Tutor Lesson

Humans practiced genetics long before they understood its principles. For more than 10,000 years, selective breeding helped people domesticate plants and animals. Examples include horses, Egyptian date palms and donkeys, and sunflowers in the Americas. Corn provides a striking case: farmers repeatedly selected useful features of teosinte, a small grass from the Mexican highlands, eventually producing modern maize with much larger kernels.

Genetics has two main branches. Transmission genetics follows information between generations as genes pass from parents to offspring through gametes, or through cell division in bacteria. Molecular genetics follows information inside cells and organisms.

This information pathway is the central dogma of molecular biology. DNA also replicates so its information can be passed on when cells divide. The first part of this lesson focuses on transmission genetics and Mendel's laws of heredity.

People could cross plants for centuries without discovering general laws of inheritance because breeders mainly sought better crops and animals, not an explanation of heredity. An experiment reported by Goss in 1824 shows the limitation of that practical approach.

Goss crossed plants grown from a yellow pea and a green pea. Every pea produced by the cross was yellow. He grew those yellow peas into plants and let them self-fertilize. Some pods contained only yellow seeds, many contained both colors, and some contained only green seeds.

Green peas taken from mixed or all-green pods produced plants whose self-fertilized offspring were entirely green. Yellow peas from all-yellow pods again produced some all-yellow pods and some mixed-color pods. Goss described these outcomes with words such as “some” and “many,” but he did not report counts. Without numerical data, the inheritance pattern remained difficult to see.

Mendel began with true-breeding yellow and green pea lines. This controlled starting point let him attribute later differences to the cross itself. A cross that tracks only one character is called a monohybrid cross.

When Mendel crossed the true-breeding lines, every pea in the first filial, or F1, generation was yellow. He grew these F1 peas and allowed the plants to self-fertilize. Among roughly 8,000 F2 seeds, about 6,000 were yellow and 2,000 were green, giving a yellow-to-green ratio near 3:1.

F2 green peas bred true when self-fertilized. Mendel also grew 519 F2 yellow peas. Of those plants, 166 produced only yellow seeds, while 353 produced both yellow and green seeds. Thus, approximately one-third of the yellow F2 peas bred true and two-thirds did not.

1. An individual carries two factors for each trait.
2. A true-breeding yellow plant can be written as AA, while a true-breeding green plant can be written as aa.
3. Only one factor enters each gamete. AA plants therefore make A gametes, and aa plants make a gametes.
4. Fertilization gives each F1 offspring one factor from each parent, producing Aa.
5. The F1 peas are yellow because A is dominant and a is recessive.

An Aa plant makes equal proportions of A and a gametes. During self-fertilization, pollen and eggs combine randomly. A Punnett square displays the four equally likely combinations.

• 1/4 are AA and breed true for yellow.
• 1/2 are Aa, appear yellow, and do not breed true.
• 1/4 are aa and breed true for green.

The genotype ratio is 1 AA : 2 Aa : 1 aa. Because both AA and Aa are yellow, the phenotype ratio is 3 yellow : 1 green. Among the yellow offspring, one-third are AA and two-thirds are Aa, which also explains Mendel's breeding results.

Genotypes determine phenotypes in this example: AA and Aa produce yellow peas, whereas aa produces green peas. Gametes have genotypes too; an individual pollen grain or egg carries either A or a.

A test cross pairs a heterozygous individual, Aa, with a homozygous recessive individual, aa. The aa parent makes only a gametes, while the Aa parent makes equal proportions of A and a gametes.

This successful prediction showed that a 3:1 ratio is not produced by every cross. Mendel's law could predict different offspring ratios from different parental genotypes, making him the first to predict phenotype proportions accurately from a genetic cross.

Mendel examined seven pea-plant traits using true-breeding lines:

1. Flower color: purple or white
2. Pea color: yellow or green
3. Pea shape: round or wrinkled
4. Pod color: green or yellow
5. Pod shape: inflated or constricted
6. Flower position: axial along the stem or terminal at the top
7. Plant height: tall or short

For each monohybrid cross, the F1 displayed the dominant phenotype, and self-fertilized F1 plants produced an F2 ratio near three dominant phenotypes to one recessive phenotype. Repeating the pattern across all seven traits supported the generality of segregation.

Mendel crossed a true-breeding round, yellow line, RRYY, with a true-breeding wrinkled, green line, rryy. Round is dominant to wrinkled, and yellow is dominant to green. Every F1 pea was round and yellow with genotype RrYy.

Because each gamete receives one allele from each gene, an RrYy plant can make four gamete genotypes in equal proportions:

Self-fertilization combines four pollen types with four egg types, producing 16 possible combinations. Counting their phenotypes gives the characteristic dihybrid ratio:

• 9/16 round and yellow: at least one R and at least one Y
• 3/16 round and green: at least one R together with yy
• 3/16 wrinkled and yellow: rr together with at least one Y
• 1/16 wrinkled and green: rryy

The resulting phenotype ratio is 9:3:3:1. Within the round-yellow group, only the RRYY genotype breeds true for both dominant traits; the other round-yellow combinations are heterozygous at one or both genes. The rryy offspring are true-breeding for both recessive traits.

For an RrYy plant, receiving R rather than r does not determine whether the same gamete receives Y or y. This independence makes RY, Ry, rY, and ry equally likely and produces the observed 9:3:3:1 ratio after self-fertilization.

Consider the alternative. If R always entered gametes with Y and r always entered with y, an RrYy plant would make only RY and ry gametes. Its self-cross would produce three round-yellow offspring for every one wrinkled-green offspring, rather than 9:3:3:1. Mendel repeatedly obtained 9:3:3:1 with different pairs of traits, supporting his second law.