Introduction to the
genetics of inheritance of characteristics, work of Mendel and genetic diagrams including
Punnett squares
IGCSE AQA GCSE Biology Edexcel
GCSE Biology OCR Gateway Science Biology OCR 21st Century Science
Biology
Doc Brown's
school biology revision notes: GCSE biology, IGCSE biology, O level
biology, ~US grades 8, 9 and 10 school science courses or equivalent for ~14-16 year old
students of biology
including technical terms explained, the work on Mendel with pea plants, inherited genetic disorders,
genetic-embryonic screening
This page will help you answer questions
such as ... What is the study of genetics?
How are characteristics inherited? What is dominant gene? What is a
recessive gene? What have alleles got to do with
inheritance? What do the terms homozygous and
heterozygous mean? How to explain the terms genotype and phenotype? What do we
mean by gene expression? How do you draw monohybrid genetic diagrams? How to you
construct Punnett square? Why is Mendel's work on pea plants so important? How
do you explain the genetics of cystic fibrosis sickle cell disease anaemia?
Sub-index for this page
(a)
Introduction to genetics and
inheritance and technical terms explained
(b)
Sexual
reproduction and methods of
genetic analysis - 'model' examples explained
(c)
Examples of genetic
diagrams to explain inheritance of characteristics - work of Mendel
(d)
Some examples of inheritance
and genetic disorders
Inherited disorders: (1)
sickle cell anaemia; (2)
cystic
fibrosis; (3)
polydactyly
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(a) Introduction to genetics and
inheritance and technical terms explained
Genetics is the study of heredity
and the variation of inherited characteristics.
Genes, sections of DNA, are the means by
which characteristics are passed on
from one generation to the next in both plants and animals.
In other words, the genes you
inherit from your parents control the characteristics (phenotypes)
you develop. You can use simple genetic diagrams can be
used to show this (see
section (b)).
A single gene can code for a
single characteristic, but quite often several genes are responsible
for a characteristic of an organism - it can get very complex!
Gametes (sex cells) only have one
allele per gene, but all the other cells in an organism have two
alleles per gene.
Our knowledge of genetics enables us to
treat certain medical conditions but there are ethical considerations
in treating genetic disorders.
A gene is a shorter section of the huge DNA coiled up
molecules that make up chromosomes.
Genes exist in
alternative forms called alleles which give rise to differences in inherited
characteristics.
Particular genes control specific
characteristics e.g. most characteristics are controlled by the
coordination (interaction) of several genes but some are controlled
by one gene e.g. fur colour of mice, red-green colour blindness in
humans.
In sexual reproduction, the parents (mother
and father) produce gametes (egg and sperm reproductive cells).
Each gamete only has one copy of each
chromosome, unlike pairs of chromosomes in all other cells.
Therefore the gametes have only one version
of each gene, i.e. one allele per gene.
This is because we inherit half of our genes
from our mother and the other half from our father.
In producing offspring from fertilisation, the
chromosomes from a male gamete (sperm) mix with the
chromosomes from the female gamete (egg) to produce the full
compliment of pairs of chromosomes - two alleles for each gene.
Alleles are essentially two versions of the
same gene.
Usually you have two copies of the same
gene (two alleles), one from each parent.
Therefore eg in humans, between the two copies of the
chromosomes you can have two alleles the same (homozygous) or different
(heterozygous) for a particular gene.
Individual alleles can be 'dominant' or 'recessive'
in character and are represented in genetic diagrams or charts by upper case
letters e.g.
D
for a dominant gene or a lower case letter e.g.
d for a recessive
gene.
Remember
alleles are
versions of the same gene and are
represented by single letters in genetic diagrams.
Humans have two alleles, different versions,
of every gene in the chromosomes of your body.
If you have two alleles for a
particular gene that are the same e.g.
DD or
dd, then it is
homozygous for that characteristic trait.
If two alleles for a specific gene are
different, then they are heterozygous for that
characteristic trait e.g.
Dd.
This means you have instructions for
two different versions of a characteristic trait, but you
will only display one version of the two (only one of
the two possible phenotypes).
As we have said, if the two alleles for a gene are
different (heterozygous e.g. Dd), only one can determine the
characteristic trait. The allele for that characteristic
phenotype observed (gene expression) is called the dominant
allele (denoted by a capital letter - upper case e.g.
D).
The other allele (denoted by a small
letter - lower case) is described as a recessive allele
e.g. d.
Note that D overrides d, i.e. a dominant
allele overrides a recessive allele in all heterozygous
organisms.
So, a pair of homozygous alleles e.g. DD,
or heterozygous alleles Dd, will
both produce the dominant gene trait, BUT, a pair
of homozygous recessive alleles e.g. dd, will produce the
recessive gene trait.
In order to display a characteristic
caused by a recessive allele, both alleles must be recessive
e.g. dd.
So DD or Dd allele pairs lead to a
dominant phenotype and a dd allele pair produces the
recessive phenotype.
In total, your genotype is a combination of
all the genes-alleles you have in your chromosomes.
In your body's biochemistry, your alleles
are functioning at a molecular level (DNA/RNA) to determine the
characteristics you display - described as phenotypes - the
results of your gene-allele expressions, which can be either
dominant or recessive.
Many characteristics are controlled by a
single gene, known as single gene inheritance.
Summary of some important terms to know the meaning of, and use appropriately
in the correct context.
genotype - a 'bit of genetic code'
pairs of or individual alleles eg XX, XY, X, Y (and it is the
genotype pairs that give rise to the phenotype you observe in the
organism.
Watch out for the different allele
genotypes in parents e.g. Dd, but in gametes this becomes
D and d, (separated alleles), this is rather
important when working out the genotypes, and hence phenotypes,
of offspring.
dominant - if two alleles for a characteristic
are different (heterozygous) then only one of the alleles can determine the
nature of the characteristic - know as the dominant allele (usually shown as
a capital/upper case letter) eg a gene for height might be H, so HH or Hh
genotypes will give a tall organism. A dominant allele will override a
recessive allele.
recessive - if an allele is not dominant, it
is described as recessive (small/lower case letter), and, in order for the
recessive allele to be expressed in the phenotype observed.
You must have a
double recessive allele eg homozygous genotype hh
will give rise to a recessive phenotype.
homozygous - if a pair alleles for a
characteristic are the same on a gene eg genotype XX for phenotype female.
Homozygous alleles can be dominant or
recessive e.g. DD or dd.
heterozygous - if a pair of alleles for a
characteristic are different on a gene eg genotype XY for phenotype male.
These are typically denoted in genetics
using upper case (dominant) and lower case (recessive) letters
e.g. Aa, Dd or Pp.
phenotype - the result of 'gene expression' -
the nature of the characteristic you see eg tall, blue eyes, male etc.
gene expression - the process from the
genotypes to the observed phenotypes - the genetic results!
gamete cells are sex cells (gametes).
You need to be able to analyse and interpret patterns of
monohybrid inheritance using a genetic diagram, Punnett squares and family
pedigrees ...
and be able to calculate and analyse outcomes (using
probabilities, ratios and percentages) from monohybrid crosses.
TOP OF PAGE and
sub-index
(b) Sexual
reproduction and methods of
genetic analysis - models explained
This section is written to illustrate how to analyse the possibilities of
offspring phenotypes using both Punnett squares and genetic link
diagrams.
I suggest you first work through this section
on genetic diagrams and then start on the other sections from (c)
onwards.
You can work your way through them all now, or refer
to them while working down the rest of this page from (c) onwards.
That's up to you, BUT you must be completely familiar with the terms and
phrases:
gamete, genotype, phenotype etc. as introduced
and described above in
section (a).
Examples of using
Punnett squares and genetic diagrams to analyse the phenotypes of offspring
For alleles involving gamete genotypes D (dominant) and d
(recessive),
there are three possible genotypes: DD,
Dd and dd,
this means there are only 6 possible 'crosses' between
these genotypes:
1. DD x DD; 2. DD x Dd; 3. Dd
x Dd;
4. DD x dd; 5. Dd x dd; 6. dd x dd
All of which are all described and explained below.
Some are not very important, others are
very important when looking at inherited diseases, and other are
unlikely to happen in nature.
These are examples of
monohybrid
inheritances.
From the 'crosses' analysis with Punnett squares or
diagrams can work out the offspring phenotypes as 'dominant'
or 'recessive'.
You can think of 'dominant' as 'normal' and
recessive as 'abnormal', but take care in using such terms!
The following six diagrams show the possible
alleles of offspring from three possible genotypes.
PLEASE NOTE
The percentages of outcomes from the analysis
are only statistical probabilities, they are NOT precise
predictions.
A theoretical outcome ratio of 1 : 1 might
emerge in an experiment as 47 : 53, not 50 : 50.
A theoretical outcome ratio of 1 : 3 might
emerge in an experiment as 26 : 74, not 25 : 75
Method of constructing two types
of genetic diagrams.
Example 1. Introduction to a Punnett square genetic diagram
To find the probability of
phenotype outcomes you can construct a Punnett square deduced from
'crossing' the different genes or chromosomes.
In this case you construct a
genetic diagram or 'chart' to show the possible outcomes from gamete
pair from parent a crossed with the gamete pair from parent b.
You put the possible gametes from
parent a above the ('yellow')
square and the possible gametes from parent b down the left side of
the square.
You then fill in the matching
genotype pairings using a Punnett square.
Example 1. Parents a
and b, both homozygous phenotypes: a = 'dominant,'
b = 'dominant'
because of genotypes: a = DD, b = DD |
Comments on genotype cross
DD x DD
[Punnett square: offspring's
genotypes]
All
offspring phenotypes are 'dominant'.
Nothing else is possible!
Boringly 'normal'
All
offspring the same phenotype.
Genetic
hereditary diagram below. |
Punnett square analysis of offspring - the resulting allele
pairings |
parent a's
gametes genotypes - alleles |
D |
D |
parent b's gametes genotypes |
D |
DD |
DD |
D |
DD |
DD |
Same example 1. using circles with connecting lines genetic
diagram
You can also construct a 2nd type of genetic
diagram using circles and connecting lines.
At the top are the parents indicating the
phenotype and genotype.
Below that you show the possible gametes that
can be formed.
One gamete from parent a combines with one
gamete from parent b in fertilisation.
You then use connecting lines to show how the
chromosomes can combine.
Finally, the bottom row of circles show the
genotypes of the offspring, to which you can add the phenotype.
This is crossing two homozygous 'parent'
dominants DD.
Example 2.
Example 2. Parents a
(homozygous) and b (heterozygous) phenotypes: a = 'dominant,'
b = 'dominant' because of genotypes: a = DD, b = Dd |
Comments on genotype cross
DD x Dd
[Punnett square: offspring's
genotypes]
All
the offspring phenotypes are 'dominant', non will express the
recessive gene d.
BUT, ~50% (2/4, 1 in 2 chance) of the offspring will carry the
recessive gene d (1 in 2 won't), but non will express the recessive
gene as a phenotype.
Genetic
hereditary diagram below. |
Punnett square analysis of offspring - the resulting allele
pairings |
parent a's
gametes genotypes - alleles |
D |
D |
parent b's gametes genotypes |
D |
DD |
DD |
d |
Dd |
Dd |
This is crossing a homozygous dominant
parent DD with a heterozygous parent Dd. Genetic diagram
below.
Example 3.
Example 3. Parents a
and b, both heterozygous phenotypes: a = 'dominant,'
b = 'dominant'
because of genotypes: a = Dd, b = Dd |
Comments on genotype cross
Dd x Dd
[Punnett square: offspring's
genotypes]
~75% (3 in 4 chance) of the offspring will carry the recessive gene
d (1 in 4 won't).
~25% (1 in 4 chance) of the offspring will actually express the
recessive gene (dd effect).
A
3 : 1 ratio
of dominant : recessive gene expression of the offspring phenotypes.
Genetic
hereditary diagram below. |
Punnett square analysis of offspring - the resulting allele
pairings |
parent a's
gametes genotypes - alleles |
D |
d |
parent b's gametes genotypes |
D |
DD |
Dd |
d |
Dd |
dd |
This is crossing a pair of heterozygous
parents Dd. Genetic diagram below.
Example 4.
Example 4. Parents a
and b, both homozygous phenotypes: a = 'dominant,'
b = 'recessive'
because of genotypes: a = DD, b = dd |
Comments on genotype cross
DD x dd
[Punnett square: offspring's
genotypes]
All
offspring phenotypes are 'dominant', despite one parent's phenotype
being recessive.
All
offspring genotypes are the same (Dd),
and
all offspring are hereditary carriers of the recessive gene d.
Genetic
hereditary diagram below. |
Punnett square analysis of offspring - the resulting allele
pairings |
parent a's
gametes genotypes - alleles |
D |
D |
parent b's gametes genotypes |
d |
Dd |
Dd |
d |
Dd |
Dd |
This is crossing a homozygous dominant
parent DD with a homozygous recessive parent dd. Genetic
diagram below.
Example 5.
Example 5. Parents a
(heterozygous) and b (homozygous) phenotypes: a = 'dominant,'
b = 'recessive'
because of genotypes: a = Dd, b = dd |
Comments on genotype cross
Dd x dd
[Punnett square: offspring's
genotypes]
~50% (2/4, 1 in 2 chance) of the offspring phenotypes being
'dominant',
~50% (2/4, 1 in 2 chance) of the offspring phenotypes being
'recessive', a 1 : 1
ratio.
and
all the offspring are hereditary carriers of the recessive gene d.
Genetic
hereditary diagram below. |
Punnett square analysis of offspring - the resulting allele
pairings |
parent a's
gametes genotypes - alleles |
D |
d |
parent b's gametes genotypes |
d |
Dd |
dd |
d |
Dd |
dd |
This is crossing a heterozygous parent
Dd with a homozygous recessive parent dd. Genetic diagram below.
Example 6.
Example 6. Parents a
and b, both homozygous phenotypes: a = 'recessive,'
b = 'recessive'
because of genotypes: a = dd, b = dd |
Comments on genotype cross
dd x dd
[Punnett square: offspring's
genotypes]
All
offspring phenotypes are the same and 'recessive', all hereditary
carriers.
If
the recessive gene confers a disadvantage on an organism, it is
highly unlikely that this particular 'cross' would occur in nature!
Genetic hereditary diagram below. |
Punnett square analysis of offspring - the resulting allele
pairings |
parent a's
gametes genotypes - alleles |
d |
d |
parent b's gametes genotypes |
d |
dd |
dd |
d |
dd |
dd |
This is crossing two homozygous recessive
'parents' dd. Genetic diagram below.
TOP OF PAGE and
sub-index
(c) Some examples of genetic
diagrams to explain the inheritance of characteristics
Initially
using
examples of the investigations of Mendel into the inheritance of
characteristics by plants
A good example is to consider some of the results of
Mendel’s work which preceded the work by other scientists which links
Mendel’s ‘inherited factors’ with the chromosomes of the humble pea.
Mendel was an Austrian monk, educated in
mathematics and natural history at the University of Vienna.
Gregor Mendel, working in a humble
garden plot of his monastery in the mid 19th century, made notes that provided good experimental evidence on
how
characteristics of pea plants were passed on from generation to the next.
Mendel conducted many experiments to investigate
how characteristics of plants (particularly pea plants) were passed
on from one generation to the next.
His 'classic' investigations included
looking at the height and colour of pea plants.
His research results were published in 1866 and eventually became an
important work and foundation of the relatively modern study of genetics.
We are now able to explain why Mendel proposed the
idea of separately inherited factors. The importance of his
discoveries were not recognised until after his death because there was no
knowledge of chromosomes, genes and how DNA functions.
The principles used
by Mendel in investigating monohybrid inheritance in peas were ...
His worked involved (as far as
he could tell) crossing different pure bred pea plants of a particular
characteristic eg a particular colour or tall or short plants and then
cross-breeding the offspring e.g.
(1)
Mendel's experiment on height -
first cross
Mendel crossed a tall pea plant (in
modern notation, genotype TT) with a dwarf pea plant (genotype tt) and found
all the offspring were
tall.
Note that he used pure bred tall
or dwarf (short), which we know recognise as homozygous genotypes.
Genetic diagram fro TT x tt
Above and below are the 'modern' genetic diagram and
Punnett
square for crossing the tall pea with a dwarf pea (1st cross to give F1)
This is crossing a homozygous dominant
'parent' TT with a homozygous recessive 'parent' tt.
The TT and tt allele
plants were pure bred, i.e. no heterozygous allele pairs i.e.
no Tt pairings.
Genetic table for crossing tall pea with
dwarf pea |
Parent genotypes: TT x tt |
Gametes: T,
T, t and t (alleles) |
Genotypes of
plants |
T |
T |
t |
Tt |
Tt |
t |
Tt |
Tt |
The diagrams above and below
give a modern genetic
interpretation of Mendel's results from initially crossing a pure line of
tall pea plants with a pure line of dwarf pea plants (F1 hybrids)
From the using a Punnett square, gives 100% tall plants (genotype Tt),
but, in terms of modern genetics, they all carry the allele t for dwarf pea plants.
(2)
Mendel's second cross of two of
the tall plants from the first set of offspring (from F1 hybrids above)
Genetic diagram for Tt x Tt
This is crossing a heterozygous 'parent'
Tt with another heterozygous 'parent' Tt.
The 'modern' genetic diagram and Punnett
square for crossing two plants from the 1st cross (2nd cross to give F2
hybrids) The modern
interpretation is shown by the Punnett square and inheritance diagram
analyses below.
Genetic table for crossing tall pea
plants from the first crossing |
Parent genotypes: Tt x Tt |
Gametes: T,
t, T and t (alleles) |
Genotypes of
plants |
T |
t |
T |
TT |
Tt |
t |
Tt |
tt |
The first resulting offspring
(F1) were all tall pea plants, and these were then crossed
with each other, to give the second set of offspring (F2) shown above.
This gave approximately 75% tall plants
(genotype TT or Tt) and 25% dwarf pea plants (genotype tt)
Mendel found that the second
cross produced tall : dwarf pea plants in the approximate ratio of 3 : 1.
He therefore showed that the tall pea plant trait was
dominant over the dwarf pea plant trait.
The genetic diagrams and Punnett squares
shows why you statistically expect these results.
The ratio of tall plants to dwarf plants
(3 : 1) showed that the dominant factor was 'tall' over the 'dwarf factor'.
BUT, he also showed that under the right
circumstances, dwarf pea plants were formed and we now know this is due to
the double recessive gene combination.
From these humble, but carefully done experiments, Mendel deduced that
the height characteristics (and other characteristics) were determined by
what he called 'separate inherited factors' passed on from each parent
plant.
We now know that these 'separate inherited units'
in modern genetic theory are genes.
(3) He did
similar experiments with the colour of pea plants.
He did similar experiments with pea plants with
purple and white coloured flowers.
The PP (purple) and pp
(white) allele
plants were pure bred, i.e. no heterozygous allele pairs i.e. no
Pp pairings.
Again, the modern interpretation is
shown by the Punnett square analyses.
Genetic table for crossing
purple pea with
white pea |
Parent genotypes: PP x pp |
Gametes:
P,
P, p and p (alleles) |
Genotypes of
plants |
P |
P |
p |
Pp |
Pp |
p |
Pp |
Pp |
The diagrams above and below give a
modern genetic interpretation of Mendel's results from initially
crossing a pure line of purple pea plants with a pure line of white pea plants (Punnett
square of F1 hybrids)
This gives 100% purple plants (genotype Pp),
but, in terms of modern genetics, they all carry the allele dominant
P for purple flowers and recessive allele p for
white pea plants.
The 'modern' genetic diagram and Punnett
square for crossing two plants from the 1st cross (2nd cross to give F2
hybrids)
Genetic table for crossing purple pea
plants from the first crossing |
Parent genotypes: Pp x Pp |
Gametes:
P,
p, P and p (alleles) |
Genotypes of
plants |
P |
p |
P |
PP |
Pp |
p |
Pp |
pp |
The first resulting offspring
(F1) were all purple pea plants, and these were then crossed
with each other, to give the second set of offspring (F2) shown above.
This gave approximately 75% purple plants
(genotype PP or Pp) and 25% white pea plants (genotype pp)
Mendel found that the second
cross produced purple : white pea plants in the approximate ratio of 3 : 1.
He therefore showed that the purple flower trait was
dominant over the white flower trait.
The genetic diagrams and Punnett squares
shows why you statistically expect these results.
The ratio of purple plants to white plants
(3 : 1) showed that the dominant flower colour factor was 'purple'
over the 'white'.
So he also showed that under the
right circumstances, white pea plants were formed, and, as with the tall
and short plant sizes, we now know this is due to
the double recessive gene combination.
(4) The
outcome and importance of Mendel's experiments
and why
wasn't Mendel's brilliant work recognised at the time?
From his experiments Mendel concluded the
following:
(i) Characteristics in plants are determined by
some kind of 'hereditary units' (we now know as genes).
(ii) These hereditary units are passed from one
generation to their offspring unchanged from both parents AND one
'unit' from each parent (plant).
(iii) These 'hereditary units' can be 'dominant'
or 'recessive' -if a plant has both the 'dominant unit' and
'recessive unit', the dominant characteristic would be expressed
(the observed phenotype).
Mendel's work was so new and revolutionary
that most scientists just didn't appreciate the results of his
experiments - his results didn't fit in with any current theory of
the time!
Few, if any? other scientists seem to
doing the same sort of experiments as Mendel and then publishing
their results, so there was no independent verification of his
results.
Mid 19th century scientists had no knowledge
of modern genetics e.g. DNA, genes, chromosomes etc.
Fortunately, after his death, scientists e.g.
biologists, began to realise the significant of his work after it
was published in 1866 and linking inherited factors with genes and
chromosomes.
Using Mendel's experiments as a guide, many
experiments have been done to confirm his ideas and further
contribute to our understanding of genetics - the fundamental theory
of inheritance at the molecular level e.g.
From the late 19th century the structures
we call chromosomes were recognised and microscopes were good
enough to see how they behaved during cell division.
But, it was only early in the 20th century
that scientists realised the similarity between the way
chromosomes behaved and Mendel's 'inheritance units'
Therefore it was proposed that these
Mendelian 'units' were part of the structure of chromosomes -
these, as we now know, are genes/alleles.
Finally (sort of), in 1953, through the
work of Crick, Watson and others, the double helix structure of
DNA was worked out.
The science of genetics has advanced so
much that we now the sequence of the nucleotides (and their
bases) in the complete genome of an organism - known as
genome sequencing.
With this knowledge we can now understand
how genes work at the molecular level e.g. from DNA, via RNA,
codes for proteins and many other functions of an organism.
Scientists can use genome sequencing
to identify which parts (genes) control particular
characteristics of an organism.
This can get very complicated because
(i) most characteristics are controlled by several genes and
(ii) genetic variants interact with each other.
Footnote on (sort of): The chemistry of
genetics is developing all the time and is turning out to be far
more complicated than could ever have been envisaged back in
1953.
We are now able to test whether people
are susceptible to a particular disease or inherited
disorder. See
genetic screening.
We can modify organisms to introduce a
specific gene into their genome.
See
Genetic
engineering - making insulin gcse biology revision notes
For lots more examples of genetic analysis of
offspring see
section (b) with lots of diagrams and
explanations.
TOP OF PAGE and
sub-index
(d) Some examples of inheritance
and genetic disorders.
Know and understand that some disorders are
inherited.You need to be able to evaluate the outcomes
of pedigree analysis when screening for genetic disorders.
Examples of genetically inherited disorders are
described below, some with very serious consequences, others not so
serious.
(1) sickle cell anaemia; (2) cystic
fibrosis; (3) polydactyly;
Genomics and inherited disease
It is now known most of our characteristics
are controlled by more than one gene.
This is also true for genetically inherited
diseases.
Single-gene disorders like cystic
fibrosis comply with what is called 'Mendelian inheritance'
and genetic diagrams and Punnett squares are quite easy to work out
- as I hope you will find out below.
Most diseases with a 'genetic connection' like
diabetes, obesity and cardiovascular diseases (heart disease)
involve the interaction of many genes including non-coding sections
of the genome's DNA and environmental factors e.g. lifestyle choice
- diet and exercise.
See detailed notes on the
human genome project for notes on genetic testing ('pros and cons') and
medical treatments
TOP OF PAGE and
sub-index
(1)
Sickle cell diseases
- the most common is sickle cell anaemia
Sickle cell anaemia is a genetic (inherited) blood
disorder in which red blood cells (the carriers of oxygen around the body),
develop abnormally.
Instead of being round and flexible, the sickle red
blood cells become shaped like a crescent (hence the name 'sickle').
These
abnormal red blood cells can then clog sections of blood vessels (especially
the narrow capillaries) leading to pain. These painful effects can last from
a few minutes to several months.
The abnormal blood cells have a shorter
life-span and are not replaced as quickly as normal healthy red blood cells
leading to a shortage of red blood cells, called anaemia.
Symptoms of sickle
cell anaemia include tiredness, painful joints and muscles and
breathlessness, especially after exercise ie any extra physical exertion.
The highest frequency of sickle cell disease
is found in tropical regions, particularly sub-Sahara Africa, and
tribal regions of India and the Middle East.
Although less frequent, sickle cell disease
can occur in any population containing people whose ancestors came
from the geographical regions mentioned above - it is on the
increase in Europe due to current large scale migration from these
regions.
For sickle cell anaemia to occur in a child,
both parents must carry the recessive allele
a for sickle cell disease, but
neither is affected by it.
In the above diagram, imagine the
'arrowed' yellow band represents
one of the alleles that codes for red blood cells.
A
represents the normal dominant alleles in the pair of chromosomes
(notated as AA below in the genetic analysis).
This person is not a carrier or
sufferer of sickle cell anaemia.
B
represents a dominant normal and a defective recessive allele (notated
as Aa below in the genetic analysis).
This person is a carrier, but not
a sufferer of sickle cell anaemia because normal is dominant.
C
represents a person with a pair of defective recessive alleles (notated
as aa below in the genetic analysis).
This person is both a carrier and
sufferer of sickle cell anaemia - the double recessive gene prohibits
the production of the vital protein.
Genetic diagram and Punnett
squares for sickle cell anaemia
Genetic diagram
However, there is a 1 in 4 (25%) chance that one of their
children will be affected by this genetic disorder - refer to diagram above
and Punnett table below, which shows a double recessive allele is needed for the
offspring to be affected (genotype aa).
Note
(i) For someone to suffer from sickle cell
anaemia, they must inherit the faulty allele (a) from both
parents.
(ii) There is a 3 in 4 chance (75%) of
offspring being carriers of
the recessive allele a, but only 1 in 3 (25%) of these will actually
suffer
from sickle cell anaemia.
Punnett square genetic table for sickle cell
anaemia |
Genotypes of parents: Aa x Aa
normal but both carriers |
Gametes:
A, a, A and a (alleles) |
Genotypes
of children |
A |
a |
A |
AA |
Aa |
a |
Aa |
aa |
Five other possible parental crosses
involving the recessive allele a for sickle cell anaemia.
I've shown below the analyses for
sickle cell anaemia using a basic Punnett square of the two pairs of
gametes of the parents and the four possible genotypes of offspring
(children).
2. genotypes of parents: Aa x aa |
Comments on cross 2.
A carrier crossed with someone suffering from
sickle cell anaemia.
All the offspring will be carriers of the
recessive gene a.
2 in 4 chance (50%) of the offspring being
affected by sickle cell anaemia. |
genotypes
of children |
A |
a |
a |
Aa |
aa |
a |
Aa |
aa |
3. genotypes of parents: AA x Aa |
Comments on cross 3.
A non-carrier crossed with a carrier of sickle
cell anaemia recessive gene a.
2 in 4 chance (50%) of the offspring will be
carriers of the recessive gene a.
Non of the offspring will be affected by
sickle cell anaemia. |
genotypes
of children |
A |
A |
A |
AA |
AA |
a |
Aa |
Aa |
4. genotypes of parents: AA x aa |
Comments on cross 4.
A non-carrier crossed with someone suffering from
sickle cell anaemia.
All the offspring will be carriers of the
recessive gene a.
Non of the offspring will be affected by
sickle cell anaemia. |
genotypes
of children |
A |
A |
a |
Aa |
Aa |
a |
Aa |
Aa |
Extra note on
sickle cell anaemia:
(i) For 5. AA x AA, all offspring will be
AA not affected, similarly, for 6. aa x aa, all offspring
will be aa affected and carriers.
(ii) For couples who may carry the recessive gene,
certain crosses carry an increased risk that their child might suffer
from sickle cell anaemia.
Genetic screening for potentially harmful
alleles may inform potential parents of the risk, but this may in
itself lead to agonising decisions.
In many poorer countries genetic screening
is highly unlikely to be available.
See detailed notes on the
human genome project for notes on genetic testing ('pros and cons') and medical
treatments
TOP OF PAGE and
sub-index
(2)
Cystic fibrosis
Cystic fibrosis is a genetic disorder
of cell membranes, the
disease is passed down through families.
Cystic fibrosis can be caused by
the deletion of only three bases, but this has a dramatic effect on the
phenotype.
The faulty gene should code for a
protein that controls the movement of salt and water in and out of
cells.
Unfortunately, the protein
produced by the faulty gene doesn't work properly and leads to
excess mucous production.
Cystic fibrosis causes this thick, sticky
mucus to build up in the air passages, lungs, digestive tract, pancreas and other areas of the body
- affected people suffer from breathing and digestion difficulties
and patients are on a complex mixture of medications.
It is one of the most common chronic lung diseases in children and young
adults and sadly, it is a life-threatening disorder caused by a defective gene
which causes the body to produce abnormally thick and sticky fluid, called
mucus.
The thick mucus builds up in the breathing passages of the lungs
(causing lung infections) and in the pancreas, the organ that helps to break
down and absorb food (causing digestion problems).
The parents may be carriers of the cystic
fibrosis disorder without actually having the disorder themselves.
In the above diagram, imagine the
'arrowed' yellow band represents
the allele that codes for the essential protein required to avoid
suffering from cystic fibrosis.
A
represents the normal dominant alleles in the pair of chromosomes
(notated as FF below in the genetic analysis).
This person is not a carrier or
sufferer of cystic fibrosis.
B
represents a dominant normal and a defective recessive allele (notated
as Ff below in the genetic analysis).
This person is a carrier, but not
a sufferer of cystic fibrosis because normal is dominant.
C
represents a person with a pair of defective recessive alleles (notated
as ff below in the genetic analysis).
This person is both a carrier and
sufferer of cystic fibrosis - the double recessive gene prohibits
the production of the vital protein.
It is caused by a recessive allele
(denoted by f) of a
gene and can therefore be passed on by parents, neither of whom has the
disorder.
About 1 in 25 people carry the
recessive allele of f cystic fibrosis.
About in 3000 newborn babies have
the condition.
In order to be affected by cystic
fibrosis, you must inherit the double recessive gene ff.
Punnett square and genetic
diagram for cystic fibrosis
Punnett square genetic table for cystic fibrosis |
1. Genotypes of parents: Ff x Ff, normal but both carriers |
Gametes: F, f, F
and f (alleles) |
Genotypes
of children |
F |
f |
F |
FF |
Ff |
f |
Ff |
ff |
Cystic fibrosis is caused by a recessive
allele f (so it needs genotype ff, a double recessive allele,
for the person to suffer from cystic fibrosis.
For someone to suffer from cystic
fibrosis, they must inherit the faulty allele (f) from both
parents.
The genetic diagrams above and below show that
when both
parents are carriers of the recessive allele, but NOT affected (Ff,
heterozygous), there is a 3 in 4 (75%) chance
of having a normal child (FF non-carrier or Ff carrier) and a
1 in 4 (25%) chance of having a child with cystic fibrosis (recessive and
homozygous ff sufferer and
carrier).
Genetic diagram
Five other possible parental crosses
involving the recessive allele f for cystic fibrosis
I've shown below the analyses for
cystic fibrosis using a basic Punnett square of the two pairs of gametes
of the parents and the four possible genotypes of offspring (children).
2. genotypes of parents: Ff x ff |
Comments on cross 2.
A carrier crossed with someone suffering from
cystic fibrosis.
All the offspring will be carriers of the
recessive gene f.
2 in 4 chance (50%) of the offspring being
affected by cystic fibrosis. |
genotypes
of children |
F |
f |
f |
Ff |
ff |
f |
Ff |
ff |
3. genotypes of parents: FF x Ff |
Comments on cross 3.
A non-carrier crossed with a carrier of the
cystic fibrosis recessive gene f.
2 in 4 chance (50%) of the offspring will be
carriers of the recessive gene f.
Non of the offspring will be affected by
cystic fibrosis. |
genotypes
of children |
F |
F |
F |
FF |
FF |
f |
Ff |
Ff |
4. genotypes of parents: FF x ff |
Comments on cross 4.
A non-carrier crossed with someone suffering from
cystic fibrosis.
All the offspring will be carriers of the
recessive gene f.
Non of the offspring will be affected by
cystic fibrosis. |
genotypes
of children |
F |
F |
f |
Ff |
Ff |
f |
Ff |
Ff |
Extra note on
cystic fibrosis:
(i) For 5. DD x DD, all offspring will be
DD non affected, similarly, for 6. ff x ff, all offspring
will be ff affected and carriers.
(ii) For couples who may carry the recessive gene,
certain crosses carry an increased risk that their child might suffer
from cystic fibrosis.
Genetic screening for potentially harmful
alleles may inform potential parents of the risk, but this may in
itself lead to agonising decisions.
See detailed notes on the
human genome project for notes on genetic testing ('pros and cons') and
medical treatments
TOP OF PAGE and
sub-index
(3)
Polydactyly
Polydactyly/polydactyl – having extra
fingers or toes – is caused by a dominant allele of a gene and can therefore
be passed on by only one parent who has the disorder.
Polydactyly is a physical
condition in which a person has more than five fingers per hand or more than five toes
per foot. Having an abnormal number of digits (6 or more) can occur on its
own, without any other symptoms or disease.
See photographs on
https://en.wikipedia.org/wiki/Polydactyly
The frequency of polydactyly
varies from 5 to 19 per 10,000 population.
Polydactyly may be passed down
(inherited) in families and this trait involves only one gene that can cause
several variations.
Polydactyly is caused by the
dominant allele P (so doesn't need genotype PP, can be
Pp too).
The parent that has the defective
allele (P) will be affected by the condition of polydactyly.
Note that someone affected by
polydactyly only has to inherit one dominant gene (P) from either
parent.
The genetic diagrams below shows that
there is a 50% chance of a child suffering from polydactyly if just one of
the parents is a carrier Pp.
Genetic diagram and Punnett
squares for
polydactyly
Genetic table
1. for polydactyly |
Genotypes of parents:
Pp x pp affected and normal |
Gametes: P, p, p and p
(alleles) |
Genotypes of children |
P |
p |
p |
Pp |
pp |
p |
Pp |
pp |
The analysis of the parental cross between a
non-carrier (recessive alleles pp) and someone affected by
polydactyly (alleles Pp).
Genetic diagram
Five other possible parental crosses
involving the dominant allele P for polydactyly.
I've shown below the analyses for
polydactyly using a basic Punnett square of the two pairs of gametes of
the parents and the four possible genotypes of offspring (children).
2. genotypes of parents: PP x Pp |
Comments on cross 2.
Crossing two parents affected by polydactyly.
All the offspring will be carriers and all
affected by the dominant allele P. |
genotypes
of children |
P |
P |
P |
PP |
PP |
p |
Pp |
pp |
3. genotypes of parents: Pp x Pp |
Comments on cross 3.
Crossing two parents affected by polydactyly due
to dominant allele P.
All of the offspring will be carriers and all
affected by polydactyly. |
genotypes
of children |
P |
p |
P |
PP |
PP |
p |
Pp |
Pp |
4. genotypes of parents: PP x pp |
Comments on cross 4.
A non-carrier crossed with someone suffering from
polydactyly.
3 in 4 chance (75%) of the offspring will be
carriers AND affected by allele P.
1 in 4 chance (25%) will neither be a carrier
of, or affected by, polydactyly. |
genotypes
of children |
P |
P |
p |
Pp |
Pp |
p |
Pp |
pp |
Extra note on
polydactyly:
(i) For 5. PP x PP, all offspring will be
PP affected
(ii) For 6. pp x pp, all offspring will be
pp not affected - normal.
(iii) As far as I know, there are no
serious harmful effects of polydactyly, but the situation can be dealt
with by surgery, but this always carries its own risks.
See detailed notes
human genome project for notes on genetic testing ('pros and cons') and
medical treatments
APPENDIX
now in section (b)
TOP OF PAGE and
sub-index
Some typical learning objectives for this page
Be able to interpret genetic
diagrams, including family trees.
You may have to construct genetic diagrams of monohybrid
crosses and predict the outcomes of monohybrid crosses and be able to use
the terms homozygous (same alleles eg XX or TT) genes, heterozygous
(different alleles eg XY or Tt), phenotype (gene expression - the
outcome!) and genotype (gene type),
You should be able to interpret genetic diagrams of monohybrid
inheritance and sex inheritance but will not be expected to construct
genetic diagrams or use the terms homozygous, heterozygous, phenotype or
genotype.
Be able to predict and/or
explain the outcome of crosses between individuals for each possible
combination of dominant and recessive alleles of the same gene
Keywords: genetics inheritance of characteristics dominant
recessive genes alleles homozygous heterozygous genotype phenotype gene
expression monohybrid genetic diagram Punnett square Mendel pea plants
cystic fibrosis sickle cell disease anaemia
Sub-index of Genetics
Notes - from DNA to GM and lots
in between!
Cell division - cell cycle - mitosis, meiosis, sexual/asexual reproduction,
binary fission and cancer gcse
biology revision
DNA and RNA structure and Protein Synthesis and an
experiment to extract DNA gcse
biology revision notes
An introduction to genetic
variation and the formation and consequence of mutations
gcse biology revision notes
Introduction to the inheritance of characteristics and
genetic diagrams (including Punnett squares) including technical terms, Mendel's work and inherited
genetic disorder, genetic testing gcse biology revision notes
The human GENOME project - gene expression, chromosomes, alleles, genotype, phenotype, variations,
uses of genetic screening-testing including 'pros and cons' gcse biology revision notes
Inherited characteristics and human sexual
reproduction, genetic fingerprinting and its uses gcse biology
Genetic
engineering: uses - making insulin, medical applications, GM crops & food
security gcse biology
More complicated genetics: Sex-linked genetic
disorders, inheritance of blood groups gcse biology revision
See also section on
Cloning -
tissue culture of plants and animals gcse biology revision notes page
IGCSE revision
notes genetics KS4 biology Science notes on inheritance of characteristics GCSE biology guide
notes on dominant genes for schools colleges academies science course tutors images
pictures diagrams for recessive genes science revision notes on
cystic fibrosis sickle cell disease anaemia explained with genetic
diagrams and Punnett squares
for revising biology modules biology topics notes to help on understanding of
homozygous pairs of alleles heterozygous pairs of alleles university courses in biological science
careers in science biology jobs in the pharmaceutical industry
biological laboratory assistant
apprenticeships technical internships in biology USA US grade 8 grade 9 grade10 AQA
GCSE 9-1 biology science notes on alleles GCSE
notes on genotypes phenotypes Edexcel GCSE 9-1
biology science notes on gene expression for OCR GCSE 9-1 21st century biology science
notes on Mendel pea plants experiments OCR GCSE
9-1 Gateway
biology science
notes on monohybrid genetic diagrams Punnett squares WJEC gcse science CCEA/CEA gcse science
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