What is autosomal recessive inheritance?

• In its simplest form, it involves 1 pair of genes with the wild type gene "A" being dominant to (or canceling out) the effect of the mutant/recessive gene "a" in any combination. If the wild type/natural colour of a bird is green and the mutant form is blue, the following will be true:

                + AA = Green Bird

                + Aa = Green Bird (but it carries a blue gene)

                + aa = Blue Bird

• In the following example, one gene, we will call it "A", is responsible for colour. The "A" gene comes in 2 forms: "A" and "a". When the "A" gene is a capital "A", it is responsible for the green colour and classified as dominant. When the "A" gene is a small "a", it is responsible for the blue colour and is classified as recessive. There are always 2 copies of the "A" gene in each individual. If an individual is AA, then it is green/normal/wild type or is a homozygote (the pair of genes are the same) for the green gene. Green is the dominant form. If an individual is aa, then it is blue or in its recessive form. "aa" is a homozygote for the blue gene. If an individual is Aa, it is a heterzygote (contains a copy of each gene form). Heterozygous individuals appear green, but carry both blue and green genes.

"A" is the green gene

"a" is the blue gene

Each individual consists of two genes!

"AA" = green individual

"Aa" = heterozygous individual

"aa" = blue individual

There are 6 possible crosses:

 (1) Green (AA) X Green (AA) = all green (AA) offspring

Parent 2 (down), Parent 1 (top)

	A	A
A	AA	AA
A	AA	AA

• The first one in the cross is parent # 1 and the second one in the cross is parent # 2. All of the crosses will work the same if parent #1 is male or female and parent #2 is male or female. The colour genes are autosomal, not sex-linked (on the X or Y chromosomes).

• The 2 single A's across the top are from parent 1 and the 2 single A's down the left are from parent 2. Each individual gets 1 gene from each parent.

(2) Blue (aa) X Blue (aa) = all blue (aa) offspring

• The 4 squares with 2 letters are the possible genotypes of the offspring. Since both parents are blue, the offspring can only get the blue (a) gene from each parent.

	a	a
a	aa	aa
a	aa	aa

(3) Green (AA) X Blue (aa) = 100% heterozygous (Aa) offspring

• Each offspring gets one gene from each parent (1 green gene and 1 blue gene) therefore all of the chicks are heterozygotes (Aa).

	A	A
a	Aa	Aa
a	Aa	Aa

(4) Heterozygous (Aa) X Heterozygous (Aa) = 25% green (AA), 25% blue (aa), and 50% heterozygous (Aa)

	A	a
A	AA	Aa
a	Aa	aa

• Note: "Aa" and "aA" are genetically equivalent. "Aa" got the "A" gene from parent 1 and the "a" gene from parent 2. "aA" got the "a" gene from parent 1 and the "A" gene from parent 2.

Here's the Math!

1) Each offspring has a 50% chance of getting the "A" gene from either parent.  0.5 X 0.5 = 0.25 or 25% you multiply the probability of getting the gene from one parent by the probability of getting that same gene from the other parent.

2) the same math goes for getting the blue gene from both parents 0.5 X 0.5 = 0.25 or 25%

3) Each offspring has a 50% chance of getting the "A" gene from either parent and a 50% chance of getting the "a" gene from either parent. 0.5 X 0.5 = 0.25 or 25% But since you get both "Aa" and "aA", you multiply 0.25 by 2! 

0.25 X 2 = 0.5 or 50%

• Whenever you have a cross between 2 heterozygotes - have a big and a little letter), you will get 50% heterozygotes (both Aa and aA)!

(5) Green (AA) X Heterozygous (Aa) = 50% Green (AA) and 50 % Heterozygous (Aa)

	A	A
A	AA	AA
a	Aa	Aa

(6) Blue (aa) X Split (Aa)= 50 % Blue (aa) and 50 % Split (Aa)

	a	a
A	Aa	Aa
a	aa	aa

• crosses 1-6 are only expected probability, not what you get all of the time. Depending on deletions, mutations, additions, or masking of the genetic code, you could get very different results than what is expected.