Eyes of brown, blue, green, or gray; hair of black, brown, blond, or red—these are just a few examples of heritable variations that we may observe among individuals in a population. What genetic principles account for the transmission of such traits from parents to offspring?
One possible explanation of heredity is a “blending” hypothesis, the idea that genetic material contributed by the two parents mixes in a manner analogous to the way blue and yellow paints blend to make green. This hypothesis predicts that over many generations, a freely mating population will give rise to a uniform population of individuals. However, our everyday observations and the results of breeding experiments with animals and plants contradict such a prediction. The blending hypothesis also fails to explain other phenomena of inheritance, such as traits reappearing after skipping a generation.
An alternative to the blending model is a “particulate” hypothesis of inheritance: the gene idea. According to this model, parents pass on discrete heritable units—genes—that retain their separate identities in offspring. An organism’s collection of genes is more like a deck of cards or a bucket of marbles than a pail of paint. Like cards and marbles, genes can be sorted and passed along, generation after generation, in undiluted form.
Alleles, alternative versions of a gene. A somatic cell has two copies of each chromosome (forming a homologous pair) and thus two alleles of each gene, which may be identical or different. This figure depicts an F1 pea hybrid with an allele for purple flowers, inherited from one parent, and an allele for white flowers, inherited from the other parent
Mendel’s law of segregation. This diagram shows the genetic makeup of the generations in Figure 14.3. It illustrates Mendel’s model for inheritance of the alleles of a single gene. Each plant has two alleles for the gene controlling flower colour, one allele inherited from each parent. To construct a Punnett square, list all the possible female gametes along one side of the square and all the possible male gametes along an adjacent side. The boxes represent the offspring resulting from all the possible unions of male and female gametes.
An organism having a pair of identical alleles for a character is said to be homozygous for the gene controlling that character. A pea plant that is true–breeding for purple flowers (PP ) is an example. Pea plants with white flowers are also homozygous, but for the recessive allele (pp ). If we cross dominant homozygotes with recessive homozygotes, as in the parental (P generation) cross, every offspring will have two different alleles—Pp in the case of the F1 hybrids of our flower–colour experiment. An organism that has two different alleles for a gene is said to be heterozygous for that gene. Unlike homozygotes, heterozygotes are not true–breeding because they produce gametes with different alleles—for example, P and p in the F1 hybrids. As a result, those F1 hybrids produce both purple–flowered and white–flowered offspring when they self–pollinate.
Because of the different effects of dominant and recessive alleles, an organism’s traits do not always reveal its genetic composition. Therefore, we distinguish between an organism’s traits, called its phenotype , and its genetic makeup, its genotype . In the case of flower colour in pea plants, PP and Pp plants have the same phenotype (purple) but different genotypes
The Law of Independent Assortment
Mendel derived the law of segregation by performing breeding experiments in which he followed only a single character, such as flower color. All the F1 progeny produced in his crosses of true–breeding parents were monohybrids , meaning that they were heterozygous for one character. We refer to a cross between such heterozygotes as a monohybrid cross.
Mendel identified his second law of inheritance by following two characters at the same time. For instance, two of the seven characters Mendel studied were seed colour and seed shape. Seeds may be either yellow or green. They also may be either round (smooth) or wrinkled. From single–character crosses, Mendel knew that the allele for yellow seeds is dominant (Y ) and that the allele for green seeds is recessive (y ). For the seed–shape character, the allele for round is dominant (R ), and the allele for wrinkled is recessive (r).
Imagine crossing two true–breeding pea varieties differing in both of these characters—a parental cross between a plant with yellow–round seeds (YYRR ) and a plant with green–wrinkled seeds (yyrr ). The F1 plants will be dihybrids , heterozygous for both characters (YyRr ). But are these two characters, seed color and seed shape, transmitted from parents to offspring as a package? Put another way, will the Y and R alleles always stay together, generation after generation? Or are seed colour and seed shape inherited independently of each other?
The figure illustrates how a dihybrid cross, a cross between F1 dihybrids, can determine which of these two hypotheses is correct.
The F1 plants, of genotype YyRr, exhibit both dominant phenotypes, yellow seeds with round shapes, no matter which hypothesis is correct. The key step in the experiment is to see what happens when F1 plants self–pollinate and produce F2 offspring. If the hybrids must transmit their alleles in the same combinations in which they were inherited from the P generation, then there will only be two classes of gametes: YR and yr. This hypothesis predicts that the phenotypic ratio of the F2 generation will be 3:1, just as in a monohybrid cross.
The alternative hypothesis is that the two pairs of alleles segregate independently of each other. In other words, genes are packaged into gametes in all possible allelic combinations, as long as each gamete has one allele for each gene. In our example, four classes of gametes would be produced by an F1 plant in equal quantities: YR, Yr, yR, and yr. If sperm of the four classes are mixed with eggs of the four classes, there will be 16 (4 × 4) equally probable ways in which the alleles can combine in the F2 generation, as shown in the Punnett square. These combinations make up four phenotypic categories with a ratio of 9:3:3:1 (nine yellow–round to three green–round to three yellow–wrinkled to one green–wrinkled). When Mendel did the experiment and “scored” (classified) the F2 offspring, his results were close to the predicted 9:3:3:1 phenotypic ratio, supporting the hypothesis that each character—seed color or seed shape—is inherited independently of the other character.
Mendel tested his seven pea characters in various dihybrid combinations and always observed a 9:3:3:1 phenotypic ratio in the F2 generation. Notice in Figure 14.8, however, that, if you consider the two characters separately, there is a 3:1 phenotypic ratio for each: three yellow to one green; three round to one wrinkled. As far as a single character is concerned, the alleles segregate as if this were a monohybrid cross. The results of Mendel’s dihybrid experiments are the basis for what we now call the law of independent assortment , which states that each pair of alleles segregates independently of other pairs of alleles during gamete formation.
Strictly speaking, this law applies only to genes (allele pairs) located on different chromosomes—that is, on chromosomes that are not homologous. Genes located near each other on the same chromosome tend to be inherited together and have more complex inheritance patterns than predicted by the law of independent assortment. All the pea characters studied by Mendel were controlled by genes on different chromosomes (or behaved as though they were); this fortuitous situation greatly simplified interpretation of his multi–character pea crosses.
