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F2 seeds from F1 plant

F2 seeds: 3/ are spherical 1/4arewrinkled (3:1 ratio)

Plasma Membrane

Conclusion: There is no irreversible blending of characteristics. A recessive trait can reappear in succeeding generations.

10.3 Mendel's Experiment 1 The pattern Mendel observed in the F2 generation—X of the seeds wrinkled, X spherical—was the same no matter which strain contributed the pollen in the parental generation.

phenomenon. According to the blending theory, Mendel's F1 seeds should have had an appearance intermediate between those of the two parents—in other words, they should have been slightly wrinkled. Furthermore, the blending theory of-

GENETICS: MENDEL AND BEYOND 191

fered no explanation for the reappearance of the wrinkled trait in the F2 seeds after its apparent absence in the F: seeds.

Mendel proposed that the units responsible for the inheritance of specific traits are present as discrete particles that occur in pairs and segregate (separate) from one another during the formation of gametes. According to this theory, the units of inheritance retain their integrity in the presence of other units. This particulate theory is in sharp contrast to the blending theory, in which the units of inheritance were believed to lose their identities when mixed together.

As he worked mathematically with his data, Mendel reached the tentative conclusion that each pea plant has two units of inheritance for each character, one from each parent. During the production of gametes, only one of these paired units is given to a gamete. Hence each gamete contains one unit, and the resulting zygote contains two, because it is produced by the fusion of two gametes. This conclusion is the core of Mendel's model of inheritance. Mendel's unit of inheritance is now called a gene.

Mendel reasoned that in experiment 1, the two true-breeding parent plants had different forms of the gene affecting seed shape. The spherical-seeded parent had two genes of the same form, which we will call S, and the parent with wrinkled seeds had two s genes. The SS parent produced gametes that each contained a single S gene, and the ss parent produced gametes each with a single s gene. Each member of the F1 generation had an S from one parent and an s from the other; an F: could thus be described as Ss. We say that S is dominant over s because the trait specified by the s allele is not evident when both forms of the gene are present.

The different forms of a gene (S and s in this case) are called alleles. Individuals that are true-breeding for a trait contain two copies of the same allele. For example, all the individuals in a population of a strain of true-breeding peas with wrinkled seeds must have the allele pair ss; if S were present, the plants would produce spherical seeds.

We say that the individuals that produce wrinkled seeds are homozygous for the allele s, meaning that they have two copies of the same allele (ss). Some peas with spherical seeds—the ones with the genotype SS—are also homozy-gous. However, not all plants with spherical seeds have the SS genotype. Some spherical-seeded plants, like Mendel's F1, are heterozygous: They have two different alleles of the gene in question (in this case, Ss).

To illustrate these terms with a more complex example, one in which there are three gene pairs, an individual with the genotype AABbcc is homozygous for the A and C genes, because it has two A alleles and two c alleles, but heterozygous for the B gene, because it contains the B and b alleles. An individual that is homozygous for a character is sometimes called a homozygote; a heterozygote is heterozygous for the character in question.

The physical appearance of an organism is its phenotype. Mendel correctly supposed the phenotype to be the result of the genotype, or genetic constitution, of the organism showing the phenotype. In experiment 1 we are dealing with two phenotypes (spherical seeds and wrinkled seeds). The F2 generation contains these two phenotypes, but they are produced by three genotypes. The wrinkled seed phenotype is produced only by the genotype ss, whereas the spherical seed phenotype may be produced by the genotypes SS or Ss.

Mendel's first law says that alleles segregate

How does Mendel's model of inheritance explain the composition of the F2 generation in experiment 1? Consider first the F1, which has the spherical seed phenotype and the Ss genotype. According to Mendel's model, when any individual produces gametes, the two alleles separate, so that each gamete receives only one member of the pair of alleles. This is Mendel's first law, the law of segregation.

In experiment 1, half the gametes produced by the F: generation contained the S allele and half the s allele. In the F2 generation, since both SS and Ss plants produce spherical seeds while ss produces wrinkled seeds, there are three ways to get a spherical-seeded plant, but only one way to get a wrinkled-seeded plant (s from both parents)—predicting a 3:1 ratio remarkably close to the values Mendel found experimentally for all six of the traits he compared (see Table 10.1).

While this simple example is easy to work out in your head, determination of expected allelic combinations for more complicated inheritance patterns can be aided by use of a Punnett square, devised in 1905 by the British geneticist Reginald Crundall Punnett. This device reminds us to consider all possible combinations of gametes when calculating expected genotype frequencies. A Punnett square looks like this:

Female gametes

Female gametes

Male gametes

Male gametes

It is a simple grid with all possible male gamete genotypes shown along one side and all possible female gamete genotypes along another side. To complete the grid, we fill in each square with the corresponding pollen genotype and egg genotype, giving the diploid genotype of a member of the F2 generation. For example, to fill the rightmost square, we put in the S from the egg (female gamete) and the s from the pollen (male gamete), yielding Ss (Figure 10.4).

Mendel did not live to see his theory placed on a sound physical footing based on chromosomes and DNA. Genes are now known to be regions of the DNA molecules in chromosomes. More specifically, a gene is a portion of the DNA that resides at a particular site on a chromosome, called a locus

| A parent homozygous for the allele for spherical seeds is crossed with a parent homozygous for the allele for wrinkled seeds.

Parental (P) generation

F1 generation

F2 generation

| A parent homozygous for the allele for spherical seeds is crossed with a parent homozygous for the allele for wrinkled seeds.

Parental (P) generation

F1 generation

F2 generation

Generation

The parental gametes combine to produce F1 plants with the Ss genotype and a spherical seed phenotype.

The heterozygous F1 plant is self-pollinated.

S and s gametes combine randomly to produce two different seed phenotypes in the F2 plants, as this Punnett square shows.

10.4 Mendel's Explanation of Experiment 1 Mendel concluded that inheritance depends on factors from each parent, and that these factors are discrete units that do not blend in the offspring.

The parental gametes combine to produce F1 plants with the Ss genotype and a spherical seed phenotype.

The heterozygous F1 plant is self-pollinated.

S and s gametes combine randomly to produce two different seed phenotypes in the F2 plants, as this Punnett square shows.

10.4 Mendel's Explanation of Experiment 1 Mendel concluded that inheritance depends on factors from each parent, and that these factors are discrete units that do not blend in the offspring.

(plural, loci), and encodes a particular character. Genes are expressed in the phenotype mostly as proteins with particular functions, such as enzymes. So a dominant gene can be thought of as a region of DNA that is expressed as a functional enzyme, while a recessive gene typically expresses a nonfunctional enzyme. Mendel arrived at his law of segregation with no knowledge of chromosomes or meiosis, but today we can picture the different alleles of a gene segregating as chromosomes separate in meiosis I (Figure 10.5).

Mendel verified his hypothesis by performing a test cross

Mendel set out to test his hypothesis that there were two possible allelic combinations (SS and Ss) in the spherical-seeded F: generation. He did so by performing a test cross, which is a way of finding out whether an individual showing a dom-

Alleles of gene for seed shape ffi This site on the chromosome is the locus of the gene with the alleles S and s.

Homologous chromosomes

Alleles of gene for seed shape ffi This site on the chromosome is the locus of the gene with the alleles S and s.

Diploid Parent

Meiotic interphase

Diploid Parent

Meiotic interphase

Homologous chromosomes

Homologous Chromosomes Meiosis

^ At the end of meiosis I, the two alleles are segregated into separate daughter cells.

^ At the end of meiosis I, the two alleles are segregated into separate daughter cells.

Segregation Alleles Haploid Gametes

Four haploid gametes

Four haploid gametes

I At the end of meiosis II, each haploid gamete contains one member of each pair of homologous chromosomes, and thus one allele for each pair of genes.

10.5 Meiosis Accounts for the Segregation of Alleles Although Mendel had no knowledge of chromosomes or meiosis, we now know that a pair of alleles resides on homologous chromosomes, and that meiosis segregates those alleles.

inant trait is homozygous or heterozygous. In a test cross, the individual in question is crossed with an individual known to be homozygous for the recessive trait—an easy individual to identify, because in order to have the recessive phenotype, it must be homozygous for the recessive trait.

For the seed shape gene that we have been considering, the recessive homozygote used for the test cross is ss. The individual being tested may be described initially as S-because we do not yet know the identity of the second allele. We can predict two possible results:

► If the individual being tested is homozygous dominant (SS), all offspring of the test cross will be Ss and show the dominant trait (spherical seeds).

► If the individual being tested is heterozygous (Ss), then approximately half of the offspring of the test cross will be heterozygous and show the dominant trait (Ss), but the other half will be homozygous for, and will show, the recessive trait (ss) (Figure 10.6).

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