Page 33 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

2.2 Genetic Analysis According to Mendel 25

Figure 2.15 A dihybrid cross produces parental types for pea color and for pea shape assort independently, the
and recombinant types. In this dihybrid cross, pure-breeding allele for pea shape in a gamete carrying Y could with equal
parents (P) produce a genetically uniform generation of F 1 dihybrids. likelihood be either R or r. Thus, the presence of a particu-
Self-pollination or cross-pollination of the F 1 plants yields the lar allele of one gene, say, the dominant Y for pea color,
characteristic F 2 phenotypic ratio of 9:3:3:1.
provides no information whatsoever about the allele of the
P second gene. Each dihybrid of the F 1 generation can there-
YY RR yy rr fore make four kinds of gametes: Y R, Y r, y R, and y r. In a
large number of gametes, the four kinds will appear in an
almost perfect ratio of 1:1:1:1, or put another way, roughly
Gametes Y R y r 1/4 of the eggs and 1/4 of the sperm will contain each of the
four possible combinations of alleles. That “the different
kinds of germinal cells [eggs or sperm] of a hybrid are pro-
F (all identical) duced on the average in equal numbers” was yet another
1
Yy Rr Yy Rr one of Mendel’s incisive insights.
At fertilization then, in a mating of dihybrids, 4 differ-
F 2 1/4 1/4 1/4 1/4 ent kinds of eggs can each combine with any 1 of 4 different
kinds of sperm, producing a total of 16 possible zygotes.
Y R Y r y R y r
Once again, a Punnett square is a convenient way to visual-
ize the process (Fig. 2.15). Using the same kind of logic
1/4 Y R
YY RR YY Rr Yy RR Yy Rr previously applied to the Punnett square for monohybrid
crosses (review Fig. 2.11), each of the 16 boxes with colored
1/4 Y r peas in the Punnett square for the dihybrid cross in Fig. 2.15
YY Rr YY rr Yy Rr Yy rr
represents an equally likely fertilization event. Again, each
1/4 y R box is an equally likely outcome only because each of the
Yy RR Yy Rr yy RR yy Rr different gamete types is produced at equal frequency in

1/4 y r each parent. Therefore, using the product rule, the frequency
Yy Rr Yy rr yy Rr yy rr of the progeny type in each box is 1/4 × 1/4 = 1/16.
Each box: If you look at the square in Fig. 2.15, you will see that
1/4 × 1/4 = 1/16 some of the 16 potential allelic combinations are identical.
Type Genotype Phenotype Number Phenotypic In fact, only nine different genotypes exist—YY RR, YY Rr,
Ratio
Yy RR, Yy Rr, yy RR, yy Rr, YY rr, Yy rr, and yy rr—because
Parental Y– R– yellow round 315 9/16 the source of the alleles (egg or sperm) does not make any
difference. If you look at the combinations of traits
Recombinant yy R– green round 108 3/16 determined by the nine genotypes, you will see only four
phenotypes—yellow round, green round, yellow wrinkled,
and green wrinkled—in a ratio of 9:3:3:1. If, however, you
Recombinant Y– rr yellow wrinkled 101 3/16
look only at pea color or only at pea shape, you can see that
each trait is inherited in the 3:1 ratio predicted by Mendel’s
Parental yy rr green wrinkled 32 1/16
law of segregation. In the Punnett square, there are 12 yel-
low for every 4 green and 12 round for every 4 wrinkled. In
Ratio of yellow (dominant) to green (recessive) = 12:4 or 3:1
other words, the ratio of each dominant trait (yellow or
Ratio of round (dominant) to wrinkled (recessive) = 12:4 or 3:1
round) to its antagonistic recessive trait (green or wrinkled)
is 12:4, or 3:1. This means that the inheritance of the gene
When Mendel counted the F 2 generation of one exper- for pea color is unaffected by the inheritance of the gene for
iment, he found 315 yellow round peas, 108 green round, pea shape, and vice versa.
101 yellow wrinkled, and 32 green wrinkled. Both yellow The preceding analysis became the basis of Mendel’s
wrinkled and green round recombinant phenotypes did, in second general genetic principle, the law of independent
fact, appear, providing evidence that some shuffling of the assortment: During gamete formation, different pairs of
alleles of different genes had taken place. alleles segregate independently of each other (Fig. 2.16). The
independence of their segregation and the subsequent random
union of gametes at fertilization determine the phenotypes
The law of independent assortment observed. Using the product rule for assessing the probability
From the observed ratios, Mendel inferred the biological of independent events, you can see mathematically how the
mechanism of that shuffling—the independent assortment 9:3:3:1 phenotypic ratio observed in a dihybrid cross derives
of gene pairs during gamete formation. Because the genes from two separate 3:1 phenotypic ratios. If the two sets of