Human Sex-Linked Traits

In some cases, males are more likely to express a trait than females. For example, why are there more colorblind men than colorblind women? Certain traits are controlled by genes located on the human sex chromosomes, usually on the X chromosome. These traits are called sex-linked traits. Human sex chromosomes determine the gender of an individual, but they also carry the genes for other traits, including certain genetic diseases. This activity will help you understand how these sex-linked traits are passed on to offspring.

Hemophilia

Hemophilia is a genetic disorder caused by a mutation on the X chromosome. The recessive gene causes proteins that are necessary for clotting blood to be defective. If an individual with hemophilia gets a cut or scrape, the blood cells do not clot properly, and the resulting blood loss can be fatal. Fortunately, the disease can be treated with drugs.

To show that the genes for hemophilia are sex-linked, or linked to the sex chromosomes, we use superscripts written above the X chromosome in a person’s genotype. An XH can symbolize the dominant normal gene, and Xh represents the mutation, or the recessive gene for hemophilia.

1. Complete the following chart:

Genotype Gender Disease (Y/N)? xHxH

A carrier is someone that is ______. In other words, someone that

______, but ______.

2. Fill in with an X or a Y  Sons inherit a _____chromosome from their mother and a_____ chromosome from their father.  Daughters inherit a _____chromosome from their mother and a____ chromosome from their father. 3. Use a Punnet square to determine the probability that a carrier and a hemophilic male will have: a) a child with hemophilia… make sure to tell whether they are male or female or if they are normal or a hemophiliac or a carrier

Red-Green Color Blindness

Not being able to distinguish between the colors red and green is also a sex-linked trait. The normal dominant gene (XB) and the recessive defective gene (Xb) are linked to the X chromosome also. xB – normal xb - colorblind

1. Imagine that a colorblind male mates with a female who is not colorblind, but is a carrier (her dad was colorblind). Use a Punnett square to calculate the probability that your offspring will be normal males? Normal females? Colorblind males? Colorblind females? Carriers? (hint…

2. Diagram a cross between a normal male and a carrier female. Show all work.