Bb

B

b

1. Using the picture above, explain Mendel’s Law of Segregation.

2. Why is this Punnett

square set up incorrectly?

T s

t

S

3. Why is this Punnett

square set up incorrectly?

TT TT

Tt

Tt

• Remember, Mendel performed experiments on pea plants that only differed in one trait: HEIGHT

Di-hybrid crosses

• Mendel then decided to perform another set of crosses in which he used peas that differed from each other only in TWO traits rather than one.

Di-hybrid crossesThese experiments were called Di-hybrid crosses:

• Di means “two” - because there were only two differences

• Hybrid because the parents were different from each other (in two ways)

• Cross because he chose the parents to breed with each other

The pea plants were the same in every way, EXCEPT

PEA SHAPE (round, wrinkled) and PEA COLOR (yellow, green)

Mendel took pure-bred plants with round yellow peas and crossed them with pure-bred plants with

wrinkled green seeds.

• In pea plants:

Round is Dominant “R” and Wrinkled is Recessive “r”

Yellow is Dominant “Y” and Green is Recessive “y”

THE TWO DIFFERENCES

RR YY rr yy

• What happened with the first generation of offspring?

What happened with the second generation of offspring?

P1- Parents

F1- first generation of offspring

F2- 2nd generation of offspring

• The Law of Independent Assortment states:

genes for different traits—(for example, eye shape and eye color)—

are inherited independently of each other.

The Law of Independent Assortment

Bb bb

rr RRBb

Rr

• A Punnett square for a dihybrid cross will need to be four boxes on each side for a total of 16 boxes.

Dihybrid crossesDihybrid crosses

Punnett Square of Dihybrid Cross

Gametes from RrYy parentRY Ry rY ry

Ga

me

tes

from

RrY

y p

are

nt

RY

Ry

rY

ry

RRYY RRYy RrYY RrYy

RRYy RRYy RrYy Rryy

RrYY RrYy rrYY rrYy

RrYy Rryy rrYy rryy

• Remember with the Law of Segregation in a mono-hybrid cross, you had to separate the alleles

because you can only pass ONE ALLELE FOR EACH TRAIT on to your offspring.

TT Tt

Tt tt

T t

T

t

In a di-hybrid, you still only pass one allele PER trait to your offspring….

RR YY

But remember: in a di-hybrid you have two

traits….

R Y R Y R Y R Y

Let’s practice one together, then you will do one on your own.Male parent is Heterozygous for Brown eyes and Homozygous recessive for

almond eyes:

Female parent is Homozygous recessive for Blue eyes and Heterozygous for Round eyes:

Bb rr

bb Rr

B r B r b r b r

b R

b R

b r

b r

Bb Rr bb RrBb Rr bb Rr

Bb rr bb rrBb rr bb rr

Bb Rr bb RrBb Rr bb Rr

Bb rr bb rr bb rrBb rr

Head, C-curly, c-straight

Eyes, S-slanted, s-straight

Father, CcSs (Curly hair, Slanted eyes)M

oth

er,

CcS

s (C

urly

hai

r, S

lant

ed e

yes)

CS Cs cS cs

CS

Cs

cS

cs

CCSS CCSs CcSS CcSs

CCSs CCss CcSs Ccss

CcSS CcSs ccSS ccSs

CcSs Ccss ccSs ccss