Probability and Punnett Square

Probability: likelihood that a specific event will occur. Principle of probability: used to predict the outcomes of genetic crosses.

Two important points to remember with probabilities: Past outcomes do not affect future events Probabilities predict the average outcome of many events

Punnett Squares: diagrams that model genetic crosses. can be used to predict and compare the genetic variations that will result from a cross.

Homozygous: Organisms that have two identical alleles for a particular trait—TT,tt Heterozygous: Organisms that have two different alleles for the same trait--Tt

Monohybrid Crosses STEP  STEP  STEP  STEP  Cross two Alaskan huskies that are heterozygous for brown eyes (Bb). STEP  BB = brown eyes Bb = brown eyes bb = blue eyes B b female gametes Genotypes % 25% BB, 50% Bb, 25% bb Ratios 1BB:2Bb:1bb STEP  B b male gametes BB Bb Cross Bb x Bb STEP  STEP  Bb bb Phenotypes % 75% Brown eyed, 25% Blue eyed Ratio 3 Brown eyed: 1 Blue eyed

DIHYBRID CROSSES (R r) (Gg) F.O.I.L First Outer Inner Last RG Rg rG rg Cross: RrGg X RrGg EXTRA STEP - Finding the possible gametes STEP  rG rg (R r) (Gg) F.O.I.L First Outer Inner Last RG Rg

Incomplete Dominance In incomplete dominance, neither allele is Dominant-blending of traits when two different alleles for the same trait occur together. Colors blend together heterozygous individuals = 3rd phenotype red pink white

Incomplete Dominance Four O’ Clocks, if you cross a red RR (which is always pure) with a white WW (that is also always pure) , you get a pink RW (which is always hybrid / heterozygous RW RW RW RW

Incomplete Dominance In another flower, if red RR and blue BB flowers are crossed, they produce a 3rd purple RB flower What would be the genotype ratio and phenotype ratio if you crossed two purple flowers? http://www.gwu.edu/~darwin/BiSc150/One/rose.GIF

Cross of two purple flowers RB X RB What are gamete possibilities? genotype ratio phenotype ratio Can you have a heterozygous red or hybrid blue flower? R B RR RB R red purple BB RB B purple blue http://www.gwu.edu/~darwin/BiSc150/One/rose.GIF

Codominance Both alleles contribute to the phenotype of the organism by showing up simultaneously (at the same time) in heterozygous individuals. In cattle and horses, if you cross a pure red (RR) with a pure white (WW), you get (RW) which produces the color roan.

Codominance These cattle or horses actually have both red and white hairs intermixed, or are spotted. Roan is a third phenotype. If you cross a roan with a white… R W RW WW W roan white http://www.ccs.k12.in.us/chsteachers/Amayhew/Biology%20Notes/beyond%20dominance%20notes_files/image004.jpg RW WW W roan white

Codominance Andalusian chickens also show this pattern of inheritance. If you cross a black (BB) chicken With a white (WW) chicken You get black+white speckled (BW) chicken