non-mendelian genetics gene expression that varies from typical mendelian ratios
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Non-Mendelian Genetics
Gene expression that varies from typical Mendelian ratios
Mendelian Genetics Review
– Two possible alleles for a single gene– Dominant variation (B)– Recessive variation (b)
– When the dominant allele is present in the genotype, it is always expressed in the phenotype
– Recessive allele is only expressed when gene is homozygous recessive
Mendelian Genetics Review
Non-Mendelian Genetics
– Some traits don’t follow the dominant/recessive rules outlined by Mendel’s Laws of Inheritance
– Non-Mendelian Patterns of Inheritance: – Co-dominance– Incomplete dominance– Multiple alleles– Lethal genes– Polygenetic traits
Incomplete Dominance
Genotype of individual is heterozygous Alleles are not completely dominant over the
other Phenotype of heterozygote is a blending of
the two homozygous phenotypes Example: Snapdragons
Incomplete Dominance: Snapdragons
R = red R’ = white
RR = red flowerR’R’ = white flowerRR’ = pink flower
Incomplete Dominance: Snapdragons
R R
R’ RR’ RR’
R’ RR’ RR’
R R’
R RR RR’
R’ RR’ R’R’
RR X R’R’ RR’ X RR’
Co-dominance
– Genotype of individual is heterozygous– Full expression of both alleles– Phenotype of heterozygote is neither
dominant or recessive– Example: Roan Cattle, Speckled Chicken
Co-dominance: Roan Cattle
W W
R RW RW
R RW RW
RR X WW
Co-dominance: Roan Cattle
R W
R RR RW
W RW WW
RW X RW
Alternative NotationHomozygous Dominant (Red) RR
Homozygous Recessive (White) WW
Heterozygous (Roan) RW
Colour-determining Allele C
Homozygous Dominant (Red) CRCR
Homozygous Recessive (White) CWCW
Heterozygous (Roan) CRCW
Multiple Alleles
Genes can have many possible alleles, but an individual can only carry 2 of those alleles max One from each parent
Example: ABO Blood types with 3 alleles
Multiple Alleles: ABO Blood Types
What is blood made of? Red blood cells
Contain hemoglobin, transport O2 and CO2 throughout the body
White blood cells Fight infection
Platelets Help blood to clot
Plasma Fluid which contains salts and various kinds of proteins
Multiple Alleles: ABO Blood Types
Determining blood type Protein molecules found on RBC and in
blood plasma determine blood type on an individual Antigens are located on the surface of blood
cells Antibodies are in the blood plasma
Multiple Alleles: ABO Blood Types
Different Blood Groups Blood Group A
IA IA + IA i or AA + AO Have A antigens on the surface of red blood cells
and B antibodies in blood plasma
Blood Group B IB IB + IB i or BB + BO Have B antigens on the surface of red blood cells
and A antibodies in blood plasma
Multiple Alleles: ABO Blood Types
Different Blood Groups Blood Group AB
IA IB or AB Have both A and B antigens on the surface of the
red blood cells and no A or B antibodies
Blood Group O i i or OO Have neither A or B antigens on the surface of red
blood cells, but have both A and B antibodies in blood plasma
Multiple Alleles: ABO Blood Types
Blood Transfusions The transfusion will work if a person who is going to
receive blood has a blood group that doesn’t have antibodies against the donor’s antigens
Blood Group Genotype Antigens AntibodiesCan give blood to
Can receive blood from
AB A and B
A A
B B
O None
Lethal Alelles
- Some alleles are lethal when present as homozygous
- Can apply to dominant or recessive allele/trait- Example: Coat colour in mice, Creeper gene in
Chickens
Lethal Genes: Mice Coat Colour
- Lethal genes were first observed in mice while studying coat colour gene
- Expected phenotype ratio was 3:1 from a heterozygote cross
- Observed phenotype ratio was 2:1
Lethal Gene: Mice Coat Colour
- After several test crosses,- Yellow mice were found to be heterozygotes- White mice were homozygous recessive- No homozygous dominant yellow
mice were obtained from crosses
Lethal Genes Creeper Gene
- Lethal genes have also been observed in chickens,- When developing embryo contains two copies of a
recessive allele, the embryo dies in the eggshell- Chick heterozygous with creeper allele will survive
Polygenic Traits
− When more than two alleles and multiple genes control the expression of a trait− Examples: Height, skin colour, and hair colour in
humans
Polygenetic Traits
– Traits like height display a continuous distribution where phenotypes vary along a continuum
Polygenic Expression- Workers A, B, and C carry out
steps for painting a design on a poster
- Like genes a, b, and c are the instructions
- Worker A puts paint in the tray; a tells it how
- Worker B adds dye to the pain; b tells it what colour
- Worker C paints a design on the poster; c tells it what design
Polygenic Expression- If looking at this as a genetic
pathway, the workers and instructions would be invisible
- The only part we would see is the output, the poster- The phenotype
Polygenetic Traits
- Epistasis- When the effect of one gene is dependent on the
presence of one or more “modifier genes”- Often the proteins the genes code for work
together in the same processes
Polygenic Expression: Epistasis
- Adding epistasis:- A version (or allele) of a is broken
so it contains no instructions- Worker A couldn’t put paint into
the tray- Workers B + C still do their jobs- Output/Phenotpe: Blank Poster
- The broken version of a is epistatic to b and c
The important aspect of epistasis is that it doesn’t just influence the phenotype, it hides the output of another gene or genes.
Polygenic Expression: Epistasis
Epistasis: Lab Colour
Black Brown YellowBBEEBBEeBbEEBbEe
bbEEbbEe
BBeeBbee
Yellow (Dudley)bbee
Epistasis: Lab Colour
Yellow Yellow (Dudley)
BBeeBbee bbee
Exploring Epistasis
- http://ansci.cornell.edu/usdagen/epistasis.html - Recessive masking epistasis- Dominant masking epistasis- Modifying epistasis
1. Definition/description2. Example3. 2 Punnett Squares
- Parental cross: AABB x aabb- F1 cross- Summarize phenotypes of offspring based on epistatic pattern
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