Heredity

Unit 4/ Chapter 12
 Heredity
 The science of heredity is called genetics.
 It studies the sources of genetic variation and how the
characteristics of an organism are inherited, that is passed
from generation to generation.
 Genetic crosses follow simple mathematical rules and can
be explained using diagrams.
 In this chapter, you will learn how to interpret genetic
crosses and find out about the inheritance of some
conditions caused by faulty genes.
 Genes & Alleles
 Genes are sections of DNA that control the
production of proteins in a cell
 Each protein contributes towards a particular
body feature.
 Sometimes the feature is visible, such as eye
colour or skin pigmentation.
 Sometimes the feature is not visible, such as
the type of haemoglobin in red blood cells or
the type of blood group antigen on the red
blood cells.
 Genes & Alleles

 Some genes have more than one form.

 E.g. the genes controlling certain facial features
 The gene for earlobe attachment has two forms, which
produce the characteristics 'attached earlobe' and 'free
earlobe'.
 These alternative forms of the gene are called alleles.
 Homologous chromosomes carry genes for the same features
in the same sequence, but the alleles of the genes may not be
the same
 The DNA in the two chromosomes is not quite identical
 Suppose that, for the gene controlling earlobe
attachment, a person has one allele for attached
earlobes and one for free earlobes.
 What happens?
 Is one earlobe free and the other attached?
√ Neither.
√ In this case, both earlobes are free.
√ The 'free' allele is dominant  means that it will show
its effect, whether or not the allele for 'attached' is
present.
 Genes & Alleles
 The allele for 'attached' is called
recessive.
 The recessive allele will only show up
(be expressed) in the appearance of
the person if there is no dominant
allele present.
 The scientific way to say that a gene
'shows up' in the appearance of a
person is to say that it is 'expressed'.

The laws of genetics are the same in most organisms.

Genetic Crosses
 We usually show the dominant allele of a gene with a capital
letter (e.g. A) and the recessive allele with the corresponding
small letter (a).
Consider the coats of guinea pigs
- Several features of their coats are controlled by single genes
with dominant and recessive alleles.
- E.g, short hair is dominant to long hair, straight hair is
dominant to curls and the presence of rosettes is dominant to
smooth hair.
- Let us look at the genetics of the last characteristic.
Genetic Crosses
 The allele for rosettes (R) is
dominant over the allele for smooth
hair (r).
 There are three possible
combinations of alleles
Genetic Crosses
There are some new terms here that need to be explained:
 The genotype is the genetic make-up of an organism (RR, rr or
Rr).
 The phenotype is the appearance of an organism (rosettes or
smooth hair).
 Homozygous means that the two alleles of the gene are the
same (RR or rr).
 Heterozygous means that the two alleles of the gene are
different (Rr).
Genetic Crosses
 Remember that the dominant allele always shows itself (is
expressed) if it is present.
 This means that the heterozygote and dominant homozygote
have the same phenotype — both RR and Rr guinea pigs have
rosettes.
 The recessive allele is only expressed in the recessive
homozygote (rr).

The noun from the word homozygous is 'homozygote'. You can

say that an animal is homozygous or 'a homozygote'. Similarly,
an animal can be heterozygous or 'a heterozygote'.
Constructing Genetic Diagrams
 We can use the symbols for alleles to explain how they
are passed on to the offspring of a cross between two
animals.
 If we are dealing with one gene, this is called a
monohybrid cross.
 These diagrams depend on the fact that meiosis
separates each pair of homologous chromosomes, so a
gamete only receives one copy of each allele.
 The alleles are brought back together again at
fertilisation.
Constructing Genetic Diagrams
 We will look at diagrams of some crosses.
 Firstly, consider a cross between two guinea pigs, one
that is homozygous dominant and one that is
homozygous recessive
 Constructing Genetic Diagrams
 Now look at what happens when two of the heterozygous
animals from the Fl generation are crossed
 If you construct a genetic diagram like this, using arrows,
it is easy to make mistakes.
 A better way is to use a diagram called a Punnett square
Constructing Genetic Diagrams
 The Punnett square produces the same expected ratio in
the F2 — 3 with rosettes : 1 smooth.
 There is an important point here. These are only the
expected ratios, based on the laws of probability  if
the cross produced a large number of offspring, the
probability is that their genotypes would be in the ratio
IRR : 2Rr : Irr or their phenotypes would be in the ratio 3
with rosettes : 1 smooth.
Constructing Genetic Diagrams
 If you carried out the same cross lots of times with many
pairs of heterozygous guinea pigs, you could produce
400 baby guinea pigs.
 With a large number like this, the most likely outcome
would be that you would get 300 animals with rosettes
and 100 with smooth coats.
 However, the laws of probability mean it is quite likely
that the numbers would be 307 : 93, or 295 : 105 —
close to the expected ratio, but not exactly the same.
Constructing Genetic Diagrams
 With a small number of offspring, the ratios are likely to
be different from the expected values.
 Just by chance, a pair of heterozygous guinea pigs could
produce four babies with rosettes, or even four smooth
ones.
 Fertilisation of an egg by a sperm is a random event, so it
is impossible to be certain about the outcome.
Ratios and probabilities can be written as a percentage
or as a decimal fraction of 1.
For example, a 3 : 1 ratio can be written as 75% : 25% or
0.75 : 0.25. A 50% probability can be written as 0.5.
 PEDIGREES
 Writing out a genetic cross is a useful way of showing how
genes are passed through one or two generations, starting
from the parents.
 However, if we want to show a family history of a genetic
condition, we need more than this.
 We can use a diagram called a pedigree  a 'family tree'
showing the inheritance of a gene.
 Polydactyly is an inherited condition in which a person
develops extra digits (fingers or toes) on the hands or feet.
 It is determined by a dominant allele.
 The recessive allele causes the normal number of digits to
develop.
 PEDIGREES
 If we use the symbol D for the polydactyly allele and d
for the normal-number allele, the possible genotypes
and phenotypes are:
-DD — person has polydactyly (has two dominant
polydactyly alleles)
-Dd — person has polydactyly (has a dominant
polydactyly allele and a recessive normal allele)
-dd — person has the normal number of digits (has two
recessive, normalnumber alleles).
 PEDIGREES
A pedigree contains a lot of information. In this example:
i. four generations are shown (individuals are arranged in
four horizontal lines)
ii. individuals 4, 5 and 6 are the children of individuals 1 and 2
(a family line connects each one directly to 1 and 2)
iii. individual 4 is the first-born child of 1 and 2 (the first-born
child is shown to the left, then the second bom to the right
of this, then the third born and so on)
iv. individuals 3 and 7 are not children of 1 and 2 (no family
line connects them directly to 1 and 2)
 PEDIGREES
v. individuals 3 and 4 are father and mother of the same
children — as are 1 and 2, 6 and 7, 8 and 9, 12 and 13, 14
and 15 (a horizontal line joins them).
 It is usually possible to work out which allele is dominant
from a pedigree.
 You look for a situation where two parents show the
same feature and at least one child shows the
contrasting feature.
 In Figure 12.7, individuals 1 and 2 both have polydactyly,
but children 4 and 6 do not.
 PEDIGREES
There is only one way to explain this:
 The normal alleles in 4 and 6 can only have come from
their parents (1 and 2), so 1 and 2 must both carry
normal alleles.
 1 and 2 show polydactyly, so they must have polydactyly
alleles as well.
 If they have both polydactyly alleles and normal alleles
but show polydactyly, the polydactyly allele must be the
dominant allele.
 PEDIGREES

 Now that we know which allele is dominant, we can work

out most of the genotypes in the pedigree.
 All the people with the normal number of digits must have
the genotype dd (if they had even one D allele, they would
show polydactyly).
 All the people with polydactyly must have at least one
polydactyly allele (they must be either DD or Dd).
 From here, we can begin to work out the genotypes of the
people with polydactyly.
 PEDIGREES
 we need to remember that people with the normal
number of digits must inherit one 'normal number' allele
from each parent, and also that people with the normal
number of digits will pass on one 'normalnumber' allele to
each of their children.
 From this we can say that any person with polydactyly
who has children with the normal number of digits must
be heterozygous (the child must have inherited one of
their two 'normal-number' alleles from this parent).
 PEDIGREES
 Also, any person with polydactyly who has one parent
with the normal number of digits must also be
heterozygous (the 'normal-number' parent can only have
passed on a 'normal-number' allele).
 Individuals 1, 2, 3, 16, 17 and 18 fall into one or other of
these categories and must be heterozygous.
 PEDIGREES
 We are still uncertain about individuals 5, 8 and 12. They
could be homozygous or heterozygous.
 For example, individuals 1 and 2 are both heterozygous.
 Figure 12.9 shows the possible outcomes from a genetic
cross between them.
 Individual 5 could be any of the outcomes indicated by
the shading.
 It is impossible to distinguish between DD and Dd.
 Codominance
 So far, all the examples of genetic crosses we have seen have
involved complete dominance, where one dominant allele
completely hides the effect of a second, or recessive allele.
 However, there are many genes with alleles that both
contribute to the phenotype.
 If two alleles are expressed in the same phenotype, this is
called codominance.
 The alleles are codominant.
 If a chestnut (red-brown) horse is crossed with a white horse,
all the foals resulting from the cross will be an intermediate
colour, called red roan
 Codominance
 The appearance of a third phenotype shows that there is
codominance.
 We can represent the alleles for coat colour with symbols:
R = allele for chestnut (red-brown) hair
W = allele for white hair

Note that the alleles

for chestnut and
white hair are given
different letters,
since one is not
dominant over the
other
 Codominance
 When red roan horses are crossed together, all three
phenotypes reappear, in the expected ratio:
1 chestnut: 2 red roan : 1 white
 Codominance
 In fact, most genes do not show complete dominance.
 Genes can show a range of dominance, from complete
dominance as in rosette/smooth hair in guinea pigs through
to equal dominance as in the horse coat colour, where the
new phenotype is halfway between the other two.
 ABO Blood Groups
 The inheritance of human ABO blood
groups also shows codominance.
 However, the pattern of inheritance in
blood groups is more complex than for coat
colour in horses, as three different alleles
are involved.
 When there are more than two alleles of
one gene, this is known as the inheritance
of multiple alleles.
 ABO Blood Groups
 The blood group of a person is the result of the presence
or absence of two antigens, the A antigen and the B
antigen, on the surface of the red blood cells.
 There are three alleles involved in the inheritance of
these antigens.
IA — determines the production of the A antigen
IB — determines the production of the B antigen
IO — determines that neither antigen is produced.
 A person inherits two alleles of the gene. The alleles IA
and 1 B are codominant, but P is recessive to both.
Parents who are
heterozygous for
blood group A and
blood group B
could produce
four children, each
with a different
blood group .
(Figure 12.13)
In Figure 12.14, what are the blood groups of individuals 5 and 8?
 ABO Blood Groups
 Individual 12 has two 10 alleles (since she is blood group O),
so she must inherit one of them from individual 5.
 Individual 1 1 must inherit her IA allele from individual 5, as
her other parent is blood group B.
 Individual 5 therefore has the genotype IAP and so must be
blood group A.
 Individuals 7 and 8 produce children with blood group A and
blood group B.
 Individual 7 is blood group O and so both the IA and 1 B
alleles must come from individual 8.
 Individual 8 is blood group AB.
 Sex Determination
 Our sex – Male /Female – not under the control of a single
gene
 It is determined by the X and Y chromosomes – the sex
chromosomes
 All human cells contain 44 non-sex chromosomes.
 All cells of females (except the egg cells) also contain two X
chromosomes
 All cells of males (except the sperm) also contain one X and
one Y chromosomes.
 Our sex is effectively determined by the presence or
absence of the Y chromosome.
 Sex Determination
 The inheritance of sex follows the
pattern shown in Figure 12.16.
 In any one family, however, this
ratio may well not be seen.
 Remember that predicted genetic
ratios usually only happen when
large numbers of offspring are
involved.
 Overall, the ratio of male to
female births is 1:1
Sex-linked Genes
 The sex chromosomes do not only determine sex.
 They also carry genes for other characteristics.
 These are called sex-linked genes.
 The Y chromosome is smaller than the X chromosome,
so it contains fewer genes.
 This means that for some genes, a male will only have
one allele of a pair present (on his X chromosome).
Haemophilia
 When a healthy person's skin is cut, a clot forms. This
prevents loss of blood and entry of bacteria
 Clotting is a complex process, involving many chemicals in
the blood.
 The commonest type of haemophilia is caused by a gene
mutation that affects the production of one of these
chemicals — a protein in the blood plasma.
 Without this protein, the blood of a person with
haemophilia does not clot.
 They may need blood transfusions after minor injuries, and
injections of the missing clotting factor.
Haemophilia
 The allele for haemophilia is recessive, and is given the
symbol h.
 The allele for normal blood clotting is dominant (H).
 Since the gene is found only on the X chromosome, a
man needs to inherit only one allele of the gene to have
the disease.
 The genotype for this is shown as XhY — notice there is
no allele for this gene on the Y chromosome.

There are several types of haemophilia. They all have a genetic

cause and are due to a lack of blood-clotting factors. Two forms
are due to a sex-linked gene on the X chromosome.
Haemophilia
 A woman, with two X chromosomes, would need to
inherit two copies of the faulty allele, which is shown as
XhXh.
 If a woman has only one copy of the haemophilia allele
(XHXh), she will not have the disease, because of the
presence of the dominant H allele on one of her X
chromosomes.
 However, she can pass the haemophilia allele to her
children, so she is called a carrier.
 Haemophilia
 Boys normally inherit the recessive allele from a carrier
mother.
 This means it is possible for two healthy parents to have a son
with haemophilia
 For a girl to have haemophilia, she would have to be the
daughter of a haemophiliac father and a carrier mother.
 This is possible, but much less likely.
 This means that haemophilia is much more common in boys
than in girls.
 The most common form of haemophilia affects about one in
5 000-10 000 males.
RED-GREEN COLOUR BLINDNESS
• inherited in the same way as haemophilia
• more common in boys than in girls.
• About 6% of boys have the condition, compared with 0.4% of
girls.
• The normal (dominant) allele of the gene causes a protein to
be produced that forms the pigment in the cones of the eye
that detects green light
• The mutant (recessive) allele does not cause this pigment to
be formed.
• People without the normal allele cannot tell the difference
between similar shades of green and red
RED-
GREEN
COLOUR
BLINDNESS
 Cystic Fibrosis
 Mucus is a fluid that is produced by cells in glands
throughout the body  acts as a lubricant.
 E.g. in the airways leading to the lungs, in the lining of
the digestive system, in the reproductive organs
 Cystic fibrosis (CF) is a disease caused by a mutated gene
that affects the production of mucus.
 The gene has a high mutation rate, and if the mutation
occurs in the cells producing eggs or sperm, the faulty
allele may be passed to a person's children.
 Cystic Fibrosis
 The dominant allele of this gene allows the production
of normal mucus.
 The mutated recessive allele results in the production of
a thick, sticky (viscous) mucus.
 This does not flow easily like normal mucus and causes
many problems.
 In the lungs, it is not easily removed by the cilia lining
the airways.
 The build-up of mucus causes difficulties with breathing
and gas exchange.
 Cystic Fibrosis
 Bacteria become trapped in the mucus and cause chest
infections.
 CF causes a wide range of problems in other parts of the
body.
 Because of the symptoms, people with cystic fibrosis
often die young.
 However, modern treatments with drugs and other
medications, as well as advances in physiotherapy, have
greatly increased the average lifespan of people with CF.
 Cystic Fibrosis
 Gene therapy may offer a cure in the future.
 For a person to be affected by CF, they must inherit a
recessive allele from each parent, so both the mother
and father must carry at least one recessive allele.
 Most commonly, each parent will be heterozygous for
the CF condition
 There is a 1 in 4 (25%) chance of a child from these
parents developing cystic fibrosis.
Cystic Fibrosis
 Using Gene Therapy To Treat
Cystic Fibrosis
 Gene therapy is a technique by which a
patient can be treated for a genetic disorder
by having a functioning gene transferred into
their DNA.
 It is possible to use a harmless virus as a
'carrier' or vector for transferring the gene.
 Research has been carried out in which
scientists aimed to place a copy of the gene
that produces normal mucus into affected
cells.
 Using Gene Therapy To Treat Cystic
Fibrosis
 Gene therapy has been tried in laboratory animals and
in trials on human volunteers.
 The results so far have been disappointing, mainly
because few cells have been found to take up the virus
and express the gene — the lungs' defence system
against infection stopped the virus from entering the
affected cells.
 Using Gene Therapy To Treat Cystic
Fibrosis
 In 2015 another method was tried, using a different
vector  using minute droplets of lipid enclosing the
gene, which were sprayed into the patient's lungs.
 This method showed a small but significant
improvement in lung function.
 Research in the use of gene therapy to treat cystic
fibrosis continues, in the hope that one day it may
provide a cure for this terrible disease.
Gene and Environment produce Variation
 Identical twins are formed from the same zygote.
 When the zygote divides by mitosis, the first two cells formed
do not stay together, but separate.
 Each cell then continues to divide and produces an embryo.
 Because they each come from the same zygote, they are
genetically identical.
 Identical twins usually look very alike, and often develop
similar talents.
 However, they never look exactly the same.
 The differences are due to their environment affecting their
appearance.
Gene and Environment produce Variation
 Both genes and environment have an effect on human
variation.
 E.g. adult human body mass (weight) or height.
 A person's growth is affected by many genes.
 There are genes that influence protein synthesis in
muscles, bone development, production of hormones, etc.
 These will all have an effect on the growth of the body, but
growth will take place only if the person has access to a
healthy balanced diet.
 Their weight will largely depend on environmental factors
— in this case, availability of food.
 Gene and Environment produce Variation
 Skin colour is inherited but it is also affected by the environment
 exposure to the sun increases the amount of melanin in the skin
 One of the most controversial issues in human variation is how
much of human intelligence is genetic or environmental.
 Does an intelligent child get her genes for intelligence from her
parents, or is much of it a result of the environment in which she
grew up?
 This is sometimes called the nature/nurture argument.
 However, it is certainly true that a child's intellectual
development is affected by access to books and a good education,
as well as a healthy diet and good medical care.
 Look ahead - Dihybrid Inheritance
 For your International GCSE, you only have to deal with the
inheritance of single genes (monohybrid inheritance). If
you study biology beyond this level, you will probably have
to construct diagrams to show the inheritance of two
genes (called dihybrid inheritance).
 For example, guinea pigs can have short hair or long hair,
and dark eyes or pink eyes.
 The allele for short hair (H) is dominant to the allele for
long hair (h), and the allele for dark eyes (E) is dominant to
the allele for pink eyes
 Look ahead - Dihybrid Inheritance
 If a guinea pig that is homozygous dominant for both hair length
and eye colour (HHEE) is crossed with one that is homozygous
recessive for both features (hhee), all the offspring in the Fl
generation will be heterozygous for both characteristics.
 They will all have short hair and dark eyes
 Look ahead - Dihybrid Inheritance
 Note that in a dihybrid cross, each gamete contains two alleles
— one for each gene.
 Assuming the genes for hair length and eye colour are on
different chromosomes, then gametes with four possible
combinations of alleles can be produced from the Fl genotype
(HhEe).
 They are (HE), (He), (hE) and (he).
 The combinations are a result of meiosis
 Look ahead - Dihybrid Inheritance
 If two Fl guinea pigs are crossed, the possible outcomes in the
F2 are as shown in Figure 12.24.
 Look ahead - Dihybrid Inheritance
 The expected ratio of phenotypes in the F2 is:
 9 short hair, dark eyes : 3 short hair, pink eyes : 3 long
hair, dark eyes : 1 long hair, pink eyes
 9 : 3 : 3 : 1 is the usual ratio obtained from a dihybrid
cross between two double heterozygotes.
“Thank You”

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