BIOLOGY 12 Sex-Linked Traits

in humans, sex is determined by the 23rd pair of chromosomes known as “sex chromosomes” XX = female XY = male X: ~ 1400 genes Y: ~200 genes

Chromosome Map

Sex-Linked Traits

for genes on the X chromosome: females will have two alleles, with the dominant masking the recessive males will show the one allele they have

Sex-Linked Traits examples: red-green colour blindness male pattern baldness hemophilia

Red-Green Colour Blindness The inability to distinguish the differences between certain colors. The most common type is red-green color blindness, where red and green are seen as the same color. Usually Ishihara Plates are used to test color vision. They are made of dot patterns composed of primary colors. These dot patterns represent a symbol that is superimposed on a background of randomly mixed colors. The test can determine certain abnormalities in a person's color vision.

Hemophilia

Practice hemophilia is a sex-linked recessive disorder. show the genotypic and phenotypic ratios for a normal heterozygous female crossed with a male with hemophilia note: hemophilia is the inability for the blood to clot

Practice H = normal h = haemophilia XHXH XhXh XhXh XHXhXHXh XhXhXhXh YXHYXHYXhYXhY

Practice Phenotype: 50% of the females will be normal 50% of the females will have hemophilia. 50% of the males will be normal 50% of the males will have hemophilia. Genotype: 25% XHXh 25% XhY 25% XHY 25% XhXh

History Thomas Hunt Morgan studied fruit flies

Fruit Fly Eye Colour GenotypesPhenotypes X1X1X1X4X1X1X1X4 Female with red eyes. X4X4X4X4 Female with white eyes. X1YX4YX1YX4Y Male with red eyes. Male with white eyes. CharacterTraitAlleles Eye ColorRed Eyes 1 White eyes4