Genes and Inheritance [6]: Genetic Questions (A Level Biology)

Uploaded by freeeschool on 02.09.2012

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 Page…1