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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.