Sex-linked Traits Ch. 10.5

Sex chromosomes Humans- 23rd pair of chromosomes- X,Y Eggs- Have an X chromosome. Sperm- Half have an X chromosome, half have a Y chromosome. Fathers determine gender(sex) of offspring. Males- XY Females- XX 50% chance of having a girl or boy offpring.

Sex-linked genes Any gene on a sex chromosome- X or Y. Majority are on X chromosome. Examples- Eye color in fruit flies- on the X chromosome Red color is Normal and Dominant, XR White color is Recessive. Xr Females must have 2 recessive alleles to have white eyes. Xr Xr Males need just 1 recessive allele ( they have only 1 X chromosome) Xr Y to have white eyes.

Eye Color in Fruit flies

Human X-linked recessive traits. Hemophilia- bleeding disorder Red-green color blindness. more males with the disorder compared to females. Females can be carriers- have one recessive allele- do not have the disorder, but can pass on the recessive allele to offspring. Mothers pass trait on to sons. Pedigree- Chart used to record and trace the occurrence of a trait in a family. Squares- males Circles- females. Colored shapes- individuals that show the trait. Parents are connected by horizontal lines(marriage line) Children beneath them. Generations – designated by Roman Numerals ( I, II, III etc) Individuals are Numbered left to right in each generation.

Sex-linked recessive pedigree skips generations , see more often in males.