Sex Chromosomes and Linked Genes

A normal human cell (autosome) contains 46 chromosomes. 22 pairs of regular chromosomes and one pair of sex chromosomes. These sex chromosomes are designated by the letters ___X___ and ______Y___. A human male contains an ___X___ and a ____Y____, while the female contain two ______XX_____'s. During the reproductive process there is a fifty - fifty chance that a male or female will be born. see below: 

Parents are:

X X

X XX XX

Y XY XY

• . Certain other abnormal conditions may develop in offspring from certain genes attached to the sex chromosome X. These are called sex-linked genes. Examples of this are the bleeders disease called Hemophilia and colorblindness.

• Hemophilia is called the bleeders disease. Here a person could bleed to death if the bleeding is not stopped. People with this disease inherited it from their parents. There is no cure for the disease, but an infected person can receive the chemical that is missing to complete the clotting process.

http://library.thinkquest.org/16985/hemophiliamain.htm

• Phenotypes and Genotypes for blood clotting:

• Phenotypes (male) Genotypes (male) Phenotypes (female) Genotypes (female)

• Normal XHY Normal XHXH

• Hemophiliac XhY Carrier XHXh

• Hemophilic XhXh

• 1. What is a carrier? A female appears normal but has one recessive X-linked gene (and one normal X gene)

• 2. Is there such a person called a carrier male? ______NO_______. Explain. If a male has a sex-linked gene it will always show because he only needs one copy to have it show.

• 3. Mate a carrier female with a normal male in the space below.

X Xh

X XXnormal

XXh

carrier

Y XYnormal

XhYhemophiliac

• a). What are the chances of these parents having normal sons? _____50%_______.

• b). What are the chances of these parents having normal daughters? _____100%_

• Colorblindness is inherited by the same process. Here the person infected with the disease cannot distinguish red colors from green colors.

• 4. Mate a colorblind male with a normal female in the space below.

Parents are:

X XXc XXc XXc

Y XY XY• a). What are the chances of these parents

having normal sons? 100%.

• b). What are the chances of these parents having normal daughters? 100%.