Download 11-2Probability and PunneTt Squares


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).

Genotypic Ratios
 25% 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 Ratios
 75% 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.

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.