1. Extending Mendel's Laws
A question that breeders often have is
"what is the genotype of an organism that 
displays the dominant phenotype?"

2. 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.

3. 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

4. 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

5. 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 b Bb b Bb bb Bb bb 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

6. 25% T t TT Tt tt 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
25%
25%
Dominance and recessiveness explain some simple forms of inheritance. 
For most traits, however, inheritance patterns are more complex.

7. 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

8. 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

9. So far we have discussed traits that involve only two alleles per gene
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

10. 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.

11. There are six possible ways to pair the alleles, six possible genotypes.

12. Blood Type Marker on rbc's Antibodies Present Can give blood to
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 
rbc's
Antibodies 
Present
Can give 
blood to
Can receive 
blood from A
A A B
A, AB
A, O
B B
B, AB
B, O AB
AB AB
None
A, B, AB, O O

13. 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

14. Homework
pg. 145 #1, 5, 6, 7, 11