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

2. 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.

3. 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.

4. 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.

5. 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.

6. 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.

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