Mendel's Seed Color Trials

Among Mendel's pea collection he noticed two varieties that differed only in the color of the seeds in the pods. One variety had yellow seeds and the other had green seeds. Today if you visit the grocery store and go to the dry beans section you can see these two colors of split peas sold in bags for making pea soup. Note: the peas are not actually cut or split, but the seed coats have been removed so that the cotyledons fall apart. The colors we are observing, then, are not "seed" colors, but rather cotyledon colors.

Mendel's stock plants are true-breeding thanks to their long-term obligate self-crossing for many generations. Here is a diagram of the two varieties that Mendel started with. We call these two stocks the P (parental) generation:

P:    
Green Pea
  X  
Yellow Pea

What you see above is the "look" of the peas and this is called the phenotype. There is a green phenotype on the left and a yellow phenotype on the right. Mendel grew up the seeds above until the plants flowered, tore open the flowers, and cross pollinated them. He allowed them to produce pods and observed the resulting seeds. Here is what he found in the pods; we call them the F1 (first filial) generation:

F1:    

Obviously all five F1 peas are yellow (show the yellow phenotype). The green parent was true-breeding so it could only have given the green attribute to the offspring. The yellow parent was true-breeding so it could only have given the yellow attribute. Obviously these F1 peas have both a green and a yellow attribute, yet they are all yellow in color! What does this tell you in terms of dominance and symbolic logic to use?

Dominance

Because of later results, Mendel knew that peas are diploid (have two copies of the gene for each trait) and that when different versions of that gene (now known as alleles) are found together, one allele may express fully and the other not. The allele that expresses itself fully in the F1 is called dominant and the allele that does not express itself in the F1 is called recessive. Many people get quite confused about dominance... always remember it means only this: "When two alleles go walking, the dominant one does the talking!" So which allele is dominant in the cross above, green or yellow?

Yellow of course is observed in all five F1 peas, so yellow is dominant to green. The green allele is therefore recessive.

Symbolic Logic

Because Mendel had discovered diploidy in peas, he knew that each pea carried two alleles for each genetic trait. Mendel used symbolic logic and his knowledge of statistics to observe patterns of inheritance. For this cross we will use two conventions of symbols that have become quite standard. The gene will be represented by a single letter of the alphabet (A-Z) and the letter we choose is the one represented by the recessive allele.

So in our problem, we will use the letter g for green! We will never use the letter y for yellow in our symbolic logic. Please learn this! We use only ONE letter of the alphabet and it is the one letter for the RECESSIVE allele.

Of course the letter g comes in two forms: the capital or upper-case G, and the small or lower-case g. The convention we use is that the upper-case symbol stands for the dominant allele, and the lower-case symbol stands for the recessive allele. So in our problem, we will use G for the dominant yellow allele and we will use g for the recessive green allele.

Because peas are diploid, each plant will have two alleles. There are three possible combinations of alleles (genotypes) for diploid pea plants: GG, gg, and Gg. The genotypes GG and gg are called homozygous because the zygote (-zygous) that gave rise to those individuals got the same (homo-) allele from both the sperm and the egg during syngamy. To distinguish these two genotypes, we call GG "homozygous dominant" (its phenotype will be yellow) and we call gg "homozygous recessive" (its phenotype will be green). The Gg genotype is called heterozygous because the zygote received different (hetero-) alleles from the egg and sperm. The Gg genotype will produce seeds that are yellow because when these two alleles go walking, the yellow allele does the talking.

So, here is how the cross above is represented symbolically using these two conventions.

P:    
gg
homozygous
recessive
  X  
GG
homozygous
dominant

How did we know that both parental peas were homozygous? These original stocks are true-breeding which means that the two alleles in each of these parents are identical. The parent on the left has two copies of the recessive green allele and so is green; if it is self-crossed, all the progeny would be green. The parent on the right has two copies of the dominant yellow allele and so is yellow; if it is self-crossed, all the progeny would be yellow. Both parents are true-breeding.

So, here is how the progeny from the cross above are represented symbolically:

F1:    
Gg
heterozygous

Gg
heterozygous

Gg
heterozygous

Gg
heterozygous

Gg
heterozygous

Obviously all five F1 peas have the yellow phenotype. They each got a g allele from the left parent because it was all it could give. They each got a G allele from the right parent, because it was all it could give. So the F1 peas all have the heterozygous genotype. You might notice I chose to put the G before the g in the F1 genotypes; the sequence does not matter at all, but it sometimes helps to put any dominant allele first so you remember the impact of the genotype on the phenotype.

On to the Next Generation

Mendel was not satisfied with this at all. He had crossed a green with a yellow and got all yellow progeny. What had happened to the green? Was it lost forever? To demonstrate the validity of his logic, Mendel had to show that the green allele was still present in his F1 peas. This was easily accomplished by letting nature take its course. Since peas are naturally self-pollinating, all Mendel had to do was plant the five F1 yellow peas in a marked place in his garden, allow them to grow, and let the bees do the work. When pods formed on the plants, he observed the self-cross progeny. These peas are called the second filial generation (F2).

Here is how we would document the cross:

F1:    
Gg
heterozygous
  X   F1:    
Gg
heterozygous

...and the F2 progeny are found inside the cells of this table:

possible sperm
Gg
possible
eggs
G
GG
homozygous
dominant

Gg
heterozygous
g
Gg
heterozygous

gg
homozygous
recessive

This kind of table was not developed by Mendel, but rather by Punnett many years after Mendel. This "Punnett Square" was a tabular explanation that biologists of the early 20th century could understand. The statistical explanation Mendel used in his papers was not understood by 19th century biologists, explaining why his work was "lost" until after Mendel's death.

Each column header in the table represents the possible sperm alleles donated by the pollen-parent. Each row header in the table represents the possible egg alleles donated by the maternal plant. In the case of this cross, the one parent is both paternal and maternal!

The important observation here is that the green seed phenotype reappeared in the F2 generation. By mating two heterozygotes (yellow F1s), one-fourth of the offspring statistically should be green. Mendel did this cross repeatedly and counted up his progeny; he understood the need for replication and a large sample size for statistical inference! This was truly unique for scientists of his day. He always got a 3:1 ratio of yellow to green offspring. Great! The logic worked!

Testing the Hypothesis

Again, Mendel was gratified to get this 3:1 ratio even if he counted many different crosses, but this was not good enough yet! Applying the scientific method I think you can see how to test Mendel's ideas. If his hypothesis is correct, the Punnett square predicts that the three-fourths of the progeny are yellow but these are not all alike. Of the yellow F2 progeny, one-third should be homozygous and two-thirds should be heterozygous. But remember you cannot "see" the genotype...only the yellow phenotype. This provided a "blind" test opportunity for his hypothesis. By mating the yellow F2s in a certain way, Mendel deduced that he could find out which ones are homozygous and which are heterozygous. He called the necessary cross a test cross.

A test cross is achieved when an F2 with the dominant phenotype (in our case, yellow) but with unknown genotype (GG or Gg) is mated with a homozygous recessive individual (in our case, green with genotype gg).

Here are the two possible crosses when a yellow F2 of unknown genotype is mated with a green double-recessive:

If the unknown yellow F2 is GG:

F2:    
GG
homozygous
dominant
  X  
gg
homozygous
recessive

These are the predicted progeny inside the cells of this table:

possible sperm
gg
possible
eggs
G
Gg
heterozygous

Gg
heterozygous
G
Gg
heterozygous

Gg
heterozygous

If the unknown yellow F2 is Gg:

F2:    
Gg
heterozygous
  X  
gg
homozygous
recessive

The predicted progeny are found inside the cells of this table:

possible sperm
gg
possible
eggs
G
Gg
heterozygous

Gg
heterozygous
g
gg
homozygous
recessive

gg
homozygous
recessive

One thing that you should notice immediately is that the test cross results tell you the genotype of the unknown. If the results are all yellow progeny (in other words, all dominant phenotype), the unknown yellow F2 pea is likely homozygous (GG, in other words, all dominant in genotype). If the results are half yellow and half green progeny (in other words, half dominant phenotypes), the unknown yellow pea is heterozygous (Gg, in other words, half-dominant, half recessive). What you observe in the progeny of a test cross is the genotype of the unknown! This power of this feature cannot be overstated; it is an incredibly useful tool for Mendelian genetics!

When Mendel actually did a large number of these test crosses of his yellow F2s, he determined that, indeed, there is a 2:1 ratio of heterozygous yellow F2s to homozygous yellow F2s. These results provided the critical scientific verification of his hypothesis.

I would now invite you to try a more hands-on approach, with my interactive page on Mendel's Seed Shape Trials. This will help you to be ready for the Genetics laboratory exercise!

 

This page © Ross E. Koning 1994.

 

 

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