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.