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MONOHYBRID CROSS v2
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Introduction
A gamete is the egg or sperm cell that is produced by meiosis. A gamete contains the haploid
number of chromosomes (in a human this number is 23). In each of these chromosomes are a
number of different genes, each of which confer a different trait to an organism.
In genetics, we represent each copy of a gene for a trait with a letter from the alphabet. For
example, each of the gametes mentioned above would have one copy of a gene for eye color in
one of its chromosomes. Lets say this copy of the eye color gene is a brown eye color gene. We
call this single copy of the gene an allele (uh-LEEL) and we can represent this allele with the
letter E.
After the haploid egg is fertilized with a haploid sperm, however, the result is a zygote, which is
diploid. This zygote will now have not one copy of the eye color gene, but two, one from each
gamete. If both the egg and sperm have contributed brown eye color alleles, then we would
represent this situation with two letters, EE.
When we are referring to the particular genes an organism has for a certain trait, we are referring
to its genotype. The genotype of our example organism is EE. When we are referring to the
physical trait that results from this set of genes an organism has, we are referring to its
phenotype. The phenotype of our example organism is Brown Eyes.
From the example given above, it is easy to see how two brown eyed parents can give birth to
brown eyed children. But how do these same parents give birth to blue eyed children? What
color eyes will the child of a brown eyed parent and a blue eyed parent have? Read on to find
out...
Problem Set-up
First you will see how to set up a cross between a blue eyed and a brown eyed parent. This cross
is called a monohybrid cross because it involves only one trait, eye color (later on we will
consider more complicated dihybrid crosses which show the inheritance of two traits).
In setting up a problem you should always have a key. A key represents the genes you are
dealing with, i.e. a brown eyed gene or a blue eyed gene. Any letter may be used but the
dominant must be expressed with a capital letter, such as A, D, or Z, and the recessive with the
same letter but lower case, such as a, d, or z.
You will understand what dominant and recessive mean after the example, so keep going for
now.
Example 1
Key:
A cross between a brown eyed male and a blue eyed female, given that brown eye
color is dominant to blue eye color.
E = brown eye color
e = blue eye color
(note that blue eye color is still represented by the letter e,
but a lower case one...this is very important)
Our male has a genotype of EE, while our female has a genotype of ee.
Monohybrid Cross
page 1
The male's gametes (sperm) however, are represented simply by E, since they each only have
one copy of the eye color gene. Likewise the female's gametes (eggs) are represented by e.
Male genotype: EE
Female genotype: ee
Male gametes: E and E
Female gametes: e and e
Now make a Punnett Square (PUH-net) to show the results of this cross:
1)
E
(put other
parent's
gametes here)
E
(put one parent's gametes here)
e
e
2) Now fill in the small squares by moving each gamete allele into the appropriate box, as
shown.
E
E
e
Ee
Ee
e
Ee
Ee
Each small square represents the genotype of an
offspring. In this case they are all alike (Ee) but this
is not always the case.
**Now in this case, all offspring will have the genotype Ee, meaning a copy of both the blue and
brown eye color genes. What will the eye color of an Ee individual be? BROWN, because, as
you were told at the beginning of the example, the brown eye color allele is DOMINANT to the
blue eye color allele. The blue eye color allele is RECESSIVE to the brown allele.
When both alleles are present, the dominant allele is the one that is expressed or seen.
It is important to realize that each parent contributes one gamete to the zygote. In this case, the
gametes contain alleles E and e. The zygotes then contain the alleles Ee.
3) Can you fill in this practice Punnett Square?
B
B
B
Note: When a dominant allele is present, it always
is written before the recessive allele.
ex) Write Ee, not eE.
b
Answer on next page -- Don't look until you are finished!
Monohybrid Cross
page 2
Answer:
B
B
B
BB
BB
b
Bb
Bb
Important Genetics Vocabulary
In doing a cross we refer to the original cross as the P (parent) generation (EE x ee).
The results of this cross (the children) are all Ee and are known as the F1 (filial) generation.
The offspring of the F1 generation are referred to as the F2 generation.
What do you think the offspring of the F2 generation are called? ______________________
When both alleles are alike for one trait, EE or ee, the genotype is described as homozygous.
When the alleles of the genotype are different, Ee, the genotype is described as heterozygous.
(How to remember? Homo- refers to "same" and hetero- refers to "different.")
When assigning letters, the dominant trait is referred to with a capital letter, E, and a recessive
trait with the small letter, e. In referring to the genotype EE, we would call it homozygous
dominant; ee would be homozygous recessive; and Ee would be the heterozygous genotype.
Also recall that in talking about traits we frequently refer to phenotypes (what a trait physically
looks like) in addition to the genotypes (specific genetic makeup such as EE, Ee, ee). Going
back to the cross we just made, we can make the following assignments:
In pea plants tall height is dominant over short height (and short is recessive to tall).
Key:
H = tall
h = short
(to assign gene symbols, you may select any letter if none is given)
So,
HH = Tall (homozygous dominant)
Hh = Tall (heterozygous)
hh = Short (homozygous recessive)
Question 1: What is the phenotype for a plant of genotype Hh? ___________________
Question 2: Can you have a heterozygous short plant? ____ Why or why not?
(see answer sheet at end of packet)
____________________________________________________________________________
Sample Cross #1: Write out the phenotypes for a cross between a homozygous tall male plant
and a homozygous short female plant.
Key: ___ = Tall
___ = Short
P generation
Tall
x
Short
(fill in genotypes)
____
x
____
(fill in gametes)
__, __
__, __
Now fill in the Punnett square using the parent genotypes you figured out above.
(check answer sheet at end of packet when you are done)
Monohybrid Cross
page 3
Sample Cross #1 (continued)
If you understand the cross from the previous page, your results should have come out with 4 out
of 4 (4/4) or 100% heterozygous tall plants, such as Hh.
These offspring are the F1 generation (the first filial generation). We would describe them as:
Genotype: 100% Hh
Phenotype: 100% tall
Now, take the F1 generation and cross it with itself. This will give you the F2 generation.
Hh
x
Hh
This time, the gametes that are produced by meiosis have the alleles H and h from the male and
H and h from the female.
Hh
x
H and h
Hh
H and h
(F1 genotypes)
(F1 gametes)
Arrange these on the Punnett Square:
H
h
H
HH
Hh
h
Hh
hh
The important thing to realize here is that a parent can
contribute either an H or an h, but NOT both. And
since either one or the other must be given, there is a 1/2
chance of H and an 1/2 chance of h being given.
In a physical sense, an Hh female will have an H in 1/2 of
her eggs and an h in the other 1/2 of her eggs (the choice of
which egg gets fertilized is random). Likewise, 1/2 the
sperm from an Hh male will have an H allele and the other
1/2 of his sperm will have an h.
Again, when the F1 generation is crossed with itself, the result is the F2 generation.
The results of this cross are:
F2 GENOTYPE
F2 PHENOTYPE
1/4 HH (homozygous dominant)
Tall
75% Tall
1/2 Hh (heterozygous)
Tall
1/4 hh
Short
Monohybrid Cross
(homozygous recessive)
page 4
25% Short
Sample Cross #2: Cross a heterozygous tall plant with a homozygous short plant.
Key:
___ = Tall
P generation
Tall
x
Short
___ = Short
(fill in genotypes)
____
x
____
(fill in gametes)
__, __
__, __
(fill in Punnett Square)
Question 3: What % of the offspring are tall? ___________________________
Question 4: What % of the offspring have homozygous genotypes? ________________
(check answer sheet for answers)
Monohybrid Cross
page 5
Monohybrid Cross ANSWER SHEET
Question 1: The phenotype is Tall.
Question 2: No, because the presence of a dominant allele blocks the expression of h.
Sample Cross #1
Key:
_H_ = Tall
P generation
Tall
x
Short
_h_ = Short
(genotypes)
_HH_
x
_hh_
(gametes)
_H_, _H_
h
h
H
Hh
Hh
H
Hh
Hh
_h_, _h_
Sample Cross #2 (problem on page 5)
Key:
_H_ = Tall
P generation
Tall
x
Short
_h_ = Short
(genotypes)
_Hh_
x
_hh_
(gametes)
Question 3: 50% Tall plants (1/2 Hh)
Question 4: 50% (1/2 hh)
Monohybrid Cross
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_H_, _h_
h
h
H
Hh
Hh
h
hh
hh
_h_, _h_