Hybrids vs OP varieties [Was: Genetic engineering is a Good Thing?]

Ian Mackay mac at lionseed.demon.co.uk
Mon Sep 21 16:20:21 EST 1998

In article <UFtgsAADokB2Ewrd at upthorpe.demon.co.uk>, Oz
<Oz at upthorpe.demon.co.uk> writes

>I'm still somewhat unclear why it's so hard to produce homozygous maize.
>Surely it would only require a few decades of breeding, if that.

This has been looked at. Empirically, yields of the best inbreds rise
over time, but yields of the best hybrids rise at the same rate and
remain ahead. However, it is the case that the more recent inbreds yield
more than the earlier hybrids. 

The reason for this picture depends partly on mode of gene action, and
partly on the number of genes controlling yield. The gene action bit is
that heterozygotes at most genes controlling yield must yield better
than the average of the two parents. It is not neccessary that the
heterozygote yields more than the best homozygous parent, and there is
experimental evidence that it doesn't. Then if there are only a handfull
of genes controlling yield, it should be fairly easy, as you suggest, to
get an inbred which yields as much as the parent. However, in this case
the inbred would represent the best available line there was, and no
further progress through classical plant breeding would be possible. If
there are many segregating genes which control yield, then although
there is a finite probability that an inbred will be found which yields
more than the best hybrid, this probability is sod all. Also, the
greater the number of segregating genes, the longer response to
selection will last before genetic variation runs out. 

So since progress continues to be made, and since F1s continue to
maintain their advantage over parents, this suggests that there are a
lot of genes controlling yield and/or the generation of new variation
through (natural) mutation occurs at a sufficiently high rate to
maintain genetic variation and keep the F1s ahead.

There are a number of assumptions and simplifications in this
explanation, but I think it is essentially correct.

Ian Mackay

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