The Principle Of Segregation

The first experiments were monohybrid crosses. True-breeding parents that differed in only one of the seven characteristics were mated. Plants from a true-breeding

Table 1 Expected results of an experiment to illustrate the principle of segregation

Parent 1

Parent 2

Offspring

Parental generation

Fi generation

generation

Phenotype

Genotype

Gametes produced

Offspring

Phenotype

Genotype

Gametes produced

Offspring

Smooth seed SS

All S

Smooth seed Ss

Wrinkled seed ss

All s

Smooth seed Ss

Obtained by multiplying the frequencies and combining the alleles in the gametes

All Ss (all smooth seed)

(3/4 smooth seed:V4 wrinkled seed)

smooth-seeded line were mated with plants from a true-breeding wrinkled-seeded line. The offspring (F1) were all smooth-seeded. However, when the F1 plants were self-fertilized, they produced 5474 smooth seeds and 1850 wrinkled seeds. Mendel recognized that this was close to a 3:1 ratio. He repeated this experiment with each of the other six characteristics and, in each case, the results were close to a 3:1 ratio. He concluded, from these results, that there was a genetic determinant that existed in pairs, one from each parent. We now refer to this genetic determinant as a gene. The true-breeding parents had two copies of the same gene and the F1 offspring had one allele from each parent. The dominant (smooth) allele masked the recessive (wrinkled) allele in the F1. The F2 individuals could be divided into those that showed the recessive allele (received the recessive allele from both parents) and those that showed the dominant allele (either received the dominant allele from both parents, or the dominant allele from one parent and the recessive allele from the other parent). Offspring receive genes from parents via the gametes.

From this Mendel deduced the principle of segregation. This principle states that the two members of a gene pair segregate (separate) from each other during the formation of gametes. As a result, half of the gametes carry one member of each gene pair and the other half carry the

Table 2 Expected results of an experiment to illustrate the principle of independent assortment

Parent 1

Parent 2

Offspring

Parental generation

generation

generation

Phenotype

Genotype Gametes produced Offspring

Phenotype

Genotype Gametes produced Offspring

Smooth yellow seed SSYY All SY

Smooth yellow seed SsYy

Wrinkled green seed ssyy All sy

Smooth yellow seed SsYy

Obtained by multiplying the frequencies and combining the alleles in the gametes

All SsYy

(all smooth yellow seed)

1/16 SSYY:1/8 SSYy:1/16 Ssyy: 1/8 SsYY:1/4 SsYy:1/8 Ssyy: 1/16 ssYY:1/8 ssYy:1/16 ssyy (9/16 smooth yellow:3/16 smooth green:3/16 wrinkled yellow:1/16 wrinkled green)

other member of each gene pair. This is illustrated in Table 1.

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