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

2. Objectives
Students will be able to:
Relate dominant-recessive patterns of inheritance in autosomal chromosomes to genetic disorders.
Describe patterns of inheritance in sex-linked traits.

3. Two copies of each autosomal gene affect phenotype.
Mendel studied autosomal gene traits, like hair texture.

4. Mendel’s rules of inheritance apply to autosomal genetic disorders.
A heterozygote for a recessive disorder is a carrier.
Disorders caused by dominant alleles are uncommon.
(dominant)

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. Review
How are dominant-recessive patterns of inheritance in autosomal chromosomes related to genetic disorders?
What are the patterns of sex-linked traits?
How are Mendel’s observations related to genes on autosomes?
Why are males more likely than females to have sex-linked genetic disorders?
How are autosomal traits, including recessive genetic disorders that are carried in a population, related to Mendel’s observations of heredity?
Describe how sex-linked genes are expressed differently in males and in females.
How might a scientist determine whether a trait is sex-linked by observing the offspring of several genetic crosses?
How is the expression of sex-linked genes both similar to and different from the expression of autosomal genes?
Scientists hypothesize that over millions of years, the Y chromosome has lost genes to the X chromosome. During what stages of meiosis might the Y chromosome have transferred genes to the X chromosome? Explain.