A couple are both phenotypically normal, but their son suffers from hemophilia, a sex-linked recessive disorder.
What fraction of their children are likely to suffer from hemophilia?
What fraction are likely to be carriers?

Verified step by step guidance

1. Step 1: Understand the inheritance pattern of hemophilia. Hemophilia is a sex-linked recessive disorder, meaning the gene responsible for the condition is located on the X chromosome. Males (XY) only have one X chromosome, so if they inherit the defective gene, they will express the disorder. Females (XX) need two copies of the defective gene to express the disorder; if they have one defective gene, they are carriers.
2. Step 2: Determine the genotypes of the parents. Since the couple is phenotypically normal, the mother must be a carrier (X^HX) because their son has hemophilia. The father is normal (X^HY) because males cannot be carriers of X-linked traits.
3. Step 3: Set up a Punnett square to predict the offspring's genotypes. Place the mother's alleles (X^H and X) on one axis and the father's alleles (X^H and Y) on the other axis. This will allow you to calculate the probabilities of different combinations of alleles in their children.
4. Step 4: Analyze the Punnett square results. The possible offspring genotypes are: X^HX^H (normal female), X^HX (carrier female), X^HY (normal male), and XY (male with hemophilia). Determine the fraction of children with each genotype.
5. Step 5: Interpret the results. Based on the Punnett square, calculate the fraction of children likely to suffer from hemophilia (XY) and the fraction likely to be carriers (X^HX). Express these fractions as probabilities or percentages.

Key Concepts

Sex-Linked Inheritance

Sex-linked inheritance refers to genes located on the sex chromosomes, particularly the X chromosome. In humans, males have one X and one Y chromosome, while females have two X chromosomes. Disorders like hemophilia are often recessive and linked to the X chromosome, meaning that males are more likely to express the disorder if they inherit the affected X, while females may be carriers if they have one affected X and one normal X.

Recessive Alleles

Recessive alleles are variants of a gene that do not manifest in the phenotype unless two copies are present. In the case of hemophilia, the allele responsible for the disorder is recessive, meaning that a male with one affected X chromosome will express hemophilia, while a female must have two affected X chromosomes to show symptoms. This concept is crucial for understanding how traits are passed on and expressed in offspring.

Punnett Square

A Punnett square is a diagram used to predict the genetic outcomes of a cross between two parents. It helps visualize the possible combinations of alleles that offspring can inherit. By setting up a Punnett square for the couple in the question, we can determine the likelihood of their children being affected by hemophilia or being carriers, providing a clear method to analyze inheritance patterns.