Genetics: Mendelian Traits

Biology Tutorial

Genetics: Mendelian Traits


Genetics is the study of how traits appear in different generations. A trait can be any characteristic, as long as it is inherited. Hair color, eye color, and colorblindness are all genetic. Height is affected by genetics. Something like a tattoo is not inheritable and is not a genetic trait. Gregor Mendel, a monk living in the 1800s, is the Father of Genetics, and we use his name to discuss traits with the simplest form of inheritance.

Sample Problem

Mendel used pea plants for his experiments, and you are likely to see questions on tests in biology using this example. The flowers of pea plants can be red or white. (Sometimes you we see examples say purple or white. We’re using red.) He called them true-breeding if they only produced flowers of the same color when crossed with another flower of the same color. That is, red x red flowers produce new red flowers. White x white flowers produce new white flowers.

When Mendel crossed a red true-breeding flower with a white true-breeding flower, all of the resulting flowers were red. He called this the F1 generation.

When Mendel crossed two of the red flowers from the F1 generation together, what proportion of red:white flowers did he get?


The most important idea to remember in genetics is that traits are decided by genetic information (genes) carried on chromosomes of DNA. Every chromosome has a partner. For example, humans have 46 chromosomes, but we only have 23 different types of chromosomes. Each of those 23 are different lengths with different genes coding for different traits. To decide what the trait’s appearance, called a phenotype, will be, you have to determine what two forms of the genes, called alleles, are present on each chromosome of the pair. You normally do not need to know which chromosome pair the gene is on in introductory biology. (See Genetics: Sex-Linked Traits if you’re curious about when we need to know!)

So, we know that we have two chromosomes, so we must have two alleles. An allele is simply a form of the gene, so we could have two of the same allele or two different ones. We must have two. For Mendelian traits, we represent the allele by a capital or lower case letter. In this case, we will use R for red flowers and r for white flowers.

A true-breeding red flower would have the genetic combination of alleles RR. This is called its genotype. A true-breeding white flower would have the genetic combination of allele rr. This is also a genotype. Both are homozygous genotypes, because they have two of the same alleles.

To predict their offspring, we make a Punnett square. This is simply a box with four spaces inside. You must separate the alleles, because only one allele is passed on to the offspring from one parent. Otherwise, the offspring would end up with more chromosomes than their parents! Think back to meiosis.

    R     R      - this is the red flower

r   Rr    Rr

r   Rr    Rr

The white flower goes on the side. It is alright if you put them in the opposite order, but the genotype of one parent must be on the same side!

Write capital letters first. The allele at the top of box goes down. The allele at the side of the box goes over.

The possible offspring are inside: Rr, Rr, Rr, Rr.
These are all the same genotype, which matches what Mendel found. Remember, we just crossed a red flower and a white flower to make the F1 generation with all red flowers.

These red flowers are different from a true-breeding red flower. They are heterozygous; they have two different alleles.

Let’s look at the cross between two red flowers of the F1 generation.

    R    r

R   RR   Rr

r   Rr   rr

The possible offspring are RR, Rr, Rr, rr.
If we made a genotypic ratio, it would be 1:2:1, because we have 1 RR, 2 Rr, and 1 rr.
However, the question is asking about the phenotype, the red:white flowers.

RR and Rr are both red. Recall, RR is a true-breeding red flower. We saw earlier that Rr is red. This means we have 3 chances for red flowers.

rr is the genotype for a true-breeding white flower. We have 1 chance for white flowers.

Our ratio of red:white flowers is 3:1. This will always be your ratio when crossing two heterozygous individuals if they have Mendelian inheritance.

In Mendelian inheritance, one allele is stronger than the other. In this case, it was the R, which caused red flowers. The r allele is only expressed if we do not have the R allele.

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