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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. 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

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Page 1: Allele Expression Allele expression not always as simple as dominant alleles overriding recessive ones. Alleles of a single gene may interact together

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

Page 2: Allele Expression Allele expression not always as simple as dominant alleles overriding recessive ones. Alleles of a single gene may interact together

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

Page 3: Allele Expression Allele expression not always as simple as dominant alleles overriding recessive ones. Alleles of a single gene may interact together

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

Page 4: Allele Expression Allele expression not always as simple as dominant alleles overriding recessive ones. Alleles of a single gene may interact together

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?

Page 5: Allele Expression Allele expression not always as simple as dominant alleles overriding recessive ones. Alleles of a single gene may interact together

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

Page 6: Allele Expression Allele expression not always as simple as dominant alleles overriding recessive ones. Alleles of a single gene may interact together

Possiblefertilizations

WhiteRoanRoanRed

CRCR CRCW CWCWCRCW

Offspring

Example problem1. Cross two heterozygous (roan) shorthorn cattle

CWCR CRCWGametes

Roan cowRoan bull

CRCWCRCW

Parents X

Page 7: Allele Expression Allele expression not always as simple as dominant alleles overriding recessive ones. Alleles of a single gene may interact together

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

Page 8: Allele Expression Allele expression not always as simple as dominant alleles overriding recessive ones. Alleles of a single gene may interact together

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.

Page 9: Allele Expression Allele expression not always as simple as dominant alleles overriding recessive ones. Alleles of a single gene may interact together

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

Page 10: Allele Expression Allele expression not always as simple as dominant alleles overriding recessive ones. Alleles of a single gene may interact together

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

Page 11: Allele Expression Allele expression not always as simple as dominant alleles overriding recessive ones. Alleles of a single gene may interact together

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

Page 12: Allele Expression Allele expression not always as simple as dominant alleles overriding recessive ones. Alleles of a single gene may interact together

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

Page 13: Allele Expression Allele expression not always as simple as dominant alleles overriding recessive ones. Alleles of a single gene may interact together

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

Page 14: Allele Expression Allele expression not always as simple as dominant alleles overriding recessive ones. Alleles of a single gene may interact together

• 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.

Page 15: Allele Expression Allele expression not always as simple as dominant alleles overriding recessive ones. Alleles of a single gene may interact together

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

Page 16: Allele Expression Allele expression not always as simple as dominant alleles overriding recessive ones. Alleles of a single gene may interact together

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?


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