Learning from the Fossil Record
Grade 8 Science
Part 1 of Performance Assessment Task – Skull Model
1.
How is it possible for the recessive short skull and jaw trait to change to a dominant long skull
and jaw trait and stay that way permanently?
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Since long skull and jaw are dominant over short skull and jaw, since all the organisms in the
deepest fossil layer have short skulls and jaws, they must be homozygous recessive (tt) , for
short skull and jaw.
In the next fossil layer, there were some organisms with short skulls and jaws and some with
long skulls and jaws. The ones with short skulls and jaws were offspring of the organisms in the
deepest fossil layer. ( tt x tt = all tt offspring ) The organisms with long skulls and jaws cannot be
explained genetically because they require at least one dominant trait (Tt or TT). Since this is
not possible, coming from homozygous recessive parents the reason for this trait must be due to
a change in the genes.
In the next fossil layer, all the organisms had the dominant long skull and jaw trait. This most
likely came about by the cross between the TT and tt organisms resulting in all the offspring
coming out to be Tt. It could have been a cross between Tt and tt but some of the offspring
probably would have come out tt (short skull and jaw). Therefore the TT and tt cross is more
likely.
In the final fossil layer, all the organisms had the dominant long skull and jaw trait again. This
had to come about due to the Tt x Tt cross resulting ion 75 % having the dominant trait (long
skull and jaw) and 25% recessive trait (short skull and jaw). For some reason the short skull and
jaw organisms could not survive beyond birth (probably due to something in the environment
where they needed long jaws and skulls) leaving only the dominant organism (TT or Tt) to live to
adulthood and continue breeding. Eventually all organisms would end up with TT only, since the
chance of having t became inferior because they could not survive beyond birth.
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a. Does the reasoning behind the answer make sense and is it possible?
b. Is the reasoning behind the answer supported by data?
c. Where reasoning cannot be found using genetics, is a reasonable prediction made?
d. Is the report organized in a logical manner so the reader can understand the explanation?
2. How is it possible for the recessive long, skinny tail to change to dominant short, thick tail trait
and stay that way permanently?
 In the deepest fossil layer, all the organisms had long, skinny tails. This must mean that all the
organisms had to be homozygous recessive (tt) because the long skinny tails is the recessive
trait. If the organism had a large T in their genotype it wouldn’t have a long and skinny tail
because the short and thick tail is dominant.
Learning from the Fossil Record
Grade 8 Science
 In the next fossil layer, some organisms had long and skinny tails and some had short, fat tails.
The ones who had long skinny tails were offspring of the organisms in the deepest fossil layer.
The other organisms that were found had the short and fat tails and the only way to explain
this is the fact that some of the organisms in the deepest fossil layer evolved. It is not possible
for the organisms in the deepest layer to have offspring that have short fat tails because they
are homozygous recessive and it impossible for them to pass a large T to their offspring so the
only explanation is evolution.
 In the next fossil layer, all the organisms had short, fat tails. This means that the all of the
organisms in the previous fossil layer that had short, fat tails were the parents of these
organisms. The long, skinny tailed organisms either evolved or died out. This must mean these
short, fat tailed organisms in this fossil layer are homozygous dominant (TT) because they
either evolved from the skinny, long tailed organisms or the short, fat tailed organisms in the
previous fossil layer.
 In the last fossil layer, all the organisms still had short, fat tails. This must mean that these
organisms were the offspring of all the organisms in the previous fossil layer. These organisms
stay this way permanently so they must be TT or homozygous dominate.
3. How is it possible for the dominant long front legs trait to change to a recessive short front legs
trait and then turn into flippers?
 In the deepest layer all the fossils contained long, front legs. The long, front legs were the
dominate gene, and since the next layer contains long, front legs and short, front legs the
genotype of the fist layer has to be Ll in order to make the next layer of offspring.
 In the next fossil layer some have long, front legs and some had short font legs; because the
short, front legs are recessive there genotype would have to be ll. The long, front legs are the
dominate trait so they can have the genotype combination of Ll, or LL.
 In the next fossil layer, all the organisms had front flippers. This means that the organisms
evolved to have front flippers from front legs. This means that their genotypes are ff. There is
no way to explain this through genetics.
 In the last fossil is layer, all the organisms still had front flippers. This means that these
organisms are the offspring of the organisms in the previous fossil layer. This also means that
these offspring are pure (ff). This is like the previous fossil layer.
4.
How is it possible for the dominant long back legs trait to change to a recessive short bag legs
trait and then disappear completely?

In the deepest fossil layer, the organisms all had long back legs. This means, because long
back legs are dominant, that these organisms are all homozygous dominant (BB) or
heterozygous (Bb).

In the next fossil layer all the organisms have long back legs. This must mean they are the
offspring of the organisms in the previous layer. This also means they are homozygous
Learning from the Fossil Record
Grade 8 Science
dominant (BB) because some of the organisms would have showed the recessive trait, short
back legs.

In the next fossil layer all the organisms had short back legs. There is no way to explain this
through genetics, so there has to be another explanation because a pure organism with the
dominant trait can’t give birth to an organism that is homozygous recessive.

In the next fossil layer all the organisms had no back legs. There is no way to explain this
through genetics because an organism with short legs can’t produce offspring with no legs.
5. How is it possible for the dominant no flukes on tail trait to change to a recessive flukes on tail
trait and stay that way permanently?