V

Conclusion:

Phenotypic ratio

9 round, yellow:3 round, green:3 wrinkled, yellow :1 wrinkled, green

Conclusion:

Phenotypic ratio

9 round, yellow:3 round, green:3 wrinkled, yellow :1 wrinkled, green

3.11 Mendel conducted dihybrid crosses.

tion indicates that the alleles for each locus separate, and one allele for each locus passes to each gamete. The gametes produced by the round, yellow parent therefore contain alleles RY, whereas the gametes produced by the wrinkled, green parent contain alleles ry. These two types of gametes unite to produce the F1, all with genotype RrYy. Because round is dominant over wrinkled and yellow is dominant over green, the phenotype of the F1 will be round and yellow.

When Mendel self-fertilized the F1 plants to produce the F2, the alleles for each locus separated, with one allele going into each gamete. This is where the principle of independent assortment becomes important. Each pair of alleles can separate in two ways: (1) R separates with Yand r separates with y to produce gametes RY and ry or (2) R separates with y and r separates with Y to produce gametes Ry and rY. The principle of independent assortment tells us that the al-leles at each locus separate independently; thus, both kinds of separation occur equally and all four type of gametes (RY, ry, Ry, and rY) are produced in equal proportions (<Figure 3.11b). When these four types of gametes are combined to produce the F2 generation, the progeny consist of 9/16 round and yellow, 3/16 wrinkled and yellow, 3/16 round and green, and 1/16 wrinkled and green, resulting in a 9:3:3:1 phenotypic ratio (^Figure 3.11c).

The Relation of the Principle of Independent Assortment to Meiosis

An important qualification of the principle of independent assortment is that it applies to characters encoded by loci located on different chromosomes because, like the principle of segregation, it is based wholly on the behavior of chromosomes during meiosis. Each pair of homologous chromosomes separates independently of all other pairs in anaphase I of meiosis (see Figure 2.18); so genes located on different pairs of homologs will assort independently. Genes that happen to be located on the same chromosome will travel together during anaphase I of meiosis and will arrive at the same destination—within the same gamete (unless crossing over takes place). Genes located on the same chromosome therefore do not assort independently (unless they are located sufficiently far apart that crossing over takes place every mei-otic division, as will be discussed fully in Chapter 7).

Concepts

The principle of independent assortment states that genes coding for different characteristics separate independently of one another when gametes are formed, owing to independent separation of homologous pairs of chromosomes during meiosis. Genes located close together on the same chromosome do not, however, assort independently.

Applying Probability and the Branch Diagram to Dihybrid Crosses

When the genes at two loci separate independently, a dihy-brid cross can be understood as two monohybrid crosses. Let's examine Mendel's dihybrid cross (RrYy X RrYy) by considering each characteristic separately (< Figure 3.12a). If we consider only the shape of the seeds, the cross was

Rr X Rr, which yields a 3:1 phenotypic ratio (3/4 round and /4 wrinkled progeny, see Table 3.2). Next consider the other characteristic, the color of the endosperm. The cross was Yy X Yy, which produces a 3:1 phenotypic ratio (3/4 yellow and 1/4 green progeny).

We can now combine these monohybrid ratios by using the multiplication rule to obtain the proportion of progeny with different combinations of seed shape and color. The proportion of progeny with round and yellow seeds is 3/4 (the probability of round) X 3/4 (the probability of yellow) = 9/16. The proportion of progeny with round and green seeds is 3/4 X / = 3/16; the proportion of progeny with wrinkled and yellow seeds is /4 X 3/4 = 3/16; and the

Round, yellow

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