What a Punnett Square Can Do for You 1. WHAT ARE THE FACTORS THAT DETERMINE THE EXPRESSION OF...

What a Punnett Square Can Do for You

1. WHAT ARE THE FACTORS THAT DETERMINE THE EXPRESSION OF TRAITS?

2. HOW CAN INHERITANCE BE MODELED?©Copyright 2014 by www.cpalms.org

Punnett Square Article Scenario

You are a gardener and you were given two pea plants. The person who gave them to you said that one plant has the genotype Gg and the other plant has a genotype of gg. You decide to plant them close together to make sure that they will breed with each other and produce offspring. So . . .

How can inheritance be modeled?©Copyright 2014 by www.cpalms.org

1. The trait being studied in this scenario is pea color.

It takes two alleles to determine some traits, like pea color.

©Copyright 2014 by www.cpalms.org

An allele is a different form of the same gene.

A dominant allele is a hereditary unit that blocks the expression of another allele.

2a. The dominant allele expresses for the color green and is represented by the capital letter G.


A recessive allele is a hereditary unit that is blocked by a dominant allele.

2b. The recessive allele expresses for the color yellow and is represented by the lower case letter g.


Your Punnett Square Should Look Like This G g

g

g

• The alleles of one parent’s genes (mother) are separated into gametes during meiosis. The possible gametes are written on the top side.

• The alleles of the other parent’s genes (father) are separated into gametes during meiosis. The possible gametes are written along the left side.



g

g

• A genotype is the alleles that contain the DNA that expresses for a particular trait.

• 3. The mother’s genotype is Gg.

• 4. The father’s genotype is gg.



g

g

• When an organism has two different alleles for a trait, it is called heterozygous.

• 3. In this case, the genotype Gg is said to be heterozygous.



g

g

• When an organism has two of the same allele, it is called homozygous.

• 4. In this case, the genotype gg is homozygous recessive.



g

g

• A phenotype is the observable expression of a trait.

• The genotype controls the phenotype.

• 5. The mother’s phenotype is green peas.

• 6. The father’s phenotype is yellow peas.



g Gg

g

• Each offspring receives one allele from the mother and one allele from the father.

• When using a Punnett Square NEVER move gametes in diagonal lines. Always move gametes vertically or horizontally.



g Gg

g

• The first offspring received a dominant allele from mom and a recessive allele from dad.

• This offspring’s genotype is Gg; therefore, its phenotype is green colored peas.



g Gg gg

g

• The second offspring received a recessive allele from mom and a recessive allele from dad.

• This offspring’s genotype is gg; therefore, its phenotype is yellow colored peas.



g Gg gg

g Gg

• The third offspring received a dominant allele from mom and a recessive allele from dad.

• This offspring’s genotype is Gg; therefore, its phenotype is green colored peas.



g Gg gg

g Gg gg

• The fourth offspring received a recessive allele from mom and a recessive allele from dad.

• This offspring’s genotype is gg; therefore, its phenotype is yellow colored peas.



g Gg gg

g Gg gg

7.


Figuring Probability

8. The possible genotypes for the offspring in this cross are:

Gg = heterozygous

gg = homozygous recessive



G g

g Gg gg

g Gg gg

• There is a 50%, or one in two, chance that the mother will pass on the first allele G.

• There is a 50%, or one in two, chance that the mother will pass on the second allele g.

G 1/2

g1/2

g1/2

Gg gg

g1/2

Gg gg ©Copyright 2014 by www.cpalms.org


G g

g Gg gg

g Gg gg

• There is a 50%, or one in two chance, that the father will pass on the first allele g.

• There is a 50%, or one in two chance, that the father will pass on the second allele g.

G 1/2

g1/2

g1/2

Gg gg

g1/2

Gg gg ©Copyright 2014 by www.cpalms.org

Figuring Probability G

1/2g

1/2

g1/2

Gg1/4

gg1/4

g1/2

Gg1/4

gg1/4

• This means that each offspring has a 25%, or one in four, chance to inherit a particular allele combination from mom and dad.

• ½ x ½ = ¼ ©Copyright 2014 by www.cpalms.org


1/2g

1/2

g1/2

Gg1/4

gg1/4

g1/2

Gg1/4

gg1/4

• 9. The probability that the offspring will inherit the genotype Gg is 50%, or two in four, chance.

• ¼ + ¼ = ½ ©Copyright 2014 by www.cpalms.org


1/2g

1/2

g1/2

Gg1/4

gg1/4

g1/2

Gg1/4

gg1/4

• 10. The probability that the offspring will inherit the genotype gg is 50%, or two in four, chance.



11. The possible phenotypes for the offspring in this cross are:• Green Peas =

(controlled by the genotype Gg)

• Yellow Peas = (controlled by the genotyhpe gg)©Copyright 2014 by

www.cpalms.org


1/2g

1/2

g1/2

Gg1/4

gg1/4

g1/2

Gg1/4

gg1/4

• 12. The probability that the offspring will inherit the phenotype of green pea color is 50%, or two in four, chance.

• ¼ + ¼ = ½



1/2g

1/2

g1/2

Gg1/4

gg1/4

g1/2

Gg1/4

gg1/4

• 13. The probability that the offspring will inherit the phenotype of yellow pea color is 50%, or two in four, chance.


Ratios G 1/2

g1/2

g1/2

Gg1/4

gg1/4

g1/2

Gg1/4

gg1/4

• The ratio of genotypes is shown like this (Gg : gg)

• 14. So, the genotype ratio would be 2 : 2, which is reduced to 1 : 1.


Ratios G 1/2

g1/2

g1/2

Gg1/4

gg1/4

g1/2

Gg1/4

gg1/4

• The ratio of phenotypes is shown like this (Green : yellow)

• 15. So, the phenotype ratio would be also 2 : 2, which is reduced to 1 : 1.


Exit Ticket

1. How do you use a Punnett Square?


Guided Practice Problem #1

Tongue rolling is a genetic trait. The ability to roll the tongue is expressed by a dominant trait. Suppose you have a friend whose parents are heterozygous for rolling their tongues. Answer the following questions about the parents and possible offspring.


1. What is the trait being studied in this scenario?

The ability to roll the tongue is the trait being studied in this scenario.


2. Indicate the dominant and recessive alleles and assign an appropriate letter to each allele:

A. The dominant allele is a rolling tongue. We will use the letter R.

B. The recessive allele is not a rolling tongue. We will use the letter r.


3. What is the genotype of Parent #1 (mother)?

Genotype shown as letters: Rr

Scientific term for this genotype: Heterozygous


4. What is the genotype of Parent #2 (father)?

Genotype shown as letters: Rr



5. What is the phenotype of Parent #1 (mother)?

The phenotype of the mother is a rolling tongue.


6. What is the phenotype of Parent #2 (father)?

The phenotype of the father is a rolling tongue.


7. Draw a Punnett Square to represent the offspring the parents above are likely to have. R r

R RR Rr

r Rr rr©Copyright 2014 by www.cpalms.org

8. What are the possible genotypes of the offspring?

The possible genotypes of the offspring are:RR = Homozygous dominantRr = Heterozygousrr = Homozygous recessive


9. What are the possible phenotypes of the offspring?

The possible phenotypes of the offspring are:Rolling Tongue (controlled by the genotype

RR or Rr)

No Rolling Tongue (controlled by the genotype rr)


10. What is the probability that the offspring will have a homozygous dominant genotype?

The probability of having an offspring with a homozygous dominant genotype is ¼ or 25%.


11. What is the probability that the offspring will have a heterozygous genotype?

The probability of an offspring inheriting a heterozygous genotype is ½ or 50%.


12. What is the probability that the offspring will have a homozygous recessive genotype?

The probability that an offspring will have a homozygous recessive genotype will be ¼ or 25%.


13. What is the genotype ratio for the possible offspring?

The genotype ratio set-up is (RR : Rr : rr).

The genotype ratio would be 1 : 2 : 1.


14. What is the probability that the offspring will have a phenotype of a rolling tongue?

The probability of having an offspring with a rolling tongue phenotype is ¾ or 75%.


15. What is the probability that the offspring will have a phenotype of a non-rolling tongue?

The probability of an offspring inheriting a non-rolling tongue phenotype is ¼ or 25%.


16. What is the phenotype ratio for the possible offspring?

The phenotype ratio set-up is

(rolling tongue : non-rolling tongue).

The phenotype ratio would be 3 : 1.


17. What is the probability that your friend will be able to roll his/her tongue?

There is a ¾ or 75% chance that my friend will be able to roll his/her tongue.



Complete the following questions if you bought a pair of pea plants on sale at the local nursery. The clerk at the store told you that the tall stemmed pea plant’s genotype is homozygous dominant. The other plant’s genotype is homozygous recessive and has a short stems.



The height of the plant (stem length) is the trait being studied in this scenario.


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2. Indicate the dominant and recessive genes and assign an appropriate letter to each allele:

A. The dominant gene is a tall height. We will use the letter T.

B. The recessive allele is a short height. We will use the letter t.



Genotype shown as letters: TT

Scientific term for this genotype: Homozygous dominant



Genotype shown as letters: tt

Scientific term for this genotype: Homozygous recessive



The phenotype of the mother is a tall height.



The phenotype of the father is a short height.


7. Draw a Punnett Square to represent the offspring the parents above are likely to have. T T

t Tt Tt

t Tt Tt©Copyright 2014 by www.cpalms.org


The only possible genotype of the offspring is:Tt = Heterozygous



The possible phenotypes of the offspring are:Tall Height (controlled by the genotype Rr)



The probability of having an offspring with a homozygous dominant genotype is 0/4 or 0%.



The probability of an offspring inheriting a heterozygous genotype is 4/4 or 100%.



The probability that an offspring will have a homozygous recessive genotype will be 0/4 or 0%.



The genotype ratio set-up is (Tt).

The genotype ratio would be 1 for the heterozygous genotype.In other words, all of them.


14. What is the probability that the offspring will have a phenotype of a tall height?

The probability of having an offspring with a tall height phenotype is 4/4 or 100%.


15. What is the probability that the offspring will have a phenotype of a short height?

The probability of an offspring inheriting a short height phenotype is 0/4 or 0%.




(tall height : short height).

The phenotype ratio would be 1 for tall height.In other words, all of them.


17. If you wanted to produce more short pea plants rather than tall pea plants, would you be able to with these two plants? Explain why on the back of this sheet of paper.Answer 1—If you answered no.

I would not be able to produce short pea plants with this pair of plants because one parent is homozygous dominant and can only pass on the dominant get to their offspring. This means that every offspring produced will have at least one dominant allele. So, all offspring will by tall.


17. If you wanted to produce more short pea plants rather than tall pea plants, would you be able to with these two plants? Explain why on the back of this sheet of paper.Answer 2—If you answered yes.

I would not be able to produce short pea plants in this generation; however, if I crossed two of the heterozygous offspring, I would be able to produce some short pea plants. If two heterozygous tall pea plants crossed about 1 out of every 4 plants will be short.


Exit Ticket

1. What kinds of information can you find while using a Punnett Square?



A white flowered plant is crossed with a plant that is heterozygous for purple flowers. Purple flowered plants have at least one dominant allele, while white flowered plants have no dominant alleles. Answer the following questions for this cross.



Flower color is the trait being studied in this scenario.


2. Indicate the dominant and recessive genes and assign an appropriate letter to each allele:

A. The dominant gene is the color purple. We will use the letter P.

B. The recessive allele is the color white. We will use the letter p.



Genotype shown as letters: pp

Scientific term for this genotype: Homozygous recessive



Genotype shown as letters: Pp




The phenotype of the mother is white flowers.



The phenotype of the father is purple flowers.


7. Draw a Punnett Square to represent the offspring the parents above are likely to have. p p

P Pp Pp p pp pp



The possible genotypes of the offspring are:Pp = Heterozygouspp = Homozygous recessive



The possible phenotypes of the offspring are:Purple Flowers (controlled by the genotype

Pp)

While Flowers (controlled by the genotype pp)



The probability of having an offspring with a homozygous dominant genotype is 0/4 or 0%.



The probability of an offspring inheriting a heterozygous genotype is ½ or 50%.



The probability that an offspring will have a homozygous recessive genotype will be ½ or 50%.



The genotype ratio set-up is (Pp : pp).

The genotype ratio would be 2 : 2.Reduce the ratio to 1 : 1.


14. What is the probability that the offspring will have a phenotype of purple flowers?

The probability of having an offspring with a purple flower phenotype is ½ or 50%.


15. What is the probability that the offspring will have a phenotype of white flowers?

The probability of an offspring inheriting a non-rolling tongue phenotype is ½ or 50%.




(purple flowers : white flowers).

The phenotype ratio would be 2 : 2.Reduce the ratio to 1 : 1.


17. You want to have a pea plant garden with about fifty percent white and fifty percent purple flowers. Which offspring would you cross to ensure that you would keep producing about half of each flower color every generation?

Create more Punnett Squares to help you answer this question.

p p

P Pp Pp

p pp pp

P p

P PP Pp

p Pp pp

p p

p pp pp

p pp pp


17. You want to have a pea plant garden with about fifty percent white and fifty percent purple flowers. Which offspring would you cross to ensure that you would keep producing about half of each flower color every generation? Create more Punnett Squares to help you answer this

question.

p p

P Pp Pp

p pp pp

P p

P PP Pp

p Pp pp

p p

p pp pp

p pp pp


17. You want to have a pea plant garden with about fifty percent white and fifty percent purple flowers. Which offspring would you cross to ensure that you would keep producing about half of each flower color every generation?

Explain. I would cross a heterozygous offspring with a homozygous

recessive offspring to make sure that I had half purple and half white flowers. The first cross (Pp x Pp) would create too many purple flowers and the third cross (pp x pp) would create nothing but white flowers. The second cross (Pp x pp) would create the right amount because only half of the offspring would most likely inherit the dominant allele from one parent. ©Copyright 2014 by

www.cpalms.org

Wrap Up

What are some advantages of using a Punnett Square mathematical model?

What are some disadvantages of using a Punnett Square mathematical model?


Advantages and Disadvantages of a Punnett Square Advantages

Punnett squares are a mathematical model of what could happen in the real world.

Punnett squares show you the possible genetic combinations for all offspring.

You can calculate the probability of

Genotypes

Phenotypes

Genotype ratios

Phenotype ratios

Disadvantages Punnett Squares will only predict what could happen, NOT what will happen

exactly in the real world.©Copyright 2014 by www.cpalms.org

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