Transcript Slide 1

Mendel‘s Law
of
Segregation
Gregor Mendel (1822-1884)
He was a monk and
in his spare time he
performed experiments on
garden peas, mice and
bees in the monastery
gardens.
He wished to see what
would happen when he
crossed similar organisms
with different
characteristics.
Experiments with Pea Plants
- Seed coat colour (grey or white)
- Seed shape (round or wrinkled)
- Seed colour (yellow or green)
- Pod colour (green or yellow)
- Flower position (axial or
terminal)
- Pod shape (inflated or
constricted)
- Stem length (tall or dwarf)
Cross-Pollination of Purebread
Plants
cross-pollination between
true breeding green and
yellow pods
- all F1 green
F1
Generation
Gg =
heterozygous
F2 Generation
- self-pollination of
green F1 plants
- ¾ in F2 green,
¼ yellow
- 3 : 1 ratio in pod
colour in F2
G = dominant = green
g = recessive = yellow
GG, gg =
homozygous
Monohybrid inheritance
Let the allele for round seeds be: R (dominant allele)
Let the allele for wrinkled seeds be: r (recessive allele)
Parents phenotype
genotype
Gametes
F1 generation
round seeds
RR
R
x wrinkled seeds
rr
R
r
r
gametes
gametes R
R
r
Rr
Rr
r
Rr
Rr
F1 phenotypes
100% plants producing round seeds
F1 genotypes
100% heterozygotes Rr
• F1 intercross
•
Parents phenotype
genotype
round seeds x round seeds
Rr
Rr
R
•
Gametes
•
F2 generation
Phenotype
r
R
r
gametes
gametes R
r
R
RR
Rr
r
Rr
rr
75% plants producing round seeds
25% plants producing wrinkled seeds
Genotype
25% RR
50% Rr
25% rr
Ratio
3:1 Round seeds: wrinkled seeds
Backcross
To test whether a plant producing round seeds is homozygous RR
or heterozygous Rr it can be crossed with a homozygous rr plant
If plant is homozygous dominant RR
If plant is heterozygous Rr
Parents
phenotype round x wrinkled
genotype
RR
rr
gametes
R R
r r
Parents
phenotype
genotype
gametes
Offspring
Offspring
gametes
gametes
R
R
r
Rr
Rr
r
Rr
Rr
Offspring
round x wrinkled
Rr
rr
R r
r
r
gametes
gametes
R
r
r
Rr
rr
r
Rr
rr
Offspring
phenotype
100% round
phenotype 50% round 50% wrinkled
Genotype
100% Rr
genotype
50% Rr
50% rr
Results from Mendel's Experiments
F1
Phenotyp
e
F2 Phenotypic Ratio
F2
Ratio
Round x Wrinkled
Seed
Round
5474 Round :
1850 Wrinkled
2.96:1
Yellow x Green
Seeds
Yellow
6022 Yellow :
2001 Green
3.01:1
Axial x Terminal
Flower Position
Axial
705 Axial :
224 Terminal
3.15:1
Tall x Dwarf
Plants
Tall
l787 Tall :
227 Dwarf
2.84:1
Parental Cross
Does the observed ratio match the
theoretical ratio?
• The theoretical or expected ratio of plants producing
round or wrinkled seeds is 3 round :1 wrinkled
• Mendel’s observed ratio was 2.96:1
• The discrepancy is due to statistical error
• The observed ratio is very rarely the same as the
expected ratio
• The larger the sample the more nearly the results
approximate to the theoretical ratio
• Statistical tests such as the chi-squared test can be
used to test the closeness of fit of the observed and
theoretical ratios
Mendel‘s Generalization
1. Alternative versions of genes account for
variations in inherited characters
- concept of alleles (G=green, g=yellow)
2. For each character, an organism inherits
two genes, one from each parent
- two gametes form somatic cells
- one allele comes from the mother,
one from the father
Mendel‘s Generalization
3. If the two alleles differ, then:
- dominant allele is fully expressed in the
organism's appearance (phenotype)
- recessive allele has no noticeable effect on the
organism's appearance (genotype)
4. The two genes for each character segregate
during gamete production
- ensures variation
Law of Segregation
• the pair of alleles of each parent
separate and only one allele
passes from each parent on to
an offspring
• which allele in a parent's pair of
alleles is inherited is a matter of
chance
• segregation of alleles occurs
during the process of gamete
formation (meiosis)
• randomly unite at fertilization