Lesson: Chapter - 7

Modern Explanation of Mendel’s Results

With our modern understanding of genes, chromosomes, and cellular reproduction, we can explain the biological basis of Mendel’s observations and make pretty accurate predictions about the offspring that any given cross (short for crossbreeding) will produce.

Alleles

Each of the traits that Mendel observed in his pea plants came in one of two varieties; modern science calls any gene that gives rise to more than one version of the same trait an allele. So, for example, the tall gene and the short gene are different alleles (variations) of the height gene.

Every somatic cell contains two complete sets of chromosomes, one from each parent. Now you can understand why homologous chromosomes are similar, but not identical: although they contain the same genes, they may not contain the same alleles for these genes.

Video Lesson -

Homozygous and Heterozygous

Going back to Mendel’s plants, we can now say that all of his true-breeding plants contained two of the same alleles for each of the observed genes. Tall plants in this P generation had two alleles for tallness (TT), and short P generation plants had two alleles for shortness (tt). Anytime an organism’s two alleles for a specific trait are identical, that the individual is said to be homozygous (“homo” means same) for that trait.

On the other hand, crossing the tall and short plants to produce F1 hybrids created a generation of plants with one tall allele and one short allele (Tt). An organism with two opposing alleles for a single gene is said to be heterozygous for that trait.

Genotype and Phenotype

Although the P generation of pure-breeding tall plants looked the same as their hybrid F1 offspring, the P and F1 generations did not have identical genetic makeups. The genetic makeup of a certain trait (e.g., TT, Tt, or tt) is called its genotype, while the physical expression of these traits (e.g., short or tall) is called a phenotype.

Video Lesson - Genotype and Phenotype

For any given trait, an organism’s genotype will indicate alleles from both parents, while the phenotype only indicates the allelic form that is physically expressed in that individual. This distinction between genetic makeup and physical appearance explains the apparent “disappearance” of the recessive alleles in the F1 generation. Mendel’s results for the F2 generation can also be reinterpreted in light of these new distinctions. Mendel’s results showed that 75 percent of the F2 offspring exhibited the dominant phenotype, a ratio of 3:1 dominant to recessive. But from a genetic perspective, the breakdown would actually be around 25 percent homozygous dominant (TT), 50 percent heterozygous with a dominant phenotype (Tt), and 25 percent homozygous recessive (tt)—a ratio of 1:2:1.

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