A Level Biology: Genes and Inheritance 5 Ð Inheritance Part 2
Hi! Welcome to my fifth video in the series and this one is looking at Genetic crosses.
In this video, we are going to be looking at the Genetic cross; looking at earlobe attachment
which is leading on the previous video.
In this case, we are looking at a mother with this genotype and the genotype is the genetic
make-up of an organism. The mother has one dominant allele and one recessive. The father
has one dominant and one recessive. Both of them have this phenotype unattached. What
this means is that the dominant allele must be the unattached allele. The E represents
unattached and e represents the attached ear lobe.
There is a specific name for this type of genotype where the two alleles are different
and thatÕs referred to as heterozygous. Hetero means different. If the two alleles are the
same, if they were ee, then that would be referred to as homozygous; also the same if
they were EE. Heterozygous means different. Homozygous means the same.
The fatherÕs genotype is Ee. That means that in his sperm, he only carries one of these
alleles. That is very important. Gametes or sex cells only carry half the genetic information.
They only carry one allele, in this case, earlobe attachment. The fatherÕs sperm can
either have the allele for unattached or it can have the allele for attached. The same
is true with the mother. You notice that this has been represented on this diagram. These
are the sperm, obviously, the detail and these are the eggs.
In this part of the diagram, this section here, this is known as a Punnett square and
what it shows is the different possibilities of the genotype of the offspring. What you
have are the two possibilities of the sperm. The sperm can either have the unattached ear
lobe or it can have the attached. Here you have the eggs which can have the unattached
ear lobe allele or the attached and what we are going to do is we are going to put these
together to form the genotype or the possible genotype of the offspring.
In this case, it would be EE, which would be a homozygous individual. In this case,
E from the mom and e from the father and that makes this heterozygous. In this case eE,
again this is the heterozygous genotype because the e from the egg and the E from the sperm.
In this case, ee is a homozygous individual because it has two recessive alleles. This
shows the different possibilities that are available.
Possibilities is a really key term here because by chance, you would expect the offspring
between these two individuals. You would expect a 1/4 chance for them to have this genotype;
a 1/2 chance to have these two genotypes; and 1/4 to have these genotypes.
On the face of it, you would say unlikely. What is less likely is that these two individuals
would have a child that has an attached ear lobe, but there is a 1/4 chance of that. The
overwhelming outcome should be the fact that the child should have an unattached ear lobe
because in the 3 of the 4 cases, thatÕs what the genotype indicates.
But this doesnÕt mean to say that if these individuals have three kids, that they couldnÕt
all have attached ear lobes. It would be unlikely. The first kid having that genotype would be
1/4. The second would be 1/4 and the third would be 1/4, so that would be 1/64 chance
that it is possible, but it is unlikely.
A carrier is whatÕs technically known as an individual who carries the allele for a
certain condition. In this case, if the condition is having attached ear lobes, then these two
individuals that are heterozygous with eE, these two (Ee & eE) are whatÕs technically
known as the carrier and if this was a genetic disease, sometimes we refer to this (ee) being
a sufferer. ThatÕs a 25% case you would expect this outcome.
In summary, a heterozygous individual or heterozygous means a set of alleles which are different,
in this case Ee. Homozygous is a set of alleles which are the same; with this example, EE
or ee. A carrier is an individual who carries the allele for the condition but doesnÕt
have it, so in this case, heterozygous Ee and sufferer, someone who has a particular
condition; in this case ee.
[end of audio Ð 05:13] A Level Biology: Genes and Inheritance 5 Ð
Inheritance Part 2 PageÉ1
