Re: Genetic Diversity
"But then, how did WE get started?"
IANAGeneticBiologist
If an inbreeding population can produce enough offspring then, in the wild, nature will cull those that are unsuitable or not viable. That will leave a few that are apparently suited. If enough survive then random DNA mutations will gradually increase the size of the gene pool - with nature doing the culling again.
All humans alive today apparently have a single "Eve" ancestor for their cells' mitochondria component. There has been a lot of genetic mutation in the various lines in the 100-170 thousand years since then.
The cells that produce a sperm or an egg each split their 23 pairs of chromosomes into two random sets. Therefore each sperm/egg has 23 of the original 46 chromosomes - theoretically at least 2^23 possible combinations can result. There are then other mechanisms in play that can multiply the variants.
At fertilisation the sperm's 23 chromosomes are combined with the egg's 23 to give the new set of 23 pairs of chromosomes.
The problems of inbreeding depend on how life threatening particular gene variants are.
The worst case is where a gene variant is "dominant" and will always produce its effect. That can be very harmful eg haemophilia which is only inherited by sons from the mother on a 50:50 chance.
A gene that is "recessive" needs to be inherited from both parents before it has any major effect - although a single copy may confer other properties eg Cystic Fibrosis or Sickle Cell Anaemia.
Over many generations each line of DNA will get random mutations. These may be harmful, beneficial, or a combination of effects.
Inheriting two copies of a gene for Sickle Cell Anaemia is pretty deadly - however inheriting only one copy (a "carrier") gives immunity to malaria. Similarly Cystic Fibrosis carriers have relative immunity to typhoid.
Biting the hand that feeds IT
