Inheritance Patterns

Mendel was the first scientist to develop a method for predicting the outcome of inheritance patterns. He performed his work with pea plants, studying seven traits: plant height, pod shape, pod color, seed shape, seed color, flower color, and flower location. Pea plants pollinate themselves. Therefore, over many generations, pea plants develop individuals that are homozygous for particular characteristics. These populations are known as pure lines.

In his work, Mendel took pure-line pea plants and cross-pollinated them with other pure-line pea plants. He called these plants the parent generation. When Mendel crossed pure-line tall plants with pure-line short plants, he discovered that all the plants resulting from this cross were tall. He called this generation the F1 generation (first filial generation). Next, Mendel crossed the offspring of the F1 generation tall plants among themselves to produce a new generation called the F2 generation (second filial generation). Among the plants in this generation, Mendel observed that three-fourths of the plants were tall and one-fourth of the plants were short.

Mendel’s laws of genetics

Mendel conducted similar experiments with the other pea plant traits. Over many years, he formulated several principles that are known today as Mendel’s laws of genetics. His laws include the following:

    1.  Mendel’s law of dominance: When an organism has two different alleles for a trait, one allele dominates.

    2.  Mendel’s law of segregation: During gamete formation by a diploid organism, the pair of alleles for a particular trait separate, or segregate, during the formation of gametes (as in meiosis).

    3.  Mendel’s law of independent assortment: The members of a gene pair separate from one another independent of the members of other gene pairs. (These separations occur in the formation of gametes during meiosis.)

Mendelian crosses

An advantage of genetics is that scientists can predict the probability of inherited traits in offspring by performing a genetic cross (also called a Mendelian cross). To predict the possibility of an individual trait, several steps are followed. First, a symbol is designated for each allele in the gene pair. The dominant allele is represented by a capital letter and the recessive allele by the corresponding lowercase letter, such as E for free earlobes and e for attached earlobes. For a homozygous dominant individual, the genotype would be EE; for a heterozygous individual, the genotype would be Ee; and for a homozygous recessive individual, the genotype would be ee.

The next step in performing a genetic cross is determining the genotypes of the parents and the genotype of the gametes. A heterozygous male and a heterozygous female to be crossed have the genotypes of Ee and Ee. During meiosis, the allele pairs separate. A sperm cell contains either an E or an e, while the egg cell also contains either an E or an e.

To continue the genetics problem, a Punnett square is used. A Punnett square is a boxed figure used to determine the probability of genotypes and phenotypes in the offspring of a genetic cross. The possible gametes produced by the female are indicated at the top of the square, while the possible gametes produced by the male are indicated at the left side of the square. Figure 9-1 shows the Punnett square for the earlobe example.


Figure 9-1     An example of a Punnett square.

Continuing, all of the possible combinations of alleles are considered. This is done by filling in each square with the alleles above it and at its left. This is done as shown in Figure 9-2.

From the Punnett square, the phenotype of each possible genotype can be determined. For example, the offspring having EE, Ee, and Ee will have free earlobes. Only the offspring with the genotype ee will have attached earlobes. Therefore, the ratio of phenotypes is three with free earlobes to one with attached earlobes (3:1). The ratio of genotypes is 1:2:1 (1 EE : 2 Ee : 1 ee).


Figure 9-2     The Punnett square is used to determine the probabilities of the genotypes and phenotypes in the offspring of a genetic cross.