Question

Hemophilia, a genetic blood disorder, is caused by a recessive sex-linked gene. A phenotypically normal couple had a son with hemophilia. What is the probability that their next child, if a girl, would also have hemophilia?

Short answer

The probability that their next child, if a girl, would also have hemophilia is 25%.

Step-by-step solution

Identifying the parents' genotypes

In a sex-linked gene, the hemophilia gene in question is located in the X chromosome. Males have one X and one Y chromosome (XY), whereas females have two X chromosomes (XX). Since the father is phenotypically normal, he must have no faulty hemophilia gene in his X chromosome: XY The woman has a normal phenotype too, but her son does have hemophilia. That means she is a carrier, and her genotype is XhX, where 'Xh' denotes the recessive hemophilia gene. So the parents' genotypes are: Mother: XhX Father: XY

Setting up a Punnett square

A Punnett square is a diagram that is used to predict the probability and ratio of genotypes of offspring from a particular cross or breeding event. We will set up a Punnett square to represent the possible genotypes of the next child. X Y _______________ Xh | XhX | XhY | _________ X | XX | XY | _________

Interpreting the Punnett square

The Punnett square now shows us the genotypes of the possible offspring from this couple: - XhX: female child with a carrier mother (25% chance) - XhY: male child with hemophilia (25% chance) - XX: normal female child (25% chance) - XY: normal male child (25% chance)

Calculating the probability

The exercise asks for the probability of a girl with hemophilia. There is only one possible genotype for this scenario, which is XhX (carrier mother). The chance of having a girl with such genotype is 25% (1 out of 4 possibilities in the Punnett square). So, the probability that their next child, if a girl, would also have hemophilia is 25%.

Key concepts

hemophilia

Hemophilia is a genetic disorder where blood doesn't clot properly. This can lead to excessive bleeding from even minor injuries. The condition is primarily due to a defect in one of the genes located on the X chromosome, which plays a crucial role in blood clotting.

Since hemophilia is linked to this specific chromosome, it often affects males more frequently. Men have just one X chromosome, so if it's defective, they will likely exhibit symptoms of hemophilia. In contrast, females have two X chromosomes. This means they can carry the defective gene on just one chromosome and not show symptoms if the other X chromosome is normal.

However, females can still be carriers of the disorder and pass it on to their children, which is an important aspect of understanding genetic inheritance. Being aware of family history can help in identifying those who might be carriers of hemophilia.

sex-linked genes

Sex-linked genes are those located on the sex chromosomes, specifically the X or Y chromosomes in humans. These chromosomes determine our biological sex, with females having two X chromosomes (XX) and males having one X and one Y chromosome (XY).

When a gene associated with a particular condition is found on these chromosomes, it is termed a "sex-linked gene." The majority of sex-linked genes are found on the X chromosome since it carries more genes than the Y chromosome. Disorders like hemophilia and color blindness are examples of conditions caused by mutations in these genes.

It's also important to note that males are more likely than females to express X-linked recessive disorders. This is because they possess only one X chromosome. Thus, any mutation that occurs there can result in the full expression of the disorder.

Punnett square

The Punnett square is a simple and widely used tool in genetics to predict the genotypes of offspring from a genetic cross. Named after Reginald C. Punnett, this diagram helps clarify the probability of inheriting specific traits.

To create a Punnett square, you draw a grid and input the gametes of each parent along the top and side of the square. By filling in the grid, you can see all possible combinations of parental alleles and thus the potential genotypes of their offspring.

In our hemophilia example, the mother is a carrier (XhX) while the father has normal wild-type alleles (XY). The Punnett square shows the likelihood of each child inheriting a particular genotype. This makes it a vital tool in analyzing the probability of inheritance patterns for various genetic disorders.

recessive genes

Recessive genes are those whose traits are not expressed unless two copies of the gene are present. A single dominant allele can mask the presence of a recessive gene. In the context of genetic disorders, if a disorder is caused by a recessive gene, then an individual must inherit two copies of that faulty gene to display the disorder.

In the case of hemophilia, the gene responsible is recessive and located on the X chromosome. While males are directly impacted if they inherit this gene because they have only one X chromosome, females generally require two copies of the recessive gene (one on each X chromosome) to exhibit the symptoms.

This mechanism of genetic inheritance explains why carriers exist. Carriers have one normal and one affected gene, typically not displaying symptoms but capable of passing the recessive gene to their children. Understanding how recessive genes work helps unravel the complexities of genetic disorders and inheritance.