Transcript Lecture 15

Nondisjunction
A. Failure to separate homologous chromosomes during Meiosis
1. nondisjunction leads to aneuploidy
2. nondisjunction of autosomal chromosomes:
a. monosomics
2. nondisjunction of autosomal chromosomes:
b. trisomics
if one of the 5 smaller ones, somewhat viable
13, 15, 18, 21, 22
trisomy 21 and 22 may be survivable into adulthood
3. nondisjunction of sex chromosomes:
Klinefelter syndrome: XXY
Turner syndrome: XO
YY gametes (sperm)  XYY
XX gametes  XXX
OY  not viable
Mutations
A. Random changes in genes
1. rare
2. usually recessive
B. Hemophilia
- loss of any of 12 clotting proteins (genes)
1. recessive
2. 2 important clotting genes are sex-linked
C. Sickle-cell disease (anemia)
1. caused by defective hemoglobin
mutation of one allele causes a single amino acid change
C. Sickle-cell disease (anemia)
2. those homozygous for the sickle-cell allele will have
the disease
C. Sickle-cell disease (anemia)
3. those heterozygous are basically normal
pleiotropy: when a gene produces multiple effects
advantage in resisting malaria
D. Tay-Sachs
1. caused by a recessive allele
2. leads to deterioration of the nervous system (8 months)
3. increased incidence in certain populations
certain Jewish populations
in U.S. : 1 in 27 people of Jewish descent carriers
1 in 250 in general population
Cystic fibrosis
PKU
Inbreeding increases the likelihood of recessive disorders.
Dominant disorders:
Huntington’s disease
Achondroplasia
Pedigrees:
Amniocentesis: performed between weeks 14 and 20.
CVS: performed as early as 8 weeks.
Blood tests:
1. alpha-fetoprotein (AFP)
high in neural tube defect
low in Down syndrome
2. triple screen (now quad screen)
- can even indicate trisomy 18 risk
3. these tests are not diagnostic, only reflective of risk