Explanation Of Gene Action As Related To Physiological

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ones. The idea of the average affect is, therefore, a clear and simple
one. We have in mind these additive effects when we call a gene ':good"
or "bad",, or when we speak of genes "for:: or genes "against" some character.
We can rarely, if ever, compUte the average effect of a gene
directly because we do not know enough about _its frequency or its actual
effects in all sorts of combinations. Instead, we estimate the amount
of additive variation present by examining the resemblances between
different kinds of relatives. The following table will show for parent
and offspring and for full sibs what fraction of each of these different
kinds of effects would be included in the resemblance between such relatives,
if sire and dam are not related, The fractions become more
complicated where sire and dan are related and some inbreeding, therefore,
has occurred. Of the two numbers describing the kind of effect,
the first is the number of nonallelic genes needed and the second is the
number of pairs of allelic genes. Thus 2, 1 signifies effects such as
those of AbCc or AAbc; while l, 1 signifies joint effects of such combinations
as AaB, aCc, dDd, etc.
Kind of effect Number of genes Amount of this effect which is
needed to produce common to:
the effect Parent & offsprin_ Full sibs
_dditlve i, 0 i 1/2 1/6
Dominance 0ri 2 --- 1/4
EPistatic 2,0 2 I/4 i/4
3,o 3 1/8 1/8
I,i 3 --- i/8
4, 0 4 1/16 1/16
2, i 4 --- 1/16
O, 2 $ --- 1/16
5,0 5 1/32 1/32
3,i 5 --- 1/32
1,2 5 --- 1/32
6, 0 6 1/64 1/64
4,1 6 --- 1/64
2,2 6 --- 1/64
O, 3 6 --- 1/64
_ etc. etc. etc. I etc.