Part 1: Review of Inheritance

Presenting a brief review of inheritance. In humans, brown eyes are dominant over blue (except in rare circumstances). We’ll use them as our example. Traits are usually represented by letters. Uppercase is dominant, and lowercase is recessive. Brown eyes =BB, and blue eyes = bb. You have two because you inherit one from each parent. For the biology review, we’ll keep it simple, although eye colour is more complex in reality. Two same letters, BB or bb, indicate the person inherited two of the same gene for eye colour from each parent. This is referred to as homozygous (meaning the same).

	B	B
b	Bb	Bb
b	Bb	Bb

	B	B
b	Bb	Bb
b	Bb	Bb

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Say one parent gives a B and a recessive or lowercase b. The parent has brown eyes but has a hidden recessive b (Bb). This is because B is dominant and overrides the recessive gene b, and the eyes appear brown. If a parent is Bb and they have children with a blue-eyed parent, 50% of the children may have brown eyes, and 50% may have blue. The parents with one dominant and one recessive gene are called heterozygous or “split” for the trait Bb.

	B	b
b	Bb	bb
b	Bb	bb

	B	b
b	Bb	bb
b	Bb	bb

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INCOMPLETE DOMINANCE

“Incomplete Dominance” is where the combination of traits from each parent results in an intermediate colour. In the case of carnations, breeding white and red carnations result in pink flowers; the traits are blended.

	Cr	Cr
Cw	CrCw	CrCw
Cw	CrCw	CrCw

If we take the flowers from above, all pink, and breed them together (second generation), we find the genes align the same way as the parents. C^rC^r= red, C^wC^w= white, and C^wC^r =pink.

	Cr	Cw
Cr	CrCr	CrCw
Cw	CrCw	CwCw

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CODOMINANCE

Some carnations exhibit “Codominance where the trait from each parent is observed without blending, so instead of pink, the flower appears red and white.