1. Unit 8: Mendelian Genetics 8
Unit 8: Mendelian Genetics 8.6 Other Patterns of Inheritance: Sex-Linked Traits

2. Humans have 22 pairs of autosomes and 1 pair of sex chromosomes.
Males are XY
Males donate their Y chromosome to their sons only and their X chromosome to their daughters.
They determine the sex of the child!
Females are XX
Females donate one or the other of their X chromosomes to their sons and daughters.
Chromosomes

3. Sex-Linked The gene is carried on a sex chromosome (x or y)
Occur most often in males because they only receive one X chromosome from their mother.
Most of these genes are recessive, which means mom can be a carrier and pass the trait to her offspring
A carrier is a female who is heterozygous for a trait and can pass the trait on to her
offspring but does not show
symptoms of the trait.
Examples:
Hemophilia
Red-green color blindness
Sex-Linked

4. Using a Punnett Square for Sex-Linked
Determine the genotypes of the parents
Mom is a carrier: Xx heterozygous
Mom is NOT a carrier: XX homozygous
Mom shows trait: xx homozygous recessive
Dad “normal”: XY
Dad with trait: xY
Place the traits onto the Punnett square as you have before
Cross the traits in the square
Determine the genotype and phenotype of the offspring
Male = XY, xY
Female = XX, Xx
Using a Punnett Square for Sex-Linked

5. Example with Sex-Linked
Carol is a “carrier” for a recessive sex-linked trait that causes red- green color blindness. Carol has a baby with Bill, who is normal.
a. parent genotype
b. offspring genotype
c. offspring phenotype
d. proportion of male offspring
e. Proportion of offspring with red-green color blindness.

6. Parents:
The McBride’s: type O and AB (1 Punnett)
The Gonzales’: type A and B (4 Punnetts)
The Pepronis: type A and O (2 Punnetts)
Children:
Baby A is type AB
Baby B is type O
Baby C is type B
1. Who are the parents of baby A? A: Gonzales 2. Who are the parents of baby B? A: Pepronis 3. Who are the parents of baby C? A: McBride
Practice In a hospital, 3 babies were born simultaneously during a thunderstorm. The lights went off and the babies were mixed up. The hospital wants to match the baby with its parents by looking at the blood types. Can you help?