Gg

Gg

Gg

homozygous green homozygous yellow

Phenotype For

parents (GG x gg)

F-i generation (all green—Gg)

Figure 13.11 A monohybrid cross between a green-podded pea plant and a yellow-podded pea plant. Green (G) is dominant; yellow (g) is recessive.

parents (GG x gg)

F-i generation (all green—Gg)

Figure 13.11 A monohybrid cross between a green-podded pea plant and a yellow-podded pea plant. Green (G) is dominant; yellow (g) is recessive.

the genetic basis of uncharacterized traits. A common strategy is to generate a monohybrid cross. In this scenario, a single trait is studied. A cross is made between two true-breeding parents differing for that trait, producing an Fj generation. Then, these Fj plants are intercrossed to produce an F2 generation. As an example, we will examine a cross between a homozygous green-podded pea plant and a homozygous yellow-podded one (Fig. 13.11). The genotype of the homozygous dominant parent (green-podded) is GG; the genotype of the homozygous recessive parent

(yellow-podded) is gg. You will recall from Chapter 12 that gametes have only one member of each pair of homologous chromosomes. The gametes of the green-podded parent (GG), therefore, will be G; the gametes of the yellow-podded parent (gg) will be g. No matter which egg unites with which sperm of the other parent, all the zygotes of this cross will be heterozygous, with the genotype Gg. This means that all individuals in the F1 generation will have the green-podded phenotype, and all will be heterozygous.

The next step is the actual monohybrid cross. Members of the F1 generation are crossed with each other (or self-pollinated) to produce an F2 generation. All of the F1 plants have the same phenotype (green-podded) and genotype (Gg). Each F1 plant produces two kinds of gametes (G and g) in roughly equal numbers. When the gametes are produced, half will be G, and half will be g. A G egg may unite with a G sperm, producing a GG zygote. Alternatively, purely at random, the G egg may unite with a g sperm producing a Gg zygote. The same type of random combinations occur with g eggs so that either Gg or gg zygotes are produced. When all the offspring of a large number of such crosses are counted, a genotypic ratio of 1 GG: 2 Gg: 1 gg is produced. The phenotypic ratio will be approximately three green-podded plants (GG and Gg) to one yellow-podded plant (gg). These four equally possible F2 offspring are shown at the bottom of Figure 13.11.

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  • Ferumbras
    What is the genotype of a homozygous green podded plant?
    8 years ago

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