GENETICS

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Monohybrid inheritance 4/10

Objectives:

*Define monohybrid inheritance

**Explain examples of monohybrid crosses

Starter:

What is monohybrid inheritance?

Inheritance of a single characteristic i.e inheritance of alleles of one gene

Gregor Mendel was a school teacher in Austria who had a passion for research.

He is known as the father of genetics.

Mendel did many experiments with peas.

We are only going to focus on 3 experiments.

Experiment 1: He crossed pure bred tall pea plants with pure bred small pea plants.

Experiment 2: He crossed the plants resulting from the first experiment together.

Experiment 3: He crossed the plants resulting from the first experiment with pure bred small pea plants.

Draw punnet square or stick diagram to show the phenotype and genotype of each experiment

T tT t

T T tt

Tt Tt Tt Tt

Tall Tall Tall Tall

Parents

Sex cells

Offspring

All of the offspring are tall and have inherited the gene for smallness.

Let’s look at the how the idea works in detail. Experiment 1

F1 generation

T tt T

T t tT

TT Tt tT tt

Tall Tall Tall Small

Parents

Sex cells

Offspring

25% of the offspring should be small i.e 1 in 4.

Experiment 2

F2 generation

T tt t

T t tt

Tt Tt tt tt

Tall Tall small small

Parents

Sex cells

Offspring

1:1 tall : small.

Test cross Experiment 3

Tall (pure) X Small (pure) ALL Tall (?)

Tall (?) X Tall (?)

What has happened to the characteristic for small ?

75% Tall (?) and 25% Small (?)

The characteristic for small has resurfaced !

Tall (?) X Small (pure) 50% Tall (?) and 50% Small (?)

Therefore the characteristic for small must have been passed on.

It also suggests that ‘Tallness’ is dominant over ‘Smallness’.

Let’s summarize the results of the experiments

Gametes

F1 Generation

Phenotype

F1 genotype F2 Generation Phenotype

3:1

Test cross

You can see in above 2 crosses in F1 recessive allele is not expressed but expressed in F2. So it is not possible to know the genotype of tall from its phenotype when you do a test cross to determine genotype

Tall (?) X Small (pure) 100% Tall (?)

Mendel came up with the idea of ‘hereditary units’ (later called genes).

These units (genes) control characteristics and are inherited from generation to generation.

How did he use this idea to explain the results of his experiments ?

How can the ratios be explained ?

Mendel further proposed that a characteristic is controlled by 2 factors which are inherited.

(These factors later become known as genes.)

Let’s call the gene for Tallness ‘big’ T

Let’s call the gene for Smallness ‘little’ t

T

t

Mendel also proposed that the 2 factors which control a characteristic are separated when sex cells are made.

Eg if a plant has the genes T and t for height, then when the sex cells are made:

50% of the sex cells will have the T gene

The other 50% of the sex cells will have the t gene

A plant which has the genes T and t will be tall because ‘Tallness’ is dominant.

Tall (TT) X Small (tt) ALL Tall (Tt)

Tall (Tt) X Tall (Tt)

The characteristic for small has been inherited but not expressed.

75% Tall (?) and 25% Small (t)

Tall (Tt) X Small (t) 50% Tall (Tt) and 50% Small (t)

What have all the Tall plants inherited ?

Why are 1 in 4 plants small ?

The Tall plants must have inherited the characteristic for small.

More evidence that ‘Tallness’ is dominant over ‘Smallness’.

Let’s use the idea of genes to try and explain the results of the experiments.

The theory, using the idea of pairs of genes controlling characteristics can account for the observations.

Each pair of genes is separated when sex cells are made.

When the sex cells unite (fertilisation), the pairs of genes are restored BUT now the combination may be different.

Some genes are dominant and some are recessive.

A genes which control a characteristic are called an alleles.

X YX X

X X YX

XX XY XX XY

female male female male

Parents

Sex cells

Offspring

50% of the offspring should be male and 50% should be female, eg a ratio of 1:1.

female male

Determination of gender:

The End