3.3.2 Punnett Squares

Drill

Gene Symbols coloring page

Objectives

Identify the P, F1, and F2 generations of a genetic cross using a Punnett Square.

Interpret and predict the results of a genetic cross.

Determine the genotypes of parents if given the genotypes of the offspring.

Key Questions

What are true-breeding and hybrid organisms?

How do geneticists use the principles of probability?

How do geneticists use punnett squares?

Vocabulary

F1 generation

F2 generation Genetic cross Hybrid P generation Probability Punnett square True-breeding

Probability

The likelihood that a particular event will occur is called probability. As an example of probability, consider an ordinary event like flipping a coin. There are two possible outcomes: The coin may land heads up or tails up. The chances, or probabilities, of either outcome are equal. Therefore, the probability that a single coin flip will come up heads is 1 chance in 2. This is 1/2 or 50 percent.

How is coin flipping relevant to genetics? The way in which alleles segregate is completely random, like a coin flip.    The principles of probability can be used to predict the outcomes of genetic crosses.

Proportions, Ratios, and Percentages

¼ is what ratio? What ratio describes this data? 32 baby

mice total, 18 are brown, 10 are white, 8 are gray, and 2 are tan

1/3 = ___ %

Punnett Squares

The gene combinations that might result from a genetic cross can be determined by drawing a diagram known as a Punnett square.

The letters in the Punnett square represent alleles: capital letters for dominant alleles and lowercase letters for recessive alleles.

Practice filling in each square

Information to Include in Punnett Square Problems

For each Punnett Square problem, you should always:– Underline the recessive allele (lowercase letter)– Label the male’s genotype and female’s genotype– Include the % of each genotype– Include the % of each phenotype

Punnett Practice Question 1

In ducks, spitting when you talk is dominant to not spitting. Donald is homozygous dominant and Daisy is homozygous recessive. If they have 8 ducklings, how many will be spitters?

Punnett Practice Question 2

Gimpy fins are the result of a genetic disorder caused by a recessive allele. Nemo has a gimpy fin. His parents, Marlin and Coral, have normal fins. What are the most likely genotypes of Marlin and Coral?

Punnett Practice Question 3

Cruella Deville wants to determine how spots are passed from one generation of dalmation to the next. She observed 101 dalmations and organized her data on your handout.

Which of these explains how two spotted parents could produce a puppy with no spots?

– Both parents are heterozygous and produce a homozygous recessive puppy

– Both parents are homozygous recessive and produce a homozygous dominant puppy

– Both parents are heterozygous and produce a heterozygous puppy

– Both parents are homozygous dominant and produce a homozygous recessive puppy

Punnett Practice Question 4

Based on the data from question 3, the allele for spots in dalmations is most likely…

dominant heterozygous recessive sex-linked

Punnett Practice Question 5

In scientists, bad eyesight is dominant to good eyesight. Dexter is heterozygous and his sister, Deedee, is homozygous dominant. What are the genotypes of their parents?

Punnett Practice Question 6

In ogres, the allele for bushy eyebrows (B) is dominant. The allele for shrubby eyebrows (b) is recessive. Fiona is heterozygous for this trait and marries Shrek, who has shrubby eyebrows. What is the probability that their first child will have bushy eyebrows?

Punnett Practice Question 7

In toys, the allele for talking (T) is dominant. The allele for not being able to talk (t) is recessive. Mr. Potato Head (TT) and Mrs. Potato Head (Tt) have four tatertots. Which of these is the predicted ratio of the tatertots who can talk to the tatertots who cannot talk?

Other Activities

Exploring Life Activity 10.2 Exploring Life Activity 10.3 Bikini Bottom Genetics Lab Simulations: Mendelian Inheritance Activity Pisum sativum BCR Galactosemia BCR in Groups

– Each student answers BCR individually– Each group shares their answers– The group writes a response together by including the best aspects of

each individual’s answer Galactosemia BCR Scoring Activity Widow’s Peak BCR Scoring Activity Bikini Bottom Genetics 2 Gummy Bear Genetics Family Reunion in a Dish Lab

Assessment

Meiosis and Punnett Squares Quiz

Homework

Black Rabbit BCR

Closure

How are the principles of probability used to predict the outcome of genetic crosses?

How are Punnett squares used? What information should you include for

EVERY Punnett square problem? 3 more Punnett square practice problems on

next slides

Paramecium Example

In paramecium, long cilia are dominant to short cilia.

What letters will you use?• L for long cilia • l for short cilia

What are the possible genotypes for long cilia?• L and L l

What is the possible genotype for short cilia?• l l

Shallow Hal Punnett Square

Mauricio is a man with a tail nub. He marries Bethany, a woman who does not have a tail nub.

If 25% of their offspring have tail nub, what is Mauricio’s genotype?

N n n n

N n n n

N n

n

n

Kermit Example

In frogs, green skin is dominant to brown skin. After Miss Piggy leaves Kermit, he meets Melinda, a brown frog.

If Kermit is homozygous dominant, what percentage of their tadpoles will be green?

F f F f

F f F f

F F

f

f

100% Ff (genotype)

100% green (phenotype)

HSA Practice

The presence or absence of freckles is determined by one gene. The allele for freckles (F) is dominant and the allele for the absence of freckles (f) is recessive.

A couple has several children. All of the children have freckles because their parents' genotypes can only produce children with freckles.

Which of these are most likely the genotypes of the two parents?

A     Ff and ff    B     FF and ff   

C     Ff and Ff    D     ff and ff

In horses, the allele for straight hair (B) is dominant to the allele for curly hair (b). Which of these sets of parents can produce offspring with curly hair?

– A     a heterozygous male with straight hair and a homozygous female with straight hair   

– B     a homozygous male with curly hair and a homozygous female with straight hair   

– C     a heterozygous male with straight hair and a heterozygous female with straight hair   

– D     a homozygous male with straight hair and a homozygous female with straight hair