Nondisjunction
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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.

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Nondisjunction

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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


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