NAME:
MLS PERIOD:
DATE:
DIRECTIONS: Answer all questions using complete sentences. When making Punnett squares, make the squares at least
4 cm on each side for single trait crosses and at least 8 cm on each side for two-trait crosses. Complete sentences are
NOT required when answering questions about Punnett square combinations unless otherwise noted.
GENE INTERACTIONS
INDEPENDENT ASSORTMENT
After establishing that genes segregate during the formation of gametes (reproductive cells), Mendel began to explore
the question of whether they do so independently. In other words, does the gene that controls one trait have anything to
do with the gene that controls a different trait? For example, does the gene that determines whether a seed is round or
wrinkled in shape have anything to do with the gene for seed color? Must a round seed also be yellow? To answer these
questions, Mendel first crossed purebred plants that produced round yellow seeds with purebred plants that produced
wrinkled green seeds.
The Two-Factor Cross: F1. In this cross, the two kinds of plants would be symbolized like this:
Round yellow seeds
Wrinkled green seeds
RRYY
rryy
Because two traits are involved in this experiment, it is called a two-factor cross. As you examine the cross, keep in mind
that you are looking at the kind of seeds the plant produces. These seeds are not necessarily the same as the seeds from
which the plants grew.
The plant that bears round yellow seeds produces gametes that contain the alleles R and Y, or RY gametes. The
plant that bears wrinkled green seeds produces ry gametes. An RY gamete and an ry gamete combine to form a fertilized
egg with the genotype RrYy. Thus only one kind of plant will show up in the F 1 generation — plants that are heterozygous,
or hybrid, for both traits (see figure 1). What is the phenotype of the F 1 plants? That is, what will the seeds produced
by the F1 plants look like? Because we know that round and yellow are dominant traits, we can conclude that the F 1 plants
will produce seeds that are round and yellow. Remember that the concept of dominance tells us that the dominant traits
will show up in a hybrid, whereas the recessive traits will seem to disappear.
rryy
RY
ry
ry
ry
ry
RrYy
RrYy
RrYy
RrYy
RY
RrYy
RrYy
RrYy
RrYy
RY
RrYy
RrYy
RrYy
RrYy
RY
RrYy
RrYy
RrYy
RrYy
RRYY
Figure 1. When an individual that is
homozygous dominant for two traits
is crossed with an individual that is
recessive for the same two traits,
all of the offspring are heterozygous
for those two traits. (Note that the
dominant allele is always written
first.)
This cross does not indicate whether the genes assort, or segregate, independently. However, it provides the
hybrid plants needed for the next cross — the cross of F1 plants to produce the F2 generation. The seeds from the F2
plants will show whether the genes for seed shape and seed color have anything to do with one another.
The Two-Factor Cross: F2. What will happen when F1 plants are crossed with each other? If the genes for seed shape
and color are connected in some way, then the dominant R and Y alleles (which came from one parent) and the recessive r
and y alleles (which came from the other parent) will be segregated as matched sets into the gametes. Thus, the
gametes could only contain one of two possible gene combinations: RY or ry, as seen in figure 2.
RrYy
RY
ry
RY
RRYY
RrYy
ry
RrYy
Figure 2. If the genes f or two traits
are connected in some way , only two
combinations of the traits are possible in the of f spring.
RrYy
rryy
If the genes are not connected, then they should segregate independently, or undergo independent assortment. This
produces four possible types of gametes: RY, Ry, rY and ry. In addition, if the genes assort independently, some of the
seeds produced by the F2 plants will have new combinations of traits — they may be wrinkled and yellow or round and
green.
This two-factor cross is examined in figure 3. Now that we have four possible gamete types (and sixteen possible
offspring types) the square is especially useful. If the genes for seed shape and seed color are inherited independently,
then the seeds produced by the offspring should be in the same proportions as that predicted by the Punnett square.
RrYy
RY
Ry
RY
Ry
rY
ry
RRYY
RRYy
RrYY
RrYy
RRYy
RRyy
RrYy
Rryy
RrYy
F generation
2
rY
RrYY
RrYy
rrYY
ry
RrYy
Rryy
rrYy
rrYy
Figure 3. If the genes f or
two traits are not linked,
then they will undergo
independent assortment.
This results in new
combinations of the traits.
rryy
Mendel actually carried out this exact experiment, and his results were very close to the ratio predicted by the
Punnett square in figure 3. From these results Mendel concluded that genes could segregate independently during the
formation of gametes. (As we shall see later, there is an important exception to independent assortment. Genes located
on the same chromosome are linked and may not undergo independent assortment.)
1.
2.
3.
Using the Punnett square in figure 3, list the genotype, genotype abbreviation, phenotype and predicted fraction of
offspring for each different gene combination.
What is the phenotypic ratio of the offspring that results from crossing two individuals that are hybrid for two
nonlinked traits?
Make a Punnett square that shows all possible gamete combinations and all possible offspring for a cross between a
woman who is hybrid for widow's peak and short fingers (WwSs) and man who has a continuous hairline and short
fingers (wwSs).
a.
What fraction of the offspring will be homozygous recessive for both traits?
b.
What fraction of the offspring will have the widow's peak with long fingers phenotype?
c.
What fraction of the offspring will have the identical genotype as the mother?
WHAT CAUSES DOMINANCE?
Dominance is the simplest example of how genes interact with each other. We have already learned that the effects of
the dominant allele are seen even when the recessive allele is present. But what causes dominance?
A gene is a section of DNA on a chromosome, and the DNA codes for a polypeptide, or string of amino acids. In
many cases, the dominant allele codes for a polypeptide that works, whereas the recessive allele codes for a polypeptide
that does not work. For example, suppose that the allele B codes for an enzyme that makes a black pigment in a mouse's
fur and allele b codes for a defective enzyme that cannot make the pigment. A mouse that has the genotype bb will have
white fur because it lacks the enzyme that makes the black pigment. But a mouse that has the genotype BB or Bb will
have black fur because it possesses the enzyme that makes the black pigment. Although each cell in the Bb animal has
just one copy of the functioning allele, that single copy can code for thousands of molecules, each of which can code for
thousands of enzymes. This is the reason the B allele is dominant over the b allele.
INCOMPLETE DOMINANCE
In 1760 the German scientist Josef Kölreuter reported on experiments in which he cross white carnations (rr) with red
carnations (RR). Kölreuter found that all of the offspring from his crosses had pink flowers (Rr). In other words, the
hybrids had a phenotype that was intermediate between those of the parents. At first glance, it might appear as if the
parents' genes had blended together. But when Kölreuter crossed his pink F 1 hybrids with each other to form an F2
generation, the parents' phenotypes reappeared. In the F2 generation, 1/4 of the plants had red flowers, 1/2 had pink
flowers, and 1/4 had white flowers, a 1:2:1 ratio.
In carnations, the R allele, which codes for an enzyme that makes red pigment, is incompletely dominant over the r
allele, which codes for a defective enzyme that cannot make pigment. In incomplete dominance the active allele does not
compensate for the inactive allele, and the heterozygous phenotype is somewhere in between the homozygous phenotypes.
4.
How did Kölreuter know that the genes of the parent flowers had not blended together but that the alleles for red
color and white color still existed separately?
5.
Make Punnett squares to show the F1 and F2 generations from a P generation cross of red with white carnations.
a.
6.
For the F2 generation, give the genotypes and phenotypes of all possible offspring including the percentage
of each that would be expected.
Make a Punnett square to show the cross between a pink and a white carnation.
a.
What fraction of the offspring will be pink?
b.
What fraction of the offspring will be red?
CODOMINANCE
Many genes display codominance, a condition in which both alleles of a gene are expressed. In other words, both alleles
are active. (Remember that in incomplete dominance only one of the alleles is active.) Codominant alleles are written as
capital letters with subscripts (for example, B1 and B2) or superscripts (for example, R and R').
Codominance is seen in many organisms. For example, red hair (HR is codominant with white hair (HW) in cattle and
horses. Cattle that have the genotype HRHW are roan, or reddish brown, colored because their coats are a mixture of
red and white hairs. This is different from incomplete dominance because neither allele is inactive. The hair is not pink;
instead, each individual hair is either red or white. Much the same thing happens in certain varieties of chickens. Black
feathers (FB) are codominant with white feathers (FW). Erminette chickens (FBFW) are speckled black and white.
7.
Make a Punnett square to show the possible offspring of erminette chickens.
a.
What percent of the offspring will be erminette chickens?
MORE PRACTICE WITH A TWO-FACTOR CROSS:
8.
Dimples and freckles are dominant traits in people. A person who has dimples and freckles marries someone who
does not have freckles or dimples. This couple produces a child that does not have dimples nor freckles. What is
the genotype of all persons concerned? (Use a Punnett square to prove your answer.)
POLYGENETIC INHERITANCE
Let's consider skin color. Skin color results from a pigment called melanin. The more melanin a person has the darker
their skin. Melanin protects the skin from the UV radiation, and those people whose ancestors evolved in areas with high
concentrations of UV (equator) will have more melanin as this is an advantageous trait in those areas. If you have lighter
skin, your ancestors most likely evolved in areas of lower UV radiation. Your skin must absorb UV to make Vitamin D so it
would be advantageous to have lighter skin in areas with less intense UV rays.
Two or more genes may affect the same trait in an additive fashion. When a person with the most melanin (darkest)
person has children with a person with the least melanin, the children's skin color is middle coloration. Two individuals
with middle coloration can produce children who range in skin color from darkest to lightest. This can be explained if we
assume that there are two genes that control skin color and that only alleles indicated with a capital letter contribute
equally to skin color. (In actuality, skin color is controlled by more than two genes.)
Darkest Coloration
Dark Coloration
Middle Coloration
Light Coloration
Lightest Coloration
=
=
=
=
=
AABB
AABb or AaBB
AaBb or AAbb or aaBB
Aabb or aaBb
aabb
9.
A woman with dark coloration (AABb) marries a man with light coloration (Aabb). Make a Punnett square to show all
possible genetic combinations of their gametes.
a.
What fraction of their children could have dark coloration?
b.
What fraction of their children will have the exact same genotype as the father?
c.
What fraction of their children will have the same phenotype as the father?
10.
Using the two-gene combinations shown above, what crosses will produce a child with darkest skin whose
grandparent was lightest in coloration? Show Punnett squares for the grandparents, parents and kids to prove your
answers.
11.
What is the darkest child that could result from a mating between a light-colored individual and a lightest-colored
individual? Use a Punnett square to prove your answer.