Karyotypes & Chromosomal Disorders Journal Notes

Human Karyotype

Karyotypes photograph of one’s chromosomes, grouped in pairs of homologous chromosomes by size Humans have 1 pair of sex chromosomes (XX or XY) and 22 pairs of autosomes (chromosomes that are not sex chromosomes).

Chromosome Abnormalities All abnormalities result from an error during meiosis, called “nondisjunction” non – dis – junction The problem arises when during meiosis there is a failure of homologous chromosomes to separate.

Non-disjunction of chromosomes

Nondisjunction of chromosomes If non-disjunction occurs, one gamete gets both chromosomes, the other gamete gets nothing.

Analyzing Karyotypes: 1. Are there 46 chromosomes? 2. Are there 2 identical chromosomes in each pair of autosomes and 2 sex chromosomes? 3. Are there any rearrangements between chromosomes or large, obvious deletions?

Karyotypes DO NOT show: Individual DNA strands or genes or the DNA sequence The number of genes in a chromosome The presence of gene mutations

6 Chromosome Disorders caused by Nondisjunction Down syndrome (trisomy 21) Edwards syndrome (trisomy 18) Klinefelters syndrome (trisomy XXY) XYY syndrome (trisomy XYY) XXX syndrome (trisomy X) Turners syndrome (monosomy XO) **Nearly all other nondisjunctions lead to death of embryo or child before adulthood!

Chromosome Abnormalities Down Syndrome – Extra copy of chromosome 21. Also called “Trisomy 21” Have 47 chromosomes. One of only a few trisomy disorders in which the child has high probability of living to adulthood Short stature Mental retardation Reduced life span Characteristic features

Down Syndrome Lauren Potter from “Glee”

Klinefelter Syndrome XXY – Male 47 Chromosomes Sterile trisomy of the sex chromosomes 47 Chromosomes Sterile Low testosterone Taller than average Often don’t know they have it unless tested.

Klinefelter Syndrome XXY

XYY Syndrome Males with extra Y. 47 chromosomes Taller than average Normal development – never know they have it unless tested. May have sterility problems

XXX Syndrome Females with extra X 47 chromosomes Taller than average Often develop normally – never know they have it unless tested. May have sterility problems

Turner Syndrome XO females. Missing 2nd sex chromosome. 45 chromosomes This is the only monosomy in which the embryo survives. If any other chromosomes are missing then embryo/fetus does not survive. 45 chromosomes Sterile Shorter than average various other characteristic symptoms

Turner Syndrome - XO