Monohybrid Cross

§ Crossing two organisms in order to compare one trait.

What is a PUNNETT SQUARE?

•  A tool to predict the probability of certain traits in offspring that shows the different ways alleles can combine

•  A chart that shows all the possible

combinations of alleles that can result when certain genes are crossed

What is a PUNNETT SQUARE?

•  Letters stand for dominant and recessive alleles

•  An uppercase letter stands for a

dominant allele •  Lowercase letters stand for recessive

alleles

Using a PUNNETT SQUARE

To set up a Punnett square, draw a large square, and then divide it into 4 equal sections (also squares). It should look something like this:

Using a PUNNETT SQUARE

Now you need two parents to cross, you must know the genotype of each parent

For example, a red flower (genotype Rr)

and a white flower (genotype rr). Rr x rr

Using a PUNNETT SQUARE

Place one of the parents on top, and one on the left. You should get a something similar to this:

Using a PUNNETT SQUARE Finally, take each letter in each column and

combine it with each letter from each row in the corresponding square. You should now have a picture close to this:

3.

Using a PUNNETT SQUARE

•  The possible genotype combinations of the offspring are:

Rr, Rr, rr, and rr •  We now know that the probability of the

offspring having a red phenotype is 2/4 or 50% or a white phenotype 2/4 or 50%.

Punnett  Square  Practice    

The parents in this cross are _____________ Homozygous Heterozygous

Heterozygous

If G is dominant for green pods and g is recessive for yellow pods, what percentage of the offspring will have green pods? _______% 75

GG Gg

Gg gg

G g G g

4.

B b B b

The genotype of the offspring in the red box is _______

bb

The offspring in the red box is homozygous heterozygous

homozygous recessive

5.

B b Bb

In guinea pigs black fur (B) is dominant over brown fur (b). The Punnett square for a cross between a HETEROZYGOUS black guinea pig and a PURE brown guinea pig would look like:

This is the Correct setup Heterozygous = Bb Pure brown = bb

B B b b

B b b b

B b B B

Another name for pure is _________________ homozygous

6.

What are the possible phenotypes of their offspring? (% and color)

Red throats (R) are dominant over white (r) throats in Goonie birds. Make a cross between a PURE RECESSIVE and a HETEROZYGOUS Gooney Bird.

50% red throats 50% white throats

Rr rr Rr rr

R r

r

r

7.

What are the possible phenotypes of their offspring? (% and color)

Black eyes (B) are dominant over red eyes (b) in rats. Make a cross between two HETEROZYGOUS rats.

75 % black eyes 25% red eyes

B b B b

BB Bb

Bb bb

8.

Which of the following genotypes could be from a HETEROZYGOUS organism? TT Rr Ww bb XX Ss

Rr Ww Ss

What kind of cross is this? Pure dominant X pure recessive Heterozygous X heterozygous Heterozygous X homozygous Hybrid recessive X hybrid pure Heterozygous X Homozygous

Another name for heterozygous is _______________ hybrid

9.

What is the probability the offspring will have straight tails?

In Pigs curly tails (T) are dominant over straight tails (t). Make a cross between a HOMOZYGOUS DOMINANT and a HOMOZYGOUS RECESSIVE Pig.

0% All will be Curly tailed (Tt)

Tt Tt Tt Tt

T T

t

t

10.

Which of the following genotypes could be from a HOMOZYGOUS RECESSIVE organism? TT bb Rr Ww tt XX aa

bb tt aa

A a A a

The genotype of the offspring in the red box is _______ homozygous heterozygous

heterozygous

What percentage of the offspring from this cross will show the dominant characteristic?

¾ (75%)

11.

G g G GG Gg

g gg

G is dominant for green pea pods. g is recessive for yellow pea pods. What is the genotype of the offspring in the blue box?

Gg

What is the phenotype of the offspring in the blue box?

Green pea pods

12.

What will the offspring be like? (% and color)

Purple (P)is the dominant color for monsters. Yellow is recessive (p). Make a cross between a HOMOZYGOUS purple and a yellow monster.

100% purple

Pp Pp Pp Pp

P P p p

13.

What genotype would a PURPLE dad have to be to make a yellow monster baby with a Purple (Pp) mom monster? – Remember Purple is DOMINANT

P p ? ?

Baby is pp

Dad is Pp Dad needs to have: a little p to give one to the baby a big P to be purple himself

Which of the following is NOT TRUE?

Genotype determines phenotype Alleles are different forms of the same gene. Genotype is the way the genes make you look. Organisms with different genotypes

can have the same phenotype.

T

T

F

T Tt and TT both look tall