Download Exceptions to Mendel`s Principles

Probability and Punnett
Squares
Copy what is in yellow 
Probabililty
 The
likelihood that a particular event
will occur.

Flip a coin…what is the probablity it will land on
heads? Does your probablility change depending on
how many times you flip the coin?
 Principles
of probability can be used
to PREDICT the outcome of genetic
crosses.

But… Probability cannot predict the
exact or precise outcome of an event!
Punnett Square
 Diagram
showing gene combinations that
might result from a genetic cross.
 Law of Segregation: the separation of
alleles during gamete formation.
 Monohybrid Cross: punnett square cross
looking at one trait.
 Dihybrid Cross: Punnett square cross
looking at two traits
Ratios: Add up to 4!!
 Phenotypic
ratio: physical appearance of
offspring
: # of recessive

# of dominant

Ex: 2 white flowers : 2 red flowers
 Genotypic
ratio: combinations of letter
pairings


# homozygous : # heterozygous : # homozygous
dominant
recessive
Ex: 1 RR : 2 Rr : 1 rr
Law of Segregation
 states
that allele pairs separate or
segregate during gamete formation, and
randomly unite at fertilization.
 There are four main concepts related to
this principle.





A gene can exist in more than one form.
Organisms inherit two alleles for each trait.
When gametes are produced (by meiosis), allele pairs separate
leaving each cell with a single allele for each trait.
When the two alleles of a pair are different, one is dominant and
the other is recessive.
Punnett Square Practice #1
 What
would be the outcome of crossing a
heterozygous tongue rolling parent with a
homozygous non tongue rolling parent?
The ability to roll your tongue is dominant.
 Heterozygous
= Aa
 Homozygous = aa
Punnett Square Practice #2
 What
would be the outcome of crossing a
heterozygous long haired cat with a
homozygous long haired cat?
 Long Hair is the Dominant Trait
 Heterozygous
= Ll
 Homozygous = LL
Punnett Square #3
 What
would be the outcome of crossing a
homozygous dominant tongue roller with a
non tongue roller?
 Homozygous
 Non
Dominant ?
Tongue Roller (recessive) ?