1. Sex-Linked Inheritance

2. Human Chromosomes Two groups of chromosomes:
Autosome: a chromosome that does not determine gender.
Humans have 22 pairs or 44 chromosomes.
Sex chromosome: a chromosome that determines gender.
Humans have one pair of sex chromosomes.

3. Sex Chromosome Characteristics
Humans have two possible sex chromosomes. X Y
XX is the female genotype
XY is the male genotype

4. Characteristics of the X chromosome
Contains many active genes that affect many different body systems.
One X chromosome is required for life.
When more than one copy of the X chromosome is present, the body turns off all but one copy in each cell.

5. Characteristics of the Y chromosome
Very small chromosome with few active genes.
Contains genes that trigger male development in fetuses.
The presence of a Y chromosome means the person is male.

6. Effects of sex chromosomes on inheritance
Traits on the X chromosome show difference dominance patterns in males and females.
In sex-linked punnett squares, the gender of the offspring is important.

7. Sex-Chromosomes in Females
Females can have three different possibilities for genotypes.
XBXB – homozygous dominant
(dominant trait)
XBXb – heterozygous
(dominant trait; carriers)
XbXb – homozygous recessive (recessive trait)

8. Carriers Women who have XBXb genotype are called carriers.
Carriers show the dominant trait, but can pass the recessive trait on to their children.

9. Sex Chromosomes in Males
Males have two possible genotypes.
XBY – Dominant (dominant trait)
XbY – Recessive (recessive trait)
Note: The Y chromosome does not have the same genes as the X chromosome. There is no genotype with the Y chromosome.