Re: Why does twining skip a generaton?

Renee,

In humans twins occur under two circumstances. If the mother produces two eggs instead of just one at ovulation and if both are fertilized by different sperm, then there will be two pregnancies. These will be true brothers and/or sisters. They just happened to be conceived at the same time. They will not be identical because each sperm and egg get a different arrangement of the parents chromosomes.

In the second situation a single egg is fertilized by a single sperm. The fertilized ovum divides into two cells, then 4, then 8, then 16, then 32, then 64 and then some of these cells begin to change into different structures until an entire baby is formed. If for some reason at the 4,8 or 16 cell stage the cells separate and form two cellular stuctures each of which continues to form a baby, both babies will be identical. They will have the same genetic composition.

Identical twins are an accident of cellular division and it occurs sporatically through all humans. Fraternal twins resulting from double ovulation are more prevalent in certain races and there may be a familial tendency for this to happen. If a woman has a strong tendency to ovulate more the one egg each cycle then she will be at risk for several sets of twins/triplets/etc.It may also just be accidental in many case. It can also be caused by medicines that cause hyperovulation and multiple pregnancie. Women on "fertility" drugs hyperovulate and have multiple gestations.

In neither case is twining known to skip generations. Identical twins occur sporatically. In the most simple terms, if the familial tendency for hyperovulation followed strict mendelian inheritance and were a recessive gene, then only those who were homozygous for the recessive gene would be a twin. A woman who was a twin would be tt, if she mated with a homozygous non twin male, TT then all the offspring would be heterozygous non-twins, Tt. These would all carry the trait. It one of the females with Tt mated with a heterozygous male, Tt, the the offspring would be TT, Tt, Tt and tt. The tt would be a twin. So in that circumstance you could say it skipped a generation. In general, familial twining is not observed to follow such strict Mendelian rules.