Inbreds | Corn Breeding: Types of Cultivars

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)

This animation has no audio. It is a video of the original animation and is not interactive. The interactive version of corn crossing is available here.

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. A₁A₂genotype), how many of these plants are expected to be homozygous?

48

Incorrect: 1/4 of 256 is 64 heterozygotes, so 256-64 = 192 homozygotes.

64

Incorrect: 1/4 of 256 is 64 heterozygotes, so 256-64 = 192 homozygotes.

96

Incorrect: 1/4 of 256 is 64 heterozygotes, so 256-64 = 192 homozygotes.

256

Incorrect: 1/4 of 256 is 64 heterozygotes, so 256-64 = 192 homozygotes.

None of the above

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

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.