1. KEY CONCEPT The chromosomes on which genes are located can affect the expression of traits.

2. Two copies of each autosomal gene affect phenotype.
Mendel studied autosomal gene traits, like hair texture.
An autosome is any of the numbered chromosomes, as opposed to the sex chromosomes. Humans have 22 pairs of autosomes and one pair of sex chromosomes (the X chromosome and the Y chromosome).

3. A heterozygote for a recessive disorder is a carrier.
Mendel’s rules of inheritance apply to autosomal genetic disorders. He used pea plants.
A heterozygote for a recessive disorder is a carrier.
Disorders caused by dominant alleles are uncommon.
(dominant)
Hetero – different
Homo- same

4. An example of a dominant single gene disorder is Huntington’s disease (A progressive neurodegenerative disorder, caused by a mutation in a single gene called huntingtin (HD), which usually develops in middle to late adult life. The mutation results in neuronal cell death in select areas of the brain and causes uncontrollable muscle spasms.), which is a disease of the nervous system.

5. Males and females can differ in sex-linked traits.
Genes on sex chromosomes are called sex-linked genes.
Y chromosome genes in mammals are responsible for male characteristics.
X chromosome genes in mammals affect many traits.

6. Male mammals have an XY genotype.
All of a male’s sex-linked genes are expressed.
Males have no second copies of sex-linked genes.

7. Female mammals have an XX genotype.
Expression of sex-linked genes is similar to autosomal genes in females.
X chromosome inactivation randomly “turns off” one X chromosome.

8. Examples of X-linked recessive disorders are hemophilia - disorder characterized by slow and inefficient formation of blood clots leading to prolonged bleeding and spontaneous internal bleeding and muscular dystrophy - an inherited disorder that gradually causes the muscles to weaken.
Are X linked recessive disorders more common in males or females?
X-linked recessive disorders are much more common in males than females because two copies of the mutant allele are required for the disorder to occur in females, while only one copy is required in males.