probability & genetic crosses. probability if you roll one die, what’s the probability of...

Probability & Genetic Crosses

ProbabilityIf you roll one die, If you roll one die, what’s the what’s the probability of probability of rolling a 6?rolling a 6?

If you pull one card If you pull one card from the deck, what’s from the deck, what’s the probability of the probability of getting the ace of getting the ace of hearts?hearts? If you flip a coin, If you flip a coin,

what’s the what’s the probability of probability of landing on heads?landing on heads?

1/6

(1 out of 6)

1/52

(1 out of 52)

1/2

(1 out of 2)

A Genetic Cross

Gregor Mendel (1865) discovered that probability could be used to predict the results of a genetic cross (mating)

If you know parent genotypes, you can predict offspring genotypes

A Genetic Cross If you know parent genotypes, you can

predict offspring genotypes

Example: Brown hair (B) is

dominant to blond hair (b). What are the possible

genotypes for their offspring?

bb Bb

♀ Bb x ♂ bb

Bb, Bb, bb, bb

A Genetic Cross If you know parent genotypes, you can

predict offspring genotypes

Example: Brown hair (B) is

dominant to blond hair (b). What is the probability that

the Incredibles’ next child will have brown hair?

bb Bb

♀ Bb x ♂ bb

Bb, Bb, bb, bb

2/4 (50%) chance of brown hair

Practice In pea plants, smooth peas (S) are

dominant to wrinkled peas (s). A homozygous smooth pea is

crossed with a wrinkled pea.

♀ _____ x ♂ _____

Practice In pea plants, smooth peas (S) are

dominant to wrinkled peas (s). A homozygous smooth

pea is crossed with a wrinkled pea.

Determine the possible offspring genotypes & phenotypes.

♀ SS x ♂ ss

Ss, Ss, Ss, Ss

100% chance of smooth peas

Practice In guinea pigs, dark fur (D) is

dominant to light fur (d). Cross a heterozygous dark guinea

pig with a light guinea pig.

♀ _____ x ♂ _____

Practice In guinea pigs, dark fur (D) is

dominant to light fur (d). Cross a heterozygous

dark guinea pig with a light guinea pig.

Determine the possible offspring genotypes & phenotypes.

♀ Dd x ♂ dd

Dd, Dd, dd, dd

50% chance of dark fur50% chance of light fur

A Genetic Cross If you know offspring genotypes,

you can predict parent genotypes

Example: Brown eyes (B) are

dominant to blue eyes (b). If Dash’s genotype is bb,

what are his parents’ genotypes?

bb

? ? bb Bb

Practice In guinea pigs, dark fur (D) is

dominant to light fur (d).

Two dark-fur guinea pigs produce a light-fur offspring.

What is the genotype of the offspring?

dd

Practice In guinea pigs, dark fur (D) is

dominant to light fur (d).

Two dark-fur guinea pigs produce a light-fur offspring.

What must the parent genotypes be?

dd

? ? Dd Dd

Punnett Squares Created by Reginald Punnett (1900) to

make predicting the outcome of genetic crosses easier

show all the possible

outcomes of a genetic

cross show the probability

of each outcome

25% 25%

25% 25%

Punnett Squares show all the possible outcomes of a genetic cross show the probability of each outcome

Example: In dogs, short

hair (S) is

dominant to

long hair (s). Determine the possible

offspring phenotypes.

25% 25%

25% 25%

♀

Ss

♂ SS

S

s

S S

100% chance of short hair

SS

Ss

SS

Ss

Why Use a Punnett Square?

Most scientists study MANY traits at one time

It gets VERY confusing without a Punnett Square!

Practice In humans,

straight toes (S) is dominant to curled toes (s).

Determine the possible results for a cross between a recessive male & heterozygous female.

25% 25%

25% 25%

♀

Ss

♂ ss

50% chance of straight toes

50% chance of curled toes

S

s

s s

Ss Ss

ss ss

Practice In pea plants, yellow color (Y) is

dominant to green (y).

Determine the possible

results for a cross of

2 heterozygous

plants.

25% 25%

25% 25%

♀ S

s

♂ Ss

S s

S

s

SS Ss

Ss ss

75% chance of yellow peas

25% chance of green peas

Special Cases

Incomplete dominance Neither allele is dominant Heterozygous = in BETWEEN phenotypes

Example:

CarnationsCRCR =

CWCW =

CRCW = ?

Special Cases

Codominance Neither allele is dominant Heterozygous = BOTH phenotypes

Example:

CowsHWHW =

HBHB =

HWHB = ?