St Michael’s RC School
Department of Science
Chromosomes, genes and alleles
Differences in the characteristics of different individuals of the same kind may
be due to differences in:

the g_______ they have inherited (genetic causes);
 the conditions in which they have developed (e______________________
causes) or a combination of both.
The nucleus of a cell contains c____________________. Chromosomes carry
g________ that control the characteristics of the body. Each
c_________________ carries a large number of genes.
Many genes have different forms called alleles, which may produce different
c___________________ .
In body cells the c__________________ are normally found in pairs.
Body cells divide to produce additional cells during g____________ or to
produce replacement cells.
1
2
3
Questions
Why do young organisms resemble
their parents?
What are gametes?
5
How is inherited information
carried between parents and their
young?
Give two reasons why individuals of
the same species are different
from each other.
Where are chromosomes found?
6
What do chromosomes do?
7
What are alleles?
8
What are found in pairs in all body
cells except gametes?
4
ANSWERS
1
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St Michael’s RC School
Department of Science
Which human characteristics show a simple pattern of inheritance?
What sex human beings are, and whether or not they inherit certain diseases,
show a very simple pattern of inheritance.
In human body cells, one of the 23 pairs of c________________ carries the
genes which determine sex. In f___________ the sex chromosomes are the
same (XX). In m_________ the sex chromosomes are different (XY).
9
Questions
Draw a diagram to show the sex
chromosomes that would be present
in a testis cell.
10
Draw a diagram to show the sex
chromosomes that would be present
in an ovary cell.
11
Draw the two gametes that on fusion
would produce a female offspring.
12
Draw the two gametes that on fusion
would produce a male offspring.
ANSWERS
2
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St Michael’s RC School
Department of Science
How are characteristics other than gender controlled?
Certain characteristics are controlled by one pair of g________ . Some genes have
two different forms called alleles. e.g. the allele for brown eyes and the allele for
____________________ .
An allele which controls the development of a characteristic when it is present on only
one of the c__________________ is a dominant allele.
e.g. B = the allele for brown.
So the pair of alleles BB = __________ and the pair of alleles Bb = _________
An allele which controls the development of characteristics only if it is present on
both of the c_______________________ is a recessive a__________ . e.g. b = the
allele for blue so the pair of alleles bb = _____________________
Questions
ANSWERS
13
What is an allele?
14
What is meant by a dominant allele?
15
What is meant by a recessive allele?
16
If T = the allele for tongue rolling and t is the allele for non-tongue rolling, what
are the characteristics of people with the following pairs of alleles.
TT
Tt
tt
3
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St Michael’s RC School
Department of Science
Inherited disorders
Huntington's disease is caused by a d____________ allele of a gene and can
therefore be passed on by only o_____ parent who has the disorder.
Cystic fibrosis It is a disorder of cell membranes. It must be inherited from
both parents. It is caused by a r____________ allele of a gene and can
therefore be passed on by p__________, neither of whom has the
d____________ . Draw a diagram to prove this.
Sickle-cell anaemia is a disorder of the r_______ blood cells. It reduces the
oxygen-carrying capacity of blood. It is caused by a recessive a___________.
Being a carrier of the allele can be advantageous in countries where malaria is
prevalent.
1
Questions
What is cystic fibrosis?
ANSWERS
2
What causes cystic fibrosis?
3
What is Sickle-cell anaemia?
4
What causes Sickle-cell anaemia?
5
For which disease is it an advantage
to be a carrier?
6
Which disease can be passed on by
only one parent?
7
Which two diseases can be passed
on by two healthy parents?
4
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St Michael’s RC School
Department of Science
Chromosomes and alleles.
If both chromosomes in a pair contain the same a__________ of a gene, the
individual is homozygous for that gene. e.g. BB or bb.
If the chromosomes in a pair contain different alleles of a g_________ , the
individual is heterozygous for that gene. e.g. Bb.
1
2
Questions
How do we describe the situation where both
alleles of a gene are identical on a pair of
chromosomes?
If an individual is described as being heterozygous
for a gene, what does this mean?
ANSWERS
Answer the questions below on lined paper. You must set out the diagram in the
standard way ….. parents characteristics: parents genes: gametes genes:
offspring genes; offspring characteristics.
3
Draw a diagram to show the outcome of a cross between an individual who is
a carrier of cystic fibrosis and a healthy individual who is homozygous.
4
Draw a diagram to show the outcome of a cross between an individual who is
a carrier of cystic fibrosis and a individual who has cystic fibrosis.
5
Draw a diagram to show the outcome of a cross between tow individuals who
are carriers of cystic fibrosis.
6
Draw a diagram to show the outcome of a cross between an individual who is
a carrier of sickle cell anaemia and a healthy individual who is homozygous.
7
Draw a diagram to show the outcome of a cross between an individual who is
a carrier of sickle cell anaemia and a individual who has sickle cell anaemia.
8
Draw a diagram to show the outcome of a cross between tow individuals who
are carriers of sickle cell anaemia.
5
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St Michael’s RC School
Department of Science
Genetic engineering
Genes control the production of all proteins in c______ .
Genes from the c_______________ of humans and other organisms can be 'cut
out' and transferred to b______________ cells. The transferred g_______
continues to make the same protein in the bacterial cell.
By culturing bacteria on a l__________ scale, commercial quantities of
p__________ can be produced.
This process of g_________ e___________________ is used in the
manufacture of drugs and hormones including human i___________ which is
used to treat people with the medical condition diabetes mellitus.
Genes can also be transferred to the cells of animals or p_________ at an early
stage in their d________________ so that they develop with desired
characteristics. e.g. resistance to disease.
You should be able to make informed judgements about the economic, social,
and ethical issues concerning genetic engineering.
1
Questions
What do genes control?
2
Where are genes found?
3
Why are bacteria used in genetic
engineering?
4
Give one example of a hormone that
can be produced on a large scale by
genetic engineering.
What are the concerns about genetic
engineering?
5
ANSWERS
6
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St Michael’s RC School
Department of Science
Find and copy a diagram to illustrate genetic engineering. Identify 1-4 below on the
diagram.
1. In genetic engineering, scientists use enzymes to cut out a piece of DNA that
contains a gene of interest. For example, the gene controlling insulin production.
2. The chromosome of the bacteria is treated with the same enzyme and joins onto
the human DNA.
3. The engineered chromosome is replaced in the bacterial cell.
4. A large number of daughter cells can be cultured and the human gene continues
to make insulin (a protein) which can be extracted for human use.
Questions
5
Put the following labels onto the diagram: bacterial cell, human cell, human
chromosome, nucleus, bacterial DNA, engineered chromosome, cell division.
6
Why do genetic engineers
use enzymes?
7
What do you understand
by ‘the engineered
chromosome’?
8
Why is the engineered
chromosome replaced
back into the bacterial
cell?
What concerns do people
have about genetic
engineering?
ANSWERS
9
7
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St Michael’s RC School
Department of Science
Evolution by Natural Selection
Individual organisms within a particular s__________ may show a wide range of
variation because of differences in their g________.
The population of the species will be controlled by predation, d____________
and competition. As a result of this large numbers of individuals within the
population will d____ .
Those individuals with characteristics most suited to the e________________
are more likely to survive and b________ successfully
Those
genes which have enabled these individuals to survive will be then
passed on to the next g____________________ .
This process is called n___________ s____________ and it gives rise to
e_____________ .
Mutations
Most m___________ are harmful.
If mutations occur in reproductive cells, e.g. e_____ or s_______ , young may
develop abnormally or d____ at an early stage of their development.
If mutations occur in body c_______ , these may start to multiply in an
uncontrolled way and invade other parts of the body. This is called c________ .
Some mutations are neutral in their effects and, in rare cases, a mutation may
i_____________ the chances of survival of an organism and any offspring
which inherit the m__________ gene. An example of this is the gene for ______
____ _____________ .
The gene for sickle cell anaemia is a mutant gene n the same way that the
brown eye gene is a mutant gene.
8
d:\116103805.doc
St Michael’s RC School
Department of Science
1
Questions
Why do individuals of a species vary
from one another?
2
Give three ways in which the size of a
population may be limited ?
3
Which individuals in a population are
most likely to survive ?
4
What must happen to ensure that
advantageous genes are passed on
to future generations?
5
What are the effects of a mutation in
reproductive cells?
6
Which mutations are useful?
7
Why are a large number of African
people heterozygous for the gene for
sickle- cell anaemia?
ANSWERS
9
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