Inbreds

In the lesson, Corn Breeding: Lessons from the Past of this series, an inbred (sometimes called an inbred line or simply a line) was described as a pure-breeding strain of corn. When pollen from an inbred plant is placed on the silks either of the same plant (this is a self-pollination) or of another plant of the same inbred (this is cross-pollination) and the resulting seed is grown, all the progeny will be genetically identical to the parental inbred plant(s). The reason for this is that at every locus an inbred is homozygous (i.e.A1A1 or A2A2, etc.). An A1A1 parent (or two A1A1 parents) can generate only A1A1 progeny. (See pictures below)

A1A1 inbred makes only A1 gametes (sex cells). Both the pollen and egg (silk) will be A1
In A1A1 self pollination all the offspring are A1A1
In A1A1 x A1A1 cross pollination, all seeds are A1A1

Inbreds usually are produced by continuous generations of self-pollination. If an individual plant is heterozygous at a locus, approximately seven generations of self-pollination are needed until nearly all the progeny generated from that single plant are homozygous (Figure 1). After seven consecutive generations of self-pollination, more than 99% of the progeny one expected to be homozygous at any one locus.

Figure 1:  Effects of consecutive generations of self-pollination on the frequency of homozygous progeny

Quiz

Question

According to Figure 1, if a breeder has 256 plants produced by two generations of self-pollination from a hybrid plant (i.e. A1Agenotype), how many of these plants are expected to be homozygous?

Looks Good! Correct: 1/4 of 256 is 64 heterozygotes, so 256-64 = 192 homozygotes.

Public breeders at universities developed many of the first corn inbreds in the United States. But for the past several decades, private seed companies have developed most of the new corn inbreds that are used as parents of commercial single-cross hybrids grown in the United States (see next section). Corn inbreds produced by seed companies are regarded as private property, are usually legally protected, and generally are not available to a farmer even under a license agreement.