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Allele Expression
Allele expression not always as
simple as dominant alleles
overriding recessive ones.
Alleles of a single gene may
interact together and give
rise to phenotypes that are
dissimilar to both of the
parents.
Incomplete dominance in snapdragons
produces pink flowers from red and white parents
This may be because:
The genes are sex-linked.
Incomplete dominance
Codominance
Multiple alleles
Roan coat color in cattle is a result of
codominance between red and white alleles
Incomplete
Dominance
In cases of incomplete
dominance, neither allele
dominates and the
heterozygote is
intermediate in
phenotype between the
two homozygotes.
Examples of incomplete
dominance include flower
color in snapdragons
(right) and sweet peas,
where red and white
flowered plants cross to
produce pink flowered
plants.
Cw Cw
C rC w
CrCr
Flower Color in Snapdragons
Red flower
White flower
Parents
X
CrCr
CwCw
Gametes
Cr
Cr
Cw
Cw
Possible
fertilizations
F1 offspring
CrCw
Pink
CrCw
CrCw
CrCw
Pink
Pink
Pink
Example problems
(answer by showing Punnet squares):
1. If a white flowered plant is crossed with a red
flowered plant, what are the genotypic and
phenotypic ratios of the F1?
2. If two of the F1 offspring were crossed, what
genotypes and phenotypes ratios would
appear in the F2?
Codominance
In cases of codominance, both alleles are independently and
equally expressed in the heterozygote.
Roan (stippled red and white) coat color in cattle.
AB human blood groups.
Black and tan tabby cats
Red bull
White cow
CWCW
CRCR
Parents
X
Gametes
CR
CR
CW
CW
Possible
fertilizations
CRCW
CRCW
CRCW
CRCW
Roan
Roan
Roan
Roan
F1 offspring
Example problem
1. Cross two heterozygous (roan) shorthorn cattle
Roan bull
Roan cow
CRCW
X
Parents
Gametes
CR
CRCW
CW
CR
CW
Possible
fertilizations
CRCR
CRCW
Red
Roan
CRCW
CWCW
Offspring
Roan
White
Example Problem
2. A true breeding red parent is crossed with a
roan parent
Roan cow
Red bull
C RC W
CRCR
Parents
Gametes
X
CR
CR
CR
CW
Possible
fertilizations
CRCR
CRCW
CRCR
Red
Roan
Red
C RC W
Offspring
Roan
Multiple Alleles in Blood
Humans have 4 blood group
phenotypes: A, B, AB and O
The four common blood
groups of the human ABO
blood group system are
determined by three alleles:
IA, IB, i (however any one individual
can possess only two alleles)
IA & IB are
co-dominant and i is
recessive.
Multiple Alleles in Blood
Blood
group: AB
EXAMPLE 1:
Cross two parents, both
with AB blood type
Parent
genotypes
Gametes
Blood
group: AB
X
AB
IA
IB
AB
IA
IB
Possible
fertilizations
Children's
genotypes
Blood
groups
IAIA
IAIB
IAIB
IBIB
A
AB
AB
B
Multiple Alleles in Blood
Blood
group: B
EXAMPLE 2:
Two parents with blood
groups A and B respectively,
both heterozygous
Blood
group: A
Parent
genotypes
Gametes
X
IBi
IB
IAi
IA
i
i
Possible
fertilizations
Children's'
genotypes
Blood
groups
IAIB
AB
IBi
B
IAi
A
ii
O
XY Sex Determination
In the XY type, sex determination is based on the presence or absence of the Y
chromosome; without it, an individual will develop into a female.
XY sex determination occurs in:
Female
Male
Mammals (including humans)
Fruit fly Drosophila
Some dioecious (separate
male and female) plants
such as kiwifruit.
Females are homogametic with
two similar sex chromosomes
(XX). The male has two unlike
chromosomes (XY) and is
heterogametic.
Primary sex characteristics are
initiated by genes on the X.
‘Maleness’ is determined by the Y.
XX
Parents
Gametes
X
X
X
XY
X
Y
Possible
fertilizations
Offspring
Sex:
XX
Female
XY
XX
Male
Female
XY
Male
Sex Linkage
Genes located on the X chromosomes
are called X-linked
X
Y
Because the Y chromosome is small
and does not contain many genes, few
traits are Y-linked and Y-linked
diseases are rare.
Note the size differences between the X
and Y chromosomes. The Y lacks alleles
for many of the genes present on the X.
Examples:
D
X
X-linked traits
are
denoted
for
a
dominant
d
allele and X for a recessive allele
For most X-linked genes, the dominant form is
the healthy form.
Hemophilia Xh
Muscular dystrophy Xm
Color blindness Xĉ
Normal blood clotting XH
Non-muscular dystrophy XM
Non-color blindness XC
• A human female can be homozygous or heterozygous with
respect to sex-linked genes.
•
Female heterozygous for X-linked alleles are called carriers,
because they
don’t have the disease (they have one
good copy of the gene) but they
do “carry” the bad
allele.
• Since (normally) males only have one X chromosome,
males only have one copy of genes located on the X
chromosome.
Sex Linkage
Unaffected
father
Sex-linked traits show a
distinct pattern of inheritance.
Carrier
mother
Fathers pass sex-linked alleles
to all their daughters but not to
their sons.
X
Mothers can pass sex-linked
alleles to both sons and
daughters.
In females, sex-linked
recessive traits will be
expressed only in the
homozygous condition.
Y
XY
X
XX
X
XX
XY
In contrast, any male receiving
the recessive allele from his
mother will express the trait.
Unaffected
son
Unaffected
daughter
Carrier
daughter
Affected
son
Practice Problems
1.Suppose a color blind man fathers children with a
woman of the genotype XC XC.
What proportion of daughters would be color blind?
What proportion of sons would be color blind?
2.One of the daughters from the above problem
marries a color blind man.
What proportion of their sons will be color blind?