, one inherited from each parent. In a pedigree, affected individuals typically have unaffected parents who are carriers of the recessive allele. Understanding this pattern is crucial for predicting the genotypes of other family members based on their relationships and the known affected individual.
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Autosomal Pedigrees

Genotype Probabilities

Genotype probabilities refer to the likelihood of an individual inheriting specific alleles from their parents. In the context of autosomal recessive conditions, one must consider the genotypes of the parents and the known affected child to calculate the probabilities for siblings. This involves using Punnett squares or probability rules to determine the chances of each possible genotype for the siblings in question.
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Probability

Pedigree Analysis

Pedigree analysis is a method used to track the inheritance of traits through generations in a family. It visually represents relationships and can help identify carriers of genetic conditions. By analyzing the pedigree, one can infer the genotypes of individuals based on the presence or absence of traits, which is essential for determining the probabilities of genotypes for II-2, II-3, and II-4 in the given scenario.
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Pedigree Flowchart
Related Practice
Textbook Question

Alleles of the IGF-1 gene in dogs, encoding insulin-like growth factor, largely determine whether a domestic dog will be large or small. Dogs with an ancestral dominant allele are large, whereas dogs homozygous for the mutant recessive allele are small. Chondrodysplasia, a short-legged phenotype (as in dachshunds and basset hounds), is caused by a dominant gain-of-function allele of the FGF4 gene. The MSTN gene encodes myostatin, a regulator of muscle development. Dogs with a dominant ancestral allele of the MTSN gene have normal muscle development, while dogs homozygous for recessive mutants in the MTSN gene are 'double muscled' and have trouble running quickly. However, dogs heterozygous for the mutant allele run faster than either of the homozygotes.

You breed a pure-breeding small basset hound of normal musculature with a pure-breeding 'bully' whippet, a double-muscled large dog with normal legs.

If the F₁ of this cross is interbred, what proportion of the F₂ are expected to be fast runners and what proportion normal-speed runners?

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Textbook Question

The accompanying pedigree shows a family in which one child (II-1) has an autosomal recessive condition. On the basis of this fact alone, provide the following information.

Using A for the dominant allele and a for the recessive allele, give the genotypes for I-1, I-2, and II-1.

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Textbook Question

The accompanying pedigree shows a family in which one child (II-1) has an autosomal recessive condition. On the basis of this fact alone, provide the following information.

Using the same alleles, give the possible genotypes for II-2, II-3, and II-4.

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Textbook Question

The accompanying pedigree shows a family in which one child (II-1) has an autosomal recessive condition. On the basis of this fact alone, provide the following information.

What is the probability that all three of the children in generation II who have the dominant phenotype are Aa?

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Textbook Question

The accompanying pedigree shows a family in which one child (II-1) has an autosomal recessive condition. On the basis of this fact alone, provide the following information.

What is the chance that among the three children in generation II who have the dominant phenotype, one of them is AA and two of them are Aa? (Hint: Consider all possible orders of genotypes.)

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Textbook Question

A pea plant that has the genotype RrGgwwdd is crossed to a plant that has the rrGgWwDd genotype. The R gene controls round versus wrinkled seed, the G gene controls yellow versus green seed, the W gene controls purple versus white flower, and the D gene controls tall versus short plants. Determine the following;


What are the phenotypes of each plant?

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