Hour 2
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Transcript of Hour 2
LECTURE 2
OBJECTIVES• Explain the Mendel’s experiments on
monohybrid cross.• Explain monohybrid cross• State Mendel’s first law/law of
segregation.• Calculate the genotypic ratio (1:2:1)
and phenotypic ratio (3:1) in F2 generation
MONOHYBRID
Monohybrid• Breeding that involves a single characteristic determined by one gene. eg. : tall and dwarf plants
• The ratio of dominant to recessive phenotypes is 3 : 1.
Mendel’s Law of Segregation
Two alleles for heritable character separate
(segregate) during gamete formation and end up in
different gametes.
Mendel’s cross of pea plants differing in flower color
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Cross of pea plants differing in flower color
• All of the F1 generation are Pp heterozygotes (purple flowers)
• When two heterozygotes F1 are crossed, 3 kinds of F2 offspring are possible:
PP homozygotes (purple flowers) Pp heterozygotes (also purple
flowers) pp homozygotes (white flowers)
So, in the F2 generation, the ratio of dominant to recessive phenotypes is 3: 1But the ratio of genotypes is 1:2:1 (1 PP : 2 Pp : 1pp)
Mendel Experiment
P : Tall x Dwarf
F1: Tall
(self cross)
F2: Tall(787) : Dwarf (277)
3 : 1
TT ttP : Tall x Dwarf
G : all T all t
F1: Tall x Tall Tt Tt
G : ½ T ½ t ½ T ½ t
F2 : ¼ TT ¼ Tt ¼ tT ¼ tt
Tall Dwarf
segregation
P
G
Progeny
VV vv Vv vv
V v
Vv Vv vv
½ V ½ v v
Monohybrid inheritance in fruit fly, Drosophila melanogaster
P : normal wing x vestigial wing
G : all V all v
F1: normal wing x normal wing
½ V ½ v ½ V ½ v
F2 : ¼ VV ¼ Vv ¼ vV ¾ vv
LAW OF SEGREGATION vs. MEIOSIS
LAW OF SEGREGATION- The two alleles for each gene separate during
gamate formation.
• 1) The R and r alleles segregate at anaphase I, yielding two types of daughter cells for this locus.
• 2) Each gamete gets one long chromosome with either the R or r allele
• 3) Fertilization recombines the R and r alleles at random
A test of Mendel’s hypothesis using test cross for monohybrid
• Crossed heterozygous with homozygous recessive
• E;g cross round peas (Rr) with wrinkled peas (rr).
- Result : phenotypic ratio 1:1