Monohybrid Crosses

Mendel started with 34 varieties of peas and spent 2 years selecting those varieties that he would use in his experiments. He verified that each variety was genetically pure (homozygous for each of the traits that he chose to study) by growing the plants for two generations and confirming that all offspring were the same as their parents.

He then carried out a number of crosses between the different varieties. Although peas are normally self-fertilizing (each plant crosses with itself), Mendel conducted crosses between different plants by opening the buds before the anthers were fully developed, removing the anthers, and then dusting the stigma with pollen from a different plant.

Fig: Mendel's monohybrid cross

Fig: Mendel’s monohybrid cross

Mendel began by studying monohybrid crosses— those between parents that differed in a single characteristic.

In one experiment, Mendel crossed a pea plant homozygous for round seeds with one that was homozygous for wrinkled seeds . This first generation of a cross is the P (parental) generation. After crossing the two varieties in the P generation, Mendel observed the offspring that resulted from the cross.

In regard to seed characteristics, such as seed shape, the phenotype develops as soon as the seed matures, because the seed traits are determined by the newly formed embryo within the seed. For characters associated with the plant itself, such as stem length, the phenotype doesn’t develop until the plant grows from the seed; for these characters, Mendel had to wait until the following spring, plant the seeds, and then observe the phenotypes on the plants that germinated.

The offspring from the parents in the P generation are the F1 (first filial) generation.

When Mendel examined the F1 of this cross, he found that they expressed only one of the phenotypes present in the parental generation: all the F1 seeds were round.

Mendel carried out 60 such crosses and always obtained this result. He also conducted reciprocal crosses: in one cross, pollen (the male gamete) was taken from a plant with round seeds and, in its reciprocal cross, pollen was taken from a plant with wrinkled seeds. Reciprocal crosses gave the same result: all the F1 were round.

Mendel wasn’t content with examining only the seeds arising from these monohybrid crosses. The following spring, he planted the F1 seeds, cultivated the plants that germinated from them, and allowed the plants to self-fertilize, producing a second generation (the F2 generation).

Both of the traits from the P generation emerged in the F2; Mendel counted 5474 round seeds and 1850 wrinkled seeds in the F2. He noticed that the number of the round and wrinkled seeds constituted approximately a 3 to 1 ratio; that is, about of the F2 seeds were round and were wrinkled.

Mendel conducted monohybrid crosses for all seven of the characteristics that he studied in pea plants, and in all of the crosses he obtained the same result: all of the F1 resembled only one of the two parents, but both parental traits emerged in the F2 in approximately a 3:1 ratio.

What Monohybrid Crosses Reveal

Mendel drew several important conclusions from the results of his monohybrid crosses.

First, he reasoned that, although the F1 plants display the phenotype of only one parent, they must inherit genetic factors from both parents because they transmit both phenotypes to the F2 generation. The presence of both round and wrinkled seeds in the F2 could be explained only if the F1 plants possessed both round and wrinkled genetic factors that they had inherited from the P generation.

He concluded that each plant must therefore possess two genetic factors coding for a character. The genetic factors that Mendel discovered (alleles) are, by convention, designated with letters; the allele for round seeds is usually represented by R, and the allele for wrinkled seeds by r. The plants in the P generation of Mendel’s cross possessed two identical alleles: RR in the round-seeded parent and rr in the wrinkled-seeded parent.

A second conclusion that Mendel drew from his monohybrid crosses was that the two alleles in each plant separate when gametes are formed, and one allele goes into each gamete. When two gametes (one from each parent) fuse to produce a zygote, the allele from the male parent unites with the allele from the female parent to produce the genotype of the offspring.

Thus, Mendel’s F1 plants inherited an R allele from the round-seeded plant and an r allele from the wrinkled-seeded plant. However, only the trait encoded by round allele (R) was observed in the F1— all the F1 progeny had round seeds. Those traits that appeared unchanged in the F1 heterozygous offspring.

Mendel called dominant, and those traits that disappeared in the F1 heterozygous offspring he called recessive. When dominant and recessive alleles are present together, the recessive allele is masked, or suppressed. The concept of dominance was a third important conclusion that Mendel derived from his monohybrid crosses.

Mendel’s fourth conclusion was that the two alleles of an individual plant separate with equal probability into the gametes. When plants of the F1 (with genotype Rr) produced gametes, half of the gametes received the R allele for round seeds and half received the r allele for wrinkled seeds. The gametes then paired randomly to produce the following genotypes in equal proportions among the F2: RR, Rr, rR, rr.

Because round (R) is dominant over wrinkled (r), there were three round progeny in the F2 (RR, Rr, rR) for every one wrinkled progeny (rr) in the F2. This 3:1 ratio of round to wrinkled progeny that Mendel observed in the F2 could occur only if the two alleles of a genotype separated into the gametes with equal probability.

The conclusions that Mendel developed about inheritance from his monohybrid crosses have been further developed and formalized into the principle of segregation and the concept of dominance. The principle of segregation (Mendel’s first law) states that each individual diploid organism possesses two alleles for any particular characteristic. These two alleles segregate (separate) when gametes are formed, and one allele goes into each gamete.

Furthermore, the two alleles segregate into gametes in equal proportions. The concept of dominance states that, when two different alleles are present in a genotype, only the trait of the dominant allele is observed in the phenotype. Mendel confirmed these principles by allowing his F2 plants to self-fertilize and produce an F3 generation.

He found that the F2 plants grown from the wrinkled seeds— those displaying the recessive trait (rr)—produced an F3 in which all plants produced wrinkled seeds. Because his wrinkled-seeded plants were homozygous for wrinkled alleles (rr) they could pass on only wrinkled alleles to their progeny . The F2 plants grown from round seeds—the dominant trait—fell into two types.

On self-fertilization, about of the F2 plants produced both round and wrinkled seeds in the F3 generation. These F2 plants were heterozygous (Rr); so they produced RR (round), Rr (round), and rr (wrinkled) seeds, giving a 3:1 ratio of round to wrinkled in the F3. About of the F2 plants were of the second type; they produced only the dominant round-seeded trait in the F3. These F2 plants were homozygous for the round allele (RR) and thus could produce only round offspring in the F3 generation.

Mendel planted the seeds obtained in the F3 and carried these plants through three more rounds of self-fertilization. In each generation, of the round-seeded plants produced round and wrinkled offspring, whereas produced only round offspring. These results are entirely consistent with the principle of segregation.

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