Info

Rr

2 Rr 1 rr red red white

3 red: 1 white F2 offspring

FIGURE 1.15 Checkerboard Determination of Genotype Ratios

Checkerboard diagrams (also known as Punnett squares) are often used to determine the possible genotypes and their ratios that result from a genetic cross with two or more alleles. To construct a checkerboard diagram, place the possible alleles from one parent on the horizontal row and those from the other parent on the vertical row. Fill in the boxes with the combinations determined from the intersection of the vertical and horizontal rows. Then list the various phenotypes and add up the similar phenotypes. When adding similar phenotypes, Rr and rR, although genetically dissimilar, are equivalent phenotypically.

Sex Determination and Sex-Linked Characteristics

The genetic sex of many diploid organisms, including mammals and insects, is determined by which sex chromosomes they possess. Among mammals, possession of two X-chromosomes makes the organism a genetic female, whereas possession of one X-

sex chromosome A chromosome involved in determining the sex of an individual

X-chromosome Female sex chromosome;possession of two X-chromosomes causes female gender in mammals

FIGURE 1.16 The Rr x Rr Cross: Checkerboard Determination of Phenotypes

The parents for this cross are the F1 generation from the mating shown in Figure 1.15. At the top of the figure the possible gametes are shown for each parent flower. The arrows demonstrate how the genes distribute to give a 3:1 ratio of red to white flowers in the F2 generation. At the bottom of the figure the F2 mating is analyzed by a checkerboard to yield the same 3 : 1 ratio. Note that although no white flowers were present among the parents of this second mating, they are found in their F2 offspring.

Fi offspring

Fi offspring

Red flowers (heterozygous)

Red flowers (heterozygous)

Red flowers (heterozygous)

Red flowers (heterozygous)

Gametes f2

offspring

Gametes f2

offspring

Parent 1

gametes

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