Learning Intentions• I understand genetic terminology and can identify vital

information for a monohybrid cross. • I can identify if parents are true breeding or

homozygous by carrying out a back cross.• I can carry out a monohybrid cross to the F2 generation

and state the genotype, and phenotypic ratio produced. • I can state why the observed and predicted results of

inheritance are different.

Success Criteria• I can use the layout effectively to show a step-by-step

account of inheritance. • I can confidently determine homozygous and

homozygous alleles.• I can highlight gametes by circling them.

The patterns of inheritance have been worked out using breeding experiments.

Breeding experiments involve the mating together or CROSSING of two organisms in order to study their offspring.

The next three slides shows three generations of white and three generations of black mice.

P Represents the parents

F1 Represents the first generation

F2 Represents the second generation

XPparents

XF1first generation

F2second generation

Crossing True-breeding White Mice

XPparents

XF1first generation

F2second generation

Crossing True-breeding Black Mice

GenerationExample 1

Colours of Mice(phenotypes)

Example 2Colours of Mice(phenotypes)

P

F1

F2

The offspring of the white parents were all white.

When members of this generation were crossed with others from this generation then all of their offspring would also be white.

The offspring of the black parents were all black.

When members of this generation were crossed with others from this generation then all of their offspring would also be black.

When the same characteristic is passed repeatedly from generation to generation

then we say the organism is.

An animal is not always crossed with another animal of the same phenotype.

Look at the next slide.

XF1first generation

XPparents

F2second generation

Crossing two different true breeding animals

Generation Phenotype(s) Colour(s)

P

F1

F2

Black

And

White

All Black

Black

And

White

Perhaps you would expect that when you cross a true-breeding white mouse with a true-breeding black mouse you would get something in between, eg. a

GREY mouse.

However, all of the mice in the F1 generation are the same colour – black.

Black is said to be DOMINANT

White is said to be RECESSIVE

From this is clear that the black mice in the F1 generation are not true-breeding. Since the black mice in this generation

produce both Black and White offspring.

The Monohybrid Cross

This involves looking at a single characteristic to see how it is passed

from generation to generation.

A lot of important work in genetics has been done by studying inheritance in fruit flies.

One characteristic which can be used to illustrate inheritance is wing type in Fruit

Flies.

These can be NORMAL or VESTIGIAL.

Normal wings

Fruit fly with

normal wings

Fruit fly with

vestigial wings

Wing type is controlled by a pair of genes.

Each gene can be one of two types or ALLELES.

N = normal wingsn = vestigial wings

Normal wing is dominant over vestigial wing.

Vestigial wing is said to be recessive.

There are three possible genotypes but only two possible phenotypes.

NN = normal wings

Nn = normal wings

nn = vestigial wings

If we cross true breeding normal winged flies with true breeding vestigial winged flies it is

possible to work out the genotypes and phenotypes of the offspring.

NN X nnParent (P)

N nGametes

Genotype all Nn

Phenotype all Normal winged

All NnF1 generation

Always circle the allele found in the gamete

It is more difficult to work out the results of a cross between members of the F1 generation

but it can be done.

Nn X NnF1 generation

N or n N or nGametes

We can work out the chances of each type of

offspring using a punnett square.

Nn

N nNN NnNn nn

NN NnNn nn

NN

n

nNorma

lNorma

l

Normal

Vestigial

Genotypes NN , Nn and nnPhenotypes Normal and VestigialPhenotype Ratio 3:1

F2 Generation

Observed ‘v’ Predicted Results

• When a monohybrid cross is carried out the actual results that you obtain are not always exactly the results you predicted.

• This is because fertilisation is a random process involving the element of chance

• Producing large numbers of offspring gives more reliable results.

1. Brown eyes is dominant to blue eyes. A brown eyed man (BB) marries a blue eyed woman (bb). What colour of eyes would you expect their children to have? Why?

2. A brown eyed man (Bb) marries a brown eyed woman (Bb) and have 4 children. How many should have brown eyes and how many blue eyes? Would all the brown eyed children have the same genes?

3. Tongue rolling is dominant and non-rolling is recessive. A tongue rolling woman (Tt) marries a non-rolling man (tt). What fraction of their children should be able to roll their tongues?

4. Two tongue rollers (Tt) get married. If they have 4 children how many should be able to roll their tongue and how many should not be able to roll their tongue?

Write this as a ratio -

? Rollers : ? Non-rollers

5. In flies normal wings are dominant and short wings are recessive.

A normal winged fly (NN) is mated with a short winged fly.What genes should their offspring inherit and what should they look like?

If two of these offspring are mated what fraction of their offspring would you expect to have normal wings and what fraction short wings? Write this as a ratio of -

? Normal wings : ? Short wings

Tongue Rolling

Tongue rolling is an inherited characteristic which is controlled by a single gene.

The tongue rolling allele is dominant over the non-tongue rolling allele.

T = tongue rolling

t = non tongue rolling

The pattern of inheritance can be traced using a family tree diagram, where……

Tongue rolling female

Non tongue rolling female

Tongue rolling male

Non tongue rolling male

It is possible to work out the genotypes of individuals by knowing details a number of

generations of a family.

This is demonstrated in the following example.

tt

tt tt

tttttt

Tt

Tt Tt

Tt TtTT

TtTtTtTt

?

Tt

First identify the Homozygous

recessive individuals.

These must be

tt

We can now work out the

genotype of the male parent

The tongue rollers in the F1 generation must

be Tt

We can now work out the

genotypes of the children in family X.

Family X

Before we work out the

genotypes of family Y we can work out family

Z

Family Y

Family Z

We cannot work out the the

genotypes of the rest for certain.

However, we can make an educated

guess of the genotype of one of

them.

The genotype of the last person is impossible to

work out.