Download Probability and Punnet Squares

Probability and
Punnett Squares
Probability
Probability is the likelihood that a
specific event will occur.
For example, if you flip a coin, the
probability that it will land on heads
is 50%, or 1 out of 2.
We can use Punnett Squares to
predict the probability of genetic
outcomes.
Dominant and Recessive
Alleles
Dominant alleles are expressed in
capital letters.
Ex: T B A C X
Dominant alleles can mask recessive
alleles. This means you might have a
recessive allele, but you can see the
recessive trait if the dominant allele
is present.
Recessive alleles are represented by
lowercase letters.
Ex: t b a c x
Recessive alleles are masked by
dominant alleles.
Homozygous and Heterozygous
An organism is homozygous if both
alleles are identical (both dominant or
both recessive).
Ex: TT, BB, aa, cc, tt
An organism is heterozygous if the
alleles are different (one dominant
and one recessive).
Ex: Tt, Bb, Rr, Xx
A heterozygous organism expresses
the dominant trait, but is a carrier of
the recessive trait.
TT = homozygous dominant
tt = homozygous recessive
Tt = heterozygous
Genotype and Phenotype
Phenotype is the physical appearance
of the trait.
Ex: brown hair, blue eyes, tall, short,
etc.
Genotype is the genetic makeup of
the organism, or what the alleles
actually are.
Ex: BB, Tt, Rr, rr
Example
A plant is tall, but has heterozygous
alleles for height. What is the
genotype?
A dog is homozygous dominant for a
straight tail. What will be the
genotype if the letter that
represents tail straightness is S?
Punnett Squares
A Punnett square is a simple diagram
that can be used to predict the
outcome of a genetic cross.
Punnett squares use probability to
predict possible genotypes and
phenotypes.
In a monohybrid cross only one trait
is studied at a time.
Example:
A brown mouse (Bb) is mated with a
white mouse (bb).
What trait is dominant—brown or
white? Which is recessive?
What is the probability of producing
a white mouse?
In beagles the trait for droopy ears
(D) is dominant to the trait for perky
ears (d). What would be the
expected offspring if two
heterozygous droopy-eared dogs were
mated?
Incomplete Dominance
Some alleles are neither dominant or
recessive. In these, a blend of the
two traits is expressed.
Codominance
Codominance is situation in which both
alleles are dominant.
In a codominant individual, the
phenotypes from both alleles are
clearly expressed.
Multiple Alleles
Many genes exist in several different
forms and are said to have multiple
alleles.
A gene with more than two alleles is
said to have multiple alleles.
In human, blood type is determined
by multiple alleles. A and B are
codominant, while O is recessive.
A woman with AB blood has a child
with a man who has type O blood
(OO). What are the possible blood
types of the child?
Sex-Linked Traits
Humans have 23 pairs of
chromosomes.
We have 22 pairs homologous
chromosomes, called autosomes.
The 23rd pair determines gender.
Females have an XX genotype. Males
have an XY genotype.
The X and the Y chromosomes are not
homologous. The X chromosome is
much bigger than the Y, so there are
some genes on the X-chromosome
that are not on the Y-chromosome.
The alleles for sex-linked traits are
written as superscripts of the Xchromosome.
Xb Y
X Xb
Color-blindness is a sex-linked trait
carried on the X-chromosome.
Because boys only have one Xchromosome, they are more likely to
get color blindness. If they only get
one bad copy of the gene, they have
the disorder. Girls have to get two
bad copies of the gene to have the
disorder.
Polygenic Inheritance
Traits controlled by two or more
genes are said to be polygenic.
These traits often show a wide range
of phenotypes.
More than 4 different genes code for
skin color.