© boardworks ltd 2004 1 of 36 ks4 biology inheritance

© Boardworks Ltd 20041 of 36

KS4 Biology

Inheritance


Inheritance

Contents

All about alleles

Homozygous cross

Using a test cross

Co-dominance

Heterozygous cross


the definition of certain genetic terms,

Inheritance introduction

To understand how inheritance works you need to know:

and how to use genetic crosses to determine the characteristics of offspring.

In this unit, petal colour is used to show how characteristics are inherited in offspring.

© Boardworks Ltd 20044 of 36 © Boardworks Ltd 2004

Homologous chromosomes

In all living things, characteristics are passed on in the chromosomes that offspring inherit from their parents.

Chromosomes are matched in pairs that containone chromosome inherited from each parent.

So are the genes in a matching pair of chromosomes exactly the same?

chromosome from female parent

chromosome from male parent


Each chromosome may have a different version of a gene.

Different versions of a gene, that code for different versions of a characteristic, are called alleles.

The chromosomes in a matching pair contain the same type of genes that code for the same characteristics.

Different versions of genes

version for yellow petals

version for red petals

gene for petal colour

gene for petal colour


Pairs of alleles – homozygous

If the alleles in a matching pair are the same, they are called homozygous alleles.

allele for yellow petals


allele for red petals


What colour are the flowers with these homozygous pairs of alleles?

(Click twice on each bud to reveal the flower;

click again to close them.)


Pairs of alleles – heterozygous

If the alleles in a matching pair are different, they are called heterozygous alleles.

Which characteristic is expressed if alleles are different?



Some alleles are dominant to other forms of a gene and will always be expressed.

Which is the dominant allele in this heterozygous pair?

Which is the recessive allele in this heterozygous pair?

(Click twice on the bud to reveal the flower;

click again to close it.)


Representing alleles

Letters are used to represent different alleles.

The allele pair for each characteristic is called the genotype.

What colour are flowers with the genotype Rr?

A dominant allele is always a capital letter.

A recessive allele is always the corresponding small letter.

allele for red petals R

allele for yellow petals r

=

=


Genotypes and phenotypes

The allele pair for each characteristic is called the genotype.

The physical expression of an allele pair is the phenotype.

What are the phenotypes of these genotypes?

genotype:

phenotype:

RR Rrrr

(Click twice on each bud to reveal the flowers;



Genetic jargon

gene

allele

phenotype

genotype

Section of DNA that codes for a particular trait or characteristic.

A different form of a gene that codes for a different version of a characteristic.

A description of the pair of alleles present for a characteristic.

The physical expression of the alleles.

What do these genetic terms mean?


homozygous

heterozygous

recessive

dominant

Pair of alleles that produce a characteristic that are the same, e.g. HH.

Pair of alleles that produce a characteristic that are different, e.g. Hh.

An allele that will only be expressed when both alleles are of this type, represented by a lower case letter.

An allele that will always be expressed even when there is only one of these alleles present, represented by a capital letter.

Genetic jargon

What do these genetic terms mean?


Matching pairs – genes


Matching pairs – characteristics


Contents

All about alleles

Homozygous cross

Using a test cross

Co-dominance

Heterozygous cross

Inheritance


xphenotype:

genotype: RR rr

Homozygous means that both alleles of a gene are the same.

Red is the dominant allele for these flowers, so the alleles for petal colour are: red = R , yellow = r.

RR x rr – crossing homozygous parents

What are the possible offspring of a cross between a homozygous red flower and a homozygous yellow flower?


RR x rr

R R r r

parental genotype:

gametes:

rr

R

R

Rr Rr

RrRrF1 offspring

genotype:

RR x rr – F1 offspring

What are the phenotypes of the F1 offspring?

?


F1 genotypes:

RR x rr – F1 phenotypes

Rr Rr Rr Rr

F1 phenotypes:(Click twice on the buds

to reveal the flowers; click again to close them.)

The possible offspring of a cross between two homozygous parents are always heterozygous and so the dominant characteristic is always expressed in this generation.

RR x rrparental genotype:


Homozygous cross activity


Contents

All about alleles

Homozygous cross

Using a test cross

Co-dominance

Heterozygous cross

Inheritance


The offspring (Rr) from the first cross (RR x rr) are called the F1 generation. What happens in a cross between these offspring?

Both parent plants are now heterozygous, so the alleles in each plant are different.

F1 generation

genotype:

phenotype:X

Rr Rr

Rr x Rr – crossing heterozygous parents


Rr x Rr – F2 offspring

R r

R

r

RR Rr

Rr rr

What are the phenotypes of the F2 offspring?

?

parental genotype: Rr x Rr

R r R rgametes:

F2 offspring

genotype:


Rr x Rr – F2 phenotypes

F2 genotypes: RR Rr Rr rr



In the F2 generation, 3 of the 4 possible offspring are red. Only one offspring shows the recessive phenotype.

When two heterozygous parents are crossed, the possible offspring will always show a 3:1 ratio in favour of the dominant phenotype.

parental genotype: Rr x Rr


Heterozygous cross activity


Contents

All about alleles

Homozygous cross

Using a test cross

Co-dominance

Heterozygous cross

Inheritance


What is a test cross?

A test cross allows you to find out if an organism showing a dominant characteristic is homozygous or heterozygous for the dominant allele.

For example the genotype of a red flower could be:

RR or Rr

What could you cross a red flower with to find its genotype?


A test cross is carried out between the flower of unknown genotype and another flower whose genotype is known.

For example, a yellow flower can only have the genotype rr because it’s recessive.

So the test cross is:

x

?(RR or Rr)

rr

Carrying out a test cross


Test cross – 2 types

If the red flower is homozygous (RR) then the cross is the same as the first cross (RR x rr). All of the offspring will be heterozygous and have red petals.

What about the other possible cross between a heterozygous red flower (Rr) and yellow flower (rr)?

rr

x

?(RR or Rr)


Test cross offspring

R r r rgametes:

offspringgenotype:

r r

R

r

Rr Rr

rr rr

What are the phenotypes of these offspring?

?

Rr x rrparental genotype:


Test cross results

F2 genotypes: Rr Rr rr rr



A cross between a heterozygous parent and a recessive parent yields different types of offspring in a 1:1 ratio.

Rr x rrparental genotype:


Test cross activity


Contents

All about alleles

Homozygous cross

Using a test cross

Co-dominance

Heterozygous cross

Inheritance


RR WW

What is co-dominance?

Sometimes two alleles are equally dominant to each other.

In genetics, this is called co-dominance and means that neither allele is recessive to the other allele.

How does co-dominance affect the offspring of a cross?

For example, let’s assume that the red allele (R) and the white allele (W) are co-dominant:

x


RR x WW

R R W W

parental genotype:

gametes:

offspringgenotype:

R

R

W W

RW RW

RWRW

The alleles are co-dominant so both are expressed.What will the offspring flowers look like?

Co-dominance cross – offspring

?


RW RW RW RW

Co-dominance cross – phenotypes

offspringgenotypes:

offspringphenotypes:

(Click twice on the buds to reveal the flowers;


All the offspring flowers are pink because both the red and white alleles are expressed.

RR x WWparental genotype:


FLASH 5 – Blood groups

Co-dominance activity


Inheritance multiple-choice quiz

© boardworks ltd 2004 1 of 36 ks4 biology inheritance

Documents

allele pair

different alleles

red petals allele

yellow petals allele

dominant allele

recessive allele

heterozygous alleles

red petals r allele