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

CrossBb x Bb

B b female gametes

B

b

male gametes

BB

Bb

Bb

bbPhenotypes

%75% Brown eyed, 25% Blue eyed

Ratio3 Brown eyed: 1 Blue eyed

Genotypes%

25% BB, 50% Bb, 25% bbRatios

1BB:2Bb:1bb

STEP

STEP

STEP

STEP

BB = brown eyes Bb = brown eyes bb = blue eyes

Cross two Alaskan huskies that are heterozygous for brown eyes (Bb).

DIHYBRID CROSSESCross: RrGg X RrGg

EXTRA STEP - Finding the possible gametes

(R r) (Gg)F.O.I.LFirstOuterInner Last

STEP

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 whitepink

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

RWRW

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?

• Cross of two purple flowers RB X RB

• What are gamete possibilities?• genotype ratio

• phenotype ratio

• Can you have a heterozygous red orhybrid blue flower?

B

R

R B

BBRB

RR RBred purple

purple blue

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…

W

W

R W

WWRW

RW WWroan

roan

white

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