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