9- dihybrid cross - wordpress.com...2017/04/09 · dihybrid crosses • dihybrid cross ~ genetic...

Dihybrid Crosses

Dihybrid Crosses• Dihybrid Cross ~ genetic cross

considering 2 gene traits at the same time, each consisting of nonidentical alleles.

• i.e. Mendel crossed numerous traits repeatedly in dihybrid crosses and found he always obtained the same ratio 9:3:3:1 ~ and thus develops Law of Independent Assortment

•Law of Independent Assortment

• During gamete formation, the two alleles for one gene segregate or assort independently of the alleles for other genes.

Diploid Cells

Gamete Cells

Law of Independent Assortment

Diploid Cells

Gamete Cells

Or.....


Diploid Cells

Gamete Cells

Or.....Or.....


Diploid Cells

Gamete Cells

Or.....Or.....

etc....


Diploid Cells

Gamete Cells

So....


Dad

Diploid Cells

Gamete Cells


Dad Mom

• In humans, free earlobes are controlled by the dominant allele E and attached earlobes by the recessive allele e. The widow’s peak hairline is regulated by the dominant allele H, while the straight hairline by the recessive allele h.

• What would be the genotype and phenotype ratio of the F1 generation if a man heterozygous for both free earlobes and widow’s peak has children with a woman also heterozygous for both free earlobes and widow’s peak?

Sample Problem #1

• 1. Let “E” be dominant free earlobes let “e” be recessive attached earlobes let “H” be dominant widow’s peak let “h” recessive straight hair line

• 2. Parents P1 cross: EeHh x EeHh

• 3. Alleles: EH, Eh, eH, eh & EH, Eh, eH, eh How did

you do that?

EeHh x EeHhEeHhEeHhEeHhEeHh EeHhEeHhEeHhEeHh

Step 3:

EeHh x EeHhEeHhEeHhEeHh

EH

EeHhEeHhEeHhEeHh

Step 3:

EeHh x EeHhEeHhEeHh

EhEH

EeHhEeHhEeHhEeHh

Step 3:

EeHh x EeHhEeHh

eHEhEH

EeHhEeHhEeHhEeHh

Step 3:

EeHh x EeHh

eheHEhEH

EeHhEeHhEeHhEeHh

Step 3:

EeHh x EeHh

eheHEhEH

EeHhEeHhEeHh

EH

Step 3:

EeHh x EeHh

eheHEhEH

EeHhEeHh

EhEH

Step 3:

EeHh x EeHh

eheHEhEH

EeHh

eHEhEH

Step 3:

EeHh x EeHh

eheHEhEH eheHEhEH

Step 3:

EeHh x EeHh

eheHEhEH eheHEhEHx

P1 Cross:

Alleles for Punnet Square:

This is how you do step #3!

Step 3:

EH Eh eH eh

EH

eH

Eh

eh

EEHH EEHh EeHH EeHh

EeHH EeHh eeHH eeHh

EEHh EEhh EeHh Eehh

EeHh Eehh eeHh eehh

The Alleles of each parent is placed on the outside of your

4x4 punnet square

• EEHH ~ 1 eeHh ~ 2

• EEHh ~ 2 EEhh ~ 1

• EeHH ~ 2 Eehh ~ 2

• EeHh ~ 4 eehh ~ 1

• eeHH ~ 1

• Therefore the genotype ratio is 1:2:2:4:1:2:1:2:1 for above order

• Free earlobes & widow’s peak = 1 +2+2+4 = 9

• Attached earlobes & widow’s peak = 1+2 =3

• Free earlobes and straight hairline = 1+2= 3

• Attached earlobes and straight hairline = 1

• thus, the ratio of phenotypes is 9:3:3:1

• 1. State The phenotypes and genotype probability for a cross between a pure plant with round yellow seed and pure plant with wrinkled green seed. Round is dominant over wrinkled and yellow is dominant over green.

Practice Crosses

• Let “R” be dominant round trait let “r” be recessive wrinkled trait let “C” be dominant yellow trait let “c” be recessive green trait

• Cross P1: RRCC x rrcc

• Alleles: RC, RC, RC, RC x rc, rc, rc, rc

RC RC RC RC

rc RrCc RrCc RrCc RrCc




•RrCc - 16

•Therefore, 100% of P1 are RrCc and will be round and yellow.

Pedigree Chart ~ a graphic presentation of a family tree that permits patterns of inheritance to be followed for a single gene.

chart contains a number of symbols that identify gender, relationships between individuals and whether an individual expresses a trait or carries the

Pedigree Charts

142 Chapter 4

I

II

male

female

mating

Roman numeralssymbolizegenerations.

Arabic numbers symbolizeindividuals within a givengeneration.

Birth order within each group of offspring is drawn left to right,oldest to youngest.

identicaltwins

fraternaltwins (females)

affectedindividuals

knownheterozygotesfor autosomalrecessive

31 2

Pedigree Symbols

Sample Pedigree

I

II

III

2

1 2 3

1

54

2 31

Figure 4Squares represent males and circles representfemales. Individuals that express the trait areshown in a coloured circle or square. If it isknown, individuals that carry the allele as partof a heterozygous genotype are shown by par-tial colour or shading. A vertical line connectsparents to offspring.

From the point of view of individual III - 1, the symbols represent the fol-lowing relationships:

I - 1 = grandfather I - 2 = grandmotherII - 1 and II - 2 = aunts II - 3 = uncle II - 4 = father II - 5 = motherIII - 2 = fraternal twin sister III - 3 = brother

Practice

Understanding Concepts1. Copy Figure 5 into your notebook. Indicate whether each family

member is homozygous or heterozygous for shortsightedness, orhomozygous for normal vision.

2. If couple 4 and 5 in row II had another child, what genotype mightthe child have? (Hint: What genotype is possible but not shown inthe chart?) Would the child have normal vision or be shortsighted?

9- dihybrid cross - wordpress.com...2017/04/09 · dihybrid crosses • dihybrid cross ~ genetic...

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