Non-Standard Inheritance

Because who wants to be standard all the time?

Non-Standard Inheritance• For many genes, traits do not

appear to follow the simple rules for dominant and recessive alleles.

• This often causes individuals to show more than two different versions of a trait.

• Can you think of examples?

Non-Standard Inheritance• Three of these non-standard

ways of inheriting genes are:1. Polygenic inheritance2. Incomplete dominance3. Codominance

• (Note: other non-standard inheritance methods exist… but time is short )

Polygenic Inheritance• When a trait is controlled by 2 or

more genes (or 4 or more alleles, in other words), it is said to be polygenic.

• Each dominant allele causes cells to produce a certain product. The more dominant alleles possessed, the greater the effect– Creates a range of phenotypes.

Polygenic InheritanceEg: Skin pigmentation in humans• Controlled by many pairs of genes.• Dominant alleles cause melanin to

be produced.• Hypothetically – if it was controlled

by two genes (A and B)… AABB Very darkAABb or AaBB Dark AaBb, AAbb, aaBB Medium brownAabb or aaBb Lightaabb Very light

Incomplete Dominance• For some traits, heterozygous

individuals show a phenotype that is a blend between the dominant and recessive phenotypes

• (Or, in other words, there’s a “middle” version of the trait).

• In these cases, we say that the dominant allele is showing incomplete dominance.

Incomplete Dominance• Eg: Hair “shape”

– HH = straight hair– Hh = wavy hair– hh = curly hair

• Eg: Flower colour in snapdragons– RR = red– Rr = pink– rr = white

Codominance• When both alleles in a pair are

equally expressed (in other words, one allele is NOT dominant over the other), they are said to be codominant.

• Eg: Flower colour in Rhododendrons can be both pink and whiteat the same time.

CodominanceHuman blood groups:• Individuals with the allele for

type A blood and the allele for type B blood will show BOTH antigens (and will be AB).– A and B are codominant.

CodominanceHuman Blood Groups cont’d:• Also is an example of multiple

allele inheritance.– There are more than two

phenotypes because there are three different alleles for blood type...

– But individuals can still only have two of these alleles (one from each parent).

CodominanceHuman blood groups cont’d:• The three possible alleles for

blood type are:– IA = A antigens on red blood cells– IB = B antigens on red blood cells– i = no A or B antigens on red blood

cells• IA and IB are codominant. Both

are also dominant over i.

CodominancePhenotype Genotype

A IAIA or IaiB IBIB or IBiAB IAIB

O ii

• (Note that people with Type O blood have the recessive phenotype).

• Punnett squares can be used to solve problems with ABO blood groups, too!

Non-Standard InheritancePractice problems• A polygenic trait is controlled by three pairs of alleles.

What are the two extreme genotypes for this trait?• A child with type O blood is born to a mother with type A

blood. What is the genotype of the child? The mother? ‘what are the possible genotypes of the father?

• From the following blood types determine which baby belongs to which parents:

• Baby 1 type O Mrs. Doe type A Mrs. Jones type A• Baby 2 type B Mr. Doe type A Mr. Jones type AB

• **What is the genotype of the lightest child that could result from a mating between two medium-brown individuals?** (Tricky; not a typical question)