Fun With Mendelian Genetics
Charles Darwin’s and Alfred Wallace’s concept of natural selection explains why
variants within a population increase or decrease in number over generations. One of the
foundations of natural selection is the observation that populations are variable and that
part of this variation is the result of heredity. When a particular individual has an
increased fertility rate and produces a significantly greater number of offspring than other
organisms, that individual passes on certain traits to the next generation.
Although Darwin and Wallace recognized the relationship between the processes of
inheritance and natural selection, they were never able to define the rules of heredity.
They did not discover how new variants, the raw material of natural selection, arose (we
now know that new variants arise through a change—called a mutation—in the genetic
code). Nor did they figure out how characteristics were transmitted from generation to
generation. Although some of the basic principles of genetics were worked out during
Darwin’s and Wallace’s time, they were unaware of this new knowledge.
The key to solving one of Darwin’s and Wallace’s difficulties was solved by a
Czechoslovakian monk named Gregor Mendel (1822-1884). During the 1860s, Mendel
worked out the basic laws of inheritance, which he called the Principle of Segregation
and the Principle of Independent Assortment. He also discovered some other principles
of genetics including the concept of dominant and recessive genes.
Mendel’s First Discovery
• For every trait a person has, s/he has a PAIR OF ALLELES. Alleles are different
forms of a gene, sort of like Accords, Civics, and Preludes are different forms of
Hondas. One allele came from the person’s mom and the other from the person’s
1. My question to you is: Since all people can interlock their fingers and
thumbs together when clasping their hands (this is the “hand-clasping” trait),
and since you are a person, how many alleles do you have for the hand-
clasping trait? Write your answer in the box on the right.
• When a person produces gametes (sex cells, which are ova in females and sperm
in males), the paired alleles separate (or “segregate,” as Mendel put it) and ONE
allele goes into a sex cell. The other allele goes into a totally different sex cell.
This is known as Mendel’s Law of Segregation.
Mendel’s Second Discovery
• Some alleles are DOMINANT, others are recessive. Dominant does NOT mean
“better” or “stronger”; it simply means that if a person gets one dominant allele
and one recessive allele, only the dominant allele gets “read.” Only the protein
specified by that allele gets produced by the cell. Then, only a particular form of
a trait appears.
For example, if a person has one dominant allele for hand-clasping and one
recessive allele for hand-clasping, when that person clasps their hands together, s/
he will find that the left thumb lying over the right thumb. This means that the
person has at least one dominant allele for this particular trait. It does NOT
mean that both alleles are dominant. One of the alleles is definitely dominant,
while the other allele could be dominant or recessive.
2. Here’s your next problem: When Debbie clasps her hands
together, her right thumb lies on top of her left thumb. What are
• 2 dominants?
• 1 dominant and 1 recessive?
• 2 recessive?
Write your answer in the box on the right.
Mendel’s Third Discovery
• Some alleles are known as CO-DOMINANT. If a person inherits a co-dominant
allele from one parent and a co-dominant allele from the other parent, BOTH
alleles get “read,” and the proteins specified by BOTH alleles get produced.
For example, for the trait most commonly known as “Blood Type,” there are 3 alleles.
One of these is known as the “A” allele, and it’s dominant. Another allele is known
as the “B” allele, and it’s also dominant. The third allele is known as the “o” allele,
and it’s recessive.
Note: By convention, we use UPPERCASE letters to indicate DOMINANT alleles and
lower case letters to indicate recessive alleles. This is done for ALL alleles, not just those
for blood type.
If a person inherits an “A” allele from both parents, the person has 2 dominant alleles and
will have blood type “A.” If a person inherits a “B” allele from both parents, the person
has 2 dominant alleles and will have blood type “B.” If a person inherits an “o” allele
from both parents, the person has 2 recessive alleles and will have blood type “o.” And if
a person inherits an “A” allele from 1 parent and a “B” allele from the other, then the
person has 2 dominant alleles and will have blood type “AB”—the co-dominant
Note: When a person’s 2 alleles (for a particular trait) are the same, the person is said to
be homozygous for the trait. If the 2 alleles are different, the person is said to be
heterozygous for that trait.
3. Here’s your next problem: Debbie inherited an “A” allele from her
mother and an “A” allele from her father. Is Debbie’s blood type trait:
Write your response in the box on the right.
NOTE: By convention (that is, agreements between scientists), we use the word
phenotype when referring to the expression of a trait, and the word genotype when
referring to a person’s actual alleles.
4. Here’s your next problem: With reference to Debbie’s blood type,
what is her genotype? (Be sure to use the scientific convention of
UPPERCASE letters for dominant alleles and lowercase letters for
Write your response in the box on the right.
5. Your next problem: With reference to Debbie’s blood type, what is
Write your response in the box on the right.
Check out my representation (using scientific notation) of a female’s and a male’s
genotype with reference to the trait “tongue-rolling,” the ability to roll one’s tongue into a
long tube. This ability is the result of having a least 1 dominant allele (“T”). Both the
female and male below are heterozygous for the tongue-rolling trait.
Female genotype: Male genotype:
Tt (Heterozygous) Tt (Heterozygous)
Female phenotype: Male phenotype:
Tongue roller Tongue roller
How can we figure out all of the possible genotypes and phenotypes of the children they
could produce? The mechanics of this can be shown in a “mating grid” called a Punnett
Square. A Punnett Square is a handy device for keeping track of all of the possible ways
gametes CAN combine at fertilization. It shows the different ways alleles can be
combined and shows (and predicts) the probabilities of phenotypes and genotypes
resulting from a specific mating. It’s important to remember that a Punnett Square shows
only the probability of what might occur and NOT the actual results.
To make a Punnett Square, draw a 4-sqaure box (see below). Each of the mother’s alleles
is written down the left side of the box, one next to each square. Each of the father’s
alleles is written across the top side of the box, one over each square. And all the
possible combinations of their alleles are shown in the boxes.
T TT Tt
t Tt tt
What this shows is that a mating between 2 heterozygotes has a statistical probability of
producing a tongue-roller 75% of the time (TT, Tt, Tt) and a 25% chance of producing a
non-tongue-roller. Now let’s see what you’ve learned.
NOTE: Following convention, use a lowercase “r” to represent the recessive allele (for
right over left thumb-clasping) and an UPPERCASE “R” to represent the DOMINANT
allele (for left over right thumb-clasping) in the following genetics problem.
Your next problem: Debbie is a right thumb over left thumb hand-clasper, and her
partner is a left thumb over right thumb hand-clasper. Her partner’s mother is a right
Fill in the Punnett Square below, and then answer the questions below the Square.
Debbie Partner’s Alleles
6. What is the probability that Debbie and her partner could have a child who is a right
thumb over left thumb hand-clasper? ____________________________
7. What is the probability that Debbie and her partner could have a child who is
homozygous recessive for hand-clasping? ________________________
8. What is the probability that Debbie and her partner could have a child who is
heterozygous for hand-clasping? _____________________________
Mendel’s Fourth Discovery
• The way in which the paired alleles for one trait are segregated is TOTALLY
INDEPENDENT of the way in which paired alleles for a DIFFERENT TRAIT
are segregated. What Mendel meant by this can be best illustrated with an
The gene for earlobe type has 2 alleles: the dominant allele, “E,” and the recessive allele,
“e.” If a person is either homozygous dominant (EE) or heterozygous (Ee), s/he will have
“free” earlobes. If a person is homozygous recessive, s/he will have “attached” earlobes.
You already know about the tongue-rolling gene (T and t).
Let’s say there is a person who is heterozygous for both earlobes and tongue-rolling. Her
genotype would be (Ee) for earlobes and Tt (for tongue rolling). Her phenotype would be
free earlobes, and she could roll her tongue. What possible combinations of alleles can
show up in this person’s gametes?
First, apply the principle of segregation to EACH set of alleles. This means that every
sex cell will have EITHER and E or an e. Of course, the same applies to the R and r.
Since the distribution of the alleles into sex cells is COMPLETELY RANDOM, it’s
possible for a sex cell to contain any of the following possible combinations:
E and R OR E and r OR e and R OR e and r
This is what Mendel meant when he said that DIFFERENT traits are inherited
independently of one another. (NOTE: This is true ONLY IF the traits are determined
by alleles carried on separate chromosomes. If the alleles are carried on the same
chromosome, they tend to remain together when sex cells are formed. Genes that are
carried on the same chromosome and that are passed on together are said to be linked.
For the purposes of this class, however, we are concerned with learning basic genetics.)
Debbie has free earlobes, but her mother has attached earlobes. Debbie has a partner
whose earlobes are attached. Debbie can roll her tongue. Debbie’s partner can roll his
tongue, but both of their mothers can not roll their tongues.
9. Problem: What is the probability that they would have a child who
is both a tongue roller and has attached earlobes? (Fill in the Punnett
Square on the next page to find out!)
10. Your final problem: List the possible genotypes for a child who is
both a tongue roller and has attached earlobes.
We know that Debbie is Ee for earlobes. Why? This is because a “free” phenotype can
only happen if she has at least one Dominant allele. And we know that she has at least
one recessive allele because her mother has attached earlobes, which means her mother is
genotype ee. Likewise, we know her genotype for tongue rolling is Tt because she can
roll her tongue, but her mother can not. We know that Debbie’s partner is ee for earlobes
because he has attached earlobes. And we know that his genotype for tongue rolling is Tt
because he can roll his tongue, but his mother can not. I have put Debbie’s and her
partner’s allele combinations below. Your job is to figure out those of their possible
offspring. You do this by combining Debbie’s alleles with her partner’s. I’ve filled in
one of the possible combinations.
Debbie’s Partner’s Alleles