Lab U06


Heba Ali

II- Mendelian Genetics



The evidence that would indicate to me that the
ability to taste PTC is inherited is the number of offspring’s that are tasters
with homozygous dominant parents compared to the homozygous recessive
non-tasters. From the data, I can conclude that the ability to taste PTC is a
dominant trait. There is offspring that are tasters that have homozygous
Dominant parents. Which in comparison to the homozygous recessive parents that
had zero offspring tasters.

It was important for Snyder to verify that males
and females had similar proportions of tasters and non-tasters because Snyder
wanted to first rule out gender being a possible element in determining who was
a taster or non-taster. Snyder then continued his experiment grouping the
families by the phenotypes of the parents and got rid of the gender aspect of
his data.

Couples who can taste PTC sometimes have
children who can not taste PTC because it is possible the masking of certain
genes by other genes present. Another possible answer is that maybe one parent is
heterozygous dominant for this trait while the other parent is homozygous
dominant. This leaves a 50:50 chance that the offspring will also exhibit the
PTC gene.

The significance of the fact that couples who
can not taste PTC never have children who can is because the PTC gene is a
dominant trait. If  both parents cannot
taste PTC that means they are both homozygous recessive. So in fact, there is
not possible way that the parents would be able to pass to their offspring a
trait that they do not carry themselves. The only way they would be able to
pass down the trait is if at least one parent was heterozygous dominant and the
other was homozygous recessive. Then they would be able to have offspring who
are tasters, otherwise, there is no way a homozygous recessive couple can have
offspring who are dominant for the PTC gene.

 Based on
the data, I conclude that PTC taste blindness is a recessive trait. My evidence
for this is because if the parents who are homozygous recessive and cannot
taste are considered taste blind. Their children will also be taste blind
because like I mentioned above no child will be born with a homozygous or
heterozygous dominant PTC gene if their parents are homozygous recessive, as we
see in the data. The number of offspring for those parents who are homozygous
recessive have 0 children who are tasters.

The ratios expected from the test crosses are
either from a homozygous dominant parent crossed with a homozygous recessive
parent. The ratio would be 0:4:0. The numbers represent 0 for homozygous
dominant trait, 4 for heterozygous dominant trait, and 0 for the homozygous
recessive trait. The next test cross is with a heterozygous parent crossed with
a homozygous recessive parent. The ratio would be 0:2:2, 0 for homozygous
dominant, 2 for the heterozygous dominant trait and the last 2 is for the
homozygous recessive trait. This data does not fit either of the expected ratio
because if the parents of the offspring were tasters then according to the
ratio there should be 0 offspring who are non-tasters.