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Other Patterns of Inheritance
Exceptions to Mendel’s Rules
There are exceptions to every
rule, and exceptions to the
exceptions.
Question:
• What happens if one allele is not
completely dominant over
another?
• What if a gene has several
alleles?

Incomplete Dominance
 Incomplete Dominance: when
neither allele is completely dominant
over the other.
 Results in the heterozygous
phenotype being a blending of the
two homozygous phenotypes.
 Incomplete = Blended appearance
Incomplete Dominance

A capital letter represents the trait of each of
the extremes: R for red and W for white.
 RR
RW
WW
Incomplete Dominance
 In snapdragons, red flower color (RR) is
incompletely dominant to white (WW) flower
color. A heterozygous snapdragon is pink
(RW). Cross a pink snapdragon with a pink
snapdragon.
RW x RW
Codominance
 both alleles are
dominant!
 Heterozygous
individual (RW)
shows both
alleles
expressed.
WW
Codominance
 A capital letter represents one of
the codominant alleles (B for
black).
 A different capital letter
(W = white) represents the other
codominant allele
 The two alleles
Punnett Squares
 Make sure to label whether your Punnett
Square is Incomplete Dominance or
Codominance!
Codominance
 In horses, gray horses (GG) are codominant
to white horses (WW). The heterozygous
horses(GW) is an appaloosa horse (a white
horse with gray spots on the rump and loins).
Cross a white horse with an appaloosa
horse.
GW x WW
Codominant and Multiple Alleles
 The alleles for many human genes display
codominant inheritance.

One example is the ABO blood group,
determined by a gene with three alleles: IA, IB,
and i.


IA and IB are both dominant blood types and
therefore codominant.
i is recessive
Codominant and Multiple Alleles
 This table shows the relationship between genotype
and phenotype for the ABO blood groups.

It also shows which blood types can safely be
transfused into people with other blood types.
Codominant and Multiple Alleles
 The i allele is recessive.
 Individuals with alleles IAIA or IAi produce only
the A antigen, making them blood type A.
 Those with IBIB or IBi alleles are type B.
 Those homozygous for the i allele (ii) produce
no antigen and are said to have blood type O.
Blood Type Video
 http://www.youtube.com/watch?v=CduLOqZb
_XQ
Autosomal vs. Sex Chromosomes
 Autosomal Chromosomes (Autosomes) –
chromosomes that do not determine sex
 Sex Chromosomes – chromosomes that
determine sex

In humans –

X and Y chromosomes
Sex-Linked Inheritance
 Know the difference between autosomal
chromosomes and sex chromosomes.
 Understand sex-linked inheritance.
 Be able to create a family pedigree.
 Solve sex-linked problems using a
Punnett square
Gender Determination
in Humans
 Humans have 23 pairs of chromosomes.
 22 of them are called autosomal.
 The 23rd pair are the sex chromosomes.
 XX = Female
 XY = Male
Pedigree
XY
Marriage
XX
Mother
Father
Children
XX
XY
XY
Daughter
Son
Son
Oldest
Youngest
Red/Green Colorblindness
 The gene which allows us to distinguish
between red and green is on the X
chromosome.
XC XC
XC Xc
Xc Xc
= Female with normal vision
= Female carrier
= Female who is colorblind
Red/Green Colorblindness
 The gene which allows us to distinguish
between red and green is on the X
chromosome.
XC Y
Xc Y
= Male with normal vision
= Male who is colorblind
Pedigree
XY
c
XX
CC
Father
Mother
??
??
??
Daughter
Son
Son
Pedigree
XY
c
XX
CC
Father
Mother
XX
C c
XY
C
XY
C
Daughter
Son
Son
Using a Punnett Square
XX
Cc
XY
c
XC
Xc
Xc
XC Xc
Xc Xc
Y
XC Y
Xc Y
Genotype & Phenotype
XX
Cc
XY
c
Female Carrier
Female Colorblind
XC
Xc
Xc
XC Xc
Xc Xc
Y
XC Y
Xc Y
Male Normal
Male Colorblind
Hemophilia
XX
H H
= Female normal
XX
H h
= Female carrier
XX
h h
= Female with hemophilia
XY
H
= Male normal
XY
h
= Male with hemophilia
Review
 Autosomal??
 Pedigree - Male? Female? Marriage?
Children? Siblings?
 Why do sex-linked traits affect males
more frequently than females?
Warm Up
4/26/12
 What is the phenotypic ratio for a cross
between a mom with type AB blood and a
dad with type O blood?
 (Genotypes: _____ x ______)