Allele ExpressionAllele 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.This may be because:
The genes are sex-linked.
Incomplete dominance
Codominance
Multiple alleles
Incomplete dominance in snapdragons
produces pink flowers from red and white parents
Roan coat color in cattle is a result of
codominance between red and white alleles
Incomplete DominanceIn 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. CrCrCrCwCwCw
Possiblefertilizations
Flower Color in Snapdragons
Gametes
Cr Cr Cw Cw
Pink Pink Pink Pink
F1 offspring
CrCwCrCwCrCw
CrCw
Red flower White flower
Parents
CrCr XCwCw
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?
Possiblefertilizations
CodominanceIn 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
RoanRoanRoanRoan
CRCW CRCW CRCW CRCW
F1 offspring
CR CR CWCWGametes
White cowRed bullCRCR CWCW
Parents
X
Possiblefertilizations
WhiteRoanRoanRed
CRCR CRCW CWCWCRCW
Offspring
Example problem1. Cross two heterozygous (roan) shorthorn cattle
CWCR CRCWGametes
Roan cowRoan bull
CRCWCRCW
Parents X
X
Roan cowRed bull
Parents
Possiblefertilizations
Example Problem2. A true breeding red parent is crossed with a
roan parent
Offspring
RoanRoan RedRed
CRCR CRCW CRCWCRCR
Gametes CWCR CRCR
CRCR CRCW
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.
X
Blood
group: AB
Parent
genotypes
Blood
group: AB
Gametes IB IBIA IA
AB AB
Multiple Alleles in Blood
EXAMPLE 1:Cross two parents, both with AB blood type
BBlood
groupsABA AB
Possible
fertilizations
Children's
genotypes IAIB IBIBIAIBIAIA
Blood group: B
Blood group: A
XParent
genotypes
Multiple Alleles in Blood
EXAMPLE 2:Two parents with blood groups A and B respectively, both heterozygous
Possible
fertilizations
Children's'
genotypes IAi iiIBiIAIB
Gametes i iIB IA
IBi IAi
A OBBlood
groupsAB
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:
Mammals (including humans)
Fruit fly Drosophila
Some dioecious (separatemale and female) plantssuch as kiwifruit.
Females are homogametic withtwo similar sex chromosomes(XX). The male has two unlikechromosomes (XY) and isheterogametic.
Primary sex characteristics areinitiated by genes on the X.‘Maleness’ is determined by the Y.
XY Sex Determination
MaleFemale
Parents
Gametes
Possible
fertilizations
Offspring
Sex: Female Male Female Male
XX
XX XXXY XY
X XY
X YX X
Sex Linkage
Genes located on the X chromosomes are called X-linked
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.
X
Y
Examples:X-linked traits are denoted XD for a dominant allele and Xd for a recessive allele
For most X-linked genes, the dominant form is the healthy form.
Hemophilia Xh Normal blood clotting XH
Muscular dystrophy Xm Non-muscular dystrophy XM
Color blindness Xĉ 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.
Affected son
X Y
Sex Linkage
Sex-linked traits show a distinct pattern of inheritance.
Fathers pass sex-linked alleles to all their daughters but not to their sons.
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.
In contrast, any male receiving the recessive allele from his mother will express the trait.
Carrier daughter
XX
Unaffected daughter
XX
Unaffected son
YX
Carrier mother
X XX Y
Unaffected father
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?