Monohybrid

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Slide 1

Genetics

The study of inherited characteristics

Slide 2

Monohybrid inheritance

Let 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

Slide 3

F1 intercross

Parents phenotype round seeds x round seeds

genotype Rr Rr

Gametes

F2 generation

R

r

r

R

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

Slide 4

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

Slide 5

The use of the term F1 generation is limited to the offspring of two homozygous parents

The use of the term F2 is limited to the offspring of intercrossing the F1 generation

In all other cases the terms offspring (1) and offspring (2) should be used

The complete set of headings will be:

Parents phenotypes

Parents genotypes

Gametes

Offspring (1) genotypes

Offspring (1) phenotypes

Gametes

Offspring (2) phenotypes

Offspring (2) genotypes

What if the parents are not homozygous?

Slide 6

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

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

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