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
•Missing protein for
blood clotting.
•1:10,000 males
•Bleed to death from
minor cuts.
•Internal bleeding
from bumps and
bruises.
Muscular Dystrophy
•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:
• 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 sexdetermining region for male sexual
development.
• Even >1 X, Y causes male sexual
development.
• If this region is missing, a female embryo
develops.