Today is Monday,December 9th, 2013

Pre-ClassPredict the possible genotypes and phenotypes from the following

cross:AA x aa

In this situation, A is dominant for normal skin, while a is recessive for albinism (white skin/hair).

Other stuff: Turn in your Making Babies Activity.

In This Lesson:Punnett Squares and Monohybrid

Crosses(Lesson 2 of 6)

Today’s Agenda

• Further explore the idea of dominant/recessive alleles.

• Learn how to do problems like the pre-class MUCH faster.

• RapidTriviaKinda

• Where is this found in my book?– Academic: P. 267 and following…– Honors: P. 156 and following…

First, Mouse Genetics

• ExploreLearning has a great activity that clearly illustrates how dominant and recessive genes interact.

• In the activity, you’ll be pairing different-colored mice and seeing how the offspring turn out.

• You can pair two purebred starter individuals, or you can put the offspring of one cross into holding cages and breed them later.

• There is an accompanying Quia quiz. Also complete the questions right under the gizmo.

• [Log-in Instructions]

Back to Mendel• Mendel established two true-breeding

populations of pea plants (the parents).– True breeding means they only produce their own

phenotypes (homozygous genotypes).– The population with purple flowers produced only

purple flowers, the population with white flowers produced only white flowers.

• When Mendel crossed a purple parent flower with a white parent flower, he only got purple offspring plants (the first generation).– Which allele is dominant?

But then…

• Mendel let the first generation plants self-pollinate to form the SECOND GENERATION.

• Now here’s the drama: Out of 929 total second generation plants (he had a lot of free time, remember?), 705 were purple, and 224 were white!!!!!11

In other words…

• Parents:– All purple and all white (true-breeding/homozygous).

• First Generation:– Offspring of parents ↑ is all purple.

• Second Generation:– Offspring of first generation ↑ is mostly purple, some

white.• What gives?

Look Closer at the Numbers

# of plants % of total

Purple 705 75.9%

White 224 24.1%

Total 929

So what’s the ratio, then?

• The ratio of purple flowers to white flowers is 3:1• This number is so cool I

wasted a whole slide for it.

Why a genetic ratio is important…

• For Mendel, 3:1 was just a cool number. He used it to formulate four hypotheses that were further developed by later scientists:– For each trait, each individual has two copies of

the gene.– There is more than one allele for these genes.– When two alleles are put together, one may be

expressed, while the other may not have any visible impact on the organism’s phenotype.

– Since gametes only have one set of chromosomes, they only have one allele each. Therefore, each gamete contributes one allele during fertilization.

Back to Mendel

• As it turned out, doing that sort of cross Mendel did (in the same order) will always produce roughly the same numbers of purple/white plants, even with different total numbers of plants.

• Scientists wanted a way to predict these numbers, and to do it easily.– Their solution?

The Punnett Square

• Meet Reginald Punnett:– British Biologist– June 20, 1875 - January 3,

1967– Champion foosball player*– Created the Punnett

Square, amongst other cool discoveries.

http://www.nndb.com/people/167/000100864/reginald-punnett-1.jpg

Reginald Punnett

*Probably not true

The Punnett Square

• The Punnett Square is a tool for:–Determining which possible genotypes and

phenotypes a cross can produce.–Determining the probability a certain

genotype or phenotype will occur.

The Punnett Square

• The Punnett Square cannot:–Predict the order of offspring and their

traits.–Predict exact numbers (remember, the pea

plants weren’t exactly 75/25).–Account for mutations.

How The Square Works

• Mendel, monk that he was, didn’t know about alleles. We do, so let’s use them.

• Let’s set our allele letters [write this down]– F = Purple Flowers– f = White Flowers– I’m using F instead of P because it’s easier to tell

lower/upper case Fs apart.

How The Square Works

• Mendel’s “parent generation” was true-breeding, so they were probably homozygous.

• If that’s the case, let’s cross two homozygous individuals: one purple, one white.– FF x ff– By the way, that ___ x ___ notation is the way to write a

cross. I’m going to do that a lot.• One last thing: this cross is a monohybrid cross.– Monohybrid crosses are those that analyze only one trait.– We’re not looking at flower color and height – that would

be different.

FF x ff• To analyze this cross with a Punnett Square, first

draw a box and divide it in fourths.• Then, put one parent’s genotype across the top and

one across the side (doesn’t matter which).

f

f

F F

FF x ff• F = Dominant

allele, purple flowers

• f = Recessive allele, white flowers

F F

f

f

f

f

f

f

F FF F

• Put arrows in like so.• Now, “drag” each allele in the

direction of the arrows.• What color are the offspring?

↓ ↓

→

→

Purp

le

Purp

le

Purp

le

Purp

le

Mendel’s Parent Generation

• Remember that Mendel then took two of his resulting plants from the first generation and crossed them to make his second generation generation.– In other words, he took two heterozygous

individuals.• Now we will take two heterozygous individuals

from the first generation and cross them.

Ff x Ff• What color are the possible

offspring?

F f

F

f

F

f

F

f

F fF f↓ ↓

→

→

• F = Dominant allele, purple flowers

• f = Recessive allele, white flowers

Purp

le

Purp

le

Purp

le

Whi

te

Statistics

• Percent Chance of Purple Flowers– 75%

• Percent Chance of White Flowers– 25%

• Phenotype Ratio– Dominant : Recessive– 3:1

• Genotype Ratio– Hom. Dom. : Heterozygous : Hom. Rec.– 1:2:1

One BIG caveat…

• Remember 3:1? That magic ratio Mendel found?• The 3:1 phenotype ratio only happens when you

cross two heterozygous individuals for a trait that exhibits complete dominance.

• Try crossing Ff x ff. What’s the phenotype ratio?– 2:2 (50% Purple, 50% White)

• What about FF x ff? What’s the phenotype ratio?– 4:0 (100% Purple)

F = Purple; f = White• Using our example cross, solve this problem in

your notebook:• I’m a plant with purple flowers (heterozygous) and

my plant girlfriend (she’s got white flowers) have had 3 purple-flowered babies.

• What is the probability our fourth baby will have white flowers?– Answer: 50%.

• Remember, Punnett Squares only lead you to probability, not the order of children or any kind of guarantee.

F = Purple; f = White• Here’s another example (same genotypes):– Whew! It’s now the next season and my new plant

girlfriend and I (plants aren’t monogamous) have had a busy spring. We’ve now had 25 plant babies, all with purple flowers!

– What is the likelihood our 26th will have white flowers?– Answer: 50%.

• No matter how many offspring, as long as the parents have the same genotypes, this number will not change.

Punnett Squares Backward?

• How about we try a backwards Punnett Square problem?

• Let’s imagine two parents. One parent has brown eyes, one parent has blue eyes.– B = Brown, b = Blue

• They have a child with blue eyes.• What are the parents’ genotypes?– Bb and bb

Hey wait a second…

• Remember our guiding questions?• Suggested by a former student:– “My Dad has blue eyes. My Mom has brown

eyes. My siblings and I all have brown eyes. How did this happen?”• Dad: bb; Mom: BB or Bb (likely BB)

• Suggested by my former teacher:– “My husband and I both have brown eyes. Our

children have blue eyes. How?”• Husband: Bb; Wife: Bb

Last note…

• Notice how some people could have the allele for a certain trait but not show it?– That’s called being a carrier.

Closure and Practice

• Your assignment is to practice monohybrid crossing. I will be grading these problems closely.– Genetics Worksheets packet page 99, #19-21

ONLY.