LECTURE 3

OBJECTIVESAt the end of the lesson, students should

be able to :• Explain the Mendel’s experiment on

dihybrid cross.• Explain dihybrid cross.• State Mendel’s second law/law of

independent assortment• Calculate genotypic and phenotypic

ratios (9:3:3:1) up to F2 generation using Punnett-square method.

Eg : Drosophila and Pisum.

Dihybrid cross

• In one experiment, Mendel crossed plants from two pure-breeding strains, one tall with purple flowers, the other dwarf with white flowers.

• All the offspring in the F1 generation were tall with purple flowers, these being the dominant characteristics.

• The F1 generation were self-crossed, producing the following phenotypes and ratios in the F2 generation:

• 9 tall purple-flowered• 3 tall white-flowered• 3 dwarf purple-flowered• 1 dwarf white-flowered

• He observed that two phenotypes resembled one or the other of the parents, and the two phenotypes had combined the characteristics of both parents.

• The ratio of tall plants to dwarf plants and the ratio of purple-flowered plants to white -flowered plants were 3:1.

• This was the same ratio that occurred in the monohybrid crosses.

• He concluded from these results that the two pairs of characteristics behave quite independently of each other and this led him to formulate his second law.

Dihybrid

• Dihybrid inheritance is the inheritance of 2 characteristics, each controlled by a different gene at a different locus.

P P Q Q

Law of Independent Assortment

(Mendel’s second law)Each pair of the alleles

segregates independently of other pairs of alleles

during gamete formation

P : Tall colored x Dwarf whiteTTCC ttcc

G : all TC all tc

F1 : Tall colored x Tall coloredTtCc TtCc

G : ¼TC ¼Tc ¼ tC ¼tc ¼TC ¼Tc ¼ tC ¼tc

Segregation with independent assortment

F2 : Punnett square

¼ TC ¼ Tc ¼ tC ¼ tc

¼ TC

¼ Tc

¼ tC

¼ tc

TTCC

TTcC

tTCC

tTcC

TTCc

TTcc

tTCc

tTcc

TtCC

TtcC

ttCC

ttcC

TtCc

Ttcc

ttCc

ttcc

Tall colored

Tall white

dwarf colored

dwarf white

F2:

9 : 3 : 3 : 1

male

female

Tall colored x Dwarf whitePTtCc ttcc

G ¼TC ¼Tc ¼tC ¼tc tc

¼TtCc ¼Ttcc ¼ttCc ¼ttcc

Tall colored

Tall white

Dwarf colored

Dwarf white

LAW OF INDEPENDENT ASSORTMENT vs. MEIOSIS

LAW OF INDEPENDENT ASSORTMENT- Alleles of genes on nonhomologous chromosomes assort independently

during gamete formation.

• 1) Alleles at both loci segregate in anaphase I, yielding four types of daughter cells depending on the chromosome arrangement at metaphase I

• 2) Each gamete gets a long and a short chromosome in one of four allele combinations

• 3) Fertilization result in the 9:3:3:1 phenotypic ratio in the F2 generation

Test cross in dihybrid• Crossed F1 dihybrid with homozygous

recessive- Result : phenotypic ratio 1:1:1:1

SUMMARY :• Example of a monohybrid cross:

P generation: Homozygous tall pea plants X

Homozygous dwarf pea plants

F1 generation:

All tall pea plantsF1 tall X F1 tall

F2 generation:

About ¾ of the F2 plants will be tall

About ¼ of the F2 plants will be dwarf

• Random segregation can also be demonstrated with a testcross

• Testcross:– Cross heterozygous F1 individuals with

homozygous recessive

Dd X dd Testcross progeny

½ D All d ½ x 1 = ½ Dd Tall

½ d All d ½ x 1 = ½ dd Dwarf

Dihybrid :

P: Homozygous wild type X Vestigial ebony

F1: All wild type phenotypes,

F1 X F1

F2: 9/16 wild type phenotypes

3/16 wild type wings, ebony body

3/16 vestigial wings, wild type body

1/16 vestigial ebony

• The testcross can also be applied to independent assortment:

¼ vg+ vg e+ e (wild wing, wild body)

¼ vg+ vg e e (wild wing, ebony body)

¼ vg vg e+ e (vestigial wing, wild body)

¼ vg vg e e (vestigial wing, ebony body)