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GCSE biology: Genetics - analysis of inherited characteristics and disorders

Introduction to the genetics of inheritance of characteristics and genetic diagrams including Punnett squares

including technical terms explained, the work on Mendel with pea plants, inherited genetic disorders, genetic-embryonic screening

Doc Brown's Biology Revision Notes

Suitable for GCSE/IGCSE/O level Biology/Science courses or equivalent

 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?



Introduction to genetics and inheritance

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 Appendix).

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.

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, an allele.

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.

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.

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).

The other allele (denoted by a small letter - lower case) is described as a recessive allele.

Note that D overrides d, dominant allele overrides a recessive allele in all heterozygous organisms.

A pair of homozygous alleles e.g. DD, will also produce the dominant gene trait, but note that 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.


Sexual reproduction and methods of genetic analysis (for GCSE level)

I've written an appendix at the end of the page to illustrate how to analyse the possibilities of offspring phenotypes using both Punnett squares and genetic link diagrams.

I suggest you first work through the appendix on genetic diagrams and then start on the next section below.

You can work your way through them now, or refer to them while working down the rest of this page.

You must be completely familiar with the terms and phrases:

gamete, genotype, phenotype etc. as introduced and described above.


Some examples of genetic diagrams to explain the inheritance of characteristics

(Initially sing 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.

 

(a) 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 bread tall or dwarf (short), which we know recognise as homozygous genotypes.

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 bread, 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
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)

This gives 100% tall plants (genotype Tt), but, in terms of modern genetics, they all carry the allele t for dwarf pea plants.

 

(b) Mendel's second cross of two of the tall plants from the first set of offspring (from F1 above)

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
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.

 

(c) 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 bread, 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
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 (F1 hybrids)

This gives 100% purple plants (genotype Pp), but, in terms of modern genetics, they all carry the 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
Genotypes of plants P p
P PP Pp
p Pp pp

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% 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.

 

(d) The outcome of Mendel's experiments

From his experiments Mendel concluded the following:

1. Characteristics in plants are determined by some kind of 'hereditary units' (we now know as genes).

2. These hereditary units are passed from one generation to their offspring unchanged from both parents AND one 'unit' from each parent (plant).

3. 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).

 

Why wasn't Mendel's brilliant work recognised at the time?

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.

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 the Appendix with lots of diagrams and explanations.


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.

(a) sickle cell anaemia; (b) cystic fibrosis; (c) polydactyly; (d) *

See detailed notes on the human genome project for notes on genetic testing ('pros and cons') and medical treatments


(a) 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.

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
Genotypes of children A a
A AA Aa
a Aa aa

 

5 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


(b) 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 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.

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 genetic table for cystic fibrosis
1. Genotypes of parents: Ff x Ff, normal but both carriers
Gametes: F, f, F and f
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).

 

5 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


(c) 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 table 1. for polydactyly
Genotypes of parents: Pp x pp

affected and normal

Gametes: P, p, p and p
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).

 

5 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 on the human genome project for notes on genetic testing ('pros and cons') and medical treatments


APPENDIX

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.

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 23 : 77, not 25 : 75

 

Method of constructing two types of genetic diagrams.

1. 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.

 


Example 1. Parents a and b

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 parent a's gametes genotypes
D D
parent b's gametes genotypes D DD DD
D DD DD

2. 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.

GCSE biology hereditary genetic diagrams based on Punnett squares

This is crossing two homozygous 'parent' dominants DD.


Example 2. Parents a and b

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 parent a's gametes genotypes
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.

GCSE biology hereditary genetic diagrams based on Punnett squares

 


Example 3. Parents a and b

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 parent a's gametes genotypes
D d
parent b's gametes genotypes D DD Dd
d Dd dd

This is crossing a pair of heterozygous parents Dd.

GCSE biology hereditary genetic diagrams based on Punnett squares

 


Example 4. Parents a and b

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 parent a's gametes genotypes
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.

GCSE biology hereditary genetic diagrams based on Punnett squares

 


Example 5. Parents a and b

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 parent a's gametes genotypes
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.

GCSE biology hereditary genetic diagrams based on Punnett squares

 


Example 6. Parents a and b

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 parent a's gametes genotypes
d d
parent b's gametes genotypes d dd dd
d dd dd

This is crossing two homozygous recessive 'parents' dd.

GCSE biology hereditary genetic diagrams based on Punnett squares

 

 


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

Be able to make informed judgements about the social and ethical issues concerning the use of stem cells from embryos in medical research and treatments

Be able to make informed judgements about the economic, social and ethical issues concerning embryo screening.

Be able to demonstrate an understanding of how gene mutations change the DNA base sequence and that mutations can be:

(i) harmful - causing genetic disorders like cystic fibrosis, Downe syndrome, haemophilia and colour blindness.

(ii) beneficial - the gene expression produces an enhanced feature that makes that organism more able to survive, this is partly responsible for driving the evolution of more successful species, but not always to our benefit! e.g. bacteria genes are quite susceptible to mutations and some are becoming very resistant to antibiotics as their DNA subtly changes!

(iii) or neither ('neutral') - any faults from DNA mutations do not affect the organisms existence i.e. protein functions are not affected, no advantage is gained and no disadvantage either.

 


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


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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