Download 14-2 Human Chromosomes

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?
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.
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)
Amyotrophic Lateral
Sclerosis
Sex-Linked Genes
• Most found on the X chromosome
• 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
More Sex-Linked Genes
• *Hemophilia
• 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
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
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
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)
• Unable to reproduce