Gene Interactions Marie Cern

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Gene InteractionsMarie Cerná
Lecture No 406-H
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Mendelian genetics 1 character 1 gene Genes
are segregating independently on each
other Gene interactions1 character two or
more genes Interaction of two genes genotype
ratio as in dihybridism less phenotype classes
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Gene Interactions

• Reciprocal interactions
• Epistasis - dominant and recessive
• Inhibition
• Complementarity
• Multiplicity

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Reciprocal interactions

• Interactions without change of phenotype ratio
• F2 9 3 3 1, B1 1 1 1 1
• The identical character can occur in more various
  independent forms, which of them is determined by
  one gene.
• gene 1 ? A1 Phenotype A2 ? gene 2

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Reciprocal interactions

• Product color of paprika
• Gene 1 allele R anthocyan ? red coloring
• Gene 2 allele Cl chlorophyll degradation ?
  yellow pigment
• Phenotype 1 R-Cl- red (anthocyan)
• Phenotype 2 R-clcl brown (red green)
• Phenotype 3 rrCl- yellow
• Phenotype 4 rrclcl green (chlorophyll)

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Epistasis

• One of alleles of the epistatic gene suppresses
  phenotype manifestation of the hypostatic gene.
• It is then an unilateral relation
• - among alleles of two various genes (M gt N)
• - among alleles of more various genes (M gt N gt R
  gt S)

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Dominant Epistasis

• Dominant allele of one gene has epistatic effect.
• Dominant alleles of both genes allow the same
  precursor processing in the same direction, but
  into different final products.
• Epistatic effect will have dominant allele of
  that of both genes, which can lead by
  biosynthetic processes to more expressive form of
  a trait, and by this way will cover an effect of
  dominant allele of the hypostatic gene.

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Dominant Epistasis

• Flower color of dahlia depends on hydroxylation
  degree of colorless precursor of flavon pigment
• Gene 1 allele Y higher degree ? dark yellow
• Gene 2 allele I lower degree ? light yellow
• (ivory white)
• Phenotype 1 Y-I-, Y-ii dark yellow
• Phenotype 2 yyI- light yellow
• Phenotype 3 yyii white

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Examples of dominant epistasis in human
Determination of eye coloring - depends on type
and density of pigment in eye iris brown coloring
(melanin) gene EYCL3 BEY2 on chr.15 ?
light-brown, nut coloring gene EYCL2 BEY1 on
chr.15 genes dominant epistatic towards
lipochrome gene green coloring
(lipochrome) gene EYCL1 GEY on chr.19 ?
2nd gene gene dominant hypostatic towards
melanin gene
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Determination of eye coloring BEY gt GEY B-G-,
B-gg _brown ? intensity depends on quantity
of pigment bbG- _green bbgg _blue
(albinotic) ? inability of pigment
formation Which parents can have which
children?
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Examples of dominant epistasis in human
Determination of hair coloring - depends on type
and density of pigment in hair fiber eumelanin
dark dye - black/brown hair gene HCL3 on
chr.15 - association with eye brown
coloring gene BRHC on chr.19 - association with
eye green coloring gene dominant epistatic
towards other two genes pheomelanin
red-and-yellow dye - rusty-red hair gene RHC on
chr.4 gene dominant epistatic towards blond
gene ? gene x ? low density - blond hair
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Determination of hair coloring HCL3 (BRHC) gt RHC
gt x H-rr _black (? pigment) / brown (?
pigment) H-R- _dark-brown hhR-
_rusty-red hhrrX- _blond hhrrxx _white
(albinotic) ? inability of pigment
formation _grey ? degraded products of
pigment Which parents can have which children?
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Recessive Epistasis

• Recessive allele of one gene
• in homozygous state has epistatic effect.
• Dominant alleles of both interactive genes
  participate in multistage synthesis of the same
  final product.
• Still dominant allele of the epistatic gene
  functions in one of initial phases of
  biosynthesis, while dominant allele of the
  hypostatic gene functions not until in one of its
  later phases.

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Recessive Epistasis

• Flower color of sage depends on hydroxylation
  degree of colorless precursor of flavon pigment
  
• Gene 1 allele P lower degree ? rose coloring
• Gene 2 allele A higher degree ? violet
  coloring
• Phenotype 1 P-A- violet
• Phenotype 2 P-aa rose
• Phenotype 3 ppA-, ppaa white

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Examples of recessive epistasis in human
AB0 system of blood groups metabolite
antigens Precursor H, A H

H, B


H - (unchanged
precursor) H or h alleles are recessively
epistatic against A or B alleles hh
genotype codes the blood group 0 even in the
presence of A or B alleles hh Bombay allele
transferase H
transferase A
transferase B
hh
0
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Recessive epistasis is manifested in the case of
the gene for secretion of antigens A, B,
H Genotypes SS, Ss secret antigens into
saliva and body fluids Genotype ss does not
secret any antigens, even though they are
present in erythrocytes
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Epistasis- unilateral relation

• Dominant
• substrate
• Y
• -------gt P1
• I
• -------gt P2

• Recessive
• substrate
• B A
• -------gt P0 -------gt P

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Inhibition

• It is certain analogy of dominant epistasis.
• But, in comparison with it, inhibitive allele I
  has not another effect on phenotype than ability
  to suppress an effect of allele A.
• Feathers color of domestic fowl
• Gene 1 allele C ? red coloring
• Gene 2 allele I ? inhibits an effect of allele
  C
• Phenotype 1 C-I-, ccI-, ccii colorless
• Phenotype 2 C-ii colored

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Complementarity and Multiplicity

• genes are equal no subordination
• bilateral relation of alleles of interactive genes

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Complementarity

• is bilateral relation of alleles of interactive
  genes.
• Dominant alleles of complementary genes allow
  genesis of two or more non-replaceable
  components, which form the final product.
• Each of these components is qualitatively
  different and arises from different biosynthetic
  processes.
• For this reason replacement of any of dominant
  alleles of complementary genes for recessive one
  leads to non-formation of the final product.

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Complementarity

• Flower color of earthnut pea
• Gene 1 allele C formation of colorless
  precursor
• Gene 2 allele R formation of activation
  enzyme,
• which changes the precursor into
• colored compound
• Phenotype 1 C-R- red (anthocyan)
• Phenotype 2 C-rr, ccR-, ccrr colorless

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Multiplicity

• is bilateral relation of alleles of interactive
  genes,
• but in comparison with complementarity, each
  single dominant allele of any of these genes,
  even in itself, is sufficient for expression of a
  corresponding trait.
• To this effect these single dominant alleles are
  identical. These alleles are responsible for
  biosynthesis of identical final products, but by
  qualitatively different ways.

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Multiplicity

• Noncumulative full expression of a
  corresponding trait is caused by single dominant
  allele of given multiplicative rank and presence
  of next members of the rank no more changes
  intensity of phenotype.
• Cumulative intensity of phenotype expression is
  direct proportionally dependent on number of
  present dominant members of multiplicative rank.

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Duplicity noncumulative

• Siliqua shape of shepherds purse
• Gene 1 allele T1 normal (heart-shaped)
• Gene 2 allele T2 normal (heart-shaped)
• T1T2 normal (heart-shaped)
• Phenotype 1 T1-T2-, T1-t2t2, t1t1T2-
  normal
• Phenotype 2 t1t1t2t2 cylindrical

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Duplicity cumulative with dominancecharacter
intensity depends on gene number

• Caryopsis color of barley
• Gene 1 allele P1 brownish red coloring (half)
• Gene 2 allele P2 brownish red coloring (half)
• P1P2 dark brown coloring (maximal)
• Phenotype 1 P1-P2- maximal
• Phenotype 2 P1-p2p2, p1p1P2- half
• Phenotype 3 p1p1p2p2 null (white)

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Duplicity cumulative without dominance character
intensity depends on allele number

• Caryopsis color of wheat
• Gene 1 allele R1 pink coloring (quarter)
• Gene 2 allele R2 pink coloring (quarter)
• Phenotype 1 R1R1R2R2 dark red (maximal)
• Phenotype 2 R1R1R2r2, R1r1R2R2 red (three
  quarter)
• Phenotype 3 R1R1r2r2, R1r1R2r2, r1r1R2R2 rose
  (half)
• Phenotype 4 R1r1r2r2, r1r1R2r2 pink (quarter)
• Phenotype 5 r1r1r2r2 white (null)

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• Davenports hypothesis
• about pigment synthesis in human
• Degree of pigmentation is coded by the number of
  dominant alleles of 2 allelic pairs / genes
• black - 4 dominant alleles A1A1A2A2
• brown - 3 dominant alleles
• mulatto - 2 dominant alleles
• light brown - 1 dominant allele
• white - no dominant allele ? a1a1a2a2

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P A1A1A2A2 x a1a1a2a2 F1
A1a1A2a2 F2
A1A1A2A2 1
A1A1A2a2 2
A1A1a2a2 1
A1a1A2A2 2
A1a1A2a2 4
A1a1a2a2 2
a1a1A2A2 1
a1a1A2a2 2
a1a1a2a2 1
1 4 6 4
1 black brown
mulatto light brown white
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Bilateral allele relation of cooperated genes

• Complementarity
• alleles 2 genes
• R n S
• ? ?
• A1 A2
• ? ?
• A
• phenotype

• Multiplicity
• alleles 2 genes
• T1 ? T2
• ? ?
• A
• phenotype

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GENE INTERACTIONS - SUMMARY
interaction type phenotype cross ratio in
the F2 generation reciprocal interaction
9 3 3 1 dominant epistasis 12 3 1 recessive
epistasis 9 3 4 inhibition 13 3 complementa
rity 9 7 noncumul. duplicity with
domin. 15 1 cumul. duplicity with domin.
9 6 1 cumul. duplicity without domin.
1 4 6 4 1 Mendelian inheritance 9 3 3 1
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Significance of gene interactions in
multifactorial diseases

• the main genetic mechanism of predisposition to
  diseases
• Principle of cumulative multiplicity
• heredity of quantitative traits - polygenic
  heredity

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Significance of gene interactions in monogenic
diseases

• low penetrance
• penetrance
• probability of expression of dominant allele
  in phenotype
• - sick or healthy persons
• different expressivity
• expressivity
• intensity of phenotype manifestation
• - severe or minor clinical signs