GENETICS REVIEW PRACTICE

You can do it!!

Where do you need to start?

• Punnett square basics• Dominant-recessive practice problems• F1 and F2 problems• Incomplete dominance problems• Sex-linked problems• More practice on-line

I’m ready for the test!!

Punnett square basics1. Determine the genotypes of the parent organisms

Ex: cross two heterozygous red flowers. White is the recessive phenotype

2. Write down your "cross" (mating) Ex: Rr x Rr

3. Draw a punnett-square

4. “Split" the letters of the genotype for each parent & put them "outside" the p-square R r R r

Punnett square basics (cont.)5. Determine the possible genotypes of the offspring by filling in the p-square

R r R 25% homozygous red (RR) 50% heterozygous red (Rr) r 25% white (rr) Now you try one.Problem: If red flowers are dominant over white flowers. Which punnett square shows the cross between a homozygous red flower and a white flower? R R R r r r

r r

100% heterozygous red 50% heterozygous red 50 % white

RR Rr

Rr rr

Rr Rr

Rr Rr

Rr rr

Rr rr

Awesome!!

• What else do you need to practice?

F1 and F2 problems• If brown is dominant over white feathers in

chickens, what will the F1 and F2 ratios be when a homozygous brown chicken is mated with a white chicken?– F1 will be 100% homozygous white and F2 will be

50% heterozygous brown and 50% white– F1 will be 100% homozygous brown and F2 will be

75% brown and 25% white– All offspring in both generations will be brown

That’s not the one…Let’s look at the punnett squares:

When a homozygous brown chicken is mated with a white chicken:

F1 B B F2 B b b B

b b

100% heterozygous brown 75% brown 25% white

Bb Bb

Bb Bb

BB Bb

Bb bb

Let’s try another F1 and F2 problem

• If green is dominant over tan skin in frogs, what will the F1 and F2 ratios be when a homozygous green frog is mated with a tan frog?– All offspring in both generations will be green.– F1 will be 100% green and F2 will be 75% green an

d 25% tan– F1 will be heterozygous green and F2 will be all ta

n

Incomplete Dominance

• In carnations, red and white colors show incomplete dominance. The heterozygous form is pink. What are the expected phenotypes in a cross between two pink carnations?– 100% will be pink– 50% will be red and 50% will be white– 25% will be red, 25% will be white and 50% will be pink

That’s not the one…• Let’s look at the square. The genotype for pink

is Rr so the square is R r R r

25% homozygous red50% heterozygous pink25% homozygous white

Try another one

RR Rr

Rr rr

Incomplete Dominance

• In horses, brown and white colors show incomplete dominance. The heterozygous form is tan. What are the expected phenotypes in a cross between a brown and a tan horse?– All offspring will be tan– All offspring will be brown– 50% will be tan and 50% will be brown

Sex-linked genetics

• In humans, color blindness is a recessive, sex-linked condition. If a colorblind man marries a woman who is a carrier, what are the chances that their children will be colorblind?– All of the boys will be colorblind but none of the girls– 50% chance for any child– 50% chance for boys and 25% for girls

That’s not the one…• Let’s look at the square. • Dad’s genotype is Xr Y and mom’s is XR Xr , so

the punnett square looks like this• Xr Y• XR This girl and boy are NOT affected

• Xr This girl and boy ARE colorblind

• So there is a 50% chance that any child will be color blind regardless of gender.

XR Xr XRY Xr Xr Xr Y

Sex-linked genetics

• In humans, hemophilia is a recessive, sex-linked condition. If a man who does not have hemophilia marries a woman who has the disease, what are the chances that their children will have hemophilia?– 50% chance for any child– 100% for the boys, 0% for the girls– 100% for the girls, 50% for the boys

More practice problems available• Check out these websites for more genetics

practice problems:

http://www.hobart.k12.in.us/jkousen/Biology/psquprac.htm

http://biology.clc.uc.edu/courses/bio105/geneprob.htm

In your textbook, chapter 10 and 11 have more informationhttp://jgadow.wikispaces.com/Academic+Biology+Home