Nondisjunction

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 viable13, 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

MutationsA. Random changes in genes

1. rare2. usually recessive

B. Hemophilia- loss of any of 12 clotting proteins (genes)1. recessive2. 2 important clotting genes are sex-linked

C. Sickle-cell disease (anemia)1. caused by defective hemoglobinmutation 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

advantage in resisting malariapleiotropy: when a gene produces multiple effects

D. Tay-Sachs1. caused by a recessive allele2. leads to deterioration of the nervous system (8 months)3. increased incidence in certain populations

certain Jewish populationsin U.S. : 1 in 27 people of Jewish descent carriers

1 in 250 in general population

Cystic fibrosisPKU

Inbreeding increases the likelihood of recessive disorders.

Dominant disorders:Huntington’s diseaseAchondroplasia

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

2. triple screen (now quad screen)

3. these tests are not diagnostic, only reflective of risk

low in Down syndrome

- can even indicate trisomy 18 risk