Predicting Results Of Dihybrid Crosses

A dihybrid (die-HIE-brid) cross is a cross in which two characteristics are tracked. The offspring of a dihybrid cross are called dihybrids. Predicting the results of a dihybrid cross is more complicated than predicting the results of a monohybrid cross because more combinations of alleles are possible. For example, to predict the results of a cross in which both seed texture and seed color are tracked, you have to consider how four alleles from each parent can combine.

Homozygous x Homozygous

Suppose that you want to predict the results of a cross between a pea plant that is homozygous for round, yellow seeds and one that is homozygous for wrinkled, green seeds. In pea plants, the allele for round seeds (R) is dominant over the allele for wrinkled seeds (r), and the allele for yellow seeds (Y) is dominant over the allele for green seeds (y).

As Figure 9-12 shows, the Punnett square used to predict the results of a cross between a parent of the genotype RRYY and a parent of the genotype rryy will contain 16 boxes. Alleles are carried by the male and female gametes (pollen and ovule). The independently sorted alleles from one parent—RY, RY, RY, and RY—are listed along the left side of the Punnett square. The independently sorted alleles from the other parent—ry, ry, ry, and ry—are listed along the top of the Punnett square. Each box is filled with the letters that are above it and to the left of it outside the square. Notice that the genotype of all of the offspring of this cross will be heterozygous for both traits (RrYy); therefore, all of the offspring will have round, yellow seed phenotypes.

figure 9-12

This Punnett square shows a dihybrid cross between a pea plant that is homozygous recessive for wrinkled, green seeds (rryy) and a pea plant that is homozygous dominant for round, yellow seeds (RRYY).

Determining Genotypes Materials pencil and paper Procedure The ability to roll the tongue upward from the sides is a dominant, inherited trait. In one family, both parents and three children are tongue rollers, while one child is not. Determine the genotype and phenotype of each parent. Analysis Are the parents homozygous or heterozygous? Are the children homozygous or heterozygous?

Determining Genotypes Materials pencil and paper Procedure The ability to roll the tongue upward from the sides is a dominant, inherited trait. In one family, both parents and three children are tongue rollers, while one child is not. Determine the genotype and phenotype of each parent. Analysis Are the parents homozygous or heterozygous? Are the children homozygous or heterozygous?

rryy

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