Chromosomal Basis of Inheritance Tracing Heredity through Inheritance of Specific Chromosomes

Sex Determination Sex Chromosomes: homologous chromosomes that differ in size and genetic composition between males and females Human Chromosome X Y Size and gene number Larger 2142 genes Smaller 326 genes Genetic Composition Multiple genes unrelated to gender Single gene influences gender: TDF = testis determining factor

Sex Determination Autosome: any chromosome that is not a sex chromosome Humans have 22 pairs of autosomes and 1 pair of sex chromosomes

Human Karyotype showing homologous chromosome pairs

Phenotypic Ratio of Offspring Sex Determination Female Male XX x XY ½ X ½ X ½ X ½ Y ¼ XX ¼ XX ¼ XY ¼ XY Phenotypic Ratio of Offspring ½ Female + ½ Male

X-linked Genes Hemophilic Male Non-hemophilic Female (father is hemophilic) XhY x XHXh ½ XH ½ Xh ¼ XHXh ¼ XhXh ½ Xh ½ Y ¼ XHY ¼ XhY Phenotypic Ratio of Offspring ¼ hemophilic males + ¼ non-hemophilic males ¼ hemophilic females + ¼ non-hemophilic females

Terms Specific to Sex-linked Genes Females Homogametic: only X found in gametes Can be homozygous or heterozygous for traits on X-chromosome Males Heterogametic: either X or Y in gametes Hemizygous for traits on the X-chromosome

Patterns of Inheritance Autosomal Dominant Autosomal Recessive X-linked Dominant X-linked Recessive Y-Linked Inheritance

Pedigree for Huntington’s Disease, an Autosomal Dominant Trait

Pedigree for Albinism, an Autosomal Recessive Trait

Pedigree for Colorblindness, an X-linked Recessive Trait