Video Transcript
A cross between two pea plants
produced the following genotypes: uppercase D lowercase d uppercase FF, all
uppercase DDFF, all lowercase ddff, uppercase D lowercase d lowercase ff. The genotype of one parent plant
was uppercase D lowercase d uppercase F lowercase f. What is the genotype of the other
parent plant? (A) Uppercase D lowercase d
uppercase FF. (B) All uppercase DDFF. (C) All lowercase ddff. (D) Uppercase D lowercase d
lowercase ff.
This question asked about a cross
between two pea plants where only one genotype is known. But first we need to review what a
genotype is. Let’s remove the answer choices and
move the question up to have more room to work with.
Recall that the genotype is the
genetic makeup, or alleles, of an organism. And alleles are alternative
versions of a gene. A common example of a plant with
different alleles is peas. Peas have different alleles for
both pea color and pea shape. Peas can be yellow or green for
color and round or wrinkled for shape.
Scientists assign a letter for the
genes they are observing to keep things simple, and the uppercase letter is the
dominant trait. In this case, pea color is assigned
letter D. Uppercase D will represent the
yellow allele because yellow is dominant, while lowercase d will represent green
because green is recessive.
You may remember that dominant
alleles are always expressed if they are present in the genotype, like the examples
provided. On the other hand, a recessive
allele is an allele only observable if there is no dominant allele present, like in
the example provided.
When scientists want to examine two
genes at once, we use a different letter to represent the other trait, such as pea
shape. For example, you could use the
letter F for pea shape. Uppercase F will represent round,
and lowercase f will represent wrinkled. So a genotype just gives us a
shorthand version of the alleles an organism has for one or more genes. For example, our known parent’s
genotype is uppercase D lowercase d uppercase F lowercase f, which means they have
inherited both the dominant and recessive alleles for both of our genes. We also call organisms with
different alleles for a gene heterozygous, while organisms with the same allele for
a gene are known as homozygous.
In this question, we’re trying to
determine which offspring each parent could produce. Offspring are produced from a
combination of gametes from both parents. We can use the FOIL method to
determine all of the possible gametes for the parent plants. FOIL stands for first, outside,
inside, last. The first in FOIL would be the
first letter of the alleles for each trait. So here it would be uppercase D and
uppercase F. The outside in FOIL would be the
outside alleles for each trait. So here it would be uppercase D and
lowercase f. The inside in FOIL would be the
inside alleles for each trait. So here it would be lowercase d and
uppercase F. The last in FOIL would be the last
alleles for each trait. So here it would be lowercase d and
lowercase f.
In order to answer our question
correctly, we need to find the potential gametes for each of our possible
parents. We will leave the known parent’s
gametes at the top right.
Answer choice (A) has a genotype
which is heterozygous for trait D and homozygous dominant for trait F. Answer choice (A) can produce two
gamete options: one that has the genotype uppercase D uppercase F and one that has
the genotype lowercase d uppercase F.
Answer choice (B) is homozygous
dominant for both traits. Answer choice (B) can only produce
gametes that have the genotype uppercase D uppercase F. Therefore, it only has one gamete
possibility.
Answer choice (C) is homozygous
recessive for both traits. Just like before, answer choice (C)
can only produce one gamete possibility. The gamete for answer choice (C) is
lowercase d lowercase f.
Answer choice (D) is heterozygous
for gene one and homozygous recessive for gene two. In this case, we have two gamete
possibilities: uppercase D lowercase f or lowercase d lowercase f.
Now that we have all the gamete
possibilities, we can conduct a Punnett square diagram for each of our answer
choices. This will allow us to see the
resulting offspring. This is what our completed Punnett
square should look like, filling in the squares with the alleles from both parents
from the top and left side.
Now, we can compare these offspring
possibilities to the ones presented in the question. Can you spot which answer choice
would give us offspring that matches the one from the question?
Notice that answer choice (A) is
the only combination of alleles that can produce heterozygous D homozygous dominant
F, homozygous dominant D heterozygous F, homozygous recessive d homozygous dominant
F, and homozygous recessive d heterozygous F. Therefore, going back to our
original question, the correct answer to this problem is (A): heterozygous D
homozygous dominant F.
