Applied Biology sections 3-8 & 3-9 Pp 80-83 Sex-linked traits Applied Biology sections 3-8 & 3-9 Pp 80-83

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 traits Any trait controlled by a gene on the X or Y chromosome ( sex chromosomes). Most sex linked traits are on the X chromosome. Some human examples: Hemophilia- blood does not clot –bleeding results. Colorblindness- can’t tell the difference between certain colors.

Sex-linked recessive disorders Why do we see the disorders more often in males? Since males have only one X chromosome if it has the recessive gene for the disorder they have the disorder. For females to have the disorder they must inherit two X chromosomes (one from mom and one from dad) with the recessive gene.

Continued…. Females can be carriers- They are heterozygous Have one recessive allele Do not have the disorder but can pass on the recessive allele to offspring. Mothers pass trait on to sons. In a Pedigree- a chart showing inheritance of traits over several generations there will be more males with the disorder.

Pedigree Chart used to record and trace inherited traits in a family over generations. Key- Squares- males Circles- females. Completely shaded shapes- have the trait. Half-shaded-carriers- do NOT show the trait. Parents-connected by horizontal lines (marriage line) Children beneath them. Generations – Roman Numerals ( I, II, III etc) Individuals -Numbered from left to right in each generation.

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