Hour 3
Transcript of Hour 3
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
¼ TTCC
T TTcC
T tTCC
t tTcC
t TTCc
T TTcc
T tTCc
t tTcc
t TtCC
T TtcC
T ttCC
t ttcC
t TtCc
T Ttcc
T ttCc
t 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:
About ¾ of the F2 plants will be tallAbout ¼ of the F2 plants will be dwarf
F2 generation:
All tall pea plantsF1 tall X F1 tall
F1 generation:
Homozygous tall pea plants X
Homozygous dwarf pea plants
P generation:
• Random segregation can also be demonstrated with a testcross
• Testcross:– Cross heterozygous F1 individuals with
homozygous recessive
½ x 1 = ½ dd Dwarf½ d All d
½ x 1 = ½ Dd Tall½ D All d
Testcross progeny Dd X dd →
Dihybrid :
9/16 wild type phenotypes
3/16 wild type wings, ebony body
3/16 vestigial wings, wild type body
1/16 vestigial ebony
F2:
All wild type phenotypes,
F1 X F1
F1:
Homozygous wild type X Vestigial ebonyP:
• 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)