0.3 Genetics in clasical understanding  (Page 2/6)

Mendel summarized his findings in two laws; the Law of Segregation and the Law of Independent Assortment.

Now when we know the mechanisms of meiosis, one can conclude that the two abovementioned Mendelian laws are direct consequences of the assortment laws of chromosomes in meiotic cell division, and the Mendelian “factors” are today’s genes.

A good description of Mendel’s pea crosses and his detail experiments was presented in MITOPENCOURSEWARE ( PDF ).

Lecture 30. gene linkage

Gregor Mendel analyzed the pattern of inheritance of seven pairs of contrasting traits in the domestic pea plant. He did this by cross-breeding dihybrids ; that is, plants that were heterozygous for the alleles controlling two different traits.

Mendel then crossed these dihybrids. If it is inevitable that round seeds must always be yellow and wrinkled seeds must be green, then he would have expected that this would produce a typical monohybrid cross: 75% round-yellow; 25% wrinkled-green. But, in fact, his mating generated seeds that showed all possible combinations of the color and texture traits.

• 9/16 of the offspring were round-yellow
• 3/16 were round-green
• 3/16 were wrinkled-yellow, and
• 1/16 were wrinkled-green

Finding in every case that each of his seven traits was inherited independently of the others, he formed his "second rule", the Rule of Independent Assortment:

The inheritance of one pair of factors (genes) is independent of the inheritance of the other pair. Today we know that this rule holds only if the genes are on separate chromosomes.

Mendel was lucky in that every pair of genes he studied met one requirement or the other. The table shows the chromosome assignments of the seven pairs of alleles that Mendel studied. All of these genes showed independent assortment , and they were inherited on separate chromosomes. With the rebirth of genetics in the 20th century, it quickly became apparent that Mendel's second rule does not apply to many matings of dihybrids. In many cases, two alleles inherited from one parent show a strong tendency to stay together as do those from the other parent. This phenomenon is called linkage.

So, gene linkage is the physical relationship of genes. Specifically, linkage means that the genes are on the same chromosome and therefore do not assort independently into gametes --in humans, ovum and spermatozoa-- during meiosis.

Because of this co-transmittance, the traits associated with the genes do not segregate between two daughter cells, following crosses between the parental cells, as predicted by Mendelian genetics.

The genes of most organisms can exist in different forms, called alleles, in a population. If the organism has identical alleles of a gene on each of its homologous chromosomes, it is called homozygous. If the alleles are different, it is called heterozygous. During the cell division process, a separation of nuclear material into gametes occurs via meiosis. If an organism is heterozygous, two kinds of gametes are produced; if homozygous, it produces only one kind of gamete. At fertilization the male and female gametes combine and the random process that creates different units put the gametes into various combinations. The ratio of the appearance of the observed traits, or phenotypes, produced by the pattern of separation of the dominant and recessive genes for that trait was predicted by Gregor Mendel following painstaking work and observation of the crosses between pea plants.