What ratio results from a dihybrid cross that used two true breeding parents

The Law of Independent Assortment

Mendel’s work with single characteristics demonstrated that alleles segregate during reproduction. He went on to study the simultaneous inheritance of two seemingly unrelated characteristics. How would two pairs of alleles segregate in a dihybrid cross? he asked. To answer this question, he employed, for instance, peas differing in color and shape (Figure 5.5). First, he obtained true-breeding yellow round peas (YYRR) and true-breeding green wrinkled ones (yyrr). He then crossed these parental (P1) varieties and found that all the F1 were yellow and round (as expected for doubly heterozygous plants—YyRr—whose phenotype expresses the dominant alleles). When he self-crossed (fertilized the plant with its own pollen) the F1 plants, four phenotypes were evident in the F2: yellow round, yellow wrinkled, green round, and green wrinkled. That is, there were two parental types and two “new ones” produced in the dihybrid cross (recombinants). In one experiment, typical of many, he found 315 yellow round peas, 101 yellow wrinkled, 108 green round, and 32 green wrinkled.

From these data he proposed that segregation of one pair of alleles had no effect on the segregation of another pair—that gene pairs assort independently. Said another way, the presence of an allele for one gene has no effect on the presence of an allele for a second gene. From the Punnett square in Figure 5.5, the proof for this thesis is at hand. In the gametes of the F1 self -cross (female and male), four genotypes are expected (YR, Yr, yR, yr)—identical in female and male. Sixteen genotypes, then, are predicted in the F2. Of these, 9/16 are yellow round, 3/16 yellow wrinkled, 3/16 green round, and 1/16 green wrinkled. Note that this conforms almost exactly to the observed numbers and phenotypes of the peas in the typical experiment noted above. As important, the ratio of yellow to green, or round to wrinkled, was 12 : 4 (or 3 : 1), exactly as in the monohybrid crosses discussed earlier.

Independent Assortment

Mendel also studied crosses where he followed the segregation of two separate pairs of contrasting traits. The initial cross involved two homozygous parents that differed in two different traits represented by the cross AABB×aabb. The F1 offspring of this cross were dihybrid plants of the genotype AaBb. Mendel performed a dihybrid cross and examined the phenotypes and genotypes of F2 plants. Mendel observed that each pair of traits was inherited independently. He observed a 3:1 ratio of dominant to recessive trait when the A and B pairs of traits were considered separately, as independent crosses (Aa × Aa and Bb × Bb). When considered together in one cross (AaBb × AaBb), the combinations of traits appeared in the phenotype ratio of 9/16 with both dominant traits, 3/16 with one dominant trait, 3/16 with the other, and 1/16 with both recessive traits. This ratio is designated as Mendel's 9:3:3:1 dihybrid ratio and is based on probability events involving segregation, independent assortment, and random fertilization.

3.4 A lesson on classical genetics and the principal processes of scientific discovery

This lesson will soon be available. It addresses a number of GCSE and A-level specifications including cell division and cell cycle, genetic inheritance and an appreciation of how scientific knowledge is developed over time. It starts with examples of inherited diseases in humans, alerting to their genetic basis. Using low-cost microscopes [49], we then carry out an exercise on Drosophila mutations: students match genetic marker mutations of real flies to a set of drawings. We suggest two possible ways to continue from here: (1) We explain the practical uses of these marker mutations in contemporary research, for example in fly stock keeping. This involves application of Punnett squares and reaches into dihybrid crosses and even the use of transgenic technology and modern genetic markers by introducing chromosomes expressing green fluorescent protein. This learning outcome can also be achieved through a homework task we provide. (2) We exemplify a scientific discovery process through, step-by-step, working out with pupils how the white mutation, combined with knowledge of the time, led to the understanding that genes are located on chromosomes [50]. At the various steps, we formulate hypotheses which are then tested by applying Punnett squares. Since the white gene is located on the X-chromosome, we show pupils Ishihara plates used for testing colour blindness [51]: ∼10% of male pupils will be red-green colour blind illustrating that it is as an X-chromosomal trait and that understanding from Drosophila directly applies to humans. (3) Another possibility (which we so far only tested on extra-curricular school visits) is to explain how some marker mutations led to important scientific discoveries; for example, the important impact that the Notch mutation had on the biomedical sciences including development, stem cells and cancer [52]. To achieve differentiation, the PowerPoint file contains further flexible modules, including one on recombination and chromosomal mapping and maps.