Pea trait Form Form

Seed shape Round Wrinkled

Seed color Yellow Green

Seed coat color Colored White

Pod shape Inflated Wrinkled

Pod color Green Yellow

Flower position Axial Terminal

Stem length Tall Short

Pea plants were a good choice for Mendel’s experiments. The petals of the pea plants tightly enclose the male and female flower parts. This flower shape keeps pollen grains inside the flower in which they are produced. Thus, new pea plants form from female sex cells that are fertilized by the pollen grains of the same flower. This kind of fertilization is calledself -fertilization.

Mendel let the plants self-fertilize for many generations. He wanted to be sure that he had plants that were purebred for each trait he was studying. Purebred plants show the same form of a trait generation after generation. When Mendel was certain he had purebred plants for each of the seven traits he wanted to study, he began his experiments. He removed pollen grains from purebred plants with one form of each trait. He then placed these pollen grains in flowers of purebred plants with the opposite form of each trait.

After all the plants were crossed, he collected the seeds that formed. He called the seeds hybrids, meaning “half-breed”. In the other words, hybrid is offspring of two parents that differ in one or more heritable characters; offspring of two different varieties or of two different species.

Mendel planted these seeds and waited to see what traits would appear in the hybrid plants. In every case, the hybrid plants showed only one form of the trait; the opposite form of the trait seemed to disappear.

A cross involving one trait is called a monohybrid cross. A genetic cross between homozygous individuals results in identical genotypic and phenotypic ratios in the first generation. A cross involving a heterozygous individual and homozygous individual results in genotypic and phenotypic ratios of 1:1.

A cross involving two pairs of alleles is called a dihybrid cross. The study of this type of cross eventually caused Mendel to develop the principle of independent assortment. The same principles that govern monohybrid crosses also apply to alleles in dihybrid crosses.

According to the principle of independent assortment, each pair of alleles will segregate independently of the other pair.

Because of dominance, the nine genotypes produce just four phenotypes. The phenotypic ratio of the second generation can be expressed as 9:3:3:1.

The Principle of Dominance. Mendel’s next step was to cross the hybrid plants. The forms of the traits that disappeared in the hybrids reappeared in the offspring of the hybrids. Mendel called the form that was visible in the hybrids the dominant form, and the hidden form the recessive form.

Dominant gene: a gene that exerts its full phenotypic effect regardless of its allelic partner; a gene that masks the effect of its allele.Recessive gene: a gene whose phenotypic expression is masked by a dominant allele and so is manifest only in the homozygous condition. Heterozygotes involving recessives are phenotypically indistinguishable from dominant homozygotes.

Mendel noticed a pattern in the way that the forms of the traits appeared in the hybrid offspring. On the average, 75 percent of the plants showed the dominant form of a trait. Only 25 percent of the plants showed the recessive form of the trait. This kind of numerical pattern is called a ratio.

The principle of Segregation. Mendel’s experiments revealed that a parental trait, such as shortness, can disappear in the first generation. His experiments also showed that the same trait can reappear in the second generation in roughly a 3:1 ratio. To explain how traits can disappear and reappear in a certain pattern from generation to generation, Mendel proposed the principle of segregation.The principle of segregation states that the members of each pair of genes separate, or segregate, when gametes are formed.

The Principle of Independent Assortment. Mendel developed his two first principles through experiments involving the inheritance of a single pair of traits. He arrived at his third principle by crossing pea plants with two or more pairs of contrasting traits. He crossed a purebred plant with yellow, round seeds and a purebred plant with green, wrinkled seeds. Seeds produced from this cross were all yellow and round. This result illustrated the principle of dominance.

When these seeds grew into plants and self-pollinated, they produced 4 types of second generation seeds. The yellow, round seeds and the green, wrinkled seeds resembled the seeds of the parental generation. However, the second generation also included round, green seeds and yellow, wrinkled seeds. From this experiment, Mendel realized that two traits produced by recessive genes did not have to appear in the same offspring. For example, green color, a recessive trait, could appear with round seeds, a dominant trait. Mendel formulated the principle of independent assortment to explain this finding. The principle of independent assortment states that two or more pairs of genes segregate independently of one another during the formation of gametes. For instance, the segregation of the genes for seed color does not effect the segregation of genes for seed shape.

Today it is known that most gene pairs segregate independently only if they are located in different chromosomes. Traits determined by two genes of the same chromosomes tend to be inherited together. Mendel, however, was able to choose seven contrasting traits, each determined by a gene pair of a different pair of chromosomes.

The Punnett Square

To visualize the probable results of genetic crosses, geneticists use a grid, or chart, that shows the possible gene combinations for a cross. This grid is called a Punnett square(Chart 2.3). It is named for Reginald Punnett, the British geneticist who developed it in the early 1900s. In a Punnett square, symbols for all the possible gametes from the male parent appear across the top of the grid. Those from the female parent appear along the left side of the grid. By combining the symbol for each male gamete with the symbol for each female gamete, all the possible gamete combinations can be found.

Chart 2.3

Punnett square