1. Human Chromosomes

2. 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.

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

4. 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.

5. 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

6. Color Blindness Test Circles

7. 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

8. 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.

9. Calico Cats

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

11. 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

12. Trisomy

13. 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.