8.2 Alleles and Genes Interact to Produce Phenotypes

AP BiologyRadjewski

Mutations

• Rare inherited changes in the genetic material• Occur in genes• One allele can be mutated to a different allele

Incomplete Dominance

• Many genes have alleles that are neither dominant or recessive to one another.

• There is an intermediate phenotype (Blend)• Example – snapdragons

– RR = red– WW – white– RW = pink!

• Example: Cross a red flowered Japanese 4:00 plant with a white flowered 4:00 plant

Incomplete Dominance Example

Genotype:

100% RW

Phenotype:

100% pink

Blood Types - Codominance

• Codominance – Two alleles of a gene share dominance and are both expressed

• In humans there is a gene I that encodes an enzyme involved in the attachment of sugars to a glycoprotein on the surface of red blood cells

• There are 3 alleles of the gene: IA, IB, and IO.• The IA and IB alleles both encode active enzymes,

but the enzymes attach different sugars to the glycoprotein.

Blood Types Continued• The IO allele does not encode an active enzyme,

so no sugar attaches• When two different alleles are present, both

alleles are expressed. • The A and B glycoproteins are antigenic, which

means the recipient will mount an immune response and produce antibodies against the nonself A or B cells.

• The O glycoprotein does not provoke any immune responses. This makes people with IOIO good blood donors

Type Antigen of RBC

Antibodies in plasma

Can receive from

Can donate to

A A B O,A A, AB

B B A O.B B, AB

AB A,B None O, A, B, AB

AB

O None A,B O O, A, B, AB

Rh Factors • Rh factors are another antigen on the surface of the RBC’s• If you have this antigen/protein, you are said to be Rh+ and if

you don’t have it you are Rh-• Dangerous in pregnant women who are Rh- carrying a Rh+

child• During delivery, the blood mixes, so the mother will produce

antibodies and the first child is unharmed• But if the second child is also Rh+, then those antibodies will

attack the child’s blood cells causing severe complications– Can be treated in early pregnancy– Otherwise child will not be able to produce RBC’s and that makes

oxygen levels low and death could result.

Cross a person with bloodtype AB with a person with bloodtype O

Codominance Example IAIB x IoIo IA IB

Io

Io

IAIo IBIo

IAIo IBIo

Genotype %

50% IAIo

50% IBIo

Phenotype %

50% bloodtype A

50% bloodtype B

Epistasis

• Phenotypic expression of one trait is affected by another gene

• Example in labrador retrievers– Allele B (black) is dominant over b (brown)– Allele E (pigment deposition in hair) is dominant

to e (no deposition, so hair is yellow)• An EE or Ee dog with BB or Bb is black; one

with ee is yellow regardless of the dominant B