Question

Duchenne muscular dystrophy (DMD), marked by muscular degeneration, results from an \(\mathrm{X}\) - linked recessive gene. Thus, a female who is heterozygous for this gene and does not have the disease can be a carrier. What kind of offspring can you expect from a DMD-affected male and a carrier female? Can there be a carrier male?

Short answer

Explain your answer. Answer: No, there cannot be a carrier male in this situation. A carrier male would have the genotype X_dY, meaning they would also be affected by DMD since they only have one X chromosome, which contains the defective gene. In this scenario, males either have the affected genotype (X_dY) or the unaffected genotype (XY).

Step-by-step solution

Understand the genotypes of the parents

A DMD-affected male has the genotype X_dY, where X_d is the defective X chromosome. A carrier female has the genotype X_dX, where X is the normal X chromosome.

Set up a Punnett square

A Punnett square is a diagram that is used to predict the possible genotypes and outcomes of offspring from the cross of two genetic parents. To create our Punnett square, draw a 2x2 grid and write the possible gametes (reproductive cells) of the parents along the top and side of the grid. For the DMD-affected male, the gametes will be X_d and Y. For the carrier female, the gametes will be X_d and X.

Fill the Punnett square

Fill the boxes of the Punnett square by combining the gametes from each parent. This will give us the following combinations: - X_dX_d: affected female - X_dX: carrier female - X_dY: affected male - XY: unaffected male

Identify the offspring outcomes

Looking at the Punnett square, the following outcomes for the offspring are: 1. 50% chance of being an affected female (X_dX_d) 2. 50% chance of being a carrier female (X_dX) 3. 50% chance of being an affected male (X_dY) 4. 50% chance of being an unaffected male (XY) Keep in mind that this is a probability, so individual offspring may not follow these exact ratios, but with a large enough sample size, these ratios would be observed.

Analyze whether there can be a carrier male

A carrier male would have the genotype X_dY, meaning they would also be affected by DMD since they only have one X chromosome, which contains the defective gene. Therefore, there cannot be a carrier male in this situation.

Key concepts

Punnett square

A Punnett square is a simple graphical tool geneticists use to predict the possible genotypes of offspring from two parents. It's a 2x2 grid that helps visualize genetic combinations.
When setting up a Punnett square, you'll list one parent's alleles (variations of a gene) along one side and the other parent's alleles along the adjacent side. Each box within the grid represents a potential combination of alleles from the parents.
Here's how it works step-by-step:

• Draw a square and divide it into four smaller squares, creating a 2x2 grid.
• Write the two types of alleles from one parent across the top (one per column).
• Write the two types of alleles from the other parent along the side (one per row).
• Fill in the boxes by combining the alleles from the top and the side of each box.

This technique not only illustrates the potential outcomes of a genetic cross but also helps determine the probability of each genotype occurring in the offspring.

Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is a genetic disorder characterized by progressive muscle degeneration and weakness. It results from mutations in the DMD gene, which is responsible for producing dystrophin. Dystrophin is a protein that helps keep muscle cells intact.
DMD follows an X-linked recessive inheritance pattern, meaning the gene responsible for DMD is located on the X chromosome. Since males (XY) have only one X chromosome, they are more likely to exhibit symptoms if they inherit the defective gene. Females (XX), having two X chromosomes, are usually carriers and often do not express symptoms if they inherit a normal copy of the gene in addition to the defective one.
To understand the implications:

• Males with the defective gene (X_dY) will have DMD.
• A female with one defective gene (X_dX) typically does not show symptoms.

DMD affects approximately 1 in 3,500 male births worldwide and currently has no cure, although treatments can help manage symptoms and improve quality of life.

Genetic carrier

A genetic carrier is an individual who has one copy of a mutated gene that can lead to a genetic disorder, but does not typically show symptoms of the disorder themselves. This is especially applicable in X-linked recessive conditions like Duchenne muscular dystrophy.
In the context of DMD:

• A carrier female has one normal X chromosome and one X chromosome with the defective DMD gene (X_dX).
• These women do not manifest the disease because the normal gene compensates for the defective one.

When a carrier female and an unaffected male have children, the genetic results can vary. Sons are more likely to be affected because they inherit one X chromosome—if it’s the defective one, they will express the disorder.
It's important to note that carrier males in an X-linked recessive condition do not exist because males will express the disorder due to the absence of a second X chromosome. Understanding carrier status is crucial in genetic counseling, especially for diseases that follow this inheritance pattern.