key concept the chromosomes on which genes are located can affect the expression of traits

7.1 Chromosomes and Phenotype

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

7.1 Chromosomes and Phenotype

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.

7.1 Chromosomes and Phenotype

Two copies of each autosomal gene affect phenotype.

• Mendel studied autosomal gene traits, like hair texture.

7.1 Chromosomes and Phenotype

• 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)

7.1 Chromosomes and Phenotype

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.

7.1 Chromosomes and Phenotype

• 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.

7.1 Chromosomes and Phenotype

• 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.

7.1 Chromosomes and Phenotype

• 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.