Human Genetics Studying humans requires alternative methods – human geneticists use medical, historical and family records Human pedigrees – records extending across several generations showing genetic relationships among a set of individuals

Human Pedigree tracing a dominant trait

Human Pedigree tracing a recessive trait

Human Pedigree tracing a sex-linked trait

Human genetic disorders result from mutations 1. Gene Mutations – slight changes in the chemical structure of a gene  Hundreds of human genetic disorders involve defects caused by mutations of single genes (be sure to read about in book)  Disorders caused by mutated recessive alleles: PKU, cystic fibrosis, sickle cell anemia, Tay-Sachs disease

2. Chromosome mutations – result from a gamete having too many or too few chromosomes Results from nondisjunction – failure of the homologous partners to separate during meiosis – homologous pair moves together into the same cellResults from nondisjunction – failure of the homologous partners to separate during meiosis – homologous pair moves together into the same cell

Fertilization my result in trisomy or monosomy Trisomy – presence of 3 chromosomes instead of 2 for a chromosome pair in fertilized egg Example: Down’s Syndrome (Trisomy 21) extra 21 st chromosome Klinefelter’s Syndrome (XXY) Klinefelter’s Syndrome (XXY) Monosomy – presence of only 1 of the homologous partners Example: Turner’s Syndrome

Some genetic disorders are caused by recessive genes that are sex-linked Examples include: hemophilia, color blindness, and Duchenne muscular dystrophyExamples include: hemophilia, color blindness, and Duchenne muscular dystrophy

Many birth defects and genetic abnormalities can be detected before birth Amniocentesis and Chorionic Villi Sampling can be used to get fetal cells to do a karyotype

Karyotyping Fetal cells, cells from bone marrow, skin, or blood are cultured and then treated with colchicine (arrests the cell in metaphase) Chromosomes are photographed and homologous partners are matched Karyotype – refers to both the chromosome composition of an individual and to a photomicrograph showing the chromosomes Can be used to find chromosomal disorders (Down’s syndrome) can be used to do chemical tests to find many genetic disorders such as cystic fibrosis, hemophilia, and Tay-Sachs disease