Seed size in a plant is a polygenic characteristic. A grower crosses two pure-breeding varieties of the plant and measures seed size in the F1 progeny. She then backcrosses the F1 plants to one of the parental varieties and measures seed size in the backcross progeny. The grower finds that seed size in the backcross progeny has a higher variance that does seed size in the F1 progeny. Explain why the backcross progeny are more variable.

The F1 progeny is completely heterozygous for the loci of interest since they were obtained by mating between two pure-breeding plant lines. In the next generation, the backcross progeny will have homo-zygous individuals and therefore they will be more variable, resulting from meiosis in F1 hybrids

Explanation:

An F1 resulting from the cross between two pure-breeding plant lines will produce all hybrid individuals, all of them genetically (and phenotypically) identical. Meiosis in F1 hybrids is well known to produce homo-zygous genotypes and thus increases genetic diversity in progeny. For instance, for a single locus, the expected ratio of genotypes obtained from crossing two heterozygous parents is 1:2:1, i.e., one homo-zigous dominant individual, two heterozygous individuals (genetically identical to the parents) and one homo-zygous recessive individual; while the expected phenotypic ratio is 3:1 (i.e., 3 dominant expressing: 1 recessive expressing​).