Human Genetics – Estrella Mountain Community College

Table of Contents

The human karyotype | Human chromosomal abnormalities

Human allelic disorders (recessive) | Human allelic disorders (dominant)

Sex-linked traits | Diagnosis of human genetic diseases | Radioactive probes

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There are 44 autosomesand 2 sex chromosomes in the human genome, for a total of 46. Karyotypesare pictures of homologous chromosomes lined up together during Metaphase I of meiosis. The chromosome micrographs are then arranged by size and pasted onto a sheet.

Click here for a larger picture. This picture is from The Primate Cytogenetics Network at ( http://www.selu.com/~bio/cyto/karyotypes/Hominidae/Hominidae.html).

A common abnormality is caused by nondisjunction, the failure of replicated chromosomes to segregate during Anaphase II. A gamete lacking a chromosome cannot produce a viable embryo. Occasionally a gamete with n+1 chromosomes can produce a viable embryo.

In humans, nondisjunction is most often associated with the 21st chromosome, producing a disease known as Down's syndrome (also referred to as trisomy21). Sufferers of Down's syndrome suffer mild to severe mental retardation, short stocky body type, large tongue leading to speech difficulties, and (in those who survive into middle-age), a propensity to develop Alzheimer's Disease. Ninety-five percent of Down's cases result from nondisjunction of chromosome 21. Occasional cases result from a translocationin the chromosomes of one parent. Remember that a translocation occurs when one chromosome (or a fragment) is transferred to a non-homologous chromosome. The incidence of Down's Syndrome increases with age of the mother, although 25% of the cases result from an extra chromosome from the father. Click hereto view a drawing (from Bioweb) of a karyotype of Down's syndrome.

Sex-chromosome abnormalities may also be caused by nondisjunction of one or more sex chromosomes. Any combination (up to XXXXY) produces maleness. Males with more than one X are usually underdeveloped and sterile. XXX and XO women are known, although in most cases they are sterile. What meiotic difficulties might a person with Down's syndrome or extra sex-chromosomes face?

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Human Genetics - Estrella Mountain Community College