This document discusses heredity and genetics concepts including:
1) Genes are located on chromosomes and are passed from parents to offspring. Genes come in pairs and determine characteristics.
2) Dominant genes will be expressed even if only one allele is present, while recessive genes require two copies to be expressed.
3) Crossing heterozygous parents (with one dominant and one recessive allele) results in a 3:1 ratio of offspring displaying the dominant versus recessive trait.
4) Sex is determined by X and Y chromosomes, with females having two X chromosomes and males having one X and one Y.
2. Terms you should know
• CHROMOSOME: thread of DNA, made up of a string of
genes.
• GENE: a length of DNA that is the unit of heredity and
codes for a specific protein. A gene may be copied and
passed on to the next generation.
• ALLELE: any of two or more alternative forms of a gene.
• HAPLOID NUCLEUS: a nucleus containing a single set
of unpaired chromosomes (e.g. sperm and egg)
• DIPLOID NUCLEUS: a nucleus containing two sets of
chromosomes (e.g. in body cells)
3. 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
4. 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.
5. 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.
6. 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
7. Terms you should know:
• GENOTYPE: genetic makeup of an organism in term of
the alleles present ( e.g. Tt or GG).
• PHENOTYPE: physical or other features of an organism
due to both its genotype and its environment (e.g. tall
plant or green seed)
• HOMOZYGOUS: having two identical alleles of a
particulat gene (e.g. TT or gg).Two identical homozygous
individuals that breed together will be pure-breeding.
• HETEROZYGOUS: having two different alleles of a
particular gene (e.g. Tt or Gg), not pure- breeding.
• DOMINANT: an allele that is expresed if it is present
(e.g. T or G)
• RECESSIVE: an allele that is only expresses when there
is no dominant allele of the gene present. ( e.g t or g )
8. 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
9. 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
10. 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
11. 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
12. 10
Punnett square:
•A neater way of working out the possible combinations
is to use a Punnett Square*:
1. Draw a grid
2. Enter the alleles in the gametes
3. Enter the possible combinations
B b female gametes
B BB Bb
male
gametes
These are the
b Bb bb F2 generation
13. 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
14. 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
15. 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?
16. 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 ?*
17. 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.
18. 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
19. 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)
20. 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)
21. 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
22. 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?
23. 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*
24. 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
25. 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
26. 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
27. 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
28. 26
grandparents
marriage marriage
parents
children
cystic fibrosis
What can you deduce about the genotypes of the grandparents from
this family tree?
29. 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
30. 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
31. 29
Question 1
Which of the following are heterozygous genotypes?
(a) Aa
(b) bb
(c) nn
(d) Bb
32. 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
33. 31
Question 3
Which of the following processes separates
homologous chromosomes ?
(a) mitosis
(b) cell division
(c) meiosis
(d) fertilization
34. 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
35. 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
36. 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
37. 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
38. 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
39. 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
40. 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
41. 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