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GCSE Biology: DNA, protein synthesis and mutations.


What you need to know: -

  • A gene is a section of DNA that codes for a specific protein

  • DNA (structure)

  • How the structure of DNA was discovered

  • DNA decides the order of amino acids in protein

  • Stages of protein synthesis, including transcription and translation

  • Proteins specific number and sequence of amino acids

  • Gene mutations can be harmful, beneficial or neither.

  • A gene is a section of DNA that codes for a specific protein

  • DNA (structure)



Chromosomes and their genes are made of a molecule called DNA. 


[image of DNA]


Each chromosome is a very long molecule of tightly coiled DNA.


What does DNA stand for?

DNA stands for DeoxyriboseNucleic Acid.


DNA molecules carry the code that controls what your cells are made of and what they do. 


A section of DNA is Called a gene!

Remember – an alternative form of a gene is called an allele


Genes code for proteins!


Chromosomes and their genes are made of a molecule called DNA. (deoxyribonucleic acid)


DNA is a macromolecule that carries the code that controls what your cells are made of and what they do (this is the Genetic Code). 


How the structure of DNA was discovered


1952 – Rosalind Franklin and Maurice Wilkins used X-rays to work out that DNA was Helical.


1953 Watson & Crick worked out the structure of DNA… 


[images of key DNA people]




The nucleotides are made up of a nitrogenous base, a pentose sugar and a phosphate group.


[Images of Nucleotides]


Nucleotides join together to form a chain

The phosphate group of one nucleotide bonds with the sugar of another, releasing water…

Formation of a Phosphodiester bond

[image of nucleotides joining together]


The two helix chains are weakly linked by hydrogen bonds that connect the complementary bases together; adenine bonds to thymine, and guanine bonds to cytosine.


Adenine pairs with Thymine 


Guanine pairs with Cytosine.


Hydrogen bonding


“weak Hydrogen bonds link the bases”.


[image of hydrogen bonding between bases]


Recap- DNA Structure


  • Double strand of nucleotides (complementary)


  • 4 bases A, T, G and C 


  • A=T 

  • G≡C


  • The two strands are anti-parallel


  • 2 types of bond:

    • Weak hydrogen bonds between the bases

    • Strong phosphodiester bonds between the ‘backbone’ sugars and phosphates


[image of DNA structure]


Protein synthesis

1. Stages of protein synthesis, including transcription and translation

2. Proteins specific number and sequence of amino acids


What are Proteins?

Proteins are polymers (called polypeptides) built up from amino acids.


It is the order of amino acids that determines the 3D conformational shape and function of the protein.


But… How does a cell arrange the amino acids in the correct order…?


The Triplet Code!  


[image of codon]


So, triplet base codes (codons) are needed to build each amino acid…


These triplet sequences are called CODONS.


Note: CODONS are the triplets of base letters that code for amino acids: DNA and mRNA have CODONS!


[image of DNA -> mRNA -> codons -> amino acids]



The table below shows codons and their corresponding amino acids. 

This is short sequence of bases: - AAACACTTGGTCGTG for a section of the insulin molecule. What is the order of amino acids?

[Insert Table of codons and amino acids]


  1. AAA = Phe 

  2. CAC = Val

  3. TTG = Asn

  4. GTC = Glu

  5. GTG = Glu


Some amino acids have more than 1 codon (this is  known as "Degenerate").


DNA and mRNA have Start & STOP codons.


RNA  = Ribonucleic acid


1) RNA is Found in the nucleus and throughout the cytoplasm.

2) RNA is composed of RNA nucleotides.

3) RNA is Single stranded, (but has 3 types)

        mRNA, tRNA and rRNA.

4) All 3 types of RNA are used in PROTEIN SYNTHESIS!


Messenger RNA (mRNA)


Single helical strands of several thousand nucleotides.


It is produced in the nucleus by transcription of DNA.


It contains triplet CODONS!


[image of codon chart]


[image of mRNA - showing codons]



Transfer RNA (tRNA)

Single strand folded into clover-leaf shape.


Different types of tRNA: each providing a code for one amino acid.


Used in translation


Has the Anticodon


[image of tRNA]


Ribosomal RNA (rRNA)

Made in nucleus. 

Used in translation -  “Protein synthesis”

remember! Ribosomes are the sites of protein synthesis!


[image of rRNA]


What is Transcription?

Transcription is a process that involves the transcribing (converting) of genetic information (e.g. a GENE) from DNA to RNA. 


Transcription can be thought of in 3 phases…


  1. Initiation

  2. Elongation

  3. Termination


Transcription can broken down into steps…


  1. Initiation: RNA polymerase bonds to promoter sites on DNA.

  2. Elongation: mRNA is “copied” from DNA.

  3. The growing mRNA strand protrudes from the ‘transcription bubble’.


[image of mRNA being transcribed]


4. Termination: At the end of a gene, a stop codon causes mRNA to be released and DNA rewinds.


Before The mRNA exits the nucleus via nuclear pores. The non-coding parts (introns) must be removed… This is called RNA splicing!




Introns are the non-coding sections of DNA & mRNA – introns are removed! 

(INtrons remain INside the nucleus).


Exons (are Expressed): they are the Coding sections of DNA & mRNA  (EXons EXit the nucleus).


[image of introns and exons]


Once the introns have been removed, the mRNA exits the nucleus via nuclear pores in the nuclear membrane… Translation is next…


What is Translation?

Translation is a step in protein synthesis where the genetic code carried by mRNA is decoded to produce the specific sequence of amino acids in a polypeptide (Protein) chain. 


Translation follows Transcription (in which the DNA sequence is copied (transcribed) into mRNA).


[image of translation taking place in the cytoplasm of a cell]


Translation can broken down into steps…


1) The code on the mRNA is used to assemble the amino acids in the correct order.

2) Ribosomes bind to the mRNA and allows tRNAs to attach.

3) Each codon on mRNA corresponds to a specific anticodon on the tRNA. 

4) tRNA molecules attach amino acids

5) Amino acids are joined by peptide bonds… forming polypeptides (proteins)!




Gene mutations can be harmful, beneficial or neither.


Mutation = change in the base sequence of DNA                                       

[mutations flow diagram]


Mutations change an organisms DNA (at the ‘Base level’ – i.e the bases are affected in some way. There are three types of mutation that can occur…


  1. Substetution

  2. D letion

  3. Adddition


Mutations that occur can be either: -


  1. Neutral – neither harmful or beneficial.

  2. Harmful

  3. Beneficial


  • Neutral – neither harmful or beneficial. Does not affect protein structure

  • Harmful – alters protein structure and causes detrimental effects – e.g. cystic fibrosis.

  • Beneficial – incurs an evolutionary benefit – e.g. antibiotic resistance.


The three types of mutation…


1. Substetution


Substitution is when one base is substituted for another. 


This causes the  codon for the amino acid to change, however the  rest of the protein  remains unchanged. 


If the new amino acid is similar to the original, then the protein’s structure and function could remain the same.


If the amino acid is very different then the protein could have a completely different structure or shape.


[image of substitution mutation]


2. D letion

When a base is deleted from DNA all of the following bases move down. This means that every subsequent codon is changed which will result in a very different protein at the end of replication.

The differences that occur in the gene after deletion depend on where in the gene the deletion occurs. If a deletion occurs near the end of a gene then the change can be only minor. If the change is at the beginning of the gene then there will be a far more serious change. 

[image of deletion mutation]


3. Adddition 


An addition of a base [where an extra base is included in to the genetic sequence]. Additions have a very similar effect to a deletion.

[image of addition mutation]


What are the Dangers of mutations?

The biggest danger in mutations of genes is that they change the protein that the gene codes for.



Well, Since most proteins in cells are enzymes and most changes to enzymes stop them working, mutations can have disastrous effects.



Substitutions are the least dangerous type of mutation because the protein can sometimes remain unchanged.  (Remember the degenerate code (that amino acids have more than one codon). Well if the substituted base is one that codes for the same amino acid… the protein will remain the same…



Previously you have learned about DNA and that DNA carries ‘codes’ which in turn via the process of protein synthesis (transcription and translation) make proteins… Well Enzymes are proteins… and now you need to know about the structure and function of these biological catalysts!


So what you have to know: -

  • Enzymes as biological catalysts

  • Enzymes catalyse chemical reactions – e.g DNA replication, Protein Synthesis, Digestion. 

  • Factors affecting enzyme action

  • Enzymes are highly specific for their substrate

  • Lock-and-key hypothesis

  • Enzymes can be denatured

  • Enzyme technology

  • Enzymes in food production


[practical work you should do in school/college should cover: -

Factors that affect enzyme activity…

Investigate the use of immobilised lactase / Enzymes in food production]



Previously you learned about how proteins are made… (transcription and translation)… 


So, what are Proteins? 

Protein - comes from the Greek word “proteios” meaning 1st place! (because - Proteins account for more than 50% of the dry weight of cells – and are VITAL in almost everything organisms do (especially when considering that ENZYMES are proteins!)



Proteins are Organic molecules Containing

Carbon, Hydrogen, Oxygen and Nitrogen


Proteins have many vital functions including…


  • Structural support

  • Storage

  • Signalling

  • Defense

  • Transport

  • Movement


Additionally as ENZYMES proteins regulate metabolism by regulating chemical reactions in the cell.


Humans have tens of thousands of different proteins… each with a different, specific structure & function. e.g. 


Globular proteins: like ENZYMES, ANTIBODIES and HORMONES

Fibrous proteins: like Keratin and Collagen.




So, Proteins have a variety of functions:


Enzymes: usually globular, due to tight folding and coiling of polypeptide chains. They are often soluble and are important in metabolism… (e.g. enzymes hydrolyse (breakdown) large food molecules (digestive enzymes) whilst others help make (synthesise) large molecules.

Enzymes as biological catalysts

  • Enzymes catalyse chemical reactions

  • Enzymes are highly specific for their substrate

  • Lock-and-key hypothesis


So, Enzymes are proteins. 

They are very important substances because they control the chemical reactions that happen in our bodies… 

Enzymes are known as biological catalysts - substances which speed up reactions but which do not get used up themselves. 


[image of enzyme and substrate]


Enzyme names usually end in the letters ‘ase’ for example: -

Amylase, protease and lipase.


Enzymes are specific  for what they will catalyse and Enzymes Are Reusable:


  • DNA /  RNA polymerase

  • Sucrase

  • Lactase

  • Maltase


Enzymes act as “Biological Catalysts” - they speed up chemical reactions.

How does an enzyme actually catalyses a chemical reaction?

Enzymes catalyse a chemical reaction by combining with a reactant and speeding up the reaction. 

The reactant is called the substrate. 

The enzyme has a specific indentation called an active site, which helps it to recognise that substrate.


Just like a key only fits into a specific lock, each enzyme has its own specific substrate. Once the reaction is complete and the required product has been produced, the enzyme releases itself and moves on to the next reaction.


[video - enzymes basics]

[image of lock and key ESC]

[image of activation energy graph]

There are two main types of enzymes: -

1) Those that break down large molecules into smaller ones. 

These are very important in digestion, why?

They are required to break down large food molecules into smaller ones that can be used by our cells.

2) Those that build up large molecules from small ones. These are very important for growth and repair. 


Enzyme specificity (enzymes are very specific).


[PDF task… Use the diagram to help you to explain why each enzyme will only catalyse one particular reaction… Enzymes are specific to one particular type of reaction, in this case it will only be able to bind/attach to substrate A, which fits into the active site of the enzyme].



Factors affecting enzyme action

Enzymes can be denatured

Many factors affect how well enzymes function: you need to know 3 of them…


1) Temperature

2) pH

3) Substrate Concentration


  1. Temperature


Most enzymes function best at normal Body temperature: 37°C


High temperatures will usually result in an enzymes denaturation


Most enzymes like near neutral pH (6 to 8)


Denaturation is defined as a permanent change in the tertiary (3-Dimensional) structure of a protein


When an enzyme is denatured it is no longer able to function.


[image / video of graph]


As temperature increases the rate of reaction increases up to a maximum (usually around 40°C). After this point the rate of reaction will decrease.


Q. Why do you think that the reaction goes faster as the temperature increases? (think about what happens to molecules as they warm up) 

A. The molecules involved in the reaction will have more kinetic energy as the temperature increases, so they will move about more, collide more. 

So the enzyme and substrate molecules are more likely to collide or combine with each other (and therefore react)



High temperatures will result in enzyme denaturation.

Enzymes are proteins. At high temperatures these proteins start to unravel. This changes the shape of the active site and as a result the substrate can no longer fit into it. 

When this happens the enzyme is said to be ‘denatured’. Once an enzyme is denatured it will not work.

[image of enzyme denaturation - graph explained]


2. The Effect of pH


Enzymes in the human body will work at an optimum within narrow pH ranges. pH changes beyond an enzymes optimum will result in the enzymes denaturation.

How does pH affect enzyme activity?

An enzymes catalytic activity is affected by how acidic or alkaline its environment is. 

The majority of enzymes work best in neutral conditions. However, some prefer acidic and others prefer alkaline conditions. 


[image of pH - enzymes]


Similar to temperature, the active site of an enzyme can be changed by conditions that are either too acidic or too alkaline. 


Q. Explain how pH could reduce the rate of a chemical reaction.

A. If the active site of the enzyme is changed (i.e. it is denatured) then the substrate can no longer bond/attach to the active site of the enzyme. Therefore no reaction can take place and no product produced.


3) Substrate Concentration


The more enzyme in a solution, the greater the chance that an enzyme substrate complex will form, and the greater the rate of reaction up to a maximum when all active sites are fully occupied.

[images of substrate concentration / graphs explained]


The more substrate in the solution the greater the chance of a substrate molecule finding an active site, and the faster the rate of reaction up to a maximum when all active sites are fully used.


Enzymes lower Activation energy.


Enzymes lower the activation energy for a reaction [image / video of activation energy]


Enzyme technology & Enzymes in food production

Enzymes use in sweet manufacturing…


Q. How do you get the runny centre inside a cream egg?

A. Enzymes!


To make soft centres in chocolates many manufactures add an enzyme called invertase (sucrase) which catalyses the reaction to break the sugar sucrose down.


Many Confectionary sweets are made using enzymes – specifically and enzyme called Invertase (sucrase). Invertase is produced by yeast and we can utilise this enzyme.

Invertase / sucrase breaks down sucrose into the monosaccharides  glucose and fructose.


Enzymes used in making Vegetarian Cheese


Enzymes in cheese making. Cheese is made when the enzyme chymosin. 


Chymosin acts upon milk – specifically the enzyme catalyses reactions that cause proteins in the milk (called curds) to coagulate and separate from the liquid (whey).


Chymosin was originally obtained from calves stomach tissues. However now, the enzyme is made using genetically modified bacteria. The process is more efficient and the product (Chymosin) contains less impurities and acts more predictably.


Enzymes using in Biological washing powders


Biological washing powders contain enzymes, for example:


Proteases – break down proteins into amino acids


Lipases – break down fats into glycerol and fatty acids


Carbohydrases (e.g. amylase) – break down sugars (polysaccharides, e.g. starch) into monosaccharides (e.g. glucose)


Because biological washing powders contain enzymes (biological catalysts) they work best at “optimal temperatures”.


Which means they are most effective at lower temperatures, e.g around 30OC. 

This has many other benefits:


  • Less energy is used (heating water / production of carbon emissions)

  • Dyes in fabrics are less likely to ‘run’ out of fabrics

  • Clothes are less likely to shrink in the wash

Enzymes are super important… and you will learn more about enzymes when you learn about The digestive system – e.g. Enzymes that break down carbohydrates (Carbohydrases) like the enzyme amylase which is found in the saliva in the mouth).

[You will also revisit enzymes in a little more detail when you learn /  investigate enzyme action: i.e.

  • Use of immobilized enzymes

  • Lactase will break down lactose in milk

  • Pectinase will break down pectin (a sugar found in the cell walls of plants / fruit)

  • Investigate how temperature and pH effect enzyme action ]

GCSE Biology: Genetics - Inheritance.


You’ll need to learn some new Ke­y terms used in genetics and know: -

Genes exist in alternative forms

Monohybrid Genetic Diagram / Punnett squares and family pedigrees

Calculate and analyse outcomes from monohybrid crosses

Symptoms of sickle cell disease and cystic fibrosis

Pedigree analysis screening for genetic disorders

The sex of a person is controlled by one pair of chromosomes

How the sex of offspring is determined at fertilisation (genetic diagram)

How sex-linked genetic disorders are inherited



First you should recap Cell division, mitosis and meiosis…

on the cell biology page we covered the following: -


How do cells divide to become anything at all?


Through a process called… Mitosis!


Somatic (body) cells are described as Diploid (from the Greek meaning ‘double’). 

…you will see this written as “2n”


Mitosis is the process of normal cell division. 


In mitosis, the chromosomes are copied and divided equally between the 2 new daughter cells. ..


...So each mitotic division produces 2 cells…


SO... both are diploid, & each has exactly the same genes as the parent cell!! Mitosis produces identical cells


Some cells in the human body are not diploid... 

e.g. Gametes (sex cells) contain only 1 copy of each gene as they have only 1 set of chromosomes.


These cells are Haploid and are produced by a special type of cell division called Meiosis


A male and Female gamete join  together at fertilisation...  forming a.  zygote

This 1 cell then divides  by  Mitosis to produce   a complete  new   organism…


You received one chromatid from your father. 


And one from chromatid your mother!


So, although homologous pairs contain the same genes, they do not necessarily carry the same versions of each gene.

A different version of a gene is called an allele.



This is key to understanding why organisms vary!


which brings us to inheritance…


Remember… that in all living things, characteristics are passed on in the chromosomes that offspring inherit from their parents. 


 [Image of chromosomes - genes / alleles]


So, although homologous pairs contain the same genes, they do not necessarily carry the same versions of each gene.


 Remember that A different version of a gene is called an ALLELE.


so, Each chromosome may have a different version of a gene (an Allele). 


“Different versions of a gene, that code for different versions of a characteristic, are called ALLELES”. 


Ok, In genetics, different genes (alleles) are often represented by letters, such as Aa. 


The capital letter represents a DOMINANT allele ‘A’; whereas a lowercase letter ‘a’ would be the recessive allele.


some key terminology… Terminology is so important! Make sure you learn the language of biology - understanding what a word means makes all the difference! 


Here are some key terms you have to know: - 

Heterozygous, Homozygous (dominant and recessive), Phenotype, Genotype, Allele, Gene


Homozygous (dominant and recessive) and heterozygous…




“Homo” meaning “the same” and zygous - referring to the zygote (the fertilised cell). So you can think of homozygous as having 2 of the same “letter” like 2 capital letters ‘AA’ or 2 lowercase letters ‘aa’.


Now you’ll know that a capital letter,  ‘A’ represents a dominant trait…. so 2 capital letters ‘AA’ can now be called Homozygous Dominant (2 of the same capital letters - which represent a particular trait, for example eye colour).


One of those A’s (genes/alleles) has come from mother whilst the other ‘A’ (gene/allele) has come from father - so, one of the letters represents the “sperm” and its genes, and the other represents the egg (ova) and its genes…. 


Homozygous individuals are true breeding. This means that they will always produce the same phenotype of offspring because they are not ‘hiding’ a recessive allele.


So what does heterozygous mean?

Well ‘hetero’ means “different” and again ‘zygous’ refers to the zygote (the fertilised cell).


So, in genetics and when we use letters to represent characteristics and inherited traits we can now think of heterozygous as being 2 different version of the same letter… so, a capital letter ‘A’ representing a dominant trait and a lowercase ‘a’ representing the recessive trait.


Aa = heterozygous


You know now that the “letters” ‘A’ and ‘a’ are the different version of the same gene - that is they are Alleles…


so we have:


Homozygous dominant: 2 of the same capital letters, AA, FF, HH, etc… to represent the 2 dominant genes inherited from each parent.


Homozygous recessive: 2 of the same lowercase letters, aa, ff, hh, etc… to represent the 2 recessive genes inherited from each parent.


Heterozygous: 2 different version of the same letters, ‘Aa’, Ff, Hh etc… to represent a dominant allele inherited from one parent and a recessive allele inherited from the other parent.



Now we know what the letters mean and the key terms homozygous dominant, homozygous recessive and heterozygous we can better understand what Gregor Mendel was up too when cross fertilising pea plants…


Mendel and Monohybrid inheritance: How single genes are passed on... Medal and his peas…




what is monohybrid? we can better understand this term like this: -

mono = one

hybrid = inheritance of a gene


so literally the inheritance of one gene…


ok, so… Monohybrid (single - gene) inheritance, is about the inheritance of different alleles BUT concerning a single gene. (e.g. the gene for height or the gene for flower colour)…






...Like Mendel, we’ll start with the pea plants, which have easily observable features that are controlled by a single gene;


for example Pea plants have one gene for height. 


But, the height gene has 2 alleles: 


you can represent the genes/alleles for height by using the letter ‘H’


You’’ll now that a capital ‘H’ (represents a Dominant allele, in this case the allele for being Tall). 

whilst the lowercase ‘h’ (represents a a recessive allele, in this case the alley for being short).




Pea plants are diploid and so have 2 alleles (for height). Therefore there are 3 possible genotypes:


1. The pea plant could have 2 Dominant alleles: “TT” which you know is called homozygous dominant (homo = same) (so you can say the pea plants are homozygous for T - Tallness)


2. The pea plant could have 2 recessive alleles: “tt” which you know is called homozygous recessive (homo = same) (so you can say the pea plants are homozygous for t - shortness)


3. The pea plant could have inherited 1 dominate allele and 1 recessive allele: “Tt” which you know is called Heterozygous (hetero = different).


What you must remember though is that a dominant allele will “overpower” a recessive  allele… so in this case Tt would be a Tall pea plant.


Note: more key terms!

The letters we have used, e.g. “Tt” are an organisms Genotype (the genes the organisms have inherited).


However, how the organisms looks - for example the heterozygous “Tt” pea plant looks Tall - this is its phenotype (the “Physical” outcome, or physical representation of the genotype)…


So the genotype is the genetic make up of an organism… the genes it contains, and you can use letters to represent them, such as using the letter “H” for height… HH, hh or Hh.


The Phenotype is the physical outcome - the expression of those genes… for example if the letter “H” is the dominate gene for Tall plant - you can physical see the organism, the pea plant in this case is Tall… but, you wouldn’t know if the pea plant was heterozygous or homozygous dominant (because you can’t see its genes!)


And this is exactly what Mendel was up to! (he wanted to know what genes the pea plants where passing on to their offspring after being fertilised).





Remember Pea plants are diploid - they have 2 alleles (for height in this example). 


So, Consider what happens when a homozygous Dominant plant (TT = Tall) is crossed with a homozygous recessive plant (tt = short).


[insert Punnett square]


All gametes from the Tall plant contain a T allele

All gametes from a dwarf plant contain a t allele. 


These pea plants combine at fertilisation to give offspring which have all inherited 1 Dominant gene (allele) ‘T’ from the homozygous dominate (Tall) plant, and 1 recessive allele from the homozygous recessive (short) plant ’t’. So, the genotypes for all offspring following this fertilisation must be “Tt” (hererozygous). and since the genes / alleles determine an organisms phenotype we now that all the pea plant “Look” Tall.


So we can say The 1st generation of pea plants, (known as the F1 generation), are all Tall because Tallness is determined by the Dominant Allele (T)…

However, although the pea plants all look identical (and just like the Tall parent plant), they are very different in one very important respect: they are Heterozygous and NOT homozygous!


Remember: How an organism actually looks is its called its phenotype!


Mendel wanted to know: What happens when 2 of these heterozygous pea plants are fertilised? how will they look?


So, If 2 of these heterozygous plants are crossed, half of the gametes from each parent are T and half are t, giving us 4 outcome with 3 possible genotypes in the second generation (F2)…


the genotypes are: -


1. HH, 

2. Hh, 

3. Hh and 

4. hh.


[insert Punnett square]


The first 3 genotypes are all Tall plants (phenotypically), but the fourth is a short plant (phenotypically). You can say 75 % are Tall and 25% are short…



Monohybrid inheritance in humans


Clear cut examples of monohybrid inheritance in humans are pretty rare, but often they involve a genetic disease where people inherit 1 or more faulty alleles. 


Genetic diseases are often recessive; this is because faulty alleles that fail to make an important protein can be masked by normal ones that function properly.


i.e. Recessive alleles are overpowered (masked) by Dominant Alleles.


In contrast though, some genetic diseases like Huntington's disease can be caused by DOMINANT ALLELES. The alleles concerned code for a product that actively causes damage; symptoms are not due to an allele not doing its job. Such alleles are dominant because the presence of a normal allele cannot mask the symptoms.


Some examples of monohybrid inheritance in humans: -


[insert image/table of monohybrid inheritance]


Pedigree analysis screening for genetic disorders)


Huntington's disease is a rare inherited disorder of the nervous system. it is caused by a dominant allele (we’ll represent this allele with the capital letter ‘H’. The recessive allele for this gene we can represented by lowercase letter ‘h’.


[insert “Pedigree” image]


The diagram shows the inheritance of Huntington’s disease in a family.


Use a genetic diagram to show the inheritance of the Huntington's Disease allele by the children of parents P and Q.


[images of genetic diagrams]


Q) Explain why none of the children of R and S inherited Huntington’s disease.


[insert image for Q above]


A) Both parents are unaffected; They do not have Huntington’s disease. 

The Genotype of the parents is... hh homozygous recessive (So, neither parent has the ‘H’ dominant gene/allele - therefore cannot pass on the disease).


Calculate and analyse outcomes from monohybrid crosses

What if both parents are heterozygous? Organise your results into a Punnett square: -


[insert images of Punnett square and explainations]


Monohybrid Genetic Diagram Punnett squares and family pedigrees…

Cystic fibrosis is a recessive disorder. Cross 2 carriers with the  genotypes Ff


[insert genetic diagram images]


Organise the results of the genetic cross into a Punnett square: -


[insert Punnett square / plain ratio / %]




You know that Meiosis is a special type of cell division which results in the production of Gametes (sex cells). 


“Meiosis is a special kind of cell division in which there are 2 successive divisions that result in the production of gametes – the sex cells!”


Remember sperm and egg (ova) are Haploid – having just “n” half the number of chromosomes.


Meiosis: The production of Gametes (sex cells)  


You also need to know that: -

The sex of a person is controlled by one pair of chromosomes (X and Y).

How the sex of offspring is determined at fertilisation (use a genetic diagram).


[insert genetic diagram -showing inheritance of X and Y]


Organise your results into a Punnett square: - 

[insert Punnett square showing inheritance of X and Y]


Sex linked Genetic disorders… How sex-linked genetic disorders are inherited…


You know who the sex of a person is determined by the inheritance of their X and Y chromosomes, now you need to know that Some genetic characteristics are also “sex linked” – meaning they are carried on one of the Sex Chromosomes (X or Y). 


Colour Blindness is and example of an inherited sex linked characteristic… it is caused by a faulty allele on the X chromosome… 


Since the Y chromosome is smaller than the X chromosome it carries fewer sex-linked genetic disorders! 


So, colour blindness, for example is much more common in men than in women – because men only require 1 (recessive) faulty allele – whereas women need 2.


[insert images: This genotype is rare, This genotype is more common]


[insert images of genetic diagram showing inheritance of colour blindness - sex linked]

[insert images of Punnett square showing inheritance of colour blindness - sex linked]



Sex linked Genetic disorders: 


Haemophilia (a disease where blood doesn't clot properly)  is a genetic disease that is inherited in exactly the same way as colour blindness.


Haemophilia is inherited, and is caused by a faulty allele on the X chromosome…




Recap/revise key words: - 


Homozygous: -

Pair of alleles that produce a characteristic are the same, e.g. 

Homozygous dominant = HH

Homozygous Recessive = hh 


Heterozygous: - Pair of alleles that produce a characteristic; the alleles are different, e.g. Hh. 


Dominant: -

An allele that will always be expressed even when there is only one of these alleles present, represented by a capital letter. e.g. HH or Hh. – the H (dominate allele – will be expressed) 


Recessive: - An allele that will only be expressed when both alleles are of this type e.g. hh. 


Gene: - Section of DNA that codes for a particular trait or characteristic. 


Allele: - A different form of a gene that codes for a different version of the same characteristic (i.e. different eye colour) 


Genotype: - A description of the pair of alleles present for a characteristic. 


Phenotype: - The physical expression of the alleles.

Lets look at how we can construct a monohybrid Punnett sqaure for an inherited condition known as Brachydactyly.


If the allele for Brachydactyly (having short fingers) is Dominant (B) (which it actually is!) and the allele for “normal” length fingers is recessive (b) -  then shouldn’t the frequency of short fingered people in a population become more common, with each successive generation?

Surly in time – pretty much everyone should have short fingers!?

Let’s take a look at this problem from a very simple Mendelian point of view…


Let’s say that a heterozygous Male (Bb) and a heterozygous (Bb) female have a child… what is the probability that their child will inherit the allele for short fingers (B)remember short fingers are dominant.


Well of course this is easy to consider when using a Monohybrid Punnett square: watch the video below to see how a simple monohybrid Punnett square is made...

Monohybrid inheritance (Punnett square)