Why do both parents need to have a recessive copy of a gene in order for the offspring to inherit it?
1 Answer
Because we have a double set of Chromosomes.
Explanation:
If a gene is recessive it usually is broken/inactive/damaged.
As long as you have a dominant, proper working gene on the corresponding chromosome donated by your other parent, you will survive and pass the recessive gene on to the next generation...
To review the basics of inheritance, see here and Genetics Overview.
Though not necessarily lethal anymore these days, Haemophilia is a relatively good example: This is caused by a defective gene that normally codes for FactorVIII (8), an essential factor in bloodclotting.
It resides on the X-chromosome....
Females have 2 X-chromosomes, so if one carries a defective gene, it will be compensated for by the other chromosome.
Males aren't so lucky: They receive an X-chromosome from their mother and a Y-chromosome from their father.
The Y-chromosome doesn't contain the FactorVIII gene, so if you inherit a defective one from your mother ( on the X-chromosome she contributed), you're in trouble....
Females who have a defective gene on one of their X-chromosomes won't be Haemophiliac, but they are a Carrier.
Occasionally a female CAN inherit TWO X-chromosomes that have defective FVIII-genes, and then she will be affected, but the chance is much lower...
As mentioned, in this case the gene in question resides on a sex-linked chromosome, but same goes up for Autosomal Chromosomes. In that case both males and females will be equally affected....
