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Biology I
Genetics #2: Fun with Punnett Squares!
2.14.07
I envision this worksheet as serving several purposes: to help you practice your punnett
squares, to give you a useful tool for reviewing for the quiz later on, and to review some
concepts so that they will be indelibly burned into your consciousness. Let's start at the
beginning…
Genetics:
- The branch of biology that studies biological inheritance (heredity).
Gene:
- A section of DNA that codes for a specific sequence of amino acids (polypeptide chain).
- A good working definition is to say that it codes for a specific protein.
- Mendel thought (incorrectly) that it coded for a specific trait. This definition is OK, but it
doesn't reflect what we now know about genetics.
Allele:
- These are alternate forms of the same gene created by mutations in the genetic code.
Some genes have multiple alleles, such as blood type (three alleles), but at this point let's
assume that there are just two alternate forms, or alleles, for each gene. Example: plant
height in peas is either tall or short depending on which combination of two alleles is
present. Since you get one allele from each parent (remember homologous
chromosomes?), there will be four possible combinations for a specific pair of genes. This
is represented by two letters placed next to each other (TT, Tt, or tt in this example). The
first letter represents the contribution from one parent and the second letter represents
the contribution from the other parent.
- Alleles are either dominant or recessive. A dominant allele is one that will express itself
even in the presence of the other, or recessive, allele. In contrast, a recessive allele can
be expressed only when both copies are recessive. Example: the tall (T) allele is
dominant over short (t) allele in pea plants. Notice that the dominant allele is always
expressed as a capitol letter and the recessive as a lowercase letter. In addition, it is
customary to always put the dominant allele first.
- All alleles are found at the same locus.
Locus:
- The location of a specific gene within a strand of DNA. It can be thought of as the
"address" of a specific gene. Example: The gene for plant height in peas will always be
found at the same place on the same homologous chromosomes in every pea plant in
existence!
Homozygous vs. Heterozygous:
- Organisms that have two identical alleles for a particular trait (TT or tt in this example)
are said to be homozygous for that trait. Pea plants that are homozygous dominant (TT)
-
or homozygous recessive (tt) at that locus are both purebred for plant height. In other
words, if they self-pollinate they can only produce offspring identical to themselves.
Organisms that have two different alleles for a particular trait (Tt) are said to be
heterozygous for that trait, or hybrids. They look like the homozygous dominant plant,
but have a different genetic makeup. This brings up these two more very important
words…
Genotype:
- The genetic makeup of an individual.
- We can sound more professorial by referring to genotype as the collection of alleles in an
organism.
- The genotype of each individual in a sexually reproducing species is usually unique to that
individual, with the exceptions being clones and identical twins.
Phenotype:
- The physical expression (characteristic) of an organism’s genotype (always an adjective).
- Often influenced by environmental factors (e.g. size), but sometimes it isn't (eye color).
- Three aspects: morphological (physical), physiological (biochemical), and behavioral.
- When we perceive variation it is usually on this level. Of course there is a corresponding
genetic variation bubbling under the surface.
With this handy review the following should be a breeze. Remember that meiosis segregates
alleles during gamete formation and allows the two alleles for each trait to split up and
recombine (pair up with an allele from the other parent) in the offspring. This allows the
recessive alleles to escape the dominant ones and for the recessive phenotype to re-appear
from time to time.
NOTE: All of the following examples are for pea plants and concern the alleles for height
already discussed. Use punnett squares and show all of your work.
Cross two homozygous dominant plants.
1. What is the genotype of the offspring plants? The phenotype?
Cross a hybrid plant with heterozygote.
2. What are the genotypes of the offspring? The phenotypes? Why are there three genotypes
yet only two phenotypes?
Cross a homozygous recessive with a heterozygote.
3. What are the genotypes of the offspring? The phenotypes? Do the ratios match up? Why
or why not?
4. What would happen if you crossed a purebred dominant with a homozygous recessive? Do
a punnett square.
5. How do you tell a homozygous dominant from a heterozygote? Will phenotype be enough?
Why or why not?
Let me re-introduce the testcross. Its purpose is to answer the preceding question. If you
have this situation, you can cross the organism of indeterminate genotype with a homozygous
recessive individual (tt). I want you to create two punnett squares below. One should be a
homozygous dominant with a homozygous recessive, the other a heterozygote with a
homozygous recessive.
6. What is the ratio of genotypes in the offspring of each cross? The ratio of phenotypes in
the offspring of each cross?
7. Explain in your own words the purpose of a testcross, how to do it, and how you interpret
the results. Be thorough as well as specific!
Of course organisms have more than one gene. Do these genes interact? We know that alleles
segregate during meiosis, but do they do so independently? In other words, does the gene that
controls one trait influence the expression of another? Does the fact that a pea plant is tall or
short affect the fact that the peas themselves are round or wrinkled? Let's see.
A two factor (or dihybrid) cross is a cross that involves two different genes. Therefore, it can
also involves four alleles - two for each gene (trait). In the following examples we will use tall
(T) and short (t), but now we will also consider round peas, which are dominant (R), and
wrinkled peas, which are recessive (r). Since there are four alleles, or possibilities, for
segregation during gamete formation, there needs to be a total of 16 boxes in our punnett
square! Here is an example: A homozygous dominant (TTRR) is crossed with a homozygous
recessive (ttrr). We use FOIL (first outer inner last) to determine the possible combinations of
alleles to be contributed and place them along either side of the square. Now you fill in all of
the boxes. This is really no harder than what you did before; it just takes a little longer.
8. What are the genotypic and phenotypic ratios of the offspring? Were any recessive traits
visible at all?
Hopefully all of the offspring were heterozygous (TtRr). If not, ask me why. Assuming that
you're still with me, let's move on. Now that we have hybrids we can cross them and see what
the next generation will look like. Below I want you to use FOIL to set up the sides of the
punnett square and then fill in all of the boxes. I'll even give you a little help.
9. What are the nine possible genotypes of the offspring? Which of them is most prevalent?
Least prevalent?
10. What are the four possible phenotypes of the offspring? What ratios do they occur in?
11. If independent assortment didn't occur, would you get the ratios noted above? Explain.
Also, what would happen if these two genes were on the same chromosome? Explain that
one too.