Mendelian Genetics: The Dihybrid Cross


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! If you found this video useful, I hope
you’ll consider subscribing to my channel and also checking out some of my
other videos. I put a lot of time into creating them, and I’m always looking for
new ideas for content that would be useful to you all thanks so much for
watching and please don’t forget to subscribe!

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