Mendel and the Gene Idea
Gregor Mendel’s Discoveries
Mendel’s particulate mechanism differed from the blending theory of inheritance,
since it is the gene idea. According to his model, parents pass on discrete heritable
units, genes, that retain their separate identities in offspring. This is different from
the blending hypothesis, which is the idea that genetic material contributed by the
two parents mix.
True-breeding refers to plants that produce offspring of the same variety when
they self-pollinate. Hybridization is the mating or crossing of two true-breeding
varieties. A monohybrid cross crosses two organisms that are heterozygous with
respect to a single gene off interest. The P generation is the true-breeding parents,
which are the parental generation. The F
generation, the first filial generation, is
the hybrid offspring, and allowing the F
hybrids to self-pollinate produces an F2
generation, the second filial generation.
There are four components of Mendel’s hypothesis that led him to deduce the law
of segregation. First, alternative versions of genes account for variations in
inherited characters. The alterative versions of a gene are now called alleles.
Second, for each character, an organism inherits two alleles, one from each
parent. Each somatic cell in a diploid organism has two sets of chromosomes, one
set inherited from each parent. Thus, a genetic locus is actually represented twice
in a diploid cell. Third, if the two alleles at a locus differ, then one, the dominant
allele, determines the organism’s appearance; the other, the recessive allele, has
no noticeable effect on the organism’s appearance. The final part of Mendel’s
model, now known as the law of segregation, states that the two alleles for a
heritable character separate during gamete formation and end up in different
gametes. Thus, an egg or a sperm gets only of the two alleles that are present in
the somatic cells of the organism. This segregation corresponds to the distribution
of homologous chromosomes to different gametes in meiosis.