Punnett Squares and Probability

What is a punnett square and why do we use it?

What is it? What is it? A grid system for A grid system for predicting all possible predicting all possible genotypes of offspring genotypes of offspring from two parentsfrom two parents

The axes of the grid The axes of the grid represent the possible represent the possible gamete genotypes of each gamete genotypes of each parentparent

The boxes show all The boxes show all possible genotypes of the possible genotypes of the offspringoffspring

What is a Monohybrid Cross?Showing a Cross

Aa x Aa

Monohybrid Crosses

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

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

What is it?

Incomplete Dominance

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?

What is it?

RR x WW

Co-Dominance

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

Co-Dominance ExampleWhen 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.

What is it? What does it look like?

Dihybrid Cross

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

Dihybrid Cross ExampleIn 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?

What is it? Does it look like?

Multiple Alleles (Blood Typing)

Blood types demonstrate co-dominance 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-

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?

What is it? What does it look like?

Sex-Linked Problems

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

Important things to knowIn 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 XHYRecessive male XhY

Sex-linked ExamplePerform 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 happenHow 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 ExampleColorblindness 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?