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Chapter 5: Problem 16

Club foot is one of the most common congenital skeletal abnormalities, with a worldwide incidence of about 1 in 1000 births. Both genetic and nongenetic factors are thought to be responsible for club foot. C. A. Gurnett et al. (2008. American Journal of Human Genetics \(83: 616-622\) ) identified a family in which club foot was inherited as an autosomal dominant trait with incomplete penetrance. They discovered a mutation in the PITXI gene that caused club foot in this family. Through DNA testing, they determined that 11 people in the family carried the \(P I T X I\) mutation, but only 8 of these people had club foot. What is the penetrance of the PITXI mutation in this family?

Short Answer

Expert verified
The penetrance of the PITXI mutation is approximately 72.73%.

Step by step solution

01

Understand Penetrance

Penetrance refers to the percentage of individuals carrying a particular gene mutation that also express an associated trait. In this case, it refers to the percentage of individuals carrying the `PITXI` mutation that exhibit symptoms of club foot.
02

Calculate Total Carriers with Mutation

According to the problem, 11 people in the family carry the `PITXI` mutation. This number represents the total number of carriers in the family.
03

Identify Carriers with the Trait

Out of the 11 people carrying the `PITXI` mutation, only 8 people have club foot. This represents the number of mutation carriers who are actually expressing the trait.
04

Compute Penetrance

Penetrance is calculated by dividing the number of mutation carriers exhibiting the trait by the total number of mutation carriers, then multiplying by 100 to convert it to a percentage. So, the penetrance is \( \frac{8}{11} \times 100 \approx 72.73\%\).

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Key Concepts

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

Understanding Club Foot
Club foot, medically known as talipes equinovarus, is a common congenital condition that affects the skeletal structure of a newborn's foot, causing it to turn inwards or downwards. This condition can lead to difficulties in walking if not treated properly. The exact cause of club foot is not entirely understood, but it is believed to result from a combination of genetic and environmental factors.

Some key points about club foot include:
  • Occurs in approximately 1 in 1000 births worldwide.
  • More common in males than females.
  • Can affect one or both feet.
  • May be treated effectively with physical therapy, casting, or surgery.
Recognizing club foot early in a child's life is essential for effective treatment, allowing for a better outcome as the child grows.
Autosomal Dominant Trait Explained
When a trait is described as autosomal dominant, it means that only one copy of the mutant gene is needed for the trait to be expressed. Autosomal refers to the fact that the gene is located on one of the non-sex (autosome) chromosomes, meaning both males and females are equally likely to inherit the trait.

Here are some essential features of autosomal dominant traits:
  • If a parent carries the gene, each offspring has a 50% chance of inheriting the condition.
  • Both males and females can be affected, and they can pass the gene to their offspring.
  • The trait can appear in every generation of a family.
In the context of the exercise, club foot following an autosomal dominant inheritance pattern was observed in a particular family, related to a mutation in the PITXI gene.
The Role of the PITXI Gene
The PITXI gene plays a crucial role in limb development and morphogenesis. Specifically, this gene encodes a transcription factor involved in developing the limbs, which can influence certain genetic disorders when mutated.

Key aspects of the PITXI gene include:
  • Located on chromosome 5.
  • Involved in regulating other genes important for developmental processes.
  • Mutations can lead to skeletal abnormalities, including club foot.
In the Gurnett et al. study, a specific mutation in the PITXI gene was pinpointed as a cause for the inherited form of club foot in a family's case. Understanding this connection helps researchers develop targeted treatments and interventions for those affected by similar genetic conditions.
Penetrance: What It Means
Penetrance is a concept used in genetics to describe the extent to which a particular genotype manifests in the phenotype of individuals in a population. In simpler terms, it tells us how likely it is for someone with a specific gene mutation to exhibit the trait associated with that mutation.

Key points about penetrance include:
  • Complete penetrance means every individual with the mutation shows the trait.
  • Incomplete penetrance indicates that not everyone with the mutation will show the trait, as seen in the provided example where only 8 out of 11 carriers exhibit club foot.
  • Penetrance is expressed as a percentage, calculated by dividing the number of individuals expressing the trait by the total number of those carrying the mutation, then multiplying by 100.
Understanding penetrance helps geneticists predict the likelihood of traits being passed on, offering valuable insights into the dynamics of genetic inheritance.

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Most popular questions from this chapter

Suppose that you are tending a mouse colony at a genetic research institute, and one day you discover a mouse with twisted ears. You breed this mouse with twisted ears and find that the trait is inherited. Both male and female mice may have twisted ears, but when you cross a twisted-eared male with a normal- eared female, you obtain results that differ from those obtained when you cross a twisted-eared female with a normal-eared male: the reciprocal crosses give different results. Describe how you would determine whether this trait results from a sex-linked gene, a sex-influenced gene, genetic maternal effect, a cytoplasmically inherited gene, or genomic imprinting. What crosses would you conduct, and what results would be expected with these different types of inheritance?

Long ears in some dogs are an autosomal dominant trait. Two dogs mate and produce a litter in which \(75 \%\) of the puppies have long ears. Of the dogs with long ears in this litter, \(1 / 3\) are known to be phenocopies. What are the most likely genotypes of the two parents of this litter?

White eyes is an X-linked recessive mutation in Drosophila that results in eyes that are devoid of the normal red pigment seen in wild-type flies (see Chapter 4 ). The white locus encodes an ABC transporter protein that, when functional, moves a variety of substances across the cell membrane including pigment precursors. When the transporter protein is defective due to a mutation at the white locus, the precursors are not present inside the cell and no eye pigments are produced. Mutations at the white locus also affect mating behavior and the fly's ability to recover from oxygen deprivation. a. What phenomenon is illustrated by the different phenotypic effects of mutations at the white locus? b. Propose an explanation for why mutations at the white eye locus have such differing effects as eye color, mating behavior, and physiology.

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?

A summer-squash plant that produces disc-shaped fruit is crossed with a summer-squash plant that produces long fruit. All the \(\mathrm{F}_{1}\) have disc-shaped fruit. When the \(\mathrm{F}_{1}\) are intercrossed, \(\mathrm{F}_{2}\) progeny are produced in the following ratio: \(9 / 16\) disc- shaped fruit : \(6 / 16\) spherical fruit \(: 1 / 16\) long fruit. Give the genotypes of the \(\mathrm{F}_{2}\) progeny.
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