Transcript 8.2 Alleles and Genes Interact to Produce Phenotypes

8.2 Alleles and Genes Interact to
Produce Phenotypes
AP Biology
Radjewski
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 Antibodies
RBC
in plasma
A
A
B
Can
receive
from
O,A
Can
donate
to
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
Genotype %
Io
Io
A
o
II
IBIo
IAIo
I BI o
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