M endel s law of segregation
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M endel‘s Law of Segregation. 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. Gregor Mendel (1822-1884).

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M endel‘s Law of Segregation

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M endel s law of segregation

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.

Gregor Mendel (1822-1884)


Experiments with pea plants

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 of Purebread Plants

- cross-pollination between true breeding green and yellow pods

- all F1 green

F1 Generation

Gg = heterozygous


F2 generation

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

Monohybrid inheritance

Let the allele for round seeds be: R (dominant allele)

Let the allele for wrinkled seeds be: r (recessive allele)

Parentsphenotyperound seeds x wrinkled seeds

genotypeRRrr

Gametes

F1 generation

R

R

r

r

F1 phenotypes100% plants producing round seeds

F1 genotypes100% heterozygotes Rr


M endel s law of segregation

  • F1 intercross

  • Parents phenotyperound seeds x round seeds

    genotype RrRr

  • Gametes

  • F2 generation

R

r

R

r

Phenotype75% plants producing round seeds

25% plants producing wrinkled seeds

Genotype25% RR 50% Rr 25% rr

Ratio3:1 Round seeds: wrinkled seeds


M endel s law of segregation

If plant is homozygous dominant RR

Parents

phenotype round x wrinkled

genotype RR rr

gametes

Offspring

If plant is heterozygous Rr

Parents

phenotype round x wrinkled

genotype Rr rr

gametes

Offspring

BackcrossTo test whether a plant producing round seeds is homozygous RR or heterozygous Rr it can be crossed with a homozygous rr plant

R

r

r

r

R

R

r

r

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 Terminal

Flower Position

Axial

705 Axial :

224 Terminal

3.15:1

Tall x Dwarf Plants

Tall

l787 Tall :

227 Dwarf

2.84:1

Results from Mendel's Experiments


M endel s law of segregation

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

Mendel‘s Generalization

  • 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


M endel s law of segregation

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

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


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