7 1 Chromosomes and Phenotype KEY CONCEPT The

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 Two copies of each autosomal gene affect phenotype. • Some autosomal gene traits, like hair texture, have different patterns of inheritance from Mendelian genetics. 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

7. 1 Chromosomes and Phenotype • 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) 1. Is a carrier homozygous or heterozygous? 2. What percentage of offspring is expected to be a carrier? 3. Is it possible for a CC individual to have offspring with cystic fibrosis? 4. Connect How are Mendel’s observations related to genes on autosomes?

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 sexlinked 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.

7. 1 Chromosomes and Phenotype • Female mammals have an XX genotype. – Expression of sex-linked genes is similar to autosomal gene expression in females. – X chromosome inactivation randomly “turns off” one X chromosome during development and can alter phenotypes. 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. 1 Chromosomes and Phenotype Check your understanding… • Infer Why are males more likely than females to have sexlinked genetic disorders? • Apply How might a scientist determine whether a trait is sexlinked 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.