Heredity

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Heredity
Heredity
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Recap
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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
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Dominance
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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
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Symbols
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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
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Alleles
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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
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F1
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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
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All offspring will be black (Bb)
meiosis
fertilization
sperm mother cell
ovum mother cell
zygote
meiosis
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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
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F2
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Possible combinations
Fertilization
sperms
BB
sperm mother cell
Bb
meiosis
ovum mother cell
Bb
bb
ova
zygotes
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Punnett square
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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
female gametes
BB
Bb
male gametes
These are the F2 generation
Bb
bb
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31 ratio
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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 31 ratio
For example, 6 successive litters might produce
35 black and 13 brown mice. This is a ratio of
2.71, near enough to 31
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Some terminology
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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 black. Its
genotype is BB
BB
The phenotype of this mouse is also black, but
its genotype is Bb
Bb
The phenotype of this mouse is brown. Its
genotype is bb
bb
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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?
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There are 21 white seedlings. This is a ratio of
7521 or 3.571
c
C
You would expect the cross to produce 72 green
to 24 white seedlings (31)
C
c
cc
1 CC 2 Cc and 1 cc, a ratio of 3 green to 1
white seedling
Is 3.571 near enough to 31 ?
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Sex chromosomes
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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.
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Sex ratio
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fertilization
meiosis
female
male
sperm mother cell
female
male
ovum mother cell
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Single gene effects
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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)
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ABO blood groups
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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)
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Genetic defects
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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
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Genetic counselling
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(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 fathers 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 an
affected child
What are the probabilities if both parents are
affected?
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Cystic fibrosis
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(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
A NN parent would produce only normal children
NN
Nn
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
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Sickle cell anaemia
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(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
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Carriers
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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
HbAHbS
HbSHbS
HbS
Similarly, individuals with the genotype Nn are
carriers for cystic fibrosis
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Family trees
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It is sometimes possible to work out the
genotypes of parents and to track the inheritance
of an allele by studying family trees
Parents have normal phenotypes
but produce
an affected child
For this to happen, both parents must have
heterozygous genotypes (Nn) for the
characteristic
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If one of the parents is homozygous for a
dominant allele, all the children will be affected
If one parent is heterozygous for a dominant
allele and the other is homozygous recessive,
there is a chance that half their children
will be affected
If both parents are heterozygous for a recessive
allele, there is a chance that one in four of
their children will be affected
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grandparents
marriage
marriage
parents
children
cystic fibrosis
What can you deduce about the genotypes of the
grandparents from this family tree?
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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
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d
D
D
DD
Dd
Dd
dd
d
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
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Question 1
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Which of the following are heterozygous genotypes?
(a) Aa
(b) bb
(c) nn
(d) Bb
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Question 2
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Which of these genes are alleles?
chromosomes
(a) A and A
(b) A and B
(c) B and C
(d) B and b
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Question 3
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Which of the following processes separates
homologous chromosomes ?
(a) mitosis
(b) cell division
(c) meiosis
(d) fertilization
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Question 4
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Which of the following terms correctly describes
the genotype bb ?
(a) homozygous dominant
(b) heterozygous dominant
(c) homozygous recessive
(d) heterozygous recessive
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Question 5
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What is the likely ratio of affected children
born to parents both of whom are heterozygous for
cystic fibrosis ?

1. 1 affected 3 normal

(b) 3 affected 1 normal
(c) 2 affected 2 normal
(d) all affected
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Question 6
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Which of the following phenotypes corresponds to
the Genotype IAIO ?

1. Blood group A

(b) Blood group B
(c) Blood group O
(d) Blood group AB
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Question 7
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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
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Question 8
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Which of these Punnett squares correctly
represents a cross between two heterozygous
individuals ?
(a)
(b)
aa
AA
AA
Aa
aa
AA
Aa
aa
(c)
(d)
a
Aa
Aa
AA
Aa
aa
aa
Aa
Aa
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Question 9
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A married couple has a family of 6 boys. What are
the chances that the next child will be a girl ?
(a) 61
(b) 16
(c) 31
(d) 11
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Question 10
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Which of the following is a carrier genotype
for a disease caused by a recessive gene ?
(a) nn
(b) NN
(c) Nn
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Question 11
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If normal parents have a child with cystic
fibrosis
(a) one of them must be heterozygous
(b) both of them must be heterozygous

1. one of them must be homozygous

(d) both of them must be homozygous
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Answer
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Correct
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Answer
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Incorrect