Objectives Relate dominant-recessive patterns of inheritance in autosomal chromosomes to genetic disorders. Describe patterns of inheritance in sex-linked traits.

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

Vocabulary Carrier – organism whose genome contains a gene for a certain trait or disease that is not expressed in the organism's phenotype. Sex-linked gene – gene that is located on a sex chromosome. X chromosome inactivation – process that occurs in female mammals in which one of the X chromosomes is randomly turned off in each cell.

Two copies of each autosomal gene affect phenotype. Mendel studied autosomal gene traits, like hair texture. Other examples of autosomal gene traits: Hairline Earlobes Tongue Rolling Hitch-hiker’s thumb Hairline: Dominant widows peak Earlobes: Dominant free lobes Tongue Rolling: Dominant Roller Thumb: Dominant Straight

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) Why do you think Disorders caused by dominant alleles are uncommon? Answer on your EXIT TICKET

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.

WHY? 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. WHY?

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.