KAROTYPING CHROMSOME ABNORMALITIES

* Usually done from the 15th or 16th week to the 20 th week (2 nd trimester). *Only at this point in a pregnancy is there sufficient amniotic fluid to allow some of it to be drawn off without significant danger to the fetus. * Only about.5% risk of the test causing a miscarriage.

*In chorionic villi sampling (or biopsy), a small sample of chorion cells are collected. Chorion is a membrane that develops around an embryo and contributes to the formation of the placenta. Done beginning in the 10th- 12th week (1 st trimester). * Risk of miscarriage is at least 1-3%, or 2-6 times higher than with amniocentesis.

*A band is defined as that part of a chromosome which is clearly distinguishable from its adjacent segments by appearing darker or brighter with one or more banding techniques. *The chromosomes are visualized as consisting of a continuous series of bright and dark bands. *The banding techniques fall into two principal groups: 1) Bands distributed along the length of the whole chromosome (EX: G-banding) 2) Bands stained at a restricted number of specific sites. C-banding methods do not permit identification of every chromosome in the somatic cell, but can be used to identify specific chromosomes.

*G-bands are most commonly used. Shows bright and dark bands in an alternating fashion. *They take their name from the Giemsa dye. *Dark regions are heterochromatic. *Areas with a lot of adenine and thymine *Contains no functional genes *The bright regions are euchromatic. *Areas with a lot of guanine and cytosine *Contains functional genes (make proteins)

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AUTOSOMAL ABNORMALITIES The majority of human chromosomal abnormalities occur in the autosomes. *Most common: 1. M onosomy: one copy of a chromosome -- All fetuses spontaneously abort early in pregnancy. 2. Trisomy: 3 copies of a chromosome -- Almost all fetuses die before birth

TRISOMY 21 (DOWN’S SYNDROME)

*1 in 800 live births * Approximately 85% spontaneously abort. *About 2-4% (of 350,000) are genetically mosaic. -- Some of their cells have chromosome 21 trisomy while others do not, resulting in generally milder symptoms

TRISOMY 18

Trisomy 18

Characteristic clenched hand Rocker bottom feet

SEX CHROMSOMAL ABNORMALITIES *In frequency of occurrence, they are only slightly less common than autosomal abnormalities. They are usually much less severe in their effects. The high frequency of people with sex chromosome aberrations is partly due to the fact that they are rarely lethal conditions. * Sex chromosome gross abnormalities can be diagnosed before birth by amniocentesis and chorionic villi sampling.

KLINEFELTER’S SYNDROME

1 in 500 and 1 in 1000 male births *Genotype is XXY or more rarely XXXY, XXXXY *Relatively high-pitched voices (low levels of testosterone) *Asexual to feminine body contours as well as breast enlargement (low levels of testosterone) *Comparatively little facial and body hair. *Sterile or nearly so KLINEFELTER’S SYNDROME

“SUPER-MALE” *It may be as common as 1 in 900 male births to as rare as 1 in 1500 or even 1 in 2,000. *Genotype is XYY Usually tall (above 6 feet) and generally appear and act normal During adolescence, they often are slender, have severe facial acne, and are poorly coordinated *Some researchers suggest that the high testosterone levels can make them somewhat more prone to violence and that this may cause higher rates of wife beating.

TURNER’S SYNDROME

*Genotype is XO 1 in 3,000 female infants to 1 in 5,000 4 foot 7 inches as adults, distinctive webbed necks (i.e., extra folds of skin), small jaws, and high arched palates. *If diagnosed in early childhood, regular injections of human growth hormones can increase their stature by a few inches. * Beginning around the normal age of puberty, estrogen replacement therapy can result in some breast development and menstruation.