Sex Determination and Sex Linkage

Sex Determination Recall, sex chromosomes determine the sex of an individual. The female gamete (egg) always donates an X chromosome The male gamete (sperm) donates either an X or Y chromosome XX  Female XY  Male

Sex Linked Traits In 1910, Scientist Thomas Morgan studied eye colour in fruit flies (Drosophila melanogaster)

He crossed 2 red-eyed parent flies Result? Produced white-eyed male fly Morgan thought this outcome followed Mendel’s Law of Dominance : R  Dominant (red eye colour) r  Recessive (white eye colour)

So he then crossed White-eyed male and Red-eyed female Morgan was not able to obtain a white-eyed offspring, so… he concluded that eye colour was associated with the X chromosome P.177

Human Sex Linked Traits X chromosome carries a great variety of traits that are called X-linked Y chromosome only carries a few known traits that are called Y-linked **unless specified, assume the sex-linked traits discussed are X-linked.

Many X-linked disorders or diseases are recessive, therefore we use the following: X N Normal allele X n Recessive allele Note: Y stands alone

In females, the recessive trait will be expressed if X n X n is present. If the female is heterozygous for the trait, X N X n, then the trait is not expressed but is present and will be passed on – This female is called a GENETIC CARRIER In males, the recessive trait will be expressed if X n Y is present. (**Notice, only one recessive allele needs to be inherited in order to be expressed)

Example 1: Red-Green Colour blindness (RGCB) Person that expresses (RGCB), cannot distinguish shades of red and green. Their ability to see remains normal. Red and Green pigments called opsins are found in specific cells called cones in the retina. The genes that express these opsins are located on the X chromosome

Example 1: Red-Green Colour blindness (RGCB) People with RGCB, either have A defective allele for red pigment OR A defective allele for Green pigment When one allele is defective, then the person cannot distinguish between Red and green.

RGCB is a recessive X-linked trait X B X b **Carrier X b Female Expresses RGCB X b Y Male Expresses RGCB X B Y Normal Male ** The woman is a heterozygous carrier because she carries one allele that will be passed on but will not express RGCB

Sample Problem: A man with RGCB marries a woman that is a carrier for RGCB. Determine the possibility of their son being born with RGCB. What is required? Possibility of son possessing RGCB What is given: Mother  X B X b Father  X b Y

Solution: X b Y X B X b XB YXB Y Xb XbXb Xb Xb YXb Y XBXbXBXb The son will have a 50% chance of being born with RGCB

Hemophilia A Genetic disease where blood cannot clot properly at the site of an injury Coagulation proteins are involved in the clotting of blood and become active at a wound. People with Hemophilia lack one of these coagulation proteins.

People with hemophilia also: Bruise easily May experience internal bleeding into their joints and muscles And are at risk of dying even from minor cuts Frequency: 1 in 10,000 males

Recessive X-linked disease H  normal expression h  hemophilia X H X h **Carrier X h Female with hemophilia X h Y Male with hemophilia X H Y Normal Male

Sample Problem: A man that expresses hemophilia marries a woman that does not express this disease. What is the possibility of their son expressing hemophilia? What is required? Possibility of son with hemophilia What is given? Dad  X h Y Mom  X H X H

Solution: X H X h XH YXH Y XHYXHY XhYXhY XHXHXHXH The son has no possibility of expressing hemophilia. The daughter will be a carrier of the disease.