Monohybrid crossesMonohybrid crosses

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By the end of this lesson:

You should be able to:

Know what a monohybrid crossKnow how to draw out a monohybrid crossPredict the proportions of phenotypes of the F2 offspring in a monohybrid crossKnow why there is a difference between observed and predicted ratios in monohybrid crosses

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F1 and F2 generations

When 2 parents are crossed they produce offspring.

We refer to these offspring as the First Filial generation (F1)

When we do genetics experiments, we often take individuals from the F1 generation, and cross them

Their offspring are called the Second Filial generation (F2 )

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Parents X

F1

generation

Then we cross two members of the F1 generation

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XF1

generation

F2

generation

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Monohybrid Crosses

In standard grade we have to work out theoretical crosses.

When we do this we only consider one characteristic at a time

This is called a monohybrid cross

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An example

The gene for coat colour in mice has 2 alleles Brown which is dominant White which is recessive

What GENOTYPE would a HOMOZYGOUS brown mouse have?

BB What GENOTYPE would a HOMOZYGOUS

white mouse have? bb

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Brown mouse

White mouse

Parents phenotype

Parents genotype

Gametes

F1 genotypeF1 phenotype

BB bb

All B All b

All Bb

All heterozygous brown

X

Now we cross 2 individuals from the F1 generation

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Brown mouse

Brown mouse

F1 phenotype

F1 genotype

Gametes

F2 genotype

Bb Bb

B or b B or b

Aargghh, how do we work this out?????

Use a punnett square

X

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Punnett square

B b

B BB(Brown)

Bb(Brown)

b Bb(Brown)

bb(White)

Gametes from one parent

Gametes from other parent

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Results!

So all the possible F2 generation genotypes are BB (Brown) Bb (Brown) Bb (Brown) bb (white)

The F2 phenotypic ratio would be 3 brown : 1 white

The F2 genotypic ratio would be 1 BB: 2 Bb : 1 bb

1:2:1

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Does this happen in nature?

It is important to remember that when we are working out ratios, that it is only probable as fertilisation is a random process, and the true ratio is a matter of chance.

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Work

Insert and complete the “Genetics- Monohybrid Cross” sheet from your diagram pack.

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Work Calculate the F2 generation phenotypic

and genotypic ratios of a cross between these flies- homozygous straight- winged and curved-winged fruit fly

Straight wing is dominant

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Work

Once you are confident that you understand how to write out a monohybrid cross, ask your teacher for a question sheet.

Remember to set our your answers in the correct format, as you have been shown!

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By the end of this lesson:

Can you do the following?

Do you know what a monohybrid cross is?Can you draw out a monohybrid cross?Can you predict the proportions of phenotypes of the F2 offspring in a monohybrid cross?Explain why there could be a difference between the observed and predicted ratio