As humans we don't consciously go in for selective breeding. We just follow our romantic feelings.
Usually this works out fine. However, there are occasions when people discover that one of their genes actually gives rise to an inherited disease.
It is helpful for a couple to know how likely they are to pass on these 'bad' alleles to any children they might have.
Let's look at some of the common diseases that can be inherited.
Sufferers of this disease produce a thick, sticky mucus which coats their airways and lungs. If it is not cleared by daily massage and physiotherapy, and treated with antibiotics, the person can get serious chest infections.
The cause of the disease was discovered in 1989 as being a recessive allele. This allele is carried by about 1 in 20 of people.
Let's call the recessive allele c and the dominant, normal allele C.
If a person is heterozygous (Cc) then they are a carrier but have a normal phenotype, they don't develop cystic fibrosis but can pass it on to their children.
What happens if a carrier (Cc) and a 'normal' person (CC) want to have children?
It works out that half of the children will be 'normal' and half will be 'carriers' of the cystic fibrosis gene. So none of the children will actually develop the disease.
But what happens if another two carriers wanted to have children?
There is a 1 in 400 chance that two carriers will meet and have children.
Would that make a difference to their children's chances of developing the disease?
One in four children (25%) would be cystic fibrosis sufferers and half (50%) would be carriers. Not great odds. But potential parents can be warned of them.
If the chances of two carriers having children is 1 in 400. And the chances of a child having cystic fibrosis is 1 in 4.
Haemophilia is a famous blood disease. Its fame comes from the children of Queen Victoria and their offspring.
The symptoms are that blood fails to clot. The smallest wound or tooth extraction can prove fatal. A bump will not lead to a bruise but large, internal bleeding.
Nowadays sufferers are treated with a protein extracted from the blood of donors. Regular injections of clotting factor 8 can allow the patient's blood to clot normally.
The genetic basis of the disease is that the damaged allele occurs on the X sex chromosome.
Therefore it is sex-linked.
The normal blood-clotting allele is dominant and is shown as H. The recessive allele that causes Haemophilia is shown as h. (It doesn't matter really what letter is used, just as long as everyone is clear about what you are describing).
There are five possible combinations of the defective allele with the sex chromosomes. They are shown together like this for clarity:
In 2 cases the 'h' allele is not involved and so the people have normally clotting blood.
When a woman is heterozygous for the allele she will be an unaffected 'carrier'.
A man who has the 'h' allele on his single X chromosome will have the disease.
A woman who was homozygous for the 'h' allele would have the disease, but they never develop.
What happens if a female carrier had children with a normally clotting man?
Therefore a woman carrier has a 1 in 4 chance of an affected child. Risky odds!
This inherited disease causes the red blood cells to change from their usual round shape to become pointed like a sickle.
This shape change means that they get stuck in blood vessels and cannot pick up oxygen properly from the lungs.
The allele responsible for it is a recessive one just as in haemophilia. Again you get carriers but this time the 'wrong' allele is not on a sex chromosome.
What happens if two carriers were to have children?
They would have a 1 in 4 chance of a child being a sickle cell sufferer.
Sufferers usually die young, before they can reproduce.
Why doesn't the allele just die out?
By a strange quirk being heterozygous (Ss) for the gene gives some protection against malaria.
Since the carriers escape suffering from this serious and possibly lethal disease of malaria they live and reproduce, therefore passing on the sickle cell allele to another generation.
This is also known as Huntington's Disease. Chorea means dancing, that's where we get the word choreography.
The symptoms of Huntington's chorea are a series of uncontrolled, dance-like movements which do not appear until the sufferer is in their forties. There is also a severe mental damage which gets worse with increasing age.
Unusually the disease is caused by a dominant allele. So, if you look at the case of a sufferer who has children with an unaffected person:
There is a 50% chance that any children will suffer with this disease. Very bad odds!
Any person with one gene will be a sufferer and a carrier.
If two of these had children 75% of them would get the disease! Horrible odds!
Unlike the previous examples, Down's syndrome is caused by having an extra whole chromosome.
Therefore, Down's syndrome is a mutation in which an extra chromosome 21 is passed into the same egg cell during meiosis. (The other egg cell created during the same meiotic division will have no chromosome 21 at all).
If the egg cell with two chromosomes 21 becomes fertilised, the zygote will end up with three chromosomes 21. It will have a total of 47 chromosomes instead of the usual 46. This causes Down's syndrome.
The main effects of this syndrome are that the person has a lower mental ability and are more susceptible to certain diseases. Sadly they tend to die quite young at about thirty years old.