Why is this couple marrying?
Not so good with Mendellian genetics, hmm?
Let's call a certain dominant gene A. The recessive gene is a. Each person has two copies of this gene. Most everyone is homozygous dominant--AA. Some people are carriers--Aa. They carry the recessive gene, but they don't know because it is masked by the dominant A.
If two heterozygous (Aa) parents marry, their children have a 25% chance of being noncarriers (AA), 50% chance of being carriers (Aa), and a 25% chance of expressing the recessive trait (aa).
So two people get married, go along all chipper and happy, and have a baby. After a year she is diagnosed with cystic fibrosis--she got the double-dose of the recessive trait. Thanks to modern medical treatment, she may live to 30 years. She will have respiratory and digestive problems for her entire life.
Now the parents know that they are carriers and that their future children will have a 25% chance of also having cystic fibrosis. They can either continue having kids without considering their children's health at all or decide to use contraception.
About one in twenty people is a carrier of cystic fibrosis--and most of them don't know it.
The other scenario is an autosomal dominant disease. In this case the normal phenotype is homozygous recessive (aa). A person who is heterozygous (Aa) has the disease. The dominant gene is typically so rare that homozygous dominants don't really occur (AA).
Normally with autosomal dominant diseases the person is either aware that they have the disease or is aware they may be a carrier. An example is Huntington's disease, in which a heterozygous person develops slowly worsening dementia with onset at 35-50 years. Sometimes heterozygous children develop the disease early, and it then has an accellerated timeline.
So if a man who is going to develop Huntington's in the next decade or so marries a woman with the normal genotype, their children have a 50% chance of being normal (aa) and a 50% chance of having Huntington's (Aa).
Why do you ask why they are marrying? Is it your position that people whose genotypes make child-bearing cruel and irresponsible or people who are sterile should never marry? That would be extreme!
And, honestly, is this the reason 99% of Christians use contraception?

I'll refresh your memory--we're talking about this not becaust 99% of Christians are in danger of having disabled children, but because you sidetracked us earlier by purposefully misinterpreting my earlier post to accuse me of being cold-hearted and hating disabled people. The sidetrack developed from there.
It sounds a lot like the tactic of the pro-abortion crowd.
I'm about to propose a new Godwin's Rule (
http://en.wikipedia.org/wiki/Godwin's_rule) where the first person to accuse his opponent of
A. being pro-eugenics or pro-abortion or
B. being spiritually inferior
automatically loses the argument. I think it would make things so much more civil around here!
[ July 25, 2005, 01:30 PM: Message edited by: blackbird ]