Genetic Disorders

These are caused by mutations, duplications or omissions of parts of the genetic code. Some disorders are passed on from one or both of the parents, while others (c. 33%) are spontaneous occcurrences (usually in sperm cell production). Examples are:

• Down's syndrome: three sets of chromomes instead of two.
• Klinefelter's syndrome: XXY genotype resulting in feminised males.
• Turner's syndrome: X genotype lacond second X resulting in immature sexual development of females.
• Genotype XYY: males have increased height. A link with criminality is not proven but perplexingly high frequency of XYY males in high security mental hospital settings (c. 20 times the normal rate).

Monogenic diseases are caused by a single gene defect; polygenic diseases are caused by more than one gene.

• Sickle cell anaemia is monogenic for red blood cell distortion. In Africa up to 30% of the population have sickle cell trait (Hh) and survive malaria better than those with normal (HH) or anaemic (hh) genes. 
• In adult American Indians, Australian aborigines and some other racial groups, the enzyme lactase (essential to digest lactose sugar in milk) is rare or absent.

Mutations are caused by natural or synthetic chemicals, various radiations, some viruses, plus random 'spontaneous' mutations. Chromosome abnormalities affect abour 1 in every 200 newborns. More than 3,000 genetic diseases are attributable to a single gene but occur infrequently in the general population.

• Nuclear bombings: ionising radiation is more efficient at killing cells by disrupting metabolic processes than at inducing genetic mutations. There are many cases of acute radiation sickness. A tenfold increase in leukemia in the 1960's (two decades later) shows delayed effects, but this was transient and levels fell to normal later on. There has been no sign of increased chromosomal abnormalities in the offspring of survivors.
• It seems that humans are at least 4 times as resistant as mice to genetic damage from a given dose of radiation. The only damage by radiation that can be passed on to the next generation is damage to the germ cells. Most damaged embryos result in spontaneous abortion very early in pregnancy. Life threatening damage occurs first to rapidly dividing tissues, such as those that produce red and white blood cells (hence cases of leukemia). To produce high enough levels of hamful genes, such a high dose of radiation would be needed that the victim would die of radiation poisoning before being able to reproduce.

The X chromosome contains many genes. Although women have two X chromosomes, within a few days of conception most of one X chromosome in every cell in a female body is inactivated, except the part that pairs with the Y chromosome and a few other genes. The chromosomes that shuts down are selected at random; in any organ of the body some cells will shut down the paternal X and others the maternal X. This is why women can carry a recessive gene but not suffer from it.

• Colour blindness affects males almost entirely: in Caucasian popuations 1 in 12 males but 1 in 200 femals. A single gene that describes red pigment sits alongside 1 to 5 copies of a gene for red pigment on the X chromosome (the gene for blue pigment is on chromosome 7). The genes for red and green colour and for blood (factor VIII or haemophilia) are no more than 1.1 million base pairs apart in band Xq28 (i.e. very close).
• A paper in the Lancet (1996) claimed there is evidence that several genes that determine intelligence appear to be located on the X chromosome. Boys inherit a single X from their mother; girls receive one from each parent. Boys therefore have the full advantage of bright genes from their mother, while girls may have their mother's bright genes diluted by a less intelligent father. The paper suggests that this is why males are more likely to be geniuses and that if women are weighing up men with regard to future children, they should consider the intelligence of the man's mother.
• Bodmer & McKie report that children in large families tend to have lower IQs than those from small families, that in any given family IQ generally decreases with birth order. They also note that only 3% to 4% of high IQ individuals fail to have families, while more than 30% of those with IQs under 70 have children.

Sources
Walter Bodmer & Robin McKie: The Book of Man: the quest to discover our genetic heritage. Scribner, 1995.
Suzuki & Knudtson: Genethics. Stoddart, 1990.
Article: Smart boys wear Mum's genes: Daily Mail 28 June 1996