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 flower R’R’ = white flower RR’ = pink flower

Incomplete Dominance: Snapdragons 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 RR X WW

Co-dominance: Roan Cattle RW X RW

Alternative Notation Homozygous 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 Antibodies Can give blood to Can receive blood from AB A and B A 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 Yellow BBEE BBEe BbEE BbEe bbEE bbEe Yellow (Dudley) bbee

Epistasis: Lab Colour Yellow Yellow (Dudley) BBee Bbee bbee http://ansci.cornell.edu/usdagen/epistasis.html

Exploring Epistasis http://ansci.cornell.edu/usdagen/epistasis.html Recessive masking epistasis Dominant masking epistasis Modifying epistasis Definition/description Example 2 Punnett Squares Parental cross: AABB x aabb F1 cross Summarize phenotypes of offspring based on epistatic pattern