GENE INTERACTIONS Flashcards
WHAT ARE RECIPROCAL INTERACTIONS?
Alleles of two or more genes contribute to the phenotype.
Each possible combination of these alleles has a separate phenotype
-analogue of incomplete dominance – more genes, each determining a different phenotype final phenotype is the mixture of all gene interactions without a change in the phenotype ratio
EG: F2 -> 9:3:3:1
B1 -> 1:1:1:1 (MENDELS)
EG: COLOUR OF PAPRIKA
gene 1: allele r/R -> anthocyanin (red dye) – very intense
gene 2: allele cl/Cl -> chlorophyll degradation
→degradation of chlorophyll is yellow
RCL = red (anthocyanin)
Rclcl = brown (red + green)
-> Red because of R
-> Green because of small CL and therefore no chlorophyl degradation
rrclcl = green
→ no enzymes / red color so stays green
rrCL = yellow (chlorophyll degraded)
WHAT ARE GENE INTERACTIONS?
polygenic inheritance
DEFINE MONOGENIC INHERITANCE
1 trait = 1 gene which segregate independently
DEFINE POLYGENIC INHERITANCE
1 trait = 2 or more genes which affect each other via gene interactions
WHAT ARE THE DIFFERENT TYPES OF GENE INTERACTIONS?
1) reciprocal interactions
2) epistasis (dominant and recessive)
3) inhibition
4) complementarity
5) multiplicity
WHAT IS EPISTASIS?
when two different genes contribute to a single phenotype and their effects are not merely additive
-differ greatly from mendelian ratios
-unilateral relationship
-relationship of superiority and subordination
->alleles of 2 genes (M>N)
->alleles of various genes (M>N>R>S)
WHAT IS AN EPISTATIC GENE?
one that suppresses the effect of another gene
WHAT IS A HYPOSTATIC GENE?
one whose phenotype is altered by the expression of an allele at a separate locus in an epistasis event
DEFINE DOMINANT EPISTASIS
happens when the dominant allele of one gene masks the expression of all alleles of another gene
-if 1 or 2 copies of dominant allele inherited they will have the trait
EG: A of one gene hides B of another gene and expresses its phenotype
->B allele will only be expressed when A locus contains recessive alleles
->AABB / AAbb / AaBb / Aabb produce the same phenotype because large A is present
->aaBB / aaBb / aabb produce an additional 2 phenotypes
= 12:3:1 ratio
EG: flower colour in dahlias determined by plant metabolites called flavonoids
y/Y = can hydroxylate the pigment to dark yellow
i/I = hydroxylates pigment only to light yellow
YI / Yi = Y dominates and capital I is redundant as it only produces light yellow hydroxylate that gets converted to intense yellow
yI = light yellow (no Y; so I is expressed)
yi = white
EG: human eye colour depends on the type and density of pigment in the iris
EPISTATIC = B →allows the production of melanin (brown)
HYPOSTATIC = G -> determines green colouring
BG / Bg = brown
bbG = green
bbgg = blue (no melanin pigment)
EG: hair colour depends on eumelanin and pheomelanin pigments
EPISTATIC = H
HYPOSTATIC = R
eumelanin = black, brown
pheomelanin and a little eumelanin = red, blonde
Hrr = black
HR = dark brown
hhR = red
hhrrK = blonde
hhrrkk = white
WHAT ARE THE GENES DETERMINING EYE COLOUR?
HERC2 / OCAS2 = brown or blue
gey = green or blue eyes
WHAT ARE THE GENES DETERMINING HAIR COLOUR?
eumelanin encoded by -> HCL3 / BRHC
pheomelanin encoded by -> RHC / KITLG (blonde hair - encodes a secreted gland for the receptor tyrosine kinase)
HCL3 / BRHC > RHC > KITLG
-grey hair is a result of degraded products of pigment due to environment and or age
EXPLAIN RECESSIVE EPISTASIS
when the recessive allele of one gene in a homozygous state makes the phenotypic expression of the dominant allele of another gene
-9:3:4 ratio in F2
EG: flower colour of sage depends on the hydroxylation level of a colourless flavon precursor
P = low hydroxylation -> rose
A = high hydroxylation -> violet
PA = violet
Paa = rose
ppA / ppaa = white
EG: ABO blood groups - H or h alleles are recessively epistatic against A or B alleles
hh = bombay allele -> codes blood group O even in the presence of A or B alleles
H = produces A and B alleles
EXPLAIN THE UNILATERAL RELATION OF EPISTASIS
DOMINANT = substrates made from both genes being expressed, but the phenotype of the dominant allele will only be seen, the substrate of the dominant allele suppresses the phenotypic expression of the other gene
RECESSIVE = the substrate made through the expression of the recessive allele in a homozygous state of gene A masks the phenotypic expression of the dominant allele of gene Bs
DISCUSS INHIBITION
an inhibitive allele I effects the phenotype of allele A using its ability to suppress the expression of allele A
-similar to dominant epistasis, except that dominant epistatic allele has no effect itself on the phenotype, can only inhibit the effect of hypostatic alleles
-ratio of 13:3
EG: domestic fowl chicken
allele C = red colour
allele I = inhibits an effect of allele C
PHENO 1 = CI / ccl / ccii -> colourless
PHENO 2 = Cii -> coloured
DEFINE COMPLEMENTARY
bilateral relationship between alleles of interactive genes; genes are equally expressed
-no superiority/subordination; genes operate on the same level
-dominant allow phenotypic expression or 2 or more non replaceable components which form the final product
-> each component is qualitatively different and arise from different biosynthetic processes
-> replacement of any of the dominant alleles for recessive ones causes lack of final product formation (NEED BOTH DOMINANT FORMS)
- 9:7
EG: flower colour of earthnut pea
C = colourless precursor formation
R = formation of activation enzyme which changes precursor into coloured compound
PHENO 1 = CR = red
PHENO 2 = Crr / ccR / ccrr = colourless
EXPLAIN MULTIPLICITY
bilateral relationship between alleles of interactive genes, but in comparison with complementarity, each single dominant allele of any of these genes, is sufficient for the expression of the corresponding trait
-single dominant alleles are identical, these alleles responsible for biosynthesis of identical final products but via different qualitative ways
-cumulative or non-cumulative
EXPLAIN NON CUMULATIVE MULTIPLICITY
full expression of corresponding trait is caused by a single dominant allele of a given multiplicative rank and the presence of another member of the rank does not change the intensity of the phenotype
-15:1
EG: Siliqua shape of sheperds purse
T1 = normal heart shaped
T2 = normal heart shaped
T1+T2 = normal heart shaped
PHENO 1 = T1T2 / T1t2t2 / T2t1t1 = normal
PHENO 2 = t1t1t2t2 = cylindrical
EXPLAIN CUMULATIVE MULTIPLICITY
the intensity of the phenotype is a direct proportion and dependent on the number of dominant alleles within the multiplicative gene rank
-9:6:1
EG: Caryopsis colour of barley
P1 = brownish - red
P2 = brownish - red
P1+P2 = dark brown
PHENO 1 = P1P2 = dark brown
PHENO 2 = P2p2p2 / P2p1p1 = brownish-red
PHENO 3 = p1p1p2p2 = white
DISCUSS DAVENPORTS HYPOTHESIS OF HUMAN PIGMENT SYNTHESIS
degree of pigmentation is coded by the number of dominant alleles of 2 genes
-A and B which both produce melanin
-the more dominant alleles, the darker the skin colour
-black = 4 / brown = 3 / mulatto = 2 / light brown = 1 / white = 0
-1:4:6:4:1
BILATERAL RELATIONSHIP OF INTERACTING GENES
GENE INTERACTIONS AND F2 PHENOTYPIC RATIOS
WHAT IS THE MAIN MECHANISM THAT CONTRIBUTES TO DISEASE PREDISPOSITION?
principle of cumulative multiplicity
-> heredity of quantitative traits (polygenic)
WHAT IS THE SIGNIFICANCE OF GENE INTERACTIONS IN MONOGENIC DISEASES?
low penetrance = reduced probability of the expression of a dominant allele that produces the phenotype
expressivity is different = intensity of phenotype manifestation (mild/severe clinical symptoms)
WHAT DOES THE PRINCIPLE OF CUMULATIVE MULTIPLICITY RESEMBLE?
The principle of cumulative multiplicity resembles a principle of multifactorial inheritance
GIVE AN EXAMPLE OF RECIPROCAL INTERACTIONS
peppers
GIVE AN EXAMPLE OF DOMINANT EPISTASIS
hair colour
eye colour
dahlia flower colour
GIVE AN EXAMPLE OF RECESSIVE EPISTASIS
blood groups antigens and bombay gene
colour of sage
GIVE AN EXAMPLE OF INHIBITION
hen colour
GIVE AN EXAMPLE OF COMPLEMENTARITY
earthnut pea
GIVE EXAMPLES OF MULTIPLICITY
NONCUMULATIVE: sheperds purse
CUMULATIVE WITH DOMINANCE: barley
CUMULATIVE WITHOUT DOMINANCE: human skin colour and wheat
T/F: INHIBITION IS AN ANALOGY OF DOMINANT EPISTASIS
true
