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