Lesson 18 Using Punnett Squares and Pedigrees

Bb x Bbmale / sperm

fem

ale

/ eg

gs

X

BB

Bb bb

BbB

b

B b

Remember this?

A Punnett Square is a diagram used to identify possible combinations resulting from a mating.

This married couple is considering having their first baby.

The man is heterozygous for a disease. The woman is also heterozygous for the disease.

The couple comes to you for counseling. They want you to know the chances they will have a healthy baby.

You know that the disease they carry is a recessive trait and that both parents are heterozygous.

RR Rr rrWhich of the above genotypes will you need to use?

Man x Woman

Rr RrRrR r

RR

r

r

RR rR

Rr rr?

Man x Woman

Rr RrR r

R

r

RR rR

Rr rr

Genotype %RR =Rr =rr =

25%50%25%

Phenotype %Healthy =Carrier =Diseased =

25%50%25%

The chance that the couple will have a baby that has the disease is 25%.

Let’s Practice!

HUNTINGTON’S DISEASE: dominantly inherited disease

Man x Woman

Rr rr

R r

r

r

Rr r r

Rr rr

A man has Huntington’s disease. He does not know this because Huntington’s doesn’t show symptoms until later in life, after children have been produced.

His wife does not have Huntington’s and knows that it has never been diagnosed in her family pedigree.

What are the chances their first born child will have the disease?

Dihybrid Crosses = mating of two individuals involving two traits.

Heterozygous for both traits:

AaBb x AaBbAB Ab aB ab

AB

Ab

aB

ab

A = talla = shortB = brown hairb = blond hair

Tall/Brown

Tall/blond

Short/Brown

Short/blond 9:3:3:1

Testcrossunknown x aa

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R? rr

‘ello, my name is

Reginald.I am a

magnificent red canary.

I want to make ‘de beautiful red beebies like me.

But I do not know my genetics. How can I know if my beebies

will be red?

Hi there handsome! My name’s

Gloria and I’m a purely recessive

white gal.

Maybe I can help you with

a testcross!

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R? rr

‘Dees TESTCROSS…What is it?

Hmm… I think this TESTCROSS is a good

idea.

It’s when you cross an

unknown genotype with a

homozygous recessive.

If you get any homozygous recessive babies, you know you carry the

recessive gene!

TESTCROSS = a cross between any gene pair and the homozygous recessive condition for that trait.

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R? rr

‘Eeet worked!

Now I know that I carry

the rare white gene!

Congratulations handsome!

You’re a carrier!

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I’m the jailbird.

Rr rrThe twinsRr and RrRr Rr

Later that year the babies were born…

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R?Rr

What would be the genotype needed to testcross a heterozygous purple-flowering pea plant?

a. PPb. Ppc. pp

Pedigree = family record for one specific trait.

Sex-linked traits = genes for these traits are carried on the sex chromosomes.

The gene for Red-Green colorblindness is carried on the X chromosome.

Female must get two (XX) X’s to have colorblindness.

Male must get one (XY) X to have colorblindness.

Why are most sex-linked traits carried on the X chromosome?

Analyzing a Pedigree Interactivehttp://highered.mcgraw-hill.com/sites/0072485949/student_view0/chapter3/interactive_activity.html

Reading a Pedigree

= male carrier = female carrier

= male without trait

= male with trait

= female without trait

= female with trait

= two parents

= three children

I

II

Remember this?

Another view of a pedigree…

This one is for Huntington’s Disease

Let’s Practice!

These people all have Down’s Syndrome

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Any Questions?

Assignment:Coach Book L18Textbook pg 201 Read and Summarize