Allele Expression

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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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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.

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Flower Color in Snapdragons
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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?

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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

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Example problem

• 1. Cross two heterozygous (roan) shorthorn cattle

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Example Problem

• 2. A true breeding red parent is crossed with a
  roan parent

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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.

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Multiple Alleles in Blood

• EXAMPLE 1Cross two parents, both with AB blood
  type

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Multiple Alleles in Blood

• EXAMPLE 2Two parents with blood groups A and B
  respectively, both heterozygous

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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
• 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.

Male
Female
X
Parents
X
X
X
Y
Gametes
Possiblefertilizations
Offspring
Sex
Male
Male
Female
Female
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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.

X
Y
Note the size differences between the X and Y
chromosomes. The Y lacks alleles for many of the
genes present on the X.
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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 Xc Non-color blindness XC

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• 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 dont 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.

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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.

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Practice Problems

• 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?
• One of the daughters from the above problem
  marries a color blind man.
• What proportion of their sons will be color
  blind?