4 minute read

Mendelian Genetics

The Principle Of Segregation, The Principle Of Independent Assortment, Exceptions To Mendel's Laws

Gregor Mendel (1822-1884), an Austrian monk and botanist, was curious and loved nature. He grew plants with diverse flower colors, and he cross-pollinated plant varieties to create hybrids. Mendel's fascination with "the striking regularity with which the same hybrid forms always reappeared," broadened his quest into discovering laws for inheriting any trait, not just flower color, from one generation to the next.

Mendel designed a series of experiments to learn the statistical rules governing the features that appeared in hybrids and in their offspring. Mendel identified plant varieties that exhibited the same features over many generations when the plants were allowed to self-pollinate or cross-pollinate with plants from the same variety. He chose hybrids that were fertile, so that their inherited characteristics, or traits, could be passed on to their offspring. He also made sure to exclude foreign pollen, so that outside plants did not get mixed up in his breeding experiments. Mendel chose peas as an ideal plant that had these characteristics.

Mendel obtained thirty-four varieties of peas from seedsmen, and, after two years of preparative work, he selected for study seven traits exhibited by the peas. The seven traits were: color of the seed coats (white or non-white); form of the ripe seeds (round or wrinkled); color of the seeds (yellowish orange or green); form of the ripe pods (inflated or constricted); color of the unripe pods (dark green or vivid yellow); position of the flowers (axial or terminal); and length of the stems (long or short).

Mendel carefully avoided choosing any traits, such as size and form of leaves, length of flower stalk, or size of pods, that would have generated a Figure 1. This Punnett square shows the possible outcomes, in genotypes, of a dihybrid cross (AaBb × AaBb). Reginald Punnett designed this layout to illustrate how alleles will assort according to Mendel's laws (segregation and independent assortment). The two parents are heterozygous for two traits, A and B, on nonhomologous chromosomes and carry the genotype AaBb. The four gamete types from the male and female are shown in boldface. Lowercase letters represent recessive alleles; uppercase letters represent dominant alleles. chaotic mix of forms. He chose traits that would allow plants and their off-spring to be distinctly classified.

Instead of looking at all seven traits at once, Mendel focused on one at a time. For each trait, he crossed two plant varieties to make hybrid plants. This was a monohybrid cross, because only one of the plant's many traits was studied. Mendel crossed the two chosen forms for each of the seven traits, using several hundred plants in each cross.

He found that in each case, all the first-generation offspring exhibited the same form as one of the parents, despite the hybrid having received input from two different parental varieties. Mendel called the form of the trait that appeared in these first-generation offspring dominant, as it was able to hide the other form during that generation. When the first-generation hybrid plants were allowed to self-pollinate, the hidden feature resurfaced in the next generation. Mendel called the hidden feature recessive. He further discovered that, on average, for every four offspring in the second generation, three displayed the dominant form of the trait, and one displayed the recessive form. He used these observations to suggest that each trait was governed by two "factors," one dominant and one recessive.

Mendel concluded that each plant carried two factors for every trait—either two dominant factors, two recessive factors, or one dominant and one recessive factor.

He proposed that his true-breeding parents carried two factors of the same kind. This is now defined as being homozygous. One parent plant was homozygous dominant, and the other homozygous recessive. When the parents were crossed, each offspring plant inherited one factor from each parent, but exhibited only the dominant form of the trait, even though they had received both a dominant and a recessive form. The offspring plants were hybrids, now called heterozygotes.

When these heterozygous plants self-pollinated, their offspring had an equal chance of receiving either two recessive factors, two dominant factors, or a dominant and a recessive factor. One quarter of these offspring were homozygous recessive, one quarter were homozygous dominant, one-half, were heterozygotes. Except for the one quarter that were homozygous recessives, the rest had at least one dominant factor and showed the dominant form of the trait. This explained Mendel's observation that three of every four plants showed the dominant form, and one in four the recessive.

Mendel also allowed the offspring of the heterozygous plants to self-pollinate. When he let plants with recessive features self-pollinate, only recessive features developed in their descendants, supporting the theory that they all contained only recessive factors.

When he let plants with dominant features self-pollinate, one-third gave rise to descendants that exhibited only dominant features. The other two-thirds gave rise to progeny with both dominant and recessive features, and therefore had to contain both dominant and recessive factors. Mendel tested six generations of plants and got similar results for each generation. Each generation of self-pollinating heterozygotes bore offspring, of which half were heterozygotes and half were homozygotes.

Mendel also did reciprocal crosses for each of the seven traits, switching the egg-bearing and the pollen-bearing variety to transmit the dominant and recessive features. The same ratio—three plants with dominant features for every one with recessive features—emerged from all the reciprocal crosses. Mendel concluded that a descendant had an equal chance of getting a dominant or a recessive factor (now called alleles) from either heterozygous parent, regardless of sex.

Additional topics

Medicine EncyclopediaGenetics in Medicine - Part 3