Non - Mendelian Genetics II

Mendelian Genetics Complete dominance Law of segregation Law of independent assortment One gene  one trait

Non - mendelian Genetics Codominance Incomplete dominance Multiple alleles Sex - related inheritance Polygenic inheritance Pleiotropy Epistasis

Polygenic inheritance Additive effects of two or more genes on a phenotypic character Quantitative characters vary in a population along a continuum or gradation non - contributory alleles ( abc )Due to the presence of contributory ( ABC ) and non - contributory alleles ( abc ) Expression can be affected by environmental factors

Example : –Skin pigmentation controlled by at least three separately inherited genes –Each dark skin allele contributes a unit of ‘ darkness ’ to the individual –AABBCC  very dark –aabbcc  very light

(very light) (very dark) (medium brown) Aa Bb Cc x Aa Bb Cc brown P generation F 1 generation F 2 generation

F 2 generation distributions and coin tosses Number of genes Number of alleles Number of phenotypes in F 2 Fraction of F 2 offspring containing all contributory alleles 123 OCA, 1CA, 2CA ¼ P (all CA), P(zero CA) Checkpoint 1 - Fill in the table.

F 2 generation distributions and coin tosses Number of genes Number of alleles Number of phenotypes in F 2 Fraction of F 2 offspring containing all contributory alleles 245 OCA, 1CA, 2CA, 3CA, 4CA 1/16 P (all CA), P(zero CA) Checkpoint 2 - Fill in the table.

Checkpoint 3 What proportion of the offspring will inherit exactly two contributory alleles ?

F 2 generation distributions and coin tosses Number of genes Number of alleles Number of phenotypes in F 2 Fraction of F 2 offspring containing all contributory alleles 367 OCA, 1CA, 2CA, 3CA, 4CA, 5CA, 6CA, 6CA 1/64 P (all CA), P(zero CA) Checkpoint 4 - Fill in the table.

Checkpoint 5 What proportion of the offspring will inherit exactly two contributory alleles ?

Checkpoint 6 - Express the following information in terms of n, where n is the number of alleles Number of genes Number of alleles Number of phenotypes in F 2 Fraction of F 2 offspring containing all contributory alleles 123 OCA, 1 CA, 2 CA ¼ P ( all CA ), P ( zero CA ) 245 OCA, 1 CA, 2 CA, 3 CA, 4 CA 1/16 P ( all CA ), P ( zero CA ) 367 OCA, 1 CA, 2 CA, 3 CA, 4 CA, 5 CA, 6 CA, 6 CA 1/64 P ( all CA ), P ( zero CA ) n /2 n n + 1 1/2 n

How can we determine the possible combinations of r objects ( i. e. contributory alleles ) from a set of n objects ( alleles ) without listing ( no repetition, order does not matter )? Where n = # of alleles r = number of contributory alleles

Determining the proportion of offspring with exactly the desired # of contributory alleles where n = # of alleles r = number of contributory alleles nCr2nnCr2n All possible allele combinations

Combination formula and Pascal ’ s triangle Checkpoint 7 – Draw Pascal ’ s Triangle until n = 6

Checkpoint 8 – Use Pascal ’ s triangle to determine what proportion of the offspring will have exactly three dominant alleles if a.Two genes determine the trait b.Three genes determine the trait c.Four genes determine the trait

Petal length of a plant is a polygenic trait that ranges from 4 mm to 20 mm. Out of 770 plants, only 3 of them have 4 mm petals. a.How many genes affect petal length ? b.How many phenotypes are present for petal length ? c.Give one genotype for a plant with 12 mm petals. d.Give two possible genotypes for plants with 6 mm petals e.What proportion of plants have 14 mm petals ? f.What is the phenotype of plants with 7 contributory alleles ? g.How many contributing alleles does a plant with 8 mm petals have ? Checkpoint 9

Pleiotropy ( pleion, “ more ”) 40% of cats with white fur and blue eyes are deaf Phenylketonuria (lack of enzyme that breaks down Phe) Sickle cell anemia (misshapen RBCs at low O2 concs) One gene, many phenotypic effects

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Epistasis One gene affects expression of another Effects of one gene are modified by one or several other genes Examples : –Coat color in mammals –Color of wheat grains –Fruit color in squash –Bombay phenotype in ABO blood groups in man Two genes affect coat color B  black, b  brown, E  pigment deposited, e  no pigment

A yellow dog ( Bbee ) is mated with a brown dog ( bbEe ). What is the genotypic and phenotypic ratio of the progeny ? Two genes affect coat color B  black, b  brown, E  pigment deposited, e  no pigment Checkpoint 1

Bombay phenotype