Dihybrid Crosses. A monohybrid cross involves only one trait. This is what we have been looking at so far. Ex. Only dimples, OR only tongue rolling ability.

Dihybrid Crosses

A monohybrid cross involves only one trait. This is what we have been looking at so far. Ex. Only dimples, OR only tongue rolling ability. A dihybrid cross involves two traits. Also known as two-factor cross. Ex. dimples AND tongue rolling ability.

• Dihybrid crosses illustrate Mendel’s Principle of Independent Assortment.

• The Principle of Independent Assortment states that genes for different traits segregate independently during the formation of gametes.

• New combinations of gametes are formed, creating genetic variety.

Meiosis creates genetic variety in 3 ways******

1. Crossing over

2. Independent assortment

3. creating gametes for fertilization – ultimately combining the genes of 2 organisms

ExampleEx. Cross a male heterozygous for dimples

AND tongue rolling ability with a female of the same genotype.

Presence of dimples: D= dimples d= no dimples

Tongue Rolling ability: A =able to roll a= not able

STEP 1: WRITE EQUATION

father DdAa x mother DdAa

STEP 2: GENERATE GAMETES

STEP 2: GENERATE GAMETES using the FOIL method for each parent. Make sure you get one allele for each trait in your gamete (D and A). You should always have 4 gametes, even if they repeat.

FOIL = First, Outside, Inside, Last = remember from math?

STEP 2: GENERATE GAMETES FOIL = First, Outside, Inside, Last

moms gametes for DdAa First: DA Outside: Da Inside: dA Last: da dad’s gametes for DdAa (the same as mom this

case)DA Da dA da

***STEP 2 Illustrates independent assortment, as D allele has the same chance of being in a gamete with A or a. (DA or Da equally likely).

STEP 3: 16 Box PunnettSTEP 3: DRAW a 16 box Punnett Square. COMPLETE the Punnett Square as normal. Gametes on the outside, offspring on the inside. Hint: Make sure you list the dominant allele first, and the same trait first for every box.

STEP 4: DRAW A TABLE

• LIST the possible phenotypes – there will always be 4• Assign them each a COLOR. Color the the box on the

table. Possible Phenotypes genotypes that produce the phenotype

phenotypic frequency

Dimples, Able to Roll

DDAA DDAa DdAA DdAa

9

Dimples, unable to roll

DDaa Ddaa

3

no dimples, able to roll

ddAA ddAa

3

no dimples, unable to roll

ddaa 1

• LIST all the genotypes that would produce that phenotype.

Possible Phenotypes genotypes that produce the phenotype

phenotypic frequency

Dimples, Able to Roll

DDAA DDAa DdAA DdAa

9

Dimples, unable to roll

DDaa Ddaa

3

no dimples, able to roll

ddAA ddAa

3

no dimples, unable to roll

ddaa 1

COLOR the genotypes the correct color on the Punnett square.

DA Da dA da

DA DDAA DDAa DdAA DdAa

Da DDAa DDaa DdAa Ddaa

dA DdAA DdAa ddAA ddAa

da DdAa Ddaa ddAa ddaa

• COUNT the number of boxes you have colored in the Punnett.

• RECORD this in the phenotypic frequency column.

Possible Phenotypes genotypes that produce the phenotype

phenotypic frequency

Dimples, Able to Roll

DDAADDAaDdAA DdAa

9

Dimples, unable to roll

DDaaDdaa

3

no dimples, able to roll

ddAAddAa

3

no dimples, unable to roll

ddaa 1

STEP 5

• STEP 5: RECORD the phenotypic frequency as a ratio. Make sure your numbers add up to 16!!!!

phenotypic ratio 9 dimples, able to roll :3 dimples, unable to roll : 3 no dimples, able to roll :1 no dimples, unable to roll

Independent Assortment occurs in meiosis I.

• The genes for different traits are on different chromosomes.

• Homologous pair for tongue rolling lines up independently of the homologous pair for dimples.

• Two different alignments can occur, which produce different combinations of gametes.

ORD d

A aD da A

D

A

aaD A AD

a

d d

d Da

aaD D

dA

d dA A

Gametes DA and da Gametes Da and dA

• Exception: Genes on the same chromosome do not sort independently. They are linked genes. (Of course they do not sort independently. They are physically attached!)