Genetic Crosses and Mendel’s Experiment

Geneticists use their own unique shorthand when analyzing the results from a genetic cross (a mating between two organisms with characteristics that scientists want to study). For example, they might use a letter of the alphabet to stand for each gene, capitalizing the letter for dominant alleles and using lowercase to represent recessive alleles. The same letter of the alphabet is used for each allele to show that they’re variations of the same gene.

For the cross Mendel did between tall and short pea plants the letter T can be used to represent the gene for plant height. Here, the dominant allele for tallness is shown as T, whereas the recessive allele for shortness is shown as t.

Geneticists also have special terms for describing the genes and appearance of organisms involved in a genetic cross. Here they are:

• Genotype: The combination of alleles that an organism has is its genotype. The genotypes of the two parental plants are TT and tt.
• Phenotype: The appearance of an organism’s traits is its phenotype. The phenotypes of the two parental plants are tall and short.

A tool called a Punnett square helps geneticists predict what kinds of offspring might result from a particular genetic cross. Above, the second Punnett square shows the cross between the peas of the F1 generation. Geneticists write the alleles that each parent can contribute to the offspring along the sides of the square. All possible combinations of alleles that could result in the next generation from the meeting of sperm and egg are drawn within the square.

If Mendel had used modern genetic notation and terminology, he might have analyzed his experiment like this:

• The parental pea plants are purebred, so they have only one type of allele, but each individual plant has two alleles for each gene. The tall parent’s alleles are shown as TT, and the short parent’s alleles are shown as tt. Because both of their alleles are the same, the parental pea plants are homozygous for the plant height trait (homo- means “same,” and zygous comes from a Greek root that means “together”).
• Each parental pea plant gives one allele to each offspring. Because the parentals are purebred, they can give only one type of allele. Tall pea plant parents always give a copy of the tall allele (T) to offspring, and short parents always give a copy of the short allele (t). Copies of these alleles are packaged into gametes (sperm and egg cells) as the pea plants reproduce.
• The sperm and egg of the parents combine, giving their F1 offspring two alleles for the height gene. All the F1 offspring have one copy of each allele, so their alleles are written as Tt. Because their alleles are different, the F1 pea plants are heterozygous for the plant height trait (hetero- means “other”). Although the F1 plants are heterozygous, they should all look tall because the tall allele is dominant to the short allele. This is exactly what Mendel saw — the short trait from his parentals seemed to disappear in the F1 generation.
• When F1 plants are crossed, they can each make two kinds of gametes — those that carry a dominant allele and those that carry a recessive allele. To figure out all the possible combinations of offspring the F1 plants could have, you use a Punnett square like the second one shown above. Again, geneticists write the two types of gametes each parent makes along the sides of the square, and then figure out the possible genotypes of the offspring by filling in the squares with the different combinations of gametes.
• The second Punnett square in above predicts that the F2 offspring will have three different genotypes: TT, Tt, and tt. For every one TT offspring, there should be two Tt offspring and one tt offspring. In other words, the genotypic ratio (the ratio of expected numbers of genotypes for the cross) predicted for the F2 generation is 1:2:1 for TT:Tt:tt.
• The tall allele is dominant to the short allele, so F2 plants that are TT or Tt will be tall, and only F2 plants that are tt will be short. So, the Punnett square predicts that for every three tall plants, there’ll be just one short plant. In other words, the phenotypic ratio (the ratio of expected numbers of phenotypes for the cross) for the F2 generation is 3:1 for tall:short. This is precisely what Mendel saw — for every one short plant he saw in his F2 generation, he saw about three tall ones.

When two organisms that are heterozygous for one trait are crossed with each other, that combination is called a monohybrid cross. (Mono- means “one,” and hybrid means “something from two different sources,” so a monohybrid is an organism that has two different alleles for one trait.) The cross between F1 pea plants above is an example of a monohybrid cross.