|
 As
we have already learned, the underlying
principle of genetics is the simple understanding that any trait, good or bad,
is produced by one or more pairs of alleles. One allele is provided by
each parent when the egg is fertilized.
The famous 'double helix'
(pictured at left) represents the paired DNA strands, with thousands of
connected pairs of alleles. While greatly simplified, try to imagine that one
strand is provided by each parent and each bar represents a pair of joined
alleles.
But what happens when we cross specimens possessing different
genetic traits?
|
|
|
|
|
Ab |
Ab |
 |
aB |
AaBb |
AaBb |
| aB |
AaBb |
AaBb |
Here we will cross one of our male Amelanistic (A) Cornsnakes to a female Anerythristic
Cornsnake (B).
Since each genetic trait is controlled by a different allele
pair, we will now be using two letters (A & B) to represent our two pairs of
alleles, with lower case for the recessive state and upper case for the dominant
state:
The result is shown in the Punnett square at left.
100% of our offspring are 'double heterozygous' for
amelanism and anerythrism (AaBb) and appear normal. This is because each snake is in possession of
the dominant allele to offset the other snakes recessive one for both traits.
|
|
|
|
|
AB |
Ab |
aB |
ab |
|
AB |
AABB |
AABb |
AaBB |
AaBb |
| Ab |
AABb |
AAbb |
AaBb |
Aabb |
| aB |
AaBB |
AaBb |
aaBB |
aaBb |
| ab |
AaBb |
Aabb |
aaBb |
aabb |
In order to
produce offspring which visually express both traits, we must now breed these
offspring back together.
The results are shown in the Punnett Square at
right:
-
9/16 Normal (3 are heterozygous for amelanism, 3 for anerythrism)
-
3/16 Amelanistic (2 are heterozygous for anerythrism)
-
3/16 Anerythristic (2 are heterozygous for amelanism)
-
1/16 Amelanistic & Anerythristic
As you can see, only one in sixteen offspring exhibit both genetic traits. It
is lacking in black and red pigmentation and is commonly referred to as
'Snow'. Animals such as this one (exhibiting two recessive traits) are
called double recessive.
With such limited numbers of these double
recessives being produced, it is easy to see why these animals command higher
prices.
|
 |
Back To Top