Monohybrid inheritance.

Answer the following questions in your book:

1. Write a definition for Mitosis

2. Give 3 uses for mitotic cell division
3. Why does the DNA have to duplicate
before mitosis?
Starter

4. Write a definition for Meiosis

5. What type of cell does meiosis form?
6. How does meiosis give rise to variation.
 Answers
1. Write a definition for Mitosis
Mitosis is nuclear division giving rise to genetically identical
cells.
2. Give 3 uses for mitotic cell division
Growth and development
Repair and replacement
Asexual reproduction
3. Why does the DNA have to duplicate before mitosis?
So that each new cell has identical DNA
4. Write a definition for Meiosis
Meiosis is a reduction division in which the chromosome number
is halved from diploid to haploid resulting in genetically
different cells
5. What type of cell does meiosis form?
Gametes
6. How does meiosis give rise to variation.
Forms new combinations of maternal and paternal chromosomes
 Self assessment
Each time a new egg or sperm are made, during
meiosis, a different set of chromosomes are
selected to make them. This is called
independent assortment. Each egg and sperm
from the same parents contain some
chromosomes with genes that are the same and
some chromosomes with genes that are
different. Each time a different sperm meets a
different egg new combinations of
chromosomes, with different combinations of
genes are made. This gives rise to genetic
variation.
Lesson Objective:
To develop an understanding genetic inheritance.

Learning outcomes

Be able to correctly use genetic terminology.

Be able to use genetic diagrams and Punnett squares to:

a) predict the results of monohybrid crosses and
calculate the phenotypic ratios.
b) work out and show the possible different
genotypes.

Be able to explain how to use a test cross to identify an

unknown genotype.
 Genes and alleles
All genes have different forms.
These are called alleles.

Gametes (sex cells) have only one allele but all other
cells in the body have two.

This is because we inherit half of our alleles from our

mother and half from our father.
 Genes and alleles
You have a gene for hair colour.
But there are many different hair colours, meaning that
there are different versions of the gene.
there is:

an allele for blonde hair,

an allele for brown hair,
an allele for red hair,
an allele for black hair.
 Genes and alleles
An allele is a version of a gene
 Genes and alleles
If the gene is eye colour, what are the alleles?

Amber Blue Grey

Brown

Red
green Hazel
 Genes and alleles
Some alleles are always expressed if present.
We call these dominant alleles.

Other alleles are only expressed if the

dominant allele is absent.
These are called recessive alleles.

A recessive allele is only expressed if two

copies are present (therefore no dominant allele
present).
 Genotype and Phenotype
In genetics, special terminology is used to describe
what we are talking about:
Genotype what alleles you have. Phenotype what characteristics
you have.

The alleles, or genotype, operate at a molecular level

(making proteins) to develop characteristics that can be
expressed as a phenotype.
 Genes and alleles
So, if you inherit a green eye allele from one parent and
a brown eye allele from the other, you will not see the
effects of the green allele and your phenotype will be
brown eyes.

Allele for
Allele for
Green eye
Brown eye
 Representing alleles
In genetic diagrams, letters are usually used to
represent alleles.
If the allele is dominant, it is given a capital
letter.
If the allele is recessive, it is given a lower case
letter.
Allele for
Allele for
Green eye
Brown eye

Brown = B Green = b
 Working out genotype and phenotype
If we take the eye colour gene and use the
alleles for brown (dominant) and green
(recessive) eye, there are 3 possible
combinations:

Phenotype: Brown eyes Brown eyes Green eyes

Genotype: BB Bb bb

What would be the phenotype and genotype for

each of the individuals?
 Heterozygous and Homozygous

If an organism has two

alleles for that particular
gene that are the same, Brown eyes Green eyes
then we say it is
homozygous for that trait. BB bb

Dominant Recessive

If an organism has two alleles for

that particular gene that are the
Brown eyes
different, then we say it is
Bb heterozygous for that trait.
 Inheritance terminology
Use p.200-201 to complete the table to by writing the
definitions for each of the key terms in the table:
 Inheritance terminology
Key term Definition
Genotype The genetic make-up of an organism in terms
of the alleles present
Phenotype The observable features of an organism
Homologous pair A pair of identical chromosomes
Gene A length of DNA that codes for a protein
Allele A version of a gene
Homozygous Having two identical alleles of a particular
gene
Heterozygous Having two different alleles of a particular
gene
Pure breed Two homozygous individuals that breed
together
Recessive An allele that is only expressed when there is
no dominant allele of the gene present
Dominant An allele that is expressed if it is present
Lesson Objective:
To develop an understanding genetic inheritance.

Learning outcomes

Be able to correctly use genetic terminology.

Be able to use genetic diagrams and Punnett squares to:

a) predict the results of monohybrid crosses and
calculate the phenotypic ratios.
b) work out and show the possible different
genotypes.

Be able to explain how to use a test cross to identify an

unknown genotype.
 Single gene inheritance
Characteristics that are determined by a single
gene can be studied using monohybrid crosses.
This is where you cross two parents to look at
just one characteristic.

e.g. dimples …or tongue

rolling
 Pure breed cross
Pure breed crosses are when two identical
homozygous individuals breed together:

BB bb
 Pure breed cross
Heterozygous individuals breeding do not give a
pure breed cross:

X X

BB Bb bb Bb

Bb Bb
 Predicting inheritance from Genetic diagrams of
monohybrid crosses
D = allele for dimples (dominant) d = allele for no dimples (recessive)

Parents F = First
genotype: DD X dd O = Outside
I = Inside
Phenotype: Dimples No dimples L = Last

D D d d
Gametes: Is this a pure
breed cross?

Offspring
Dd Dd Dd Dd
(F1 generation):

All of the offspring have dimples!

 Punnett Square
Another way of showing this is in a Punnett
Square:

One side shows the

gametes of one parent

The other
side shows D D
the gametes d Dd Dd
of the other
parent d Dd Dd
 Monohybrid inheritance
Complete the pure breed cross for the following
characteristic. What are the phenotypic ratios?
E = allele for unattached (dominant) e = allele for attached (recessive)

Parents
genotype:
X
Phenotype:

Gametes:

Offspring
(F1 generation):
 Monohybrid inheritance
Complete the crosses for the different monohybrid
inheritance:
 Monohybrid inheritance
Complete the pure breed cross for the following
characteristic. What are the phenotypic ratios?
E = allele for unattached (dominant) e = allele for attached (recessive)

Parents
genotype: EE X
Phenotype: Unattached Attached

Gametes: E E e e

Offspring E E
(F1 generation): e Ee Ee
e Ee Ee

All of the offspring have unattached earlobes! (100%)

 Carriers
These offspring are all heterozygous.
They all have unattached earlobes but carry the
recessive gene.

E E
e Ee Ee
e Ee Ee

These individuals are called carriers.

They do not display the phenotype but they
carry the recessive allele.
 Monohybrid inheritance
Complete the cross for two heterozygous individuals:

E = allele for unattached (dominant) e = allele for attached (recessive)

Parents
genotype: Ee X
Phenotype: Unattached Unattached

Is this a pure
Gametes: E e breed cross?
E e

Offspring E e
(F1 generation): E EE Ee
e Ee ee

3 Unattached: 1 Attached
 Monohybrid inheritance
Complete the cross for heterozygous and homozygous
dominant individuals:
E = allele for unattached (dominant) e = allele for attached (recessive)

Parents
genotype: Ee X
Phenotype: Unattached Unattached

Gametes: E e E E

Offspring E E
(F1 generation): E EE EE
e Ee Ee

All of the offspring have unattached earlobes! (100%)

 Monohybrid inheritance
Complete the cross for heterozygous and homozygous
recessive individuals:
E = allele for unattached (dominant) e = allele for attached (recessive)

Parents
genotype: Ee X
Phenotype: Unattached Attached

Is this a pure
Gametes: E e e e breed cross?

Offspring e e
(F1 generation): E Ee Ee
e ee ee

2 Unattached: 2 Attached (ratio 1:1)

 Check your learning
What would be the phenotypic ratios of
offspring for each of the following crosses?
1. Pure breed cross.
2. Two heterozygous.
3. Heterozygous and homozygous recessive
4. Heterozygous and homozygous dominant
 Self assessment
1. Pure breed cross.
All dominant phenotype (100% Heterozygous) Ratio
=1
2. Two heterozygous.
3 dominant phenotype: 1 recessive phenotype
3. Heterozygous and homozygous recessive
1:1 Dominant to Recessive phenotype
4. Heterozygous and homozygous dominant
All dominant phenotype Ratio = 1
Lesson Objective:
To develop an understanding genetic inheritance.

Learning outcomes

Be able to correctly use genetic terminology.

Be able to use genetic diagrams and Punnett squares to:

a) predict the results of monohybrid crosses and
calculate the phenotypic ratios.
b) work out and show the possible different
genotypes.

Be able to explain how to use a test cross to identify an

unknown genotype.
 Test cross
It is impossible to tell if an individual is
homozygous dominant or heterozygous, as both
inherit the dominant allele, and so the dominant
allele will always be expressed.

Phenotype:
Brown hair Brown hair

Genotype: BB Bb
 Test cross
We can do a test cross with a known individual
(homozygous recessive) and observe the ratios
of offspring.

What would the ratios for each cross be? Do a Punnett

square to show the outcome for each unknown.
Self assessment