1. 14-2 Human Chromosomes Fill-in Notes Questions to think about:
*Why are sex-linked disorders more common in males than in females?
*What is nondisjunction and what problems does it cause?

2. The Human Genome Genes are scattered throughout DNA
Not all DNA codes for a protein
(Remember introns vs. exons!)
Genes range from 3,000 to 2,000,000 base pairs
The human genome is like a library containing thousands of books.
We don’t know what all of them are, but we do know how many there are and in what order.

3. Genes & Chromosomes
Genes that are close together on the same chromosome are linked, or inherited together.
Chromosomes 21 & 22 are the smallest and best studied chromosomes.
Chromosome 22 has 545 genes
Leukemia
Neurofibromatosis (causes tumors of the nervous system)
Chromosome 21 has 225 genes
ALS (Lou Gehrig’s Disease)

4. Amyotrophic Lateral Sclerosis

5. Sex-Linked Genes Most found on the X chromosome *Colorblindness
Y chromosome is very small
Over 100 genes on X chromosome
*Colorblindness
Affects 1/10 males and 1/100 females
Males only have 1 X chromosome and only need one recessive allele to be affected
Females would need 2 copies

7. More Sex-Linked Genes *Hemophilia *Duchenne Muscular Dystrophy
Affects 1/10,000 males
2 genes on X chromosome affect blood clotting
A recessive allele can cause missing protein needed for blood clotting
*Duchenne Muscular Dystrophy
Affects 1/3,000 males
Defective version of gene on X chromosome that codes for muscle protein

9. X-Chromosome Inactivation
In females, one X chromosome can randomly be switched off
Forms a dense region in the nucleus called a Barr body
Example: Cats  Almost all calico cats are female.
One X allele for black spots
One X allele for orange spots

10. Chromosomal Disorders
Nondisjunction = chromosomes fail to separate during meiosis
Leads to abnormal number of chromosomes in gametes
*Down Syndrome
Affects 1/800 babies
3 copies of chromosome 21 (trisomy)
Delayed physical development and mild to severe intellectual disabilities

12. Sex Chromosome Disorders
*Turner’s Syndrome
Nondisjunction leads to females with only 1 X chromosome (45, X)
Unable to reproduce
*Klinefelter’s Syndrome
Nondisjunction leads to males with an extra X chromosome (47, XXY)