Introduction to the genetics of inheritance of characteristics and genetic diagrams

including technical terms terms explained, the work on Mendel with pea plants and inherited genetic disorders

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.

You can use simple genetic diagrams can be used to show this.

Our knowledge of genetics enables us to treat certain medical conditions but there are ethical considerations in treating genetic disorders.

Genes exist in alternative forms called alleles which give rise to differences in inherited characteristics.

A gene is a shorter section of the huge DNA coiled up molecules that make up chromosomes.

Alleles are essentially different versions of the same gene.

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 - see section 1.22.

In section 1.21 remember alleles are different versions of the same gene.

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).

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 phenotype short.

homozygous - if a pair alleles for a characteristic are the same on a gene eg genotype XX (phenotype female)

heterozygous - if a pair of alleles for a characteristic are different on a gene eg genotype XY (phenotype female)

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.


  • Be able to interpret genetic diagrams, including family trees.

  • HT only: 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),

    • If you are a Foundation Tier candidate, 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.

  •  Data in examination questions may be given for unfamiliar contexts.


Some examples of genetic diagrams to explain the inheritance of characteristics

A good example is to consider some of the results of Mendelís work which preceded the work by other scientists which links Mendelís Ďinherited factorsí with the chromosomes of the humble pea.

Mendel was an Austrian monk, who, working in a humble garden plot, made notes that provided good experimental evidence on how characteristics of pea plants were passed on from generation to the next. His research results were published in 1866 and eventually became an important work of the study of modern 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 (genotype TT) with a dwarf pea plant (genotype tt) and found all the offspring were tall.

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 'modern' genetic diagram and Punnett square for crossing the tall pea with a dwarf pea (1st cross to give F1)

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)

This gives 100% tall plants (genotype Tt), but 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

Genetic table for crossing tall pea plants
Parent genotypes: Tt x Tt
Gametes: T, t, T and t
Genotypes of plants T t
t Tt tt

The 'modern' genetic diagram and Punnett square for crossing two plants from the 1st cross (2nd cross to give F2)

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.

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.

Some examples of inheritance and genetic disorders.

You need to be able to evaluate the outcomes of pedigree analysis when screening for genetic disorders:


(a) sickle cell disease

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.

For sickle cell anaemia to occur in a child, both parents must carry the recessive allele for sickle cell disease, but neither is affected by it.

However, there is a 1 in 4 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).

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


(b) cystic fibrosis

Cystic fibrosis is a genetic disorder disease passed down through families. Cystic fibrosis causes thick, sticky mucus to build up in the lungs, digestive tract, and other areas of the body and is one of the most common chronic lung diseases in children and young adults. 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 of a gene and can therefore be passed on by parents, neither of whom has the disorder.

Cystic fibrosis is caused by a recessive allele f (so it needs genotype ff, a double recessive allele).

The genetic diagram shows that both parents must be carriers of the recessive allele and there is a 3/4 chance of having a normal child (FF non-carrier or Ff carrier) and a 1/4 chance of having a child with cystic fibrosis (ff sufferer and carrier).

Punnett square genetic table for cystic fibrosis
Genotypes of parents: Ff x Ff

normal but both carriers

Gametes: F, f, F and f
Genotypes of children F f
f Ff ff


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

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