Caroline and Jimmy
3/21/11
Part 2 of Performance Assessment Task – Skull Model
1.
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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?
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.
From layer to layer the organism and the environment changed. These changes weren’t just
random; they were mutations that happened because of chemicals, smoke, excessive
sunlight, pollution, drugs, or radiation. In the deepest fossil layer the short skull and jaw was
living in an environment with mostly medium trees, some smaller shrubs, and many small
creeks. But when we looked into the next fossil layer it had changed from all short skull and
jawed to some short skull and jawed and some long skull and jawed organisms. This wasn’t
a mistake, it started to change—by a mutation—because the short skull and jaw was
becoming a disadvantage in the new environment of taller grasses, small shrubs, and
swamp areas. The mutation is because of excessive sunlight, which caused it to change for
the better. This mutation didn’t change in the next two fossil layers, because it didn’t
become a disadvantage. Even though the environment did change form taller grasses,
shallow ponds, and swampy areas, to large, deep lakes, and many aquatic plants.
Jimmy Panzini and Caroline Burns
3/16/11
Science- Test Part 2
2. How is it possible for the recessive long, skinny tail trait to change
to a dominant short, thick tail trait and stay that way permanently?
2nd Fossil Layer:
This means that all should
be long but some are short.
3rd Fossil Layer:
All are short fat tailed.
Could be Tt or TT.
To ensure all short, fat
tailed it must be TT.
4th Fossil Layer:
All short, fat tailed.
The short fat tail comes from a reason that the creature needed to
change. It might be because the short tail helps the creature
maneuver around in the area that they live in.
In the deepest fossil layer the organism had a long skinny tail; and the
environment was mostly medium trees, some smaller shrubs, and many small
creeks. Then in the next fossil layer some were long skinny tailed and some were
short fat tailed. This happened because of a mutation caused by excessive
sunlight. We know this is what caused it because nothing could enter or exit the
area this species was in. And there was no pollution, smoke, chemicals,
radiation, or drugs in the area that would cause a mutation to the organism.
Then in the next two fossil layers the short fat tailed organism stayed the same.
This means that there wasn’t another mutation to change what the organism
looked like in the area of its tail. Even though the environment had changed the
organism didn’t need to change in order to live longer and reproduce more.
Jimmy Panzini and Caroline Burns
3/16/11
Science- Test Part 2
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?
2nd Fossil Layer:
Same long and a same short.
Means there can be same long
and same short legged creature.
3rd Fossil Layer:
All front flippers.
4th Fossil Layer:
 The genes are now all flipper genes.
 They all had to have evolved because there are no genes that
could possibly give the creature flippers.
In the deepest fossil layer the organism had long front legs. Then in the next fossil
layer it had some long fron legs and some short front legs. This had happened
because some of the organisms had received the recessive genes from their
parents and others had received the dominant traits. This could have happened
because the organism’s genotypes were Rr. This enables the organism to have
long front legged offspring and short front legged offspring. But then in the next
fossil layer the organism had front flippers. This was not because of inheritance, it
happened because of a mutation. One of the reasons for the mutation could
have been from excessive sunlight because as you see as time passes there is
less shade for the organism. Another reason why is that the environment
changed from being small shrubs to shallow ponds, this means that by the
organism having front flippers now it enables it to move faster in the water. Then
in the next fossil layer the organism still has front flippers and the environment has
changed to larger, deeper lakes, and many aquatic plants. The flippers still help
the organism live in the water, and move around faster. If it had legs the
organism wouldn’t be able to live as long in the water as the organism with
flippers can. There are many different reasons for why these mutations
happened but overall they are meant to give the organism a better chance of
surviving in their environment.
Jimmy Panzini and Caroline Burns
3/16/11
Science- Test Part 2
4. How is it possible for the dominant long back legs trait to change
to a recessive short back legs trait and then disappear completely?
2nd Fossil Layer:
This means that some could
have long or short back legs.
3rd Fossil Layer:
All short back legs.
tt-Means all, Tt-Means some
This allows the past to be all long
back legs and the future to have
all short back legs. Even though
it is only 25%.
4th Fossil Layer:
 No back legs.
 The creature must have had an offspring and the
chromosomes for back legs didn’t go. This would mean that
they didn’t and could have back legs.
In the fourth fossil layer there was only one mutation that occurred to the
organism and that was instead of having short back legs there are now no back
legs. This is because the environment changed to larger, deep lakes and many
aquatic plants instead of having tall grasses, shallow ponds, and swampy land.
This mutation could have come about because it allowed the organism to
move faster and easier in the water now because instead of having short back
legs it has no back legs. A connection that I can make to this is that since the
environment changed from being grassy and shallow ponds to large, deep
lakes and many aquatic plants maybe since the environment is much more
watery now from before the short back legs were useless. I can also make a
prediction that now this organism is a water creature instead of a land creature.
I can infer this because it has no back legs to walk on and has flukes in its tail to
help it move faster. So as you can see overtime an organism can change very
much just by the way the environment it is living in changes!
Jimmy Panzini and Caroline Burns
Science- Test Part 2
3/16/11
4. 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?
2nd Fossil Layer:
All no flukes on tail.
Some could be with flukes.
3rd Fossil Layer:
This enables the creature to have
had no flukes in the past but can
have them in the future.
4th Fossil Layer:
All flukes no tail.
Could be tt or Tt.
The tt would make 100% sure to
flukes on the tail. The Tt would
Enable the past not to have flukes
on the tail.
The organism had changed from the deepest fossil layer to the last fossil layer
we looked at. In the deepest fossil layer the organism didn’t have flukes on the
tail. Then in the next fossil layer it also didn’t have flukes on the tail. Even though
the environment had changed from mostly medium trees, some smaller shrubs,
and many small creeks to taller grasses, small shrubs, and swampy areas the
organism hadn’t changed to adapt to its environment. The environment didn’t
cause the organism to change so it just stayed the same. But in the next fossil
layer the organism did have flukes on the tail. The environment also changed;
instead of small shrubs it had shallow ponds. The organism had a mutation
because of excessive sunlight, and the flukes on the tail allow the organism to
live longer in the environment, and they probably help the organism maneuver
easier in its environment. Then in the last fossil layer the organism still had flukes
on the tail, even though the environment had changed to larger, deep lakes,
and many aquatic plants. The flukes on the tail probably help the organism live
in the environment, and they help the organism adapt.