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In 1983 , a sheep farmer in Oklahoma noticed in his flock a ram that possessed increased muscle mass in his hindquarters. Many of the offspring of this ram possessed the same trait, which became known as the callipyge phenotype (callipyge is Greek for "beautiful buttocks"). The mutation that caused the callipyge phenotype was eventually mapped to a position on the sheep chromosome 18 . When the male callipyge offspring of the original mutant ram were crossed with normal females, they produced the following progeny: \(1 / 4\) male callipyge, \(1 / 4\) female callipyge, \(1 / 4\) male normal, and \(1 / 4\) female normal. When the female callipyge offspring of the original mutant ram were crossed with normal males, all the offspring were normal. Analysis of the chromosomes of these offspring of callipyge females showed that half of them received a chromosome 18 with the allele encoding callipyge from their mother. Propose an explanation for the inheritance of the allele for callipyge. How might you test your explanation?

Step by step solution

01

Understanding the Phenotype Inheritance

The callipyge phenotype is an increase in muscle mass, primarily observed in the hindquarters of sheep. From the data, when callipyge males mate with normal females, the offspring are in a 1:1:1:1 ratio: callipyge males, callipyge females, normal males, and normal females.

02

Analyzing Female Offspring Inheritance

When callipyge females are crossed with normal males, all offspring appear normal. However, chromosome analysis shows half received the callipyge allele from the mother. This suggests the allele is inherited but does not express under these conditions, indicating a potential parent-of-origin effect.

03

Considering the Genetic Mechanism

Since the phenotype only appears in offspring of callipyge males crossed with normal females and not vice versa, this points to an imprinting mechanism or a similar parent-of-origin effect. Imprinting generally involves gene expression depending on whether the allele is inherited from the mother or the father.

04

Testing the Hypothesis

A test could involve crossbreeding experiments where the parent of origin for the callipyge allele is controlled. By observing whether the trait expresses itself based on the paternal or maternal inheritance, the hypothesis of imprinting can be confirmed. Additional molecular studies could look for epigenetic markers associated with imprinting on chromosome 18.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Callipyge Phenotype

The callipyge phenotype is a unique trait observed in sheep. It presents as increased muscle mass, particularly in the hindquarters. The trait was named "callipyge," which is derived from Greek, meaning "beautiful buttocks." This distinct physical characteristic is not just notable visually but also holds genetic significance.
The callipyge mutation was first noticed in a ram, and it affected many of its offspring, suggesting a strong hereditary pattern. This muscle trait is advantageous in agricultural contexts due to increased meat yield. However, understanding its genetic basis can provide insights into muscular development and genetic mutations. In genetics, callipyge is a classic case demonstrating how a mutation can affect physical traits and how these traits are passed down through generations.

Inheritance Patterns

Understanding inheritance patterns helps to determine how traits like the callipyge phenotype are passed from one generation to the next. In the case of callipyge, it's interesting to note the results of breeding experiments.
When a callipyge male mates with a normal female, the offspring appear in an even distribution of callipyge and normal males and females. This suggests a \(rac{1}{4}\) Mendelian inheritance pattern, which is typical for many simple monogenic traits.
However, when the callipyge females are crossed with normal males, all resulting offspring are normal despite some receiving the callipyge allele. This anomaly highlights the complexity of inheritance, possibly due to imprinting or other genetic mechanisms that affect gene expression. Understanding these patterns is crucial for predicting trait expression and helps reveal underlying genetic processes.

Chromosome 18

Chromosome 18 plays a key role in the expression of the callipyge phenotype. Specifically, the mutation responsible for increased muscle mass is located here. Chromosomes are structures within cells that contain DNA, the material that holds genes. Genes are sequences of DNA that encode for proteins, which, in turn, determine traits.
The location of the callipyge mutation on chromosome 18 provides a starting point for geneticists to explore and understand this trait further. By examining chromosome 18, researchers can investigate how the mutation influences muscle growth and how it interacts with other genes.
The study of this chromosome not only aids in comprehending the genetic roots of the callipyge phenotype but also contributes to broader knowledge on chromosomal behavior, genetic mutations, and trait inheritance.

Parent-of-Origin Effect

The concept of the parent-of-origin effect is central to understanding the inheritance of the callipyge phenotype. This effect suggests that the expression of a trait depends on which parent the allele is inherited from.
In the callipyge case, the phenotype only appears when the mutation is inherited from the male parent. When females with the mutation produce offspring, the trait does not manifest, even if the offspring has the mutation. This indicates that the trait is subject to imprinting, where the gene's expression is influenced by the parent who contributed it.
This parent-of-origin pattern is significant as it contradicts traditional Mendelian inheritance, where it typically does not matter from which parent a gene is inherited. Such genetic imprinting involves epigenetic processes that silence the allele from one parent, leading to unique inheritance behaviors that highlight the complexity of genetic expression.