T h e Le s s o nPunnett Squares Practice Problems

Genetics Jeopardy Review Game Quiz

Unit Lesson: Genetics

Genetics: Using Punnett Squares

Created by: Mark Dignan

1. Students should become familiar with using a Punnett Square to determine offspring genotypes and phenotypes.

2. Students should understand the differences between genotypes and phenotypes

3. Students should be able to calculate percents of different genotype combinations from the use of a Punnett Square.

4. Students should recognize the terms of dominance and recessive as they are concerned with the use of a Punnett Square

Objectives

5. Students should understand the difference between haploid and diploid number when determining the number of chromosomes within a cell.

Objectives

Early Genetics

• The study of genetics began with

observations made by Gregor Mendel.

• After noticing that the flowers of his pea plants were either violet or white, Mendel began to study the segregation of heritable traits.

Between 1856 and 1863 he cultivated and tested at least 28,000 pea plants.

Remember that Mendel worked almost 150 years ago when nobody knew about genes or even the structures (chromosomes) that carry genes.

Here are some traits observed by Mendel:

Lets consider a single gene…• A gene carries

information that determines your traits.

Traits are characteristics you inherit from your parents.

• Genes are located in chromosomes.

• Chromosomes come in pairs and there are thousands of genes in one chromosome.

Continued…• In humans, a cell’s

nucleus contains 46 individualchromosomes or 23 pairs of chromosomes.

• Half of the chromosomes come from one parent and half come from the other parent.

This is a human karyotype representing the 23 pairs of chromosomes in a male Here is the

detailed structure of a chromosome

Definitions• Allele – The different forms of a trait that

make up a gene pair within a chromosome.• Genotype - the genes of an organism for

one specific trait • Phenotype - the physical appearance of a

trait in an organism• Diploid Number - Refers to the pairs of

similar chromosomes within a cell• Haploid Number - The number referring to

half the number of chromosomes within a cell; sex cells do not have pairs of chromosomes, and are only cells that have haploid number

Definitions

• Dominant trait - Refers to a genetic feature that “hides” the recessive trait in the phenotype of an individual.

• Recessive trait - Describes a trait that is covered over (or dominated) by another form of that trait and seems to disappear.

• Homozygous - two alleles that are the same for a trait (Pure); ex: TT or tt or BB

• Heterozygous - two different alleles for a trait (Hybrid); ex: Tt or Bb or Cc

Practice• We use two letters to represent the

genotype. A capital letter represents the dominant form of a gene (allele) and a lowercase letter is the abbreviation for the recessive form of the gene (allele).

• Example below: P=dominant purple and p= recessive white

The phenotype for this flower is violet while its genotype (if homozygous) is PP.

The phenotype for this flower is white while its genotype is pp (to be white the flower must have two of the recessive copies of the allele).

Punnett Squares The Punnett square

is the standard way of working out what the possible offspring of two parents will be. – It is a helpful tool to

show allelic combinations and predict offspring ratios.

Before we go further lets discuss how

to set up a Punnett Square…

We begin by constructing a grid of two perpendicular lines.

Next, put the genotype of one parent across the top and the other along the left side.

For this example lets consider a genotype of BB crossed with bb.

It would be written as: BB x bb

B B

b

b

• Notice only one letter goes above each box

• It does not matter which parent’s genotype goes on either side.

Next, fill in the boxes by copying the column and row head-letters down and across into

the empty spaces.

B B

b B

B

B

Bb

b

b

b

b

Punnett Squares

Now that we have learned the basics of genetics lets walk through some examples using Punnett Squares.

Lets say:

W- dominant white

w- recessive violet

W wW

Parents in this cross are heterozygous (Ww).

Note: Make sure I can tell your capital letters from lowercase letters.

What percentage of the offspring will have violet flowers?

ANSWER: 25% (homozygous recessive)

Usually write the capital letter first

w

W WW w

W w w w

Red hair (R) is dominant over blond hair (r). Make a cross between a heterozygous red head and a blond.

R r

r R

R

r

rr

r

r

r

r

What percentage of the offspring will have red hair?

R r

r R

R

r

rr

r

r

r

r

50%

Let’s try some more…

In pea plants, tall pea plants (T) are dominant

over short pea plants (t). Construct a Punnett

Square for a heterozygous tall pea plant and a short pea plant.

Tt tt

Tt tt

T t

t

t

What are the percentage of phenotypes?50% tall

50% short

What are the genotype frequencies?50% Tt

50% tt

Let’s try one more…

Black eyes (R) are dominant over red eyes (r) in rats. Make a cross between a homozygous rat with black eyes and a rat with red eyes.

Rr Rr

Rr Rr

R R

r

r

What are the percentage of phenotypes?100% black

0% red

What are the genotype frequencies?100% Rr

What is the possibility of a red eye off springs?

0%

References http://www.athro.com/evo/gen/punnett.html http://www.kidshealth.org/kid/talk/qa/what_is_gene.html http://brookings.k12.sd.us/biology/ch%2011%20genetics/

punnettpractice.ppt#1 http://www.usoe.k12.ut.us/CURR/Science/sciber00/7th/gene

tics/sciber/punnett.htm http://www.biotechnologyonline.gov.au/images/

contentpages/karyotype.jpg

Punnet

t

Square

Prob

lems

PROBLEM # 1One dog carries heterozygous, black-fur traits (Bb), and its mate carries homogenous, blonde-fur traits (bb). Determine the probability of one their puppies having black fur.The problem is written as Bb x bb.

B b

b

b

B

B

b

b

b

b

b

b

PROBLEM # 1 B b

b

b

B

B

b

b

b

b

b

b

What are the genotype frequencies?50% Bb

50% bb

What are the percentage of phenotypes?50% black fur

50% blonde fur

What is the genotype ratio?

1:2

Code:

B = Black fur(Dominant)

b = Blonde fur(Recessive)

# of ways to get black fur

# of possible outcomes

PROBLEM #2In peas, the color yellow (Y) is dominant to the color green (y). What is the probability of an offspring being yellow if the parents are Yy x Yy.

Y y

Y

y

Y

Y

y

y

Y

y

Y

y

PROBLEM # 2 Y y

Y

y

Y

Y

Y

y

Y

y

y

y

What are the genotype frequencies?25% YY

50% Yy

25% yy

What are the percentage of phenotypes?25% Yellow Pure

50% Yellow Hybrid

25% green pure

What is the yellow genotype ratio?

3:1

Code:

Y = Yellow(Dominant)

y = green(Recessive)