11-2 PROBABILITY AND PUNNETT

SQUARES

1. Genetics & Probability

Probability can be used to predict the results of genetic crosses.

Probability- the likelihood that something is going to happen. In genetics expressed

as a ratio or percentage.

Ex: the probability that it will snow in March is 50:50

Punnett Squares- A tool used to determine the gene combinations that might result from a genetic cross

A capital letter is used to represent the dominant allele

A lower case letter is used to represent the recessive allele.

The principles of probability can be used to predict the outcomes of genetic crosses

This Punnett square shows the probability of each possible outcome of a cross between hybrid tall (Tt) pea plants.

Homozygous- Organisms that have two identical alleles for a particular trait organisms are true-breeding for a particular trait. Two alleles for black fur

Heterozygous- Organisms that have two different alleles for the same trait organisms are hybrid for a particular trait.One allele for black fur and one allele for white fur

Genotype- genetic make up of the organism Ex: genes for black fur and

white fur Ex: Tall (T) allele and short

(t) allele

Phenotype- the physical characteristics or what the organisms looks like. Ex: the mouse has black fur. Ex: all plants appear tall

The plants have different genotypes (TT and Tt), but they have the same phenotype (tall).

3. Probability and Segregation

Genotypic Ratios25% or 1:3 (TT to Tt or tt)

likelihood the offspring will be TT

50% or 2:2 likelihood the offspring will be Tt

25% or 1:3 likelihood the offspring will be tt

Phenotypic Ratios75% or 3:1 (tall to short)

likelihood the offspring will appear Tall

25% or 1:3 likelihood the offspring will appear short

In the F2 generation; the ratio of tall plants (TT or Tt) to short (tt) plants is 3:1.

The predicted ratio showed up in Mendel’s experiments indicating that segregation of gametes did occur.

4. Probabilities and Averages

Probabilities predict the average outcome of a large number of events.

Probability cannot predict the exact outcome of an individual event

In genetics, the larger the number of offspring, the closer the resulting numbers will get to the expected values.