Welcome back to BOGObiology! Today we're going to expand on the topic of
Mendelian Genetics by discussing something called the dihybrid cross. If
you missed out on the original video introducing the idea of Mendelian
Genetics, definitely check it out here. As I pointed out in the original video, it's
rare to have one gene to one trait match-ups. Most traits are determined by
the interaction of several genes, and there are also genes that control
multiple traits. It's important to keep this in mind as we go through the video.
When organisms reproduce, their haploid gametes (i.e. their sex cells) combine to
form diploid offspring. Since the gametes carry the genetic
information in their DNA, the offspring will inherit one set of genes from each
parent. The resulting zygote is now diploid and has genetic information from
both parents. If you look closely, you'll notice that the mother and father each
contributed a slightly different allele for the same trait on this blue
chromosome. We'll see why this is important in just a minute. To keep
things simple, let's begin by breeding some fake organisms that I call "Dots".
Unlike most creatures in the real world, dots are the ideal species for us to
study because their traits always follow the rules of Mendelian genetics
perfectly. Among other characteristics, dots can be either large or small in
size and have either a fluffy or a smooth coat. Large and fluffy are the two
dominant traits. For our first experiment, we're going to breed two dots together.
The first one is going to be homozygous for largeness and fluffiness, and the
second dot is going to be homozygous for smallness and smoothness. This first dot
is going to produce sex cells that have an allele for large size and an allele
for a fluffy coat. The second dot will produce sex cells that have alleles for
small size and a smooth coat. The first dot can't contribute a small or a smooth
allele, and the second dot cannot contribute a large or a fluffy allele. We
could probably work out the genotypes that their offspring could have, but
let's do a Punnett square anyway. On the sides of the Punnett square, we're going
to put down the possible combinations of alleles that each one of the organisms
could contribute. The first dot has only large and fluffy alleles to contribute,
so that's all we're going to put down. The second dot has only small and
smooth alleles to contribute. Because one dot can only
contribute a large L and a large F, and the other dot can only contribute a
small l and a small f every one of their offspring is going to have the genotype
"LlFf". It's going to be heterozygous for both traits.
Despite having the large and fluffy phenotype, every one of the offspring is
going to be heterozygous for size and for fluffiness. In short, they're all
going to be di-hybrids. Now let's try breeding two identically di-hybrid
organisms together and see what happens. Because these organisms have both the
large L and the small l alleles, their sex cells can contain either the large
allele or the small allele. Similarly they can contain the fluffy large F
allele or the smooth small f allele. This makes for four possible combinations of
alleles that could be in their gametes. In short, every possible combination of
upper and lowercase L and upper and lowercase F... 1, 2, 3 & 4 . We don't know
which set will be passed on when they reproduce, but we can work out the
probability. For this Punnett Square, we're going to list all the possible
combinations of alleles and the sperm cells on one side, and on the other side
we'll list all the possible combinations of alleles in the egg cells. This first
organism could have any one of these genotypes in its sex cells.
The second organism is the same. Now we can pretty easily work out all the
possible combinations of alleles that the offspring could receive. See if you
can work out all 16 squares. Okay, so here are the potential genotypes for the
offspring. Notice that some of the genotypes are much more common than
others. If we color code them, we can see there are nine possible combinations of
alleles. The most common genotype is this one right here; another di-hybrid.
Now let's check out their phenotypes. It's pretty easy to see that
the most common phenotype is for the offspring to be large and fluffy. In fact,
the offspring are created in a ratio of 9 to 3 2 3 2 1. Again, remember that the
probabilities start over every time reproduction occurs. Having one large and
fluffy offspring does not increase the chances of having a small
smooth offspring later on. So, to test out how comfortable you are with the
material, see if you can work out the genotype and phenotype possibilities for
these two organisms' offspring. If you're stuck on how to get started, remember to
put all of the possible combinations of alleles into this space right here. Here
are all the possible combinations of alleles that each parent could
contribute. Now we have all the possible genotypes for their offspring. Before you
work out the phenotypes, see if you can take an educated guess as to the ratio
of large to small and fluffy to smooth. Notice that we have a 50/50 split of
large and small offspring, but that every single one of the offspring has the
fluffy phenotype. Even though there are no smooth individuals in this generation,
because some of the offspring are heterozygous for smoothness, they might
be able to produce smooth offspring themselves. All right, that's pretty much it!
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