Transcript Punnett Squares and Probability

Punnett
Squares and
Probability
What is a punnett square
and why do we use it?
What is it?
 A grid system for
predicting all possible
genotypes of offspring
from two parents
 The axes of the grid
represent the possible
gamete genotypes of
each parent
 The boxes show all
possible genotypes of
the offspring
Monohybrid Crosses
What is a Monohybrid Cross?
 Demonstrates the
inheritance of one specific
trait
 Only contains 4 boxes. The
parent’s gametes are
separated over the boxes
 Genotypic Ratio:
1 AA: 2Aa: 1aa
Showing a Cross
Aa x Aa
Monohybrid Cross Example
Walruses are characterized by their tusks. A
heterozygous male with long tusks mates with a female
that has short tusks. The dominant trait should be
represented by (T) and the recessive trait should be
represented by (t)
 What percentage of the offspring will have short tusks?
 Write the genotypic ratio for the scenario
 Write the phenotypic ratio for the scenario
Incomplete Dominance
What is it?
A cross where the heterozygous
phenotype is somewhere between
the two homozygous phenotypes.
Neither allele is completely dominant
or recessive.
Red (RR) x White (WW)
4 Pink (RW)
Since there is no recessive
condition, all letters are capital
Incomplete Dominance Example
In a particular breed of fish, being red (R)
is incompletely dominant to being white
(W). What are the genotypes of the
parents if for every 100 offspring, 53 are
red and 47 are pink?
Co-Dominance
What is it?
A cross where both alleles of a gene
are expressed completely and
separately, neither allele is
dominant or recessive.
RR = Red cow
RW = Red and White cow (Roan)
WW = White cow
Since there is no recessive
condition, all letters are capital
RR x WW
Co-Dominance Example
When studying a certain species of lizard, you are
familiar with a yellow species and a green species.
When doing research, you find that a third species
exists that is green with yellow spots. Show the
following cross in a punnett.
Green lizard with yellow spots x Green lizard
Show the genotypic and phenotypic ratios for the
offspring.
Dihybrid Cross
What is it?
 A cross that examines the
inheritance of two different
traits
 Uses 16 boxes in total
 Parental Gametes are
separated in pairs
 Example: Genotype RrYy
Gametes: RY, Ry, rY, ry
What does it look like?
Dihybrid Cross Example
In cows, being spotted (S) is dominant to
being a solid (s) color. Also, having curved
horns (H) is dominant to having straight
horns (h).
A female who is heterozygous for both
traits is bred with a male who is
homozygous dominant for horn shape and
homozygous recessive for pattern.
What are the phenotypic and genotypic
ratios of their offspring?
Multiple Alleles (Blood Typing)
What is it?
 Blood types demonstrate codominance and the basic principles
of dominance and recessiveness
 There are four general phenotypes
for blood types: A, B, AB, and O
 Another factor that we consider in
blood typing is the Rhesus factor.
Individuals can either be Rh+ or Rh All possible phenotypes are:
A+, A-, B+, B-, AB+, AB-, O+, O-
Does it look like?
People with blood group 0 Rh
- are called "universal donors"
and people with blood group
AB Rh+ are called "universal
receivers."
Rh+ blood can never be given
to someone with Rh - blood,
but the other way around
works. For example, 0 Rh+
blood can not be given to
someone with the blood type
AB Rh -.
Multiple Allele Example
What are the potential blood types of
children for a man with heterozygous
type A blood and a woman with type
AB blood?
Show the genotypic and phenotypic
ratios
Blood Typing as a
Dihybrid Cross
Show the cross of a heterozygous A+ male and a heterozygous
B+ individual.
 Show all genotypes and all phenotypes for the possible offspring
Important things to think about:
 How is a heterozygous A or B individual’s genotype represent?
 Rh+ is dominant to Rh How will the gametes assemble in the dihybrid cross?
Sex-Linked Problems
What is it?
 Cross that demonstrates inheritance
of genes located on the sex
chromosomes
 Females have two X chromosomes,
therefore the start of their genotypes
is XX.
 Males have an X and Y chromosome,
therefore the start of their genotypes
is XY.
 50% of offspring are always male and
50% if the offspring are always
female
What does it look like?
Important things to know
In order to successfully complete the problem:
 You must make note of the sex chromosome (X) and the trait
 Example:
Sex Chromosome  XC
 Trait
 Never place a trait on a Y chromosome! Traits are only carried on the
X chromosomes.
Female Results:
 Homozygous dominant female XHXH
 Heterozygous female XHXh (expresses dominant condition or is a carrier)
 Homozygous recessive female XhXh
Male Results:
 Dominant male XHY
 Recessive male XhY
Sex-linked Example
Perform a cross between a white-eyed female fruit
fly and red-eyed male. White eyes are a recessive
condition.
 Write the phenotypic and genotypic ratio of all
offspring.
 What percent of the female offspring will have
white eyes?
Probability
The likelihood that
a particular event
will happen
How do we calculate it? (Revisit the
punnett)
(In an ideal situation) If these parent had 1
offspring, what fraction shows the chance
of the offspring being heterozygous?
½ chance of heterozygous offspring
If these parents had a second child, what
fraction shows the chance of the offspring
being heterozygous?
½ x ½ = ¼ chance of 2 heterozygous
offspring
Probability Example
Colorblindness is a sex-linked trait. A man,
whose mother was a carrier for colorblindness
marries a woman who is a carrier for
colorblindness.
What are the odds that the man is colorblind?
If he is not colorblind, what is the likelihood
that he will have 2 colorblind sons?