Genetics

alleles

Definition: An allele is a variant of a gene.

Dominant vs recessive

There are two phenotypes dominant and recessive. 

If a single dominant allele is in a couple with a recessive, the dominant gene will be the one determining which one takes place.

Calculating probability of alleles in offspring

Inorder to calculate probability of offspring having certain alleles you need to create a pummet square. This is a table where the dominant and ressive genes are written as headings.

  1. Put parent 1's alles in the header row and put parent 2's alleles in the left-most column.
  2. Then for each cell, to get the outcome, "multiply" the alleles in the corresponding header row by the alleles in the corresponding column. Write the outcome in the cell. There are 3 possible outcomes: BB, Bb , bb
  3. To calculate the probability of a given combination of phenotypes, count how many times that combination occured in the pummet square outcomes. 

Note: The capital letter is the dominant and the lower case is the recessive. 

Pummet square possibilities

Case 1: Both parents have dominant and recessive

 

B

b

B

BB

Bb

b

Bb

bb

Case 2: One parent has only dominant and other has dominant and recessive

 

B

B

B

BB

BB

b

Bb

bb

Case 3: One parent has only recessive and other has dominant and recessive

 

b

b

B

Bb

Bb

b

bb

bb

Case 4: Both parents have only dominant

 

B

B

B

BB

BB

B

BB

BB

 

Case 5: Both parents have only recessive

 

b

b

b

bb

bb

b

bb

bb

 

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