Genes answers
1) Where would you find the DNA in a cell?
(1 mark)
In the nucleus
2) Where would you find a gene
(1 mark)
On the chromosome in the nucleus
3) How many chromosomes are there in the nucleus in an animal cell?
(1 mark)
46 pairs
4) How many pairs of chromosomes are there in the nucleus of an animal cell?
(1 mark)
23 pairs
5) What is the definition of the term allele? Give an example.
(2marks)
 Different versions/form of the same gene (1),
 E.g. Allele for the brown eye characteristic or blue eye characteristic (1)
6) What do we mean by a dominant trait and a recessive trait?
(2 marks)
Note: Everyone will have 2 alleles for every characteristic in the body, one from mum and one
from dad. There is a random cehance of the trait that your parents will pass on, they won’t
always pass the dominant allele to you but if they pass you a dominant trait then that will show
inside you.
 Dominant – the trait/characteristic that will show through (1)
 Recessive – the weaker trait that will not show through if it is passed onto you alongside a
dominant trait. (1)
7) Alleles are represented with letters. How do we represent the dominant allele and the
recessive
allele?
(2 marks)
 First letter of the dominant characteristic with a capitol letter (1)
 First letter of the dominant characteristic but this time with a lower case letter (1)
8) What do we mean by the terms homozygous and heterozygous?
(2 marks)
 Homozygous – 2 alleles that are the same
 Heterozygous – 2 alleles that are different
9) What do we mean by the term genotype and phenotype?
(2+2 marks)
Give examples


Genotype – the alleles that have been inherited (1) e.g. Bb or CC or ff (1)
Phenotype – the physical characteristic that shows through (1) e.g. Blue eyes, brown
eyes, cystic fibrosis, sickle cell trait, (1)
10) Describe the following genotypes – e.g. HH = homozygous dominant
(3 marks)
a) Bb – Heterozygous
b) BB – homozygous dominant
c) bb – homozygous recessive
11) What must the genotype be if the recessive trait is going to show through? Explain your
answer (2
marks)
Homozygous recessive (1)
2 recessive alleles are needed for the characteristic to come through, if a dominant allele is
passed on
then it would show through rather than the recessive allele(1)
12) Carry out a pedigree analysis for a cross between the following genotypes Bb and bb (2
marks)
1 mark for correct genotypes in the correct box (boxes 1-4 all correct
1 mark for correct offspring (boxes 5-8 all correct)
b
(3)
b
(4)
B
(1)
Bb
(5)
Bb
(7)
b
(2)
bb
(6)
bb
(8)
13) B codes for brown eyes and b codes for blue eyes. What is the probability that the offspring
of the
cross above will have blue eyes? Represent as a ratio and a percentage and a chance out of
4
(2 marks)
Probability: 50% blue eyes or 2/4 chance
Ratio:
2: 2
Blue: Brown
14). If D codes for dark moths and d codes for white moths. What is the probability of white
moths
being produced if you had a heterozygous female with a homozygous recessive female? (4
marks)
Note: 1 mark each for correct gametes, boxes 1&2 then 3&4. 1 for correct offspring, 1 for
correct probability
D
d
Probability: 50% or 2/4
(1)
(2)
d
Dd
dd
(3)
(5)
(6)
d
(4)
Dd
(7)
dd
(8)
15a What is sickle cell disease?
(1 mark)
When the haemoglobin in the red blood cell becomes sickle shaped (1)
b What is cystic fibrosis?
(1 mark)
Genetic disorder of the cell membrane, body produces a lot of thick sticky mucus from
air
passages, gut, ducts and pancreas (1)
16a Name 2 symptoms of sickle cell disease and explain what causes these.
(4 marks)
Note: Sufferers of sickle cell disease do not have difficulties with breathing, only difficulties with
delivering oxygen.
 Tiredness (1) – caused by lack of oxygen being delivered to cells (1) (due to the sickle cell
red blood cells being stuck in the capiliiaries).
 Painful joints and muscles (1) - When sickle cells become stuck in the capillaries (1)
16b. Name 2 symptoms of cystic fibrosis and explain what has causes these.
(4 marks)
 Breathing difficulties (1), caused by production of thick sticky mucus in air passages (1)
 Lung infections (1), caused by thick mucus that has trapped bacteria not being able to move
along the air passage and out of the body or into the stomach (1)
 Malnutrition (2) – enzymes can’t work as sticky mucus blocks the ducts where organs release
enzymes so food isn’t broken down.
 Fertility problems (1) – thick sticky mucus blocking reproductive ducts
17. Why is it that someone with sickle cell disease is protected against malaria? (1)
The malaria parasite cannot live on sickle shaped cells (1)
18. If two carriers of sickle cell disease had children, what would be the likelihood of them
producing a
Normal child (non sufferer), a carrier or an affected child?
(4 marks)
Note: 1 mark each for correct gametes, one for correct offspring, 1 mark for correct presentation
of probability or ratio’s
S
(3)
s
(4)
S
(1)
SS
(5)
Ss
(7)
s
(2)
Ss
(6)
ss
(8)
19. Why do people use a pedigree analysis?
(1 marks)
Probability: Carrier: Sufferer: Normal non affected
2
: 1
: 1
To work out the probability that they might have a child that would have a particular
genetic
disorder
The pedigree anlaysis above shows the inheritance of cystic fibrosis.
20a. Look at the pedigree above. What is the genotype of Brian? How do you know?
(2 marks)
Homozygous recessive cc (1) Brian is affected by cystic fibrosis, this means that he must
have
Inherited both recessive alleles
20b. What are the chances of Sam being a carrier of cystic fibrosis?
(4 marks)
c
(3)
c
(4)
C
(1)
Cc
(5)
Cc
(7)
c
(2)
cc
(6)
cc
(8)
Probability: Carrier: Sufferer: Normal non affected
2
: 2
: 0
50% chance of being a carrier.