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Mendel‘s Law of
Segregation
Gregor Mendel (1822-1884)Gregor Mendel (1822-1884)
He was a monk andHe was a monk andin his spare time hein his spare time heperformed experiments on performed experiments on garden peas, mice and garden peas, mice and bees in the monastery bees in the monastery gardens. gardens.
He wished to see what He wished to see what would happen when he would happen when he crossed similar organisms crossed similar organisms with different with different
characteristics.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 = greeng = recessive = yellow
GG, gg = homozygous
Monohybrid inheritanceLet the allele for round seeds be: R (dominant allele)
Let the allele for wrinkled seeds be: r (recessive allele)
Parents phenotype round seeds x wrinkled seeds
genotype RR rr
Gametes
F1 generation
R R r r
F1 phenotypes 100% plants producing round seeds
F1 genotypes 100% heterozygotes Rr
gametes R Rr Rr Rrr Rr Rr
gametes
• F1 intercross
• Parents phenotype round seeds x round seeds
genotype Rr Rr
• Gametes
• F2 generation
R r rR
Phenotype 75% plants producing round seeds
25% plants producing wrinkled seeds
Genotype 25% RR 50% Rr 25% rr
Ratio 3:1 Round seeds: wrinkled seeds
gametesgametes R rR RR Rrr Rr rr
BackcrossTo 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
Parents phenotype round x wrinkledgenotype RR rrgametes
Offspring
If plant is heterozygous Rr Parents phenotype round x wrinkledgenotype Rr rrgametes
Offspring
R R r rR r r r
gametesgametes R R
r Rr Rrr Rr Rr
gametesgametes R r
r Rr rrr Rr rr
Offspring
phenotype 100% round
Genotype 100% Rr
Offspring
phenotype 50% round 50% wrinkled
genotype 50% Rr 50% rr
Results from Mendel's Experiments
Parental Cross F1
Phenotype
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 TerminalFlower Position
Axial705 Axial :224 Terminal
3.15:1
Tall x Dwarf Plants
Tall l787 Tall :227 Dwarf
2.84:1
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
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
Mendel‘s Generalization
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