Extending Mendel's Laws
A question that breeders often have is
"what is the genotype of an organism that displays the dominant
phenotype?"
For example, in a breed of dog in which the gene for black
coat is dominant (B) over the gene for red coat (b) colour, a
dog with black coat colour could have either of two possible
genotypes:
BB or Bb
To determine whether the black coat colour is homozygous
(BB) or heterozygous (Bb), it is necessary to perform a test
cross.
TEST CROSS
A specific cross to determine the unknown traits in an individual. In other words to determine
whether the organism is
Homozygous or Heterozygous
Lets revisit the example
A dog breeder thinks that his dark colour dog is carrying a gene for no red colour coat. Prove it!
Possible Genotypes
BB or Bb
Let B represent black coat colour
Let b represent red coat colour
Cross his dog with a homozygous recessive red colour coat dog
B_ X bb
B_ X bb
The phenotype of the offspring resulting from the test cross will reveal the genotype of the mystery
dog. The homozygous recessive parent can only contribute the recessive allele (b) to the offspring,
therefore the phenotypes will indicate the alleles from the other parent (the mystery dog)
B
B
B
b
Bb
Bb
b
Bb
Bb
Bb
b
Bb
b
Bb
b
bb
bb
If the mystery dog is Homozygous (BB) then it can only contribute (B) and all the
offspring would have a black coat colour
If the mystery dog is Heterozygous (bb) then it can either contribute (B) or (b) and we
would expect both colour coats
Dominance
Complete Dominance
a kind of dominance wherein the dominant allele completely masks
the effect of the recessive allele in heterozygous conditions
this type of dominance resemble Mendel's pea plants
eg. What are the chances of two heterozygous tall plants producing a short plant.
T
t
T
TT
Tt
t
Tt
tt
25%
25%
Dominance and recessiveness explain some simple forms of inheritance. For most traits, however
inheritance patterns are more complex.
Incomplete Dominance
one trait is not dominant over another, each allele has some degree of
influence
eg. Snapdragons, pink flowers occur when red and white flowers are crossed.
Let R represent Red Colour
Let W represent White
In what ratio will red, white and pink flowers occur if 2 pink flowers are crossed?
RW X RW
Similar to incomplete dominance, codominance is an inheritance pattern in which neither allele is
dominant to the other. Both alleles are completely expressed at the same time.
Codominance
a kind of dominance in which a heterozygote shows the phenotypic
effect of both alleles fully and equally.
eg. in roan cattle the expression of one allele is not masked by the expression of the other.
Homework
pg. 134 #2,3
pg. 138 #16
pg. 141 #1
So far we have discussed traits that involve only two alleles per gene. But for most genes,
more than 2 alleles exist in a population. The existence of multiple alleles means that there
maybe many possible genotypes for a particular gene, and many possible phenotypes.
Multiple Alleles
three or more forms of a gene
eg. Blood type in humans involves three alleles
IA - type A allele
IB - type B allele
i - type O allele
A person can have one of four blood types: A, B, AB, or O.
The letters refer to the two types of carbohydrates, A and B, that are found on the surface of
the red blood cells.
There are six possible ways to pair the alleles, six possible genotypes.
Blood transfusion:
Must be sure the donor's antigens (causing an immune response)
will not cause clumping with recipient's antibodies (chemical for
defense)
If the blood types are
incomnpatible, the recipient
will have a potentially fatal
immune response
Blood
Type
Marker on Antibodie
s
rbc's
Present
Can give
blood to
Can
receive
blood
from
A
A
A
B
A, AB
A, O
B
B
B
A
B, AB
B, O
AB
A, B, AB,
O
AB
AB
AB None
Eye Colour
Let B - brown eyes
Let b - blue eyes
Let G - green/hazel eyes
Let g - light coloured eyes
Lets cross Heterozygous Parents
BbGg X BbGg
Homework
pg. 145 #1, 5, 6, 7, 11