2. Recap 2
Genes control the characteristics of living
organisms
Genes are carried on the chromosomes
Chromosomes are in pairs, one from each parent
Genes are in pairs
Genes controlling the same characteristics occupy
identical positions on corresponding chromosomes
3. Dominance 3
The gene pairs control one characteristic
But they do not always control it in the same way
Of the gene pair which help determine coat colour
in mice, one might try to produce black fur and its
partner might try to produce brown fur
The gene for black fur is dominant to the gene
for brown fur
4. Symbols
4
The genes are represented by letters
The gene for black fur is given the letter B
The gene for brown fur is given the letter b
BB bb
The genes must have the same letter but the
dominant gene is always in capitals
5. 5
Alleles
The genes of a corresponding pair are called
alleles
This means alternative forms of the same gene
B and b are alleles of the gene for coat colour
B is the dominant allele
b is the recessive allele
6. 6
F1
A black male mouse (BB) is mated (crossed) with a
female brown mouse (bb)
In gamete production by meiosis, the alleles are
separated
Sperms will carry one copy of the B allele
Ova will carry one copy of the b allele
When the sperm fertilizes the ovum, the
alleles B and b come together in the zygote
7. All offspring will
be black (Bb)
B
B
B meiosis
fertilization
sperm mother cell
B B
ovum mother cell b b
b zygote
meiosis
b
b
8. 8
The offspring from this cross are called the F1 (First Filial) generation
They are all black because the allele for black coat colour is
dominant to the allele for brown coat colour
These Bb mice are called heterozygotes. Because the B and b
alleles have different effects; producing either black or brown coat
colour The mice are heterozygous for coat colour
The BB mice are called homozygotes because the two alleles
produce the same effect. Both alleles produce black coats.
The bb mice are also homozygous for coat colour. Both alleles
produce a brown coat colour
The next slide shows what happens when the two heterozygotes
are mated and produce young
9. F2 9
Fertilization Possible combinations
sperms
B
B
B B
BB
b b B
sperm mother cell b
Bb
meiosis
ovum mother cell b
B
B B
Bb
b
b
b
b
ova bb
zygotes
10. Punnett square 10
A neater way of working out the possible combinations
is to use a Punnett Square*
Draw a grid
Enter the alleles in the gametes
Enter the possible combinations
B b female gametes
B BB Bb
male
gametes
These are the
b Bb bb F2 generation
11. 3:1 ratio 11
The offspring are in the ratio of 3 black to 1 brown
Although the BB and Bb mice look identical, the Bb mice will not
breed true. When mated together there is a chance that 1 in 4 of their
offspring will be brown
This is only a chance because sperms and ova meet at random
A litter of 5, may contain no brown mice; in a litter of 12, you might
expect 3 brown mice but you would not be surprised at anything
between 2 and 5.
The total offspring from successive matings of the heterozygotes
would be expected to produce in something close to the 3:1 ratio
For example, 6 successive litters might produce 35 black and
13 brown mice. This is a ratio of 2.7:1, near enough to 3:1
12. 12
Some terminology
The offspring of the heterozgotes are the F2 generation
The genetic constitution of an organism is called its genotype
The visible or physiological characteristics of an organism are
called its phenotype
The phenotype of this mouse is BB
black. Its genotype is BB
The phenotype of this mouse is
Bb
also black, but its genotype is Bb
The phenotype of this mouse is bb
brown. Its genotype is bb
13. 13
These tobacco seedlings are the F2 generation from a cross
Between heterozygous (Cc) parents. C is the gene for chlorophyll.
cc plants can make no chlorophyll. There are 75 green seedlings present.
What is the ratio of green to white seedlings? What ratio would you expect?
14. 14
There are 21 white seedlings. This is a ratio of 75:21 or 3.57:1
C c
You would expect
CC Cc the cross to produce
C
72 green to 24 white
seedlings (3:1)
c Cc cc
1 CC 2 Cc and 1 cc,
a ratio of 3 green to 1 white seedling
Is 3.57:1 near enough to 3:1 ?*
15. Sex chromosomes 15
In most populations of animals there are approximately equal
numbers of males and females
This is the result of a pair of chromosomes; the sex chromosomes
called the X and Y chromosomes
The X and Y chromosomes are a homologous pair but in many
animals the Y chromosome is smaller than the X
Females have two X chromosomes in their cells.
Males have one X and one Y in their cells
At meiosis, the sex chromosomes are separated so the the gametes
receive only one: either an X or a Y.
16. Sex ratio 16
fertilization
meiosis
X
X female
X
X
Y Y male
Y
sperm mother cell X
X X
X female
X
X Y
X
male
ovum mother cell X
17. Single gene effects 17
Very few human characteristics are controlled by a single gene
Characteristics such as height or skin colour are controlled by
several genes acting together
Those characteristics which are controlled by a single gene
are usually responsible for inherited defects (see slide 19)
18. ABO blood groups 18
An exception is the inheritance of the ABO blood group
The IA allele produces group A The IB allele produces group B
The IO allele produces group O IO is recessive to IA and IB
The group A phenotype can result from genotypes IAIA or IAIO
The group B phenotype can result from genotypes IBIB or IBIO
The group O phenotype can result only from genotype IOIO
The AB phenotype results from the genotype IAIB
The alleles IA and IB are equally dominant (co-dominant)
19. 19
Genetic defects
Cystic fibrosis (recessive) Glands of the alimentary canal produce a
thick mucus which affects breathing, digestion and susceptibility to
chest infection
Achondroplastic dwarfism (dominant)The head and trunk grow
normally but the limbs remain short
Albinism (recessive) Albinos cannot to produce pigment in their
skin, hair or iris
Polydactyly (dominant*) an extra digit may be produced on the
hands or feet
Sickle cell anaemia (recessive)The red blood cells become
distorted if the oxygen concentration falls. They tend to block
small blood vessels in the joints
20. 20
Genetic counselling (Genetic defects)
If the genotypes of the parents are known, it is possible to
calculate the probability of their having an affected child
(i.e. one with the defect)
For example if a male achondroplastic dwarf marries a normal
woman, what are their chances of having an affected child?
The father’s genotype must be Dd. (DD is not viable)
The mother must be dd since she is not a dwarf
There is a 50% probability of their having D d
an affected child
d Dd dd
What are the probabilities if both parents d Dd dd
are affected?
21. 21
Cystic fibrosis (recessive)
If two normal parents have an affected child, they must both be
heterozygous (Nn) for the recessive allele n
A nn parent would have cystic fibrosis
N n
A NN parent would produce only normal N NN Nn
children
n Nn nn
Since the parents are now known to be
heterozygous it can be predicted that their
next child has a I in 4 chance of inheriting
the disease
This chance applies to all subsequent children*
22. 22
Sickle cell anaemia (recessive)
Hb = haemoglobin
HbA is the allele for normal haemoglobin
HbS is the allele for sickle cell haemoglobin
A person with the genotype HbSHbS will suffer from
sickle cell anaemia
A person with the genotype HbAHbA is normal
The genotype HbAHbS produces sickle cell ‘trait’ because HbA
is incompletely dominant to HbS
The heterozygote HbAHbS has few symptoms but is a ‘carrier’
for the disease
23. Carriers 23
Heterozygous recessive individuals do not usually exhibit
any disease symptoms but because their offspring may inherit
the disease, the heterozygotes are called ‘carriers’
carriers HbA HbS
HbA HbAHbA HbAHbS
HbS HbAHbS HbSHbS
Similarly, individuals with the genotype Nn are carriers for
cystic fibrosis
24. Family trees 24
It is sometimes possible to work out the genotypes of parents and
to track the inheritance of an allele by studying family trees
= normal female = affected female
= normal male = affected male
Parents have normal phenotypes
but produce
an affected child
For this to happen, both parents must have heterozygous
genotypes (Nn) for the characteristic
25. 25
AA If one of the parents is homozygous
for a dominant allele, all the children
will be affected
If one parent is heterozygous for a
Aa aa dominant allele and the other is
homozygous recessive, there is
a chance that half their children will
be affected
Aa Aa If both parents are heterozygous for
a recessive allele, there is a chance
that one in four of their children
will be affected
26. 26
grandparents
marriage marriage
parents
children
cystic fibrosis
What can you deduce about the genotypes of the grandparents from
this family tree?
27. 27
Cystic fibrosis is caused by a recessive gene
An affected person must therefore have the genotype nn
Since neither of the grandparents is affected, they must be either
NN or Nn genotypes
If they were both NN, none of their children or grandchildren could
be affected
If one was Nn and the other NN, then there is a chance that
50% of their children could be carriers Nn
If one of the carriers marries another carrier, there is a
1 in 4 chance of their having an affected child
The genotypes of the grand parents must be either both Nn or one
NN and the other Nn
28. 28
D d
D DD Dd
d Dd dd
If both parents have the Dd genotype there is a 75% chance
of their having affected children, but the DD individual is
unlikely to survive
29. 29
Question 1
Which of the following are heterozygous genotypes?
(a) Aa
(b) bb
(c) nn
(d) Bb
30. 30
Question 2
Which of these genes are alleles?
A B C
chromosomes
A b c
(a) A and A
(b) A and B
(c) B and C
(d) B and b
31. 31
Question 3
Which of the following processes separates
homologous chromosomes ?
(a) mitosis
(b) cell division
(c) meiosis
(d) fertilization
32. 32
Question 4
Which of the following terms correctly describes
the genotype bb ?
(a) homozygous dominant
(b) heterozygous dominant
(c) homozygous recessive
(d) heterozygous recessive
33. 33
Question 5
What is the likely ratio of affected children born to parents
both of whom are heterozygous for cystic fibrosis ?
(a) 1 affected: 3 normal
(b) 3 affected: 1 normal
(c) 2 affected: 2 normal
(d) all affected
34. 34
Question 6
Which of the following phenotypes corresponds to the
Genotype IAIO ?
(a) Blood group A
(b) Blood group B
(c) Blood group O
(d) Blood group AB
35. 35
Question 7
What is the expected ratio of offspring from
a black rabbit Bb and a white rabbit bb ?
(a) 3 black: 1 white
(b) 1 black: 3 white
(c) 50% white; 50% black
(d) all black
36. 36
Question 8
Which of these Punnett squares correctly represents
a cross between two heterozygous individuals ?
(a) A a (b) A a
A AA aa A AA Aa
a AA aa a Aa aa
(c) (d)
A a a a
A AA Aa A Aa Aa
a Aa Aa a aa aa
37. 37
Question 9
A married couple has a family of 6 boys.
What are the chances that the next child will be a girl ?
(a) 6:1
(b) 1:6
(c) 3:1
(d) 1:1
38. 38
Question 10
Which of the following is a ‘carrier’ genotype for a disease
caused by a recessive gene ?
(a) nn
(b) NN
(c) Nn
39. 39
Question 11
If normal parents have a child with cystic fibrosis
(a) one of them must be heterozygous
(b) both of them must be heterozygous
(c) one of them must be homozygous
(d) both of them must be homozygous
The slide shows tobacco seedlings some of which have no chlorophyll, See slide No.13
* After Dr. R.C. Punnett, Professor of Genetics in Cambridge in the first half of the 20 th century.
The 3:1 ratio is sometimes called a Mendelian ratio after Gregor Mendel (1822-84), an Austrian Monk with an interest in plant breeding and mathematics. He made experimental crosses, particularly of pea plants, to see how characteristics were inherited. He realised that although certain characteristics seemed to disappear in the first generation, they could reappear in the second generation, and so he developed the idea of dominant and recessive characteristics with the recessive characteristics being suppressed in the first generation.. His second generation crosses, involving single characteristics, produced ratios approximating to 3;1. From these results he drew accurate conclusions about the mechanism of inheritance although he knew nothing about chromosomes and genes at this time.
*There are 75 green seedlings + 21 white seedlings. This is the ‘observed result’. If the 3:1 ratio is operating, the ‘expected result’ would be 72 green and 24 white seedlings. A statistical test on these figures tells us that the difference between the observed and expected results is sufficiently small to be ignored and the 75:21 ratio is near enough to 3:1 to be confident that this is the outcome of a genetic process.
This shows that there is a 50% chance of a boy or a girl baby. Because this ratio depends on the chance meeting of X and Y gametes and human families are small, the 1:1 ratio is rarely seen, even in large families. A family with 6 boys may hope for a girl next time but there is still only a 50% chance of getting a girl. In the population as a whole, the 1:1 ratio holds good.
*Even though the allele is dominant, it may not always be fully expressed. X-rays will reveal an extra metacarpal but this may not form a separate digit so the hand looks normal, In some cases only one of the hands shows the extra digit.
The incidence of achondroplasia is about 1 in 30000. These cases arise as a result of a mutation; the affected children are born to normal parents. A mutation is a spontaneous change in a gene or a chromosome. Mutations are mostly harmful.
Even if the parents have five normal children, there is still a 1 in 4 chance of the next child being affected. If the heterozygotes could be detected before they had children, they could be counselled about the likelihood of having affected children. Families at risk (I.e. cystic fibrosis in a relative) may be offered DNA testing to look for one or more of the genes which cause the disease.
