Lesson #2: Mendel's Peas, part 1

Mendel had the good fortune to choose peas as a test organism. Peas are self-pollinating and strains of peas with certain traits only produce progenies that are the same. However, it is also fairly easy to cross pollinate peas from one strain with another strain and then let them naturally self-pollinate to look at future generations. Mendel was also lucky in that the traits he chose had clear cut alternative states.

What Mendel observed was that in the first filial, or F1 generation, one of the alternative states of each trait was present but that the alternative state that was hidden would reemerge in the next, or F2 generation. The trait that was present in the first hybrid generation was called the dominant trait and the trait that was hidden was called the recessive trait.

Mendel crossed many different lines of peas with various alternative traits and carefully counted the number of individuals that had each trait in subsequent generations. Mendel's genus was that he was able to understand the "particle" nature of inheritance and to be able to reduce the results to a ratio. What Mendel observed was that for all the traits he was studying, one alternative state would be the dominant trait in the F1 generation and that in the next generation the hidden or recessive trait would reappear in a ratio of 3 dominant plants to one recessive plant.

Here are a few of the traits that Mendel studied:

Trait F1 F2 Ratio
Round vs Wrinkled Round 5,474 Round
1,850 Wrinkled
Yellow vs Green Yellow 6,002 Yellow
2,001 Green
Long vs Short Long 787 Long
277 Short
Axial vs Terminal
Axial 651 Axial
207 Terminal

The "particles" that Mendel refereed to are now called genes. The different alternative states for each gene are called alleles. Each individual has two copies of each gene, one coming from the female or pod parent and the other coming from the male or pollen parent. Unfortunately, geneticist often use the terms genes and alleles somewhat sloppily which can sometimes result in some confusion.

To understand Mendel's basic results, lets take the example of yellow vs green cotyledons. If we say that the yellow strain is YY and the green strain is yy, then the F1 individuals will be Yy. In the case where the two alleles of a gene are the same, either YY or yy, we say the individual is homozygous for that trait and where the two alleles are different, Yy, we say that the individual is heterozygous. We say that the yellow peas have genotype YY and that green peas have genotype yy. The genotype is the actual alleles that each individual carries.

When plants undergo sexual reproduction each parent only contributes one of the alleles of each gene to the progeny. Thus, in each individual one allele for each gene is the maternal allele and the other is the paternal allele. If the plant is homozygous, then it can only produce gametes of one type. True breeding yellow peas can only produce gametes that contain the Y allele. However, since the gene is present in duplicate it actually can produce two gametes, one carrying the maternal Y allele and the other carrying the paternal Y allele. Green peas only produce gametes that carry the y allele, either the maternal or paternal y allele.

Parents:       yellow peas              green peas
Genotype:         YY                              yy

F1:                                    Yy    (Yellow)

In this simple cross we see that yellow peas can be of either genotype YY or Yy. Both of these genotypes look the same and we say they have the same phenotype. A plant's phenotype is what it looks like while its genotype is the actual allelic combination.

Now, the F1 hybrid will produce gametes of two types - Y and y. Each pollen grain can have either the Y allele or the y allele and each egg cell can have either the Y allele or the y allele. Which combines with which? Actually, its just a matter of chance. In this case we have 4 possible combinations: a Y allele from the egg combining with a Y allele from the pollen; a Y allele from the egg combining with a y allele from the pollen; a y allele from the egg combining with a Y allele from the pollen and a y allele from the egg combining with a y allele from the pollen. This can be easily represented in a checkerboard or Punnett's Square:

                        Pollen Parent
Y y
y yY yy

From this we see that three of the possible combinations; YY, Yy and yY are yellow and only one, yy is green. This fits in with what Mendel observed. What Mendel also did was to take the yellow plants in the F2 generation and let them go to a F3 generation. He observed that 1/3 of the yellow plants would breed true for yellow while 2/3 would segregate for yellow or green, which is what we would expect.

It is probably a good idea to mention one thing that may be of help in the future when trying to understand genetic ratios. In diploid plants the pod parent produces two types of gametes (even if they carry the same allele, one allele is the maternal allele and one is the paternal allele) and the same for the pollen parent, thus there is always 4 possible combinations. All segregation ratios are some multiple of 4. Four is the magical number! Sometime we will see a 1:1 ratio, but it is actually a 2:2 ratio simplified to a 1:1.

If we now backcross the F1 hybrid to the recessive parent: yy X Yy we get the following:

                              Pollen Parent
Y y
y Yy yy
y Yy yy

Now, there are two plants of genotype Yy and two of genotype yy which reduces to a ratio of 1 yellow to 1 green. This is often called a test cross. If we have a yellow pea we don't know if it is homozygous (YY) or heterozygous (Yy) because both genotypes give the same phenotype. By crossing a plant with the dominant trait to a plant with the recessive trait we can easily tell if the plant with the dominant phenotype is homozygous or heterozygous. If it is homozygous, then all the F1 progenies will be of the dominant trait; but if it is heterozygous, then the test cross progenies will segregate 1:1 for the dominant and recessive traits.