Human Chromosomes

Interesting Facts Only about 2% of the DNA in your chromosomes functions as genes. The average human gene consists of 3,000 base pairs. The largest gene has more than 2 million base pairs.

Human Genes and Chromosomes Chromosomes 21 & 22: smallest autosomes Chromosome 22: 545 genes, includes leukemia Chromosome 21: 225 genes and Lou Gehrig’s Disease

Sex-Linked Genes Genes on X & Y chromosomes. X: many sex-linked genes & >100 genetic disorders. Y: much smaller & few genes Males (1 X) all X-linked alleles are expressed, even if they are recessive.

Colorblindness X chromosome In males, only need one defective version of these genes. Red-green is most common; 1:10 men in U.S. Rare in females; 1:100 females

Color Blindness Test Circles

Hemophilia Muscular Dystrophy Missing protein for blood clotting. 1:10,000 males Bleed to death from minor cuts. Internal bleeding from bumps and bruises. Progressive weakening & loss of skeletal muscle. 1:3,000 males

X-Chromosome Inactivation In cells of females, one X is randomly turned off. Barr Body: turned off X forms a dense region. In cats, the gene for coat color is on the X. Because one X is inactive, the cat will have a mixture of colors.

Calico Cats

Chromosomal Disorders Nondisjunction: when homologous chromosomes fail to separate in meiosis. Abnormal numbers of chromosomes in gametes.

Trisomy Trisomy: Down Syndrome: three copies of chromosome 21 2 copies of autosomal chromosomes fail to separate individual is born with 3 copies of a chromosome. Down Syndrome: three copies of chromosome 21 Mild-severe mental retardation Increase susceptibility to diseases Higher frequency of birth defects

Trisomy

Sex Chromosome Disorders Turner’s syndrome (45,X) Sterile Sex organs fail to develop Klinefelter’s syndrome (47,XXY) Usually sterile Develop female characteristics Y chromosome contains a sex-determining region for male sexual development. Even >1 X, Y causes male sexual development. If this region is missing, a female embryo develops.