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.
Autosomes – chromosomes that carry genes NOT directly related to the sex of an organism.
Ex: Humans have 22 pairs of autosomes.
KEY:
Homozygous dominant – curly
Heterozygous – wavy
Homozygous recessive - straight

3. A heterozygote for a recessive disorder is a carrier.
Some autosomal genetic disorders, such as cystic fibrosis are inherited according to Mendel’s principles.
A heterozygote for a recessive disorder is a carrier.
Disorders caused by dominant alleles are uncommon.
(dominant)
Is a carrier homozygous or heterozygous?
What percentage of offspring is expected to be a carrier?
Is it possible for a CC individual to have offspring with cystic fibrosis?
Connect How are Mendel’s observations related to genes on autosomes?

4. 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.

5. Male mammals have an XY genotype.
All of a male’s sex-linked genes are expressed because males have only one copy of each type of sex chromosome.
Males have no second copies of sex-linked genes so there are no second copies of sex linked genes to mask the effects of another allele.

6. 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.
Fig. 1.4 – The female calico cats have two X chromosomes with different alleles are expressed for fur color. Both alleles are expressed in a random pattern. The male cat has only one X chromosome and its allele for fur color is expressed across the entire body.
XO = orange fur allele
Xo = black fur allele

7. Check your understanding…
Infer Why are males more likely than females to have sex-linked genetic disorders?
Apply How might a scientist determine whether a trait is sex-linked by observing the offspring of several genetic crosses?
Connect Scientists hypothesize that over millions of years the Y chromosome has lost genes to the X chromosome. During what stage of meiosis might the Y chromosome have transferred genes to the X chromosome? Explain.