B

The Mundchan View of the. World parental generation

independently transmitted and so are able to segregate independently during the formation of sex cells. This principle of independent segregation is frequently referred to as Mendel's first law.

Some Alleles Are Neither Dominant Nor Recessive

In the crosses reported by Mendel, one member of each gene pair was clearly dominant to the other. Such behavior, however, is not universal. Sometimes the heterozygous phenotype is intermediate between the two homozygous phenotypes. For example, the cross between a pure-breeding red snapdragon [Antirrhinum) and a pure-breeding white variety gives P¡ progeny of the intermediate pink color, TF these F| progeny are crossed among themselves, the resuming F2 progeny contain red, pink, and white flowers in the proportion of 1:2:1 (Figure 1-2). Thus, it is possible here to distinguish heterozy-gotes from homozygotes by their phenotype. We also see that Mendel's laws do not depend on whether one allele of a gene pair is dominant over the other.

FIGURE 1-2 The inheritance of flower color in the snapdragon. One parent is homozygous for red flowers (A4) and the other homozygous for white flowers (on), No dominance is present, and the heterozygous Ft flowers are pink The 1:2\ 1 ratio of ted. pink, and white flowers in the f 3 progeny is shown by appropriate coloring.

Principle of Independent Assortment

Mendel extended his breeding experiments to peas differing by more than one characteristic. As before, he started with two strains of peas, each of which bred pure when mated with itself. One of the strains had round yellow seeds; the other, wrinkled green seeds. Since round and yellow are dominant over wrinkled and green, the entire F, generation produced round yellow seeds. The F| generation was then crossed within itself to produce a number of Fz progeny, which were examined for seed appearance (phenotype). In addition to the two original phenotypes (round yellow; wrinkled green), two new types (recombinants} emerged: wrinkled yellow and round green,

Again Mendel found he could interpret the results by the postulate of genes, if he assumed that each gene pair was independently transmitted to the gamete during sex-cell formation. This interpretation is shown in Figure 1-3. Any one gamete contains only one type of allele from each gene pair. Thus, the gemotes produced by an Fi (RrYy) will have the composition RY, Ry, rY, or ry, but never Rr, Yy, YY, or RR. Furthermore, in this example, all four possible gametes are produced with equal frequency. There is no tendency of genes arising from one parent to stay together. As a result, the F3 progeny phenotypes appear in the ratio nine round yellow, three round green, three wrinkled yellow, and one wrinkled green as depicted in the Punned square, named after the British mathematician who introduced it, in the lower part of Figure 1-3. This principle of independent assortment is frequently called Mended 5 second law.

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