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Biology – Form 5 � Page 17 Ms. R. Buttigieg
2 DNA – the blueprint of life
See GCSE Biology Chapter 21-23 Genetics and heredity
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DNA stands for deoxyribonucleic acid. It was first isolated by Johann Friedrich Miescher
in 1868. Its structure was described by James Watson and Francis Crick in the 1950's. They
described DNA molecule as being a twisted ladder, or double helix.
�ucleic Acids - are large, complex molecules made from smaller molecules called
nucleotides (just like proteins are made from amino acids). Apart from DNA, an example
of another nucleic acid is RNA – ribose nucleic acid.
Each nucleotide is made of:
� A sugar
� A nitrogenous base
� A phosphate group
Several nucleotides can join to form polynucleotides or nucleic
acids
In DNA we find four 4 different nucleotides due to 4 different nitrogenous bases:
• Adenine (A)
• Thymine (T)
• Cytosine (C)
• Guanine (G)
These always pair in the following way
Adenine + Thymine
Cytosine + Guanine
These bases join together by hydrogen bonds and form the rungs of the twisted ladder. The
phosphates and sugar deoxyribose form the backbone.
Biology – Form 5 � Page 18 Ms. R. Buttigieg
So, the DNA molecule is made up of a number of smaller molecules called nucleotides.
These nucleotides in turn combine to form nucleic acids.
The phosphate group of one nucleotide joins to the sugar of the next. The bases attached to
the sugar groups stick out from the strand.
In DNA, two strands are bonded together to form a double helix, which is made up of 2
intertwined strands.
DNA is found in the chromosomes of every cell. These can be seen only during cell division
due to the heavy coiling of chromosomes. (from www.bioclix.org)
The role of D�A in protein synthesis.
The sequence of bases on the DNA strand determines the sequence of amino acids and therefore the
type of protein (i.e. characteristics) that will be produced.
Biology – Form 5 � Page 19 Ms. R. Buttigieg
In a DNA strand, every 3 bases on the chain, code for a particular amino acid. This is known as
the triplet code of D�A. The kind of proteins produced in the organism depend on the sequence
of bases on the DNA molecule. A gene is a sequence of nucleotides on the DNA molecule,
which code for an entire protein. The longer the protein, the longer the gene.
A gene is as a section of D�A controlling an identifiable characteristic e.g. eye colour
Structure and function of chromosomes
� Along the length of chromosomes there are many structures called genes
� Genes consist of a strand of DNA which codes for an entire protein
� The genes provide the instructions for the cell e.g. a gene may instruct the stomach to produce
the enzyme pepsin.
� A chromosome is made of 2 chromatids joined at the centromere
� Different organisms contain a different number of chromosomes.
� This number is the same in all body cells except in the gametes
(sex cells)
� Chromosomes occur in pairs having similar shape and size -
homologous
� Humans have 46.
o Body cells all have a diploid (2n) number of
chromosomes – 46 in our case
o Gametes have a haploid (n) number of chromosomes –
half the number – 23 in our case
Biology – Form 5 � Page 20 Ms. R. Buttigieg
Alleles as alternative forms of a gene.
A human egg cell and a human sperm cell both contain 23 single chromosomes. Fertilisation brings
these two sets of single chromosomes together to make 23 pairs of chromosomes in the embryo.
Each of these pairs of chromosomes contains genes inherited from the father and genes inherited
from the mother, and these genes are in pairs, both coding for the same characteristic - so you have
two genes controlling eye colour, for example. These different forms of the same gene are called
alleles (pronounced al-eel). The gene for eye colour has an allele for blue eye colour and an allele
for brown eye colour.
� If both the alleles for a particular characteristic are the same
they are called homozygous.
� If they are different from each other they are called
heterozygous.
� Alleles may be either dominant or recessive.
� A dominant allele always shows.
� A recessive allele only shows when it is on its own
(because it's on a sex chromosome) or when both the alleles
are the same (homozygous).
Summary
Biology – Form 5 � Page 21 Ms. R. Buttigieg
Biology – Form 5 � Page 22 Ms. R. Buttigieg
Mutations can occur on a chromosome or on a gene level.
Gene mutation
Can result when one base is changed with another e.g. thymine instead of cytosine.
A gene controls production of the skin pigment melanin. This protects the skin from UV light. In
albinos, the structure of the gene changes or mutates. So it no longer codes for the production of
melanin.
Cystic fibrosis is also a disease caused by a gene mutation.
Chromosome mutation
Biology – Form 5 � Page 23 Ms. R. Buttigieg
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This is important for:
� The body to grow
� Passing chromosomes from one cell to another
There are 2 main types of cell division:
1. Meiosis occurs in the reproductive organs.
2. Mitosis occurs in all the other cells (somatic cells)
Mitosis occurs in:
� Growth
� Asexual reproduction e.g. in the Amoeba
For a more detailed explanation see GCSE Biology pg. 183, figure 21.2
For a more detailed explanation of Meiosis see GCSE Biology pg. 185, figure 21.7
See GCSE Biology Chapter 21
Biology – Form 5 � Page 24 Ms. R. Buttigieg
Biology – Form 5 � Page 25 Ms. R. Buttigieg
See also the Table on GCSE Biology pg. 186
Mitosis Meiosis
One division Two divisions
Two (2) daughter cells per cycle Four (4) daughter cells per cycle
Daughter cells genetically identical
Called clones Daughter cells genetically different
Same chromosome no. as parents Chromosome no. half that of parents
Occurs in somatic cells Occurs in reproductive cells
Throughout life cycle Completed after sexual maturity
Used in growth, repair, asexual reproduction
Sexual reproduction, new gene combinations
Work out GCSE Biology pg. 186 number 1-5
1. Chromosomes occur in pairs in all cells except the gametes.
a) What are alleles?
b) Why are there 2 alleles for each character?
c) Explain what is meant by dominant and recessive alleles.
2. Red/green colour blindness is sex-linked and caused by a recessive allele.
One in 80 males are colour blind, but only 1 in 6400 females.
a) On which chromosome is the allele for colour blindness found?
b) Explain why this condition occurs far more often in males than in
females.
Biology – Form 5 � Page 26 Ms. R. Buttigieg
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� In asexual reproduction there is no variation as clones (identical cells) are produced by mitosis.
� In sexual reproduction we get new gene
combinations in individuals so all offspring are
different and have a unique genetic set. When the
sperms (haploid – 23 chromosomes) and the ova
(haploid – 23 chromosomes in each nucleus) join
together we say that fusion of gametes has occurred
to give the diploid chromosome number of 46.
This variation results from two important processes known as independent assortment and
random variation.
Independent assortment
This occurs when during meiosis half the chromosomes go to one gamete and half to the other
gamete. These go randomly so for example a gamete may get:
5 maternal and 18 paternal chromosomes
12 maternal and 11 paternal chromosomes.
Random variation
The sperm fertilising the ova does so randomly.
Sometimes harmful genes present in some sperms
or ova may never get fertilized and they are lost and
not expressed in the offspring. So in this case the
variation is not inherited.
As a result of both independent assortment and random fertilization each individual has a different
set of genes and this gives rise to variation even within a species.
� Draw a diagram to show the structure of the D�A molecule.
� What does a nucleotide consist of?
� Define: chromosome, gene, chromatid, centromere, homologous chromosomes, diploid cell.
Biology – Form 5 � Page 27 Ms. R. Buttigieg
Continuous and discontinuous variation
Variation, the small differences that exist between individuals, can be described as either
continuous or discontinuous. These differences can be due to the environment (non-inherited) or
heredity.
CO�TI�UOUS VARIATIO� - In continuous variation
there is a complete range of measurements from one
extreme to the other. Other examples include:
• Weight
• Hand span
• Length of feet
• Milk yield in cows
Continuous variation is the combined effect of
many genes and is significantly affected by
environmental influences. Milk yield in cows, for
example, is determined not only by breeding
programs but is also significantly affected by environmental factors such as pasture quality and diet,
weather, and the comfort of their surroundings.
DISCO�TI�UOUS VARIATIO� - This is where individuals fall into a number of distinct classes or
categories, and is based on features that cannot be measured across a complete range. You either
have the characteristic or you don't. Blood group is a good example: you are either one blood group
or another - you can't be in between. Another example is whether you are a male or a female.
Discontinuous variation is controlled by alleles of a single gene or a small number of genes.
Continuous variation Discontinuous variation
Continuous range of values Distinct values without intermediates
Influenced mainly by environment Influenced mainly by genes
Usually controlled by many genes Controlled by one/two genes
E.g. height, weight, intelligence, skin colour E.g. blood group, eye colour, ability to roll tongues, sex
Biology – Form 5 � Page 28 Ms. R. Buttigieg
Choose the correct answer from the following
1. Which part of a cell carries the information that is passed from one generation to the next?
A. Cell B. Nucleus C. Cytoplasm D. Chloroplast
2. Why are identical twins different from non-identical twins?
A. They look the same
B. They look different
C. They have the same genes
D. They have the same eggs
3. Which of the following is the word used to describe what happens when the nucleus of a sperm
joins with the nucleus of an egg cell?
A. Sex B. Fertilization C. Intercourse D. Reproduction
4. Which of the following statements is true about the variation?
A. Variation is caused by genes
B. Variation is caused by the environment
C. Variation can caused by both genes and the environment
D. None of the above
5. There are two types of variation. These are continuous and discontinuous variation.
Which of the following statements is false?
A. Continuous variation produces a spread of variation within a population
B. Discontinuous variation is the result of genetics alone
C. Blood group is an example of discontinuous variation
D. Discontinuous variation is influenced by the environment
6. Some types of variation are due to changes in the genetic material. What is this type of
change called?
A. Fertilization B. Mutation C. Radiation D. Sterilization
7. Some people can roll their tongues whilst others cannot. What is this an example of?
A. Genetically inherited characteristic
B. Continuous variation
C. Adaptation
D. Specialization
8. Which type of variation can be presented as a line graph?
A. Continuous B. Genetic C. Discontinuous D. Adapted
Biology – Form 5 � Page 29 Ms. R. Buttigieg
Answer these on your A4 notebook
1. In table form give 3 differences between mitosis and meiosis.
2. Give 1 similarity between mitosis and meiosis.
3. Explain (in a paragraph) why meiosis is important in sexual reproduction
4. What are mutations? Distinguish between chromosome and gene mutations and
give an example of each.
5. What are mutagens? Give an example.
6. Give 2 sources of variation in sexual reproduction.
7. Distinguish between continuous and discontinuous variation and give an example
of each.
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Some definitions you must be sure of:
1. Genotype – The genes that an organism possesses. The set of alleles inherited from both
parents.
2. Phenotype – The individual’s observed appearance.
3. Dominant allele – Always expressed. Shown by a capital letter e.g. B
4. Recessive allele – Not expressed in the phenotype when present with a dominant allele of the
same type. Shown by a small letter e.g. b
5. Homozygous genotype – the individual has inherited two identical alleles e.g. BB
(homozygous dominant) or bb (homozygous recessive)
6. Heterozygous genotype – the individuals has inherited two different alleles e.g. Bb
Genetics is the study of heredity and variation in organisms. We begin with
a study of the monohybrid cross, invented by Mendel. In a monohybrid
cross, organisms differing in only one trait are crossed.
Our objective is to understand the principles that govern inheritance in plants
and animals, including humans, by solving problems related to the
monohybrid cross.
Biology – Form 5 � Page 30 Ms. R. Buttigieg
Biology – Form 5 � Page 31 Ms. R. Buttigieg
First draw a Punnett square then fill it in to see the available possibilities
Give the percentages for possible genotypes and phenotypes
Child Phenotypes
Brown ______%
Blue ______%
Child Genotypes
Biology – Form 5 � Page 32 Ms. R. Buttigieg
Work out the following:
a. Using the symbol W = normal wing and w = short wing:
i. The genotype of a fly which is heterozygous for this character ______
ii. The possible genotypes of its gametes __________________________
iii. Work out what kind of offspring would be produced if a heterozygous fly mated
with one which was homozygous for normal wing.
b. A red flowered tulip was crossed with a white-flowered tulip. The seeds were collected and all
grew into red tulips.
i. Which colour was dominant? __________________
ii. What was the genotype of the offspring? ___________________
iii. Some of the offspring were self-fertilised. The plants produced had 3 times as
many red as white flowers. Use a Punnett square to explain this cross.
c. A Roman nose is dominant to a straight nose. A man with a Roman nose marries a woman
with a straight nose. The man’s mother also had a straight nose. Calculate the chance of one of
their children also having a straight nose.
Biology – Form 5 � Page 33 Ms. R. Buttigieg
Doing a recessive backcross
Red flowered plants can have 2 possible genotypes RR or Rr. So when you see a red flower how
can you tell which is its genotype?
One way of finding out is by crossing these red flowers with the original white flowered
homozygous recessive plants (rr). This is called a recessive back cross. If the red flowered plant is
RR, then all the offspring should be red (Rr). If however the red flowered plant is Rr, then we will
get a mixture of both red-flowered (Rr) and white flowered plants (rr)
White (rr) x Red (RR) White (rr) x Red (Rr)
Breeding True
Individuals which have the same alleles for a characteristics e.g. NN or nn are said to breed true.
When you cross two pure breeds which are different e.g. NN with nn you get hybrids. Hybrids do
not breed true.
Pedigrees – the tracing of a particular characteristic in an individual’s ancestry. Some features
persist in families, for from a pedigree you can work out the genotypes of the various individuals.
A pedigree chart is a chart which
tells you all of the known
phenotypes for an organism and its
ancestors, most commonly
humans, show dogs, and race
horses.
Biology – Form 5 � Page 34 Ms. R. Buttigieg
Codominance - Situation in which two different alleles for a genetic trait are both expressed.
An example of this is people who have an AB blood type for the ABO blood system. When they are tested, these individuals actually have the characteristics of both type A and type B blood. Their phenotype is not intermediate between the two.
� A and B are equally dominant to each other, while O is recessive to both of them.
� When A and B are present together in a genotype, both will be expressed to effect the
phenotype.
� The symbols used are: IA – allele A, I
B – allele B, i – allele O
Suppose a man who is heterozygous for blood group A marries a woman who is heterozygous for
blood group B. Give the ratio of the genotype of their children.
Other examples of codominance include coat colour in cattle (red/white/roan), and coat colour in
cats (black/orange/tortoiseshell).
Another example of codominance is sickle cell haemoglobin in humans. The gene for haemoglobin Hb has
two codominant alleles: HbA (the normal gene) and Hb
S (the mutated gene). There are three phenotypes:
HbAHb
A Normal. All haemoglobin is normal, with normal red blood cells. Hb
AHb
S Sickle cell trait. 50% of the haemoglobin in every red blood cell is normal, and 50% is
abnormal. The red blood cells are slightly distorted, but can carry oxygen, so this
condition is viable. However these red blood cells cannot support the malaria parasite, so
this phenotype confers immunity to malaria. Hb
SHb
S Sickle cell anaemia. All haemoglobin is abnormal, and molecules stick together to form
chains, distorting the red blood cells into sickle shapes. These sickle red blood cells are
destroyed by the spleen, so this phenotype is fatal.
Biology – Form 5 � Page 35 Ms. R. Buttigieg
Work these on your A4 notebook.
1. A heterozygous woman of blood group A marries a man of blood group O. Carry out a
cross to find the genotype of their children.
2. A man and a woman both having the sickle cell trait have a child. Carry out a cross to
determine the genotype of this child. What chance of surviving does this child have?
Sex determination and sex-linkage
Sex Determination
Sex is determined by the sex chromosomes (X and Y). In humans the sex chromosomes are homologous in
females (XX) and non-homologous in males (XY), though in other species it is the other way round. The
inheritance of the X and Y chromosomes can be demonstrated using a monohybrid cross:
This shows that there will always be a 1:1 ratio of males to females. Note that female gametes (eggs) always
contain a single X chromosome, while the male gametes (sperm) can contain a single X or a single Y
chromosome. Sex is therefore determined solely by the sperm and in the testes, an approximately equal
number of X and Y sperms is produced. There are techniques for separating X and Y sperm, and this is used
for planned sex determination in farm animals using IVF.
(http://www.biologymad.com/master.html?http://www.biologymad.com/GeneticsInheritance/geneticsinheritance.htm)
Sex-linkage Certain harmful recessive alleles e.g. red-green colour blindness and haemophilia are carried on
the X chromosome. Therefore if a woman inherits a harmful allele on one of her X chromosomes,
it will be masked by the dominant allele on the other X chromosome. The woman is said to be a
carrier, but is not herself affected by it. She can however pass it on to her children.
Biology – Form 5 � Page 36 Ms. R. Buttigieg
If a man inherits a defective allele on his X chromosome he will be suffering from the disease as it
cannot be masked due to the short Y chromosome. A male can never be a carrier of such sex-linked
diseases. So such diseases are more common in males than females.
A well-known example of a sex-linked characteristic is colour blindness in humans. 8% of males are colour
blind, but only 0.7% of females. The genes for green-sensitive and red-sensitive rhodopsin are on the X
chromosome, and mutations in either of these lead to colour blindness. The diagram below shows two
crosses involving colour blindness, using the symbols XR for the dominant allele (normal rhodopsin, normal
vision) and Xr for the recessive allele (non-functional rhodopsin, colour blind vision).
Normal female - XR X
R; Carrier female - X
RXr; Normal male - X
R Y; Colour blind male – X
rY
Show the possibilities when (a) a carrier female marries a normal male (b) a normal female marries a
colour blind male (c) a carrier female marries a colour blind male.
Other examples of sex linkage include haemophilia, premature balding and muscular dystrophy.
On your A4 notebook work out this problem: A man sick with haemophilia marries a normal
woman, who is not a carrier. Carry out the cross between these two to show the genotype of their
children.
Biology – Form 5 � Page 37 Ms. R. Buttigieg
Genetic problems
1. The human lung condition cystic fibrosis is controlled by a pair of alleles. Only those who
are homozygous for the recessive allele suffer from the disease. Heterozygous individuals don’t
suffer but are carriers.
i. Using the symbol N for the dominant allele and n for the recessive one, write the
genotypes of (a) the carrier and (b) someone who suffers from the disease.
ii. What percentage of children from parents who are both carriers, will be likely to be
carriers?
2. In sheep, black wool is caused by a recessive allele b and white wool by a dominant allele
B. In a flock of sheep there were some black males and a lot of heterozygous females.
i. Give the genotype of the black male sheep
ii. Give the genotype of the females.
iii. What percentage of the young would you expect to have black wool?
iv. Explain how you could find out if the genotype of a sheep with white wool was
homozygous or heterozygous.
3. The flower colour of a plant is controlled by a pair of alleles. When a red-flowered plant
was pollinated with pollen from a white flowered plant, the resulting offspring all had pink flowers.
Use the symbol CR for the allele controlling red colour and C
W for the allele controlling white
colour.
i. What is the genotype of the white flowered parent plant?
ii. Give the genotype of the pink flowers.
iii. Give the genotypes of the young produced by crossing a
red flowered parent plant with a pink flowered plant.
Give the ratio.
4. A homozygous brown-eyed man marries a homozygous blue-
eyed woman. What will be the percentage of the genotypes and
phenotypes of their children?
Biology – Form 5 � Page 38 Ms. R. Buttigieg
Genetics JL 2000-2003 Exam Questions
1. Read the following paragraph and answer the questions below:
When the mechanism of inheritance of flower colour in garden peas was investigated, red-flowered
plants were crossed with white-flowered plants. The first generation of plants all had red flowers.
However, when these red-flowered plants were allowed to self-fertilise, about 25% of the offspring
had white flowers, the remainder having red flowers.
In a similar investigation with snapdragon plants, when red-flowered plants were crossed with
white-flowered plants, the resulting first generation all had pink flowers. When these pink-flowered
plants were self-fertilised, 25% of the offspring had white flowers, 25% had red flowers and 50%
had pink flowers.
a. Suggest why the results obtained with the garden pea are different from those with the
snapdragon plants. (2)
b. Using symbols and a written explanation, account fully for the result obtained with garden peas.
(4)
c. What would be the results of interbreeding the white garden peas. (2)
d. Draw a genetic diagram, to show how the results for the snapdragon can be explained genetically.
(3)
e. Write down the genotype of the snapdragon plants with red flowers. (1)
f. Describe an experiment which you could perform to establish the genotype of the garden peas
plants with red flowers. (3)
(15 marks)
2. a. Draw and label a large diagram to show the structure of a named insect pollinated flower you
have studied. (5)
b. State the functions of 2 parts of the flower that you have labelled. (2)
c. Suggest two (2) ways by which an insect-pollinated flower might differ from a wind pollinated
flower. (2)
d. The flower colour of a certain species of plant is controlled by a single pair of alleles. When a
red-flowered plant was pollinated with pollen from a white-flowered plant, the resulting offspring
all had pink flowers.
Biology – Form 5 � Page 39 Ms. R. Buttigieg
Use the symbol R for allele controlling red colour and W for the allele controlling white colour.
(i) Give the genotype of the white-flowered parent plant.(1)
(ii) Give the genotype of the pink-flowered plant. (1)
(iii) Give the genotypes of the progeny/offspring produced by crossing the red flowered parent plant
with one of the pink-flowered plants and the ratio in which they appear. (2)
(iv) Give the genotypes of the progeny/offspring produced by self-pollination of two of the pink-
flowered plants and the ratio in which they appear. (2)
(total 15 marks)
3. The family tree below shows the pattern of inheritance of a genetic disorder.
a) Is the disease controlled by a dominant or a recessive allele (� or n)?
__________________ (1)
b) Give a reason to support your answer to ‘a’.
_____________________________________________________________________________ (1)
c) It is unlikely that the disease being referred to in the above diagram is sex linked. Give a reason.
_____________________________________________________________________________ (1)
d) Name a sex linked disease. _________________________________________ (1)
e) What must be the genotype of:
i) Elaine __________ ii) her mother ______________ iii) her father ________(3)
f) What might Simon’s genotype be? Give a reason.
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
_____________________________________________________________ (2) Total 9 marks