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

Page 251

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