Neanderthals And Humans Lived Side By Side

New Scientist ^ | 9-13-2006 | Rowan Hooper

[Dustbunny]

I believe I mentioned a nephew with Down's Syndrome.

[ahayes]

I'm glad you didn't mean it that way, it just sounded like you might consider them nonhuman!

Humans have 23 pairs of chromosomes plus a set of two sex chromosomes (having a pair of each chromosome is called being diploid). Each sperm or egg contains only one copy of each chromosome normally (this is called being haploid), so while a person normally has 46 chromosomes plus two sex chromosomes, a gamete has 23 chromosomes plus one sex chromosome. When the egg (haploid) is fertilized these are merged and the zygote (diploid) has the normal complement of genetic material.

In the process that forms gametes, meiosis, a cell that originally has 46 chromosomes and two sex chromosome (diploid) will line up each pair of chromosomes down the middle of the cell, and then pull the two chromosomes in the pair apart and start towing one of each copy towards opposite ends of the cell. Once they are towed apart the cell will split into two new cells. Occasionally something goes wrong with this process and one entire pair of chromosomes will get towed to one end. This produces a daughter cell that has two copies of the chromosome when it should have one, and a daughter cell that has no copies at all. This is called nondisjunction.

Among humans and all organisms that reproduce by sexual reproduction having two copies of each chromosome (diploid) is normal in the adult. If you were to start out with an entire population that had three copies of one chromosome, during meiosis one daughter cell would have two copies (abnormal, when chromosome 21 is involved this gamete would produce an individual with Down's syndrome) and one daughter cell would have one copy (normal). Possible offspring would have either two copies (normal), three copies (Down's syndrome with chromosome 21) or four copies (quite abnormal, mostly inviable!) Having two copies of each chromosome in the adult is the most stable situation because each gamete is made by the division of a cell into two, halving the genetic material. You can't halve three chromosomes equally. When a person has three of a set of chromosomes that person can produce children with a chromosome number for that set of two, three, or four!

Sexually reproducing animals have a wide range of chromosome numbers, some having many more chromosomes than us and some having many less. But since sexual reproduction involves going from a diploid to haploid and back to diploid, all of them have their chromosomes in pairs.

Hope this isn't too confusing, I haven't had any caffeine yet!

[wyattearp]

Also, heredity is only rarely involved in Down's Syndrome. Two people with Down's Syndrome have about as much chance of having a Down's Syndrome child as any other couple. An entire species having Down's Syndrome is basically impossible. It is a replication error, not a hereditary one.

[Redleg Duke]

I thought the Neanderthals were still around...as Socialists.

[Redleg Duke]

"Does that mean that Chelsea is sterile?"

We can only hope.