dihybrid crosses. let’s review so far, we have looked only at different types of monohybrid...

Dihybrid Crosses

Let’s review

• So far, we have looked only at different types of monohybrid crosses– the inheritance of only ONE trait at a time– Ex:

• What’s the likelihood of a pea plant producing round seeds?

• What’s the likelihood of a pea plant producing green seeds?

Let’s review

• We use a basic Punnett square and the Law of Segregation to help determine the likely outcomes of a monohybrid cross– Law of Segregation: An individual’s

alleles for a trait segregate into different gametes during meiosis so that each gamete receives only 1 allele for the trait

Segregation during meiosis

Dihybrid crosses

• Dihybrid crosses: –predict and/or observe the

inheritance of TWO traits at the same time.

–Ex: What’s the likelihood of a pea plant making seeds that are round AND green?

Mendel’s Peas (again!)Mendel had observed that the seeds of his pea plants varied in several ways – among them, whether the peas were round or wrinkled and whether they were green or yellow.

He did monohybrid crosses on both characteristics, but then decided to look at how they were inherited together….

• There are four possible combinations of the two characteristics. Peas can be:

• 1) round and yellow

• 2) round and green

• 3) wrinkled and yellow

• 4) wrinkled and green

• From his monohybrid trials, Mendel had discovered that the allele for yellow color was dominant to that for green.

• He had also found that the allele for round peas was dominant to the allele for wrinkled peas.

• Starting with individuals that were homozygous, either for both dominant characteristics or both recessive characteristics, he set off breeding again…

Phenotype Round, yellow Wrinkled, green

Genotype RRYY rryy

Gametes all RY all ry

F1 all RrYy

All of the F1 generation would be heterozygous for both characteristics, meaning that they would all be round and yellow. As with monohybrid crosses, he then crossed two of the F1 generation together…

X

Round, yellow

F1 Phenotype

Genotype RrYy

Gametes RY, Ry, rY, ry

Round, yellow X Round, yellow

RrYy

RY, Ry, rY, ry

© 2007 Paul Billiet ODWS

How does a dihybrid Punnett square look?

• With a monohybrid, two different gametes could form from one parent. A or a for example. So the Punnett square had room for 2 gametes for each parent.

• With a dihybrid cross, four different gametes are possible. So the Punnett square need to have 16 boxes (4 to a side)

RY Ry

rY ry

RY Ry

rY ry

RY

Ry

rY

ry

RY Ry

rY ry

RY Result of fertilization

Ry

rY

ry

RY Ry

rY ry

RYRRYY RRYy RrYY RrYy

Ry

rY

ry

RY Ry

rY ry

RYRRYY RRYy RrYY RrYy

Ry RRYy RRyy RrYy Rryy

rYRrYY RrYy rrYY rrYy

ryRrYy Rryy rrYy rryy

Phenotypic Ratio for F2 generation

• # of round, yellow: ______

• # of round, green: ______

• # of wrinkled, yellow: ______

• # of wrinkled, green: ______– Hint: use a different symbol to check off or

shade in each box.

RY Ry

rY ry

RYRRYY RRYy RrYY RrYy

Ry RRYy RRyy RrYy Rryy

rYRrYY RrYy rrYY rrYy

ryRrYy Rryy rrYy rryy

RY Ry

rY ry

RYRRYY RRYy RrYY RrYy

Ry RRYy RRyy RrYy Rryy

rYRrYY RrYy rrYY rrYy

ryRrYy Rryy rrYy rryy

F2

9 3 3 1: : :Round

Yellow

Round

Green

Wrinkled

Yellow

Wrinkled

Green

Mendel’s Conclusion

• Mendel’s results showed that inheritance of seed shape did NOT influence the inheritance of seed color.

• The two traits are inherited INDEPENDENTLY

• This led Mendel to develop the Law of Independent Assortment

Law of Independent Assortment

• Genes for different traits are sorted and inherited independently of each other

• The pairs of chromosomes could orientate in different ways at Anaphase 1

• Of course, if the genes (alleles) are located on the same chromosome then they will not sort independently. They are “linked” and inherited together.– Since humans have 46 chromosomes but about

30,000 genes, many genes are linked.

© 2007 Paul Billiet ODWS

F2 generation

R r

R RR Rr

r Rr rr

You can use monohybrid Punnett squares and multiply the results.

Ex: What is the probability of round, wrinkled seeds?

¾ probability of round x ¼ probability of wrinkled = 3/16 probability of round and wrinkled.

Round vs. Wrinkled

Y y

Y YY Yy

y Yy yy

Yellow vs. Green