Question

If a male hemophiliac \(\left(\mathrm{X}^{\mathrm{h}} \mathrm{Y}\right)\) is crossed with a female carrier of both color blindness and hemophilia \(\left(\mathrm{X}^{\mathrm{c}} \mathrm{X}^{\mathrm{h}}\right)\), what is the probability that a female child will be phenotypically normal? A. \(0 \%\) B. \(25 \%\) C. \(50 \%\) D. Same as for a male child

Short answer

A. 0%

Step-by-step solution

- Understand the Genetic Notations

Hemophilia is denoted by \(\text{X}^h\). Color blindness is denoted by \(\text{X}^c\). The normal allele is \(\text{X}\).

- Determine the Genotypes of Parents

The genotypes given are: Male: \(\text{X}^h\text{Y}\); Female: \(\text{X}^c\text{X}^h)\).

- Set Up the Punnett Square

Cross the male \(\text{X}^h\text{Y}\) with the female \(\text{X}^c\text{X}^h\).

- Fill in Punnett Square

Using the Punnett Square, list all possible combinations of alleles for the offspring. The combinations are \(\text{X}^h\text{X}^c\), \(\text{X}^h\text{X}^h\), \(\text{X}^c\text{Y}\), and \(\text{X}^h\text{Y}\).

- Identify Female Offspring and Their Phenotype

From the Punnett Square, identify the female offspring: \(\text{X}^h\text{X}^c\) and \(\text{X}^h\text{X}^h\). Both will be affected either by hemophilia or color blindness.

- Determine Probability of Phenotypically Normal Female

Neither of the female offspring combinations result in a phenotypically normal female.

Key concepts

Punnett Square

To understand genetics problems, like the one given, we often use a tool called a Punnett Square. This is a simple grid that helps predict the possible genetic outcomes when two organisms reproduce. By setting up and filling in a Punnett Square, we can easily visualize how alleles from each parent can combine.
Let's break it down:

• Each parent contributes one allele for each trait.
• The alleles can be either dominant or recessive.
• These combinations will then determine the traits of the offspring.

The Punnett Square for our exercise was used to predict the genetic outcomes of a male hemophiliac \( \text{X}^h \text{Y} \) mating with a female carrier of both color blindness and hemophilia \( \text{X}^c \text{X}^h \).

sex-linked traits

Sex-linked traits are traits that are determined by genes located on sex chromosomes. These chromosomes are X and Y. In many genetic problems, like the one in the exercise, these traits are often linked to the X chromosome. Since males have one X chromosome and one Y chromosome (XY) and females have two X chromosomes (XX), traits linked to the X chromosome are inherited differently in males and females.
For example:

• Males (XY) are more likely to express sex-linked traits because they only have one X chromosome. If that one X chromosome carries a recessive trait, it will be expressed.
• Females (XX), on the other hand, have two X chromosomes. For a recessive sex-linked trait to be expressed, both X chromosomes must carry the trait.

In our exercise, hemophilia and color blindness are examples of recessive sex-linked traits. The specific genetic notations used were \( \text{X}^h \) for hemophilia, \( \text{X}^c \) for color blindness, and \( \text{X} \) for a normal allele.

genotype

The term genotype refers to the genetic makeup of an organism, specifically the alleles that an individual possesses. In the exercise, the genotypes of the parents were given as:

• Male: \( \text{X}^h \text{Y} \)
• Female: \( \text{X}^c \text{X}^h \)

These notations tell us exactly which alleles for the traits are present in each parent. The term genotype contrasts with phenotype, which is the observable physical or biochemical characteristics of an organism.
For the offspring, several genotype combinations were possible based on the parent's genotypes. They are:

• \( \text{X}^h \text{X}^c \) - Female with hemophilia and color blindness
• \( \text{X}^h \text{X}^h \) - Female with hemophilia
• \( \text{X}^c \text{Y} \) - Male with color blindness
• \( \text{X}^h \text{Y} \) - Male with hemophilia

Analyzing the genotypes through the Punnett Square helped determine that neither female offspring would be phenotypically normal, thus answering the exercise question.