Chapter 14 - Mendel and the Gene Idea

A character is a heritable trait that differs between people, such as flower color. A characteristic is a variation for a character, such as the color purple or white for flowers. Another advantage of utilizing peas is their quick generation period and a big number of progeny produced from each mating.

Furthermore, Mendel was able to tightly regulate plant mating (Figure 14.2). Each pea flower includes pollen-producing stamens as well as an egg-bearing structure (carpel). Pea plants frequently self-fertilize in nature: Pollen grains from stamens settle on the carpel of the same flower.

Mendel employed very high sample sizes and meticulously recorded his findings: 705 of the F2 plants had purple blooms, whereas 224 had white blossoms. These numbers correspond to a three-to-one purple-to-white ratio (Figure 14.3).

Mendel reasoned that the heritable factor for white flowers did not vanish in the F1 plants, but was buried or disguised when the purple-flower factor was present. Purple flower color is a dominant trait, while white blossom color is a recessive trait, according to Mendel's nomenclature. The return of white-flowered plants in the F2 generation provided proof.

Mendel clipped the immature stamens of a plant before they developed pollen and then dusted pollen from another plant onto the changed flowers to achieve cross-pollination of two plants (as shown in the image attached). Each zygote that resulted was subsequently transformed into a plant embryo enclosed in a seed (pea). Mendel was always certain of the paternity of fresh seeds according to his approach.

Mendel opted to study only those traits that appeared in two different, alternate forms, such as purple or white flower color. He also made certain that he began his trials with true-breeding varieties—that is, plants that had produced only the same variety as the parent plant after many generations of self-pollination.

Mendel would cross-pollinate two contrasting, true-breeding pea types, such as purple-flowered plants and white-flowered plants, in a typical breeding experiment (see Figure 14.2). Hybridization is the mating or crossover of two true-breeding varieties.

True-breeding parents are known as the P generation (parental generation), and their hybrid children are known as the F1 generation (first filial generation, the term filial derived from the Latin word for "son"). Allowing these F1 hybrids to self-pollinate (or cross-pollinate with other F1 hybrids) results in the formation of an F2 generation (second filial generation). For at least the P, F1, and F2 generations, Mendel typically followed characteristics.

The basic patterns of heredity would have been missed by Mendel if he had halted his research at the F1 generation. Mendel's quantitative study of F2 plants resulting from thousands of genetic crossings like these enabled him to infer two essential laws of heredity, today known as the rule of segregation and the law of independent assortment.

Both purple-flowered and white-flowered plants appeared in the F2 generation, in a ratio of approximately 3:1. Therefore, the “heritable factor” for the recessive trait (white flowers) had not been destroyed, deleted, or “blended” in the F1 generation but was merely masked by the presence of the factor for purple flowers, which is the dominant trait.

If the blending model of inheritance were true, F1 hybrids from a cross between purple-flowered and white-flowered pea plants would bear pale purple flowers, a trait intermediate between the P generation and those of the P generation. Figure 14.2 shows that the experiment generated a completely different outcome: all of the F1 children had flowers that were the same color as the purple-flowered parents.

What happened to the genetic contribution of the white-flowered plants to the hybrids? If it is gone, the F1 plants will only produce purple-flowered offspring in the F2 generation. However, when Mendel let the F1 plants self- or cross-pollinate and then sowed their seeds, the white-flower characteristic returned in the F2 generation.

• The term alleles refers to alternative versions of a gene. First, alternative versions of genes account for variations in inherited characters. The gene for flower color in pea plants, for example, exists in two versions, one for purple flowers and the other for white flowers.

Each gene is a sequence of nucleotides located at a given location, or locus, along a single chromosome. However, the nucleotide sequence of the DNA at that region can change somewhat. This difference in information content might influence the function of the encoded protein, resulting in an inherited characteristic of the organism.

The purple-flower allele and the white-flower allele are two DNA sequence variants that can occur at the flower-color locus on the chromosomes of a pea plant. The purple pigment can be synthesized by the purple-flower allele sequence, but not by the white-flower allele sequence.

Second, an organism inherits two copies (i.e., two alleles) of a gene from each parent for each character. Surprisingly, Mendel drew this conclusion without knowing anything about the role, or even the existence, of chromosomes.

In a diploid organism, each somatic cell has two sets of chromosomes, one from each parent. In a diploid cell, a genetic locus is thus represented twice, once in each homolog particular pair.