Transcript Document

LECTURE
CONNECTIONS
5 | Extensions and Modifications of Basic
Principles
© 2009 W. H. Freeman and Company
5.1 Dominance Is Interaction between Genes at
the Same Locus
• Genes at the same locus – two versions of the same
gene; each version of the same gene is defined as allele.
5.1 Dominance Is Interaction between Genes at
the Same Locus
• Incomplete dominance: the heterozygote is
intermediate in phenotype between the two
homozygous.
• Codominance: the heterozygote simultaneously
expresses the phenotypes of both homozygotes.
5.2 Penetrance and Expressivity Describe How
Genes Are Expressed as Phenotype
• For some characters, the genotype does not always
produce the expected phenotype= incomplete
penetrance.
• Penetrance: percentage of individuals having a
particular genotype that express the expected phenotype
• Expressivity: the degree to which a character is
expressed
Concept Checks
1) Assume that long fingers are inherited as a
recessive trait with 80% penetrance. Two people
heterozygous for long fingers mate. What is the
probability that their first child will have long fingers?
2) Polydactyly is usually caused by a dominant allele.
If we examined 75 people having an allele for
polydactyly and found that only 50 of them were
polydactylous, the penetrance would be______
5.3 Lethal Alleles May Alter Phenotypic Ratios
• A lethal allele causes death at an early stage of
development, and so some genotypes may not appear
among the progeny.
Yellow mouse x Yellow mouse=
yellow and gray litter
(2:1)
Never a homozygous yellow
mouse.
Concept Check 2
A cross between two green corn plants yields 2/3
progeny that are green and 1/3 progeny that are white.
What is the genotype of the green progeny and the white
progeny?
5.4 Multiple Alleles at a Locus Create a Greater
Variety of Genotypes and
Phenotypes than Do Two Alleles
• Multiple alleles: For a given locus, more than two
alleles are present within a group of individuals.
• The inheritance of characteristics encoded by multiple
alleles is no different from the inheritance of
characteristics encoded by two alleles, except that a
greater variety of genotypes and phenotypes are
possible.
Genotype
Phenotype
MRMR
Restricted
MRM
Restricted
MRmd
Restricted
MM
Mallard
Mmd
Mallard
mdmd
Dusky
M R > M > md
5.4 Multiple Alleles at a Locus Create a Greater
Variety of Genotypes and
Phenotypes than Do Two Alleles
• ABO blood group
IA: encodes the A antigen
IB: encodes the B antigen
i: does not encode antigens
Dominants over i and
codominant with
each other.
Concept Check 3
What blood types are possible among the children of a cross
between a man of blood-type A and a woman of blood-type B?
•
•
•
•
could be A type with a genotype of IAIA and IAi
could be B type with a genotype of IBIB or Ibi
could be AB type with a genotype of IAIB
could also be O type with a genotype of ii
5.5 Gene Interaction Occurs When Genes at
Multiple Loci Determine a Single Phenotype
• Gene interaction: Effects of genes at one locus depend
on the presence of genes at other loci.
• Gene interaction that produces novel phenotypes
Colors in peppers results
from the relative
amount of red and
yellow carotenoids.
The Y locus encodes one
enzyme and the C
locus encodes a
different enzymes
5.5 Gene Interaction Occurs When Genes at
Multiple Loci Determine a Single Phenotype
• Gene interaction with epistasis
• Epistasis: One gene masks the effect of another
gene at a different locus.
Epistatic gene and hypostatic gene
• Recessive epistasis
•
The different coat colors are determined by interactions between
genes at two loci.
•
Dominant allele B encodes black; recessive allele b encodes
brown.
•
Allele E at a different locus allow dark pigment to be deposited,
whereas a recessive allele e prevents the deposition of dark
pigment (yellow hair).
•
The presence of genotype ee at the second locus masks the
expression of the black and brown alleles at the first locus.
5.5 Gene Interaction Occurs When Genes at
Multiple Loci Determine a Single Phenotype
• Dominant epistasis
White: W_
Green: wwyy
Yellow: wwY_
•
Allele W is epistatic to Y and y: it suppresses the expression
of these pigment-producing genes.
•
Allele W is dominant because a single copy of the allele is
sufficient to inhibit pigment production.
5.5 Gene Interaction Occurs When Genes at
Multiple Loci Determine a Single Phenotype
• Duplicate recessive epistasis
Albinism in freshwater snail Physa heterostroha can result
from the presence of either of two recessive alleles at
two different loci.
P1 aaBB X AAbb
F1
F2
AaBb
(both albinos)
(albino or pigmented?)
9/16 A_B_
Pimented?
3/16 aaB_
Albinos?
3/16 A_bb
1/16 aabb
Concept Check 4
A number of all-white cats are crossed and they produced
the following types of progeny: 12/16 all-white, 3/16 black,
and 1/16 gray. Give the genotypes of the progeny, and
which gene is epistatic?
A_B_ and AAB_: 9 + 3 = 12 give white color.
BBA_: 3 give black color.
aabb: 1 gives gray color.
“A” dominant allele is epistatic to “B” allele.