Mendelian Genetics

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 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.

The Principle of Segregation

Mendel used his observations to formulate his First Law, the Principle of Segregation. According to this principle, each gamete receives from a parent cell only one of the two alleles the parent cell carries for each trait, and the gamete has an equal chance of getting either allele. (Exceptions to the principle were described later by Thomas Hunt Morgan, an American geneticist.) When the two gametes unite during fertilization, the resulting cell contains two alleles, either identical or different, for each trait. These two alleles are referred to as the individual's genotype for the trait.

An alternative idea that other scientists during Mendel's time had was that two parental characteristics fused and blended into a single hybrid characteristic. Mendel's results showed this was not the case. His results showed, instead, that individuals inherit from their parents intact units that can leap through time. Mendel's discovery that inheritance had a particulate nature set the stage for modern advances in genetics.

The Principle of Independent Assortment

Mendel began a numerical evaluation of, respectively, two and three traits simultaneously, because he also wanted to know how different traits sorted themselves during gamete formation. He began a numerical evaluation of how two and then three traits were inherited simultaneously. The dihybrid cross involved two traits: the form of the plant's ripe seeds and the color of its interior seeds. He crossed one true-breeding variety that had wrinkled seeds and a green interior, with another that had round seeds and a yellow interior. The dihybrid cross generated offspring that all had round, yellow seeds, but the seeds' outward appearance, or phenotype, hid the offspring's heterozygous nature. The offspring contained recessive alleles for making wrinkled, green seeds, as well as the dominant alleles that generated the seeds' round and yellow appearance.

The round, yellow seeds, which were the seeds of the first filial generation, or F1, were planted, raised, and made to self-pollinate. Their progeny, the second filial generation, or F2, had four phenotypes for seed form and color, in a ratio of 9:3:3:1 (nine round and yellow, to three wrinkled and yellow, to three round and green, to one wrinkled and green).

To unmask the F2 genotypes, the next generation's wrinkled, round, yellow, or green seeds were collected. The seeds showed that there were nine different genotypes among the F2 plants. If Y represents yellow, y represents green, R represents round, and r represents wrinkled, the nine geno-types were: YyRr, YyRR, Yyrr, YYRr, YYRR, YYrr, yyRr, yyRR, and yyrr. Four of the genotypes were homozygous for both traits, four were homozygous for one trait and heterozygous for the other, and one was heterozygous for both traits.

Mendel's trihybrid cross included the trait for the color of the seed coats, which could be white or non-white, in addition to the same two traits used in the dihybrid cross. In this cross, the F2 generation had eight different combinations of seed shape, seed coat color, and interior seed color and twenty-seven different genotypes.

The existence of all these allelic combinations revealed that chance had a lot to do with what ended up in the same gamete. The chance of a descendent getting a specific seed shape and color depended on straight math, not on interaction between shape and color or another unknown influence. A ratio of three dominant to one recessive phenotype appeared for each trait, as if the other traits' alleles did not exist. The arrival of one allele inside a gamete was unaffected by the entry of another trait's allele. Mendel described this formally as "each pair of different characters in hybrid union is independent of the other differences."

The chance of a descendant getting a specific trait depends on probability, not on the interaction between traits. This is formally stated as Mendel's Second Law, or the Principle of Independent Assortment: Different traits assort (i.e. are included in gametes) independently of one another.

A Punnett square, designed by English geneticist Reginald Punnett (1875-1967) and shown in Figure 1, shows the outcomes of crosses that follow Mendel's laws. The capital letters A and B represent dominant alleles, and the lowercase letters a and b represents recessive alleles. A genotype that is heterozygous for both traits in a dihybrid cross is represented as AaBb.

Exceptions to Mendel's Laws

The seven traits that Mendel evaluated all assort independently, but not all sets of traits do. Independent assortment is true for the seven traits that Mendel evaluated, and holds generally true for traits (genes) found on non-homologous chromosomes. Any chromosome carries a collection of traits located on a long string of DNA, and the traits are therefore physically linked in a series or sequence. A non-homologous chromosome carries a unique collection of traits on a long string of DNA, that is different from the gene collection of an other non-homologue. Normal nonhomologous chromosomes are not attached to each other during meiosis, and move independently of one another, each carrying their own gene collection. Each chromosome, composed of a long string of DNA, carries a collection of genes, with each gene showing up in a particular form or type. Seven chromosomes reside within a pea gamete, and each of the traits Mendel chose to study lie on a different (non-homologous) chromosome.

Independent assortment is not true for the collection of traits that are located on a homologous chromosome. In eukaryotes, homologues come in pairs, one donated from each parent. Two homologous chromosomes carry the same collection of genes, but each gene can be represented by a different allele on the two homologues (a heterozygous individual). A gamete will receive one of those homologues, but not both. Genes or alleles that travel together on a chromosome do not show independent assortment, because they do not move independently of each other into a gamete.

Punnett and William Bateson (1861-1926), an English biologist, published the first report of gene linkage in peas. A comparison between the ratios at which certain genes were inherited and the expected Mendelian ratios showed that the traits did not assort independently.

Sometimes two traits on non-homologous chromosomes affect each other's phenotypic expression. Purple flowers, for example, occur only with the presence of at least one dominant allele from two different genes. Off-spring of two parents who are heterozygotes for both genes produce flowers that are purple or white at a ratio of nine to seven. The genotypes of the purple offspring are either aaB-or A-bb. (A dash indicates that the individual could have either a dominant or a recessive allele.)

Incomplete dominance occurs when a heterozygote has a unique phenotype. Pink flowers, for example, result when one parent is homozygous white and the other homozygous red. Neither allele hides the other, and their appearance together creates a unique intermediate phenotype.

Alleles are said to be codominant when heterozygotes express both alleles but neither affects the other's character. Individuals who have the allele for blood type A and the allele for blood type B, for example, have the characteristics of both blood types and are referred to as being of blood type AB.

Modern studies of genetic diseases use Mendel's ratios to determine whether or not genes are linked to certain chromosomes. Family histories are converted into pedigrees to help understand inheritance patterns. A disease might skip generations as expected of recessive alleles, or be linked to other traits. Deafness, white hair, and blue eyes are linked in cats, for example. A disease's symptoms might also become more severe with successive generations, as is the case with some dominant alleles.

Mendelian genetics and molecular biology together can elucidate the function of genes that are critical for development and life, in both experimental animals and human beings. Understanding of genetic processes can help to cure diseases.

see also Chromosomal Theory of Inheritance, History; Inheritance Patterns; Meiosis; Mendel, Gregor; Morgan, Thomas Hunt; Nature of the Gene, History; Probability.

Susanne D. Dyby

Bibliography

Internet Resource

Mendel, Gregor. Trans. C. T. Druery, and William Bateson. "Experiments in Plant Hybridization." (1866). MendelWeb. http://www.netspace.org/MendelWeb>.