How would one explain the special case of two normal-stature parents with many children, all of them tall? The simplest explanation for this family (in the absence of other major genetic factors influencing stature) is that two dominant genes acting synergistically together ("complementary genes") are required for tallness, with all of the children inheriting one gene from each parent. This could occur if both parents were each homozygous for only one of the two genes.
Thus, in the NPA context, the two dominant genes are those for 1) P trait, and 2) absence of A trait. One of the parents could be an N type, homozygous for the absence of A trait. The other parent could be an NPA+ type, homozygous for P trait. Thus, all of the children would necessarily have P trait and absence of A trait, and they would necessarily have to be NP types.
The N and NPA+ parents are not tall because each has only one of the two required traits.
Note that it should follow that in the general population, the non-sanguine "A type", having neither 1) P trait nor 2) absence of A trait should trend to have the shortest stature... and this seems to be the case.
We have pointed out in the past that the NP type and A type are "mirror images" of each other. That is, neither type has any of the NPA traits of the other.
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