Gregor Mendel and His Peas
Between 1854 and 1863, Gregor Mendel performed experiments using pea plants. Mendel bred tens of thousands of pea plants and discovered that traits are heritable and that recessive traits could show up in offspring after being absent for generations.
When Medel began his experiments, the major consensus was that offspring’s traits were a blending of their parents’ traits. For example, a short parent and a tall parent will have a child of medium height. Once a tall and short parent mates, their medium-sized offspring will not produce a child as short or as tall as their parents.
Huh?
For example, if you blend red clay with white clay, you end up with pink clay. If you were to blend the pink clay with white clay, you’d get a lighter pink. No matter how much white clay you added to the pink clay, you will never get pure white clay because of white and red clay’s original blending. This is the premise of the blending model.
So, is the bending model correct?
No, and Mendel’s work proved that the traits in a parent can reappear in later generations.
Red and white clay?
Yep, if clay were heritable.
So how did Mendel’s experiments prove the blending model wrong?
Mendel crossed (mated) a tall pea plant with a short pea plant, and all of the offspring were tall.
What?
That is what Mendel said, except in German.
Was?
Exactly.
Mendel thought he made a mistake and accidentally crossed two tall plants, so he mated tall and short plants again, and all of the offspring were tall.
Hat er einen weiteren Fehler gemacht?
Umm, I don’t speak German.
Sorry. Did he make another mistake?
Nope.
So, what did he do next?
He did what any scientist would do, he did more experimentation.
The next experiment is what turned the world of heredity upside-down.
Literally?
No, figuratively.
Mendel mated the tall offspring from the first experiment (F1 generation), which resulted in a ratio of 75% tall plants and 25% short plants (3:1).
Wait, how did the short plants reappear?
The tall plants from the F1 generation had the short allele, but the short trait was not expressed. (In other words, they did not have the short trait, but they did carry an allele for the short trait). Therefore, the F1 generation tall plants had a 25% chance of passing on their offspring’s short trait. And this is exactly what happened. Mating F1 generation tall plants produced 75% tall plants and 25% short plants.
Did he experiment with different pea plant traits?
Yep. Mendel repeated his experiment hundreds of times by crossing tall plants with short plants, plants with purple flowers with plants with white flowers, plants with yellow seeds with green seeds, and several other traits. With each experiment, the offsprings’ children (F2 generation) had the same 3:1 ratio of traits (75% to 25%).
Mendel proved that the blending model was incorrect and that he had discovered something that would forever change the study of genetics.
What?
Discovering how traits are heritable.
Brief Review of Genetic Terminology
Here is a brief review of some key genetics concepts:
| Term | Definition | Example |
|---|---|---|
| DNA | The genetic code; comprises four molecules written as A, G, T, and C. |  |
| Gene | A sequence of DNA on a chromosome that codes for a protein; like the gene that codes for eye color |  |
| Allele | A variation of a gene; such as blue, green, and brown eye color alleles. |  |
| Protein | A molecule that makes our traits; for example, hemoglobin carries oxygen, and melanin makes eye color. |  |
| Traits | The physical characteristics of an organism; such as eye color, liver function |  |
| Phenotype | The scientific term for a trait; such as eye color, liver function. | |
| Genotype | The scientific term for a gene/allele; such as genes that code for the eye color protein pigment and eye color variation; written using capital and lowercase letters |  |
Dominant and Recessive Traits
Mendel discovered through experimentation that:
- Some traits are dominant over other traits.
- Nondominant or recessive traits can reappear in future generations.
For example, tall pea plants are dominant over short pea plants. Therefore, crossing (mating) a true-breed tall plant and a short plant will only produce tall offspring because the tall trait dominates. However, mating two hybrid tall plants can produce both tall and short offspring.
How?
Well, it depends on the genotypes.
Dominant and Recessive Alleles
We have two alleles for every gene; one allele from mom the other from dad. The combination of the two alleles makes our genotype, which codes for our phenotype. Genotypes are written using letters, where capital letters represent dominant alleles, and lowercase letters represent recessive alleles. Let’s assume one gene codes for eye color and that brown eyes are dominant over blue eyes. The genotypes and the phenotypes are:
| Genotype | Phenotype |
|---|---|
| BB | Brown eyes |
| Bb | Brown eyes |
| bb | Blue eyes |
As you can see, a person with BB or Bb genotype will have brown eyes, and only the bb genotype will produce blue eyes.
Why?
Because the brown allele is dominant over the blue allele, and only the brown phenotype will show up when a person receives both a brown allele(B) and a blue allele (b).
When a person receives two of the same alleles (BB or bb), their genotype is homozygous. When a person receives two different alleles (Bb), their genotype is heterozygous. Since the brown allele is dominant to the blue allele, the dominant trait (brown eyes) will appear in the heterozygous state (Bb). The only way a person can have blue eyes is if they receive two recessive alleles (homozygous recessive or bb).
| Genotype Written as Words | Genotype Written as Letters | Phenotype |
|---|---|---|
| Homozygous dominant | BB | Brown eyes |
| Heterozygous | Bb | Brown eyes |
| Homozygous recessive | bb | Blue eyes |
Complete Dominance
Complete dominance occurs when the dominant allele’s phenotype shows up when a person is heterozygous for a trait. Therefore, the phenotype expressed by the heterozygous genotype will tell you what the dominant allele is. The table below shows the traits that are expressed in the heterozygous state.
| Genotypes | Dominant Phenotypes |
|---|---|
| Bb | Brown eyes |
| Tt | Tall |
| Hh | Brown hair |
| Ff | Freckles |
So, what is the dominant eye color?
Brown.
Good. What allele, B or b, codes for brown eyes?
B.
Awesome. Which two genotypes code for brown eyes?
BB and Bb.
You’ve got it. Blue eyes are recessive, so what genotype codes for blue eyes?
bb.
Yep. Alright, what genotypes code for freckles.
FF and Ff.
Good. What trait do you think the genotype ff codes for?
No freckles?
Yep. Below is a table that shows the dominant and recessive traits from the previous table.
| Dominant Genotypes and Phenotypes | Recessive Genotypes and Phenotypes |
|---|---|
| BB and Bb = brown eyes | bb = blue eyes |
| TT and Tt = tall | tt = short |
| HH and Hh = brown hair | hh = blonde hair |
| FF and Ff = freckles | ff = no freckles |
Example Problems
Frank Frankie Frankster is adopted and has never met his biological parents. He wants to know which parent gave him his blue eyes, brown hair, tall stature, and his freckles. Below is a homologous pair of Frank’s Chromosome #1, which contains the genes for all four traits.
Does frank have blonde hair?
No, his hair is brown.
Does his mom have blonde hair?
No, her hair is brown.
What if Frank’s father’s hair color?
I don’t know?
Correct. Sometimes there is not enough information given to know the genotype and phenotype of the parent. Frank’s dad could have brown hair with the genotype Hh, which means he gave Frank the recessive allele (h). But, Frank’s dad could also have blonde hair (hh), which means he could only give Frank a recessive allele (h).
So the genotypes and phenotypes are not always known?
Yep. Genetics is based on probability, so sometimes, not knowing is the correct answer.
Chapter Practice Quiz
Chapter Summary
AP Biology Prep
Click here for some AP Bio sample test questions.
Click here for a deeper look at genetics. This link contains other types of genetics (codominance, incomplete dominance) as well.