Genes and Inheritance [6]: Genetic Questions (High band Biology)

A Level Biology: Genes and Inheritance 6 – Genetic
Questions Welcome to the sixth video about Genes. Today,
we are going to be looking at Genetic questions. Today, we are going to look at how you would
answer a standard genetic question. The first part of this question: If there is a single
gene that controls eye color and the brown allele is dominant over the blue, then what
would be the possible genotype or the genetic cross between two heterozygous individuals?
Part 2: What is the likelihood of them having two blue-eyed children? There are a couple of things that you need
to establish here. This would perhaps be a typical GCSE question and they’re saying
that there are only two possible alleles: blue or brown. In reality, there are many,
many more. But for the purposes of this, we will stick with two. We first have to establish the genotypes of
the two parents and it says that they’re heterozygous. So that means that both of the
alleles need to be different. In this case, you got an allele for brown and an allele
for blue. So the letter that we will use is B and b. In the case of the mother and father,
they’re genotypes because they’re heterozygous is Bb and they both have the phenotype for
brown eyes. The phenotype, again is how it is expressed, in this case will be brown eyes.
Another type of phenotype might be blue eyes. And because parents are heterozygous and brown-eyed,
we can establish that the brown-eyed allele or the B is the dominant one. B represents
the brown allele and b, recessive is lower case, represents the blue. We will go on now to draw a Punnett square
for different possibilities available. So with the father’s gametes, because his genotype
is Bb, then his gametes can either contain B, which is the allele for brown or the b,
which is the allele for blue. The mother’s gametes are the same because she’s heterozygous,
as well. Half of her eggs will contain B, the brown allele and half of them will contain
b, for the blue allele. The offspring can have these different possibilities of genotypes. The possible genotypes that are available
are: BB, Bb, bb. You notice that I haven’t included the bB here because it is exactly
the same as this one, hence, no need to write it down. That takes care of the first part
of the question. If we go back to the second part: What is
likelihood of them having two blue-eyed children? Looking at the answer here or looking at the
Punnett square here, the likelihood of them having one child who is blue-eyed is 1/4.
Therefore, the chance of them having two blue-eyed children will be 1/4 x 1/4 which is a 1/16
chance because 4 x 4=16. Therefore, it is reasonably unlikely and you can either express
this as a fraction or you could put it as a decimal. 1/16 is 0.0625. You could change the question somewhat for
the second part. The second part of the question: What’s the likelihood of them having two
blue-eyed children? They could change this question certainly by asking this: If the
first child has blue eyes, what is the chance of their next child having blue eyes? This
is different to the second part because the second part is asking: What’s the likelihood
of them having two children in a row with blue eyes? The third part is simply asking:
Does the previous child have any bearing on any future children? The answer to that is no. Regardless of the
genotype of the previous child, it doesn’t influence the chance of the second child.
Let us get this to work. There is still, regardless of whether the first child is half blue eyes
or brown eyes, there is still, with this two parents, there is a still a 1/4 chance of
the next offspring having blue eyes. The answer to that one is 1/4 or 25%. [end of audio – 04:28]
A Level Biology: Genes and Inheritance 6 – Genetic Questions

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