Is the study of inheritance of characteristics or the study of transmission of characteristics/traits from the parents to the off springs.

Simplified illustrations:

          sexual union
parents Male         X     female
↓                            ↓
Gametes sperms                eggs
   ↘                        ↙
fertilization (Zygote)
        ↓ Growth and development
Off springs (sexually mature individual


Transmission of characters from the parents to off springs is by gametes. The male organism if animal contributes sperms and in plants pollen grains

The Female animal contributes ova and in plants it will contribute ovules. Animals have to mate to bring the two gametes together.

In human beings, they have sexual intercourse, in plants the two gametes come together by pollination (self/cross pollination). When the female and male gametes fuse, fertilization is said to have occurred. Fertilization is the fusion of the male and female nuclei to form a zygote.

In animals the zygote is formed in the fallopian tube and it begins to undergo growth (growth is an irreversible increase in the size of an organism.) Growth occurs by cell division.

The type of cell division which leads to growth is mitosis. The zygote also undergoes development. Development is the change in shape and form. Growth and development eventually lead to a sexually mature organism.

Sexual maturity in flowering plants is evidenced by the on set of flowering. In female animals its evidenced by ovulation and in male animals by sperm development.

Gametes are formed by meiosis (meiosis is the type of cell division resulting in formation of gametes and takes place in the reproductive cells while mitosis is the cell division that leads to growth of an organism and occurs in the non reproductive cells/somatic cells/body cells

Examples of somatic cells;

Liver cells, check cells, ovary cells, testis cells etc

Examples of reproductive cell;

Sortollic cells in the testis which form sperms

Follicles in the ovary that give rise to ova

The cell

With in the cell nucleus is the observable genetic material known as the chromosomes. In body cells chromosomes are found in pairs (Half from the female parent and half from the male parent.) In reproductive cells/gametes, they occur in a single set called haploid and represented by n but in body cells they occur in a double set called diploid and represented by 2n.


These are threadlike structures found in the nucleus of the cell and they contain the genetic material responsible for inheritance. They form the physical basis for inheritance since their structure can be observed under a high power microscope.

Simplified structure


Each chromosome is made up of two longitudinal strands called the chromatids.

Each chromatid has a double helical DNA molecule. The two chromatids are held together by a structure called the centromere. During cell division, the spindle fibres are attached to the centromeres.

The chromosomes are present in pairs. The pairs are called the homologous pairs (they must be similar in structure and also have the same chemical composition). A species will always have the same number of chromosomes. This is called the chromosome number and it will always be an even number. This number is called the diploid number. During gamete formation, the homologous chromosomes separate and the gametes will have only half the number of chromosomes. This number is called the haploid number. Thus the somatic or the vegetative cells of all organisms are diploid and the gametes are haploid.

Number of chromosomes per cell nucleus varies from species to species. In man there are 46 chromosomes per nucleus (23 pairs) of somatic cells and 23 chromosomes in the gametes.


Is the chemical compound responsible for inheritance of characters.

DNA is one of the nucleic acids and the other is RNA.


DNA: Deoxyribo nucleic acid (less oxygen)

RNA: Ribo nucleic acid (more oxygen)

Both are similar in composition but different in structure.

Chemical composition of DNA

  • Contains a 5 carbon sugar (ribose sugar) in its structure and there fore has five corners.
  • It has a nucleic acid. An example is the phosphoric acid
  • Nitrogen/organic base is also present. There are four types namely

i)              Adenine ii) Thymine      iii) Cytosine       iv) Guanine.

  • In both DNA and RNA adenine, Cytosine and Guanine are found.
  • Thymine is found in DNA while Uracil is found in RNA.

The following combine;

  1. Adenine (A) + Thymine (T) for DNA    or Adenine (A)   +  Uracil (U) in case of RNA.
  2. Cytosine (C) + Guanine (G)

Always the pairing of bases as indicated above is due to the matching of their structure (i.e complimentary base pairing rule)

Formation of the DNA molecule

Formed from nucleotide units to form long chains


DNA replication

It’s the ability of DNA to produce a copy of its self.

DNA replication occurs in three stages and its catalyzed by a series of enzymes.

i)              Twisted strands un wind giving two strands. The strands are still joined.

ii)             Weak bonds between the nitrogen bases will be broken down to give two separate strands.

iii)            One strand will induce the formation of another strand which is complimentary to it and the same thing will happen to the other strand.

After replicating, two new DNA molecules similar to the original DNA molecule are formed.


Why is DNA suitable for inheritance?

i)                    Because of its ability to replicate

ii)                   DNA is capable of carrying large a mount of genetic information

iii)                 DNA is a very stable chemical and therefore can not easily be changed.

Cell division

Cell division is a process which leads to cell multiplication.

It occurs in both plants and animals. Original cells which undergo division are known as parent cells and the new on ones resulting from division are known as daughter cells.

There are two types of cell division i.e Mitosis (mitotic cell division) which occurs in somatic cells and Meiosis (meiotic cell division) which occurs in reproductive cells.


Stages of mitosis

1)      Interphase (resting stage of the parent cell) During this stage the following happens to prepare a cell for nuclear division)


In this stage the cell builds up energy reserve in form of ATP

It also builds up food/nutrient reserve

Replication of DNA also takes place in the chromosomes. i.e the amount of DNA is doubled

There is synthesis/replication of new cell organelles/structures eg mitochondria, endoplasmic reticulum, centrioles, chloroplasts etc


Chromosomes become visible as long thin entangled threads.

The nucleolus begins to shrink and centrioles move to the opposite ends of the cell




Chromosomes shorten and they can be seen to comprise of 2 chromatid joined at the centromere

Nucleolus disappears

Nuclear membrane breaks up

Mendel’s contribution in genetics:

He was an Austrian and by practice he was a monk.  He carried out experiments about inheritance in plants over 120 years ago.

The 1st experiment he carried out was referred to as monohybrid inheritance. The experiment considered one type of contrasting characters at a time. Hybrid is as a result of crossing between two different characteristics. In the first experiment mendel used pure breeding seeds

E.g tall plants crossed with tall plants                                               Tall off springs only (no short plants)

He planted garden pea (Pisum sativum). The garden pea showed a variety of characteristics e.g  colour of flowers, colour of pods, height of stems, nature and texture of pods, and shaped of pods.

The pattern of transmission of different characteristics was interesting eg when a plant showing one set of characteristic is cross pollinated with that showing opposite characteristic, the first generation off spring will be showing one parent’s characteristic.

When the first generation plants are self pollinated, a mixture of both parental characteristics is shown.

For example:

Parents:                                       Tall cross pollinated with short

1st generation off springs:        All tall

1st generation off springs self pollinated

2nd Generation off springs:          ¾ tall and ¼short.

For colour of pods

Parent plants                                     Green pods  X   Yellow pods



Gametes                                      pollen grains                      ovules


1st Generation off springs            green coloured pods

The seeds from the first generation are planted again and after flowering, self pollination was carried out.


1st Generation off springs       Green pods X (self pollinated) green pods



Gametes                                      pollen grains                      ovules


2nd Generation off springs           ¾ green pod       ¼ yellow pod plants

A mixture of green poded and yellow poded plants was got.

Mendel referred to what is responsible for the characteristic as genes carried by chromosomes.

A gene is a unit of inheritance:

There fore, A gene responsible for green pods is dominant (green pod is a dominant character) and is represented by letter G and yellow pod is a recessive character and the gene is represented by letter g.

Green pods                         X                             yellow pods

Parents                                                                                                             GG            gg




                                                                                                                                     Random fertilization


1st gen            Gg               Gg                          Gg                                                Gg    All off springs green poded

1st generation off springs self pollinated.

Parents                Green pods      X             Green pods

Genotype                 Gg                                             Gg


                                            Random fertilization

2nd gen           GG      gg                         Gg                           Gg

(GG, Gg, Gg) =  ¾  2nd generations plants with greens pods and (gg) =  ¼ 2nd generation plants with yellow pods.

For height:

Tallness is dominant character and shortness is recessive character.

There fore;

Let the gene for tallness be T

Let the gene for shortness be t

There fore the genotype for the tall plant is TT and for the short plant is tt

Parents                Tall plant            X                  Short plant

Genotype                 T  T                                             t  t


                                        Random fertilization

1st generation        Tt           Tt                      Tt                       Tt   First generation plants, all tall

First generation off springs self pollinated

1st generation              Tall plant          X             Tall plant

Genotype                 Tt                                                Tt


                                             Random fertilization

2nd gen           TT        tt           Tt                            Tt

(TT, Tt, Tt) = ¾ 2nd generation off springs tall and tt = ¼ 2nd generation off springs short.

Working out fertilization using the punnet/chi square







t Tt tt





Genetic terms  

  1. 1.       Genotype: is the genetic make up of an organism. From the illustrations above (Tallness), we see 3 genes TT, Tt and tt which gives 1:2:1 as the genotypic ratio.

TT and tt are known as homozygous genes

They are called so because they were formed from fusion of the same gene.

TT is homozygous dominant (tall) and tt is homozygous recessive (short)

TT and tt are pure breeds.

Tt is known as heterozygous (tall) created from different genes fusing together. Its not a pure breed.

  1. 2.       Phenotype: is the external expression of a gene present in an organism. When expressing its self its known as an allele which is a short of allelomorph.

When not expressing its self, its simply termed as agene.

A dominant gene is one that over shadows a weaker gene known as a recessive gene.

A recessive gene is one that is over shadowed by a dominant gene.

This happens when both the recessive and dominant genes for a particular trait/ characteristic are present in an organism i.e heterozygous (Tt )

  1. 3.       Filial generation: The off springs that result from fusion of gametes in various generations eg

F1 Generation                       1st generation.

F2 Generation                       2nd  generation.

F3 Generation                       3rd  generation.

Test/back cross

Its used to determine the genotype of either homozygous dominant (TT, GG, HH) or Heterozygous (Tt, Gg, Hh). Since genotypes TT and Tt both produce tall plants, its not possible to know from the phenotype whether the tall plants are homozygous dominant or heterozygous.


The test or back cross is done by crossing the tall plants from the F1 generation with a true recessive plant (tt).

The proportion of tall and short off springs in F2 will determine the genotype in the tall F1 plants.

In case of homozygous dominant (TT), when crossed with homozygous recessive, the off springs are 100%tall

Parents  (F1)        Tall plant             X                  Short plant

Genotype                 T  T                                             t  t (Homozygous recessive)


                                        Random fertilization

2nd generation        Tt           Tt                      Tt                       Tt   2nd generation plants, all tall


An in case of Heterozygous (Tt)

Parents (F1)             Tall plant         X                  Short plant

Genotype                 T  t                                            t  t (homozygous recessive)


                                        Random fertilization

2nd generation        Tt           tt                      tt                       Tt   F2 generation plants, 50% tall and 50% short

Incomplete dominance/partial/co-dominance

This is a condition where genes controlling contrasting characteristics have equal influence when in heterozygous genotype. Such gene are said to be co-dominant genes.

E.g in hibiscus plants genes responsible for the red and white flower colours are co-dominant.

If a plant with red flowers is cross pollinated with that of white flowers, what are the possible genotype and phenotype of the F1 off springs?

Let the gene responsible for red flower colour be R and the gene for white flower colour be W. There fore the Genotype for plant with red flowers is RR and for the plant with white flowers is WW

 Parents                Red flower         X                    White flower

Genotype                 R R                                           W W


                                        Random fertilization

1st generation        RW       RW                   RW                      RW   First generation: all pink flowered plants


F1 off springs are self pollinated, state the possible genotype, phenotype, genotypic ratio and phenotypic ratio

F1 off springs    Pink flowered plant   X   Pink flowered plants

Genotype                 R W                                            RW


                                        Random fertilization

2nd generation      RR       WW                                RW                       RW

Genotype = RR, RW and WW

Genotypic ratio= 1:2:1

Phenotype = Red flower plant (RR), Pink Flower Plants (RW and RW) and white Flower plant (WW)

Phenotypic ratio 1:2:1

Blood groups in human beings

A and B are co-dominant genes but dominant over O

Genes Possible genotype Phenotype
A AA or AO Blood group A
B BB or BO Blood group B
O O Blood O
A and B AB only Blood group AB

What is the possible genotype of off springs from a marriage between a man of blood group A and a woman of blood group B?

Possible genotype of the father:  AA and AO and that of the mother: BB and BO

Parents            Mother            X                        father

Genotype                 B  B                                            A A


                                        Random fertilization

 F1generation        AB       AB                                   AB                         AB

Blood group B Blood group A
Parents Mother X Father Off springs
BB AA                → all AB
BO AA                → AB, AB, AO,AO
BB AO               → AB, AB, BO, BO
BO AO               → AB, BO, AO, O

There fore the possible blood groups of the F1 children are: A, B, AB and O

  1. In a mixed day school, Angela got pregnant and she is of blood group B, Kapere a fellow student was accused to be responsible for her condition, which he denied. Angela gave birth to bouncing baby boy of blood group O.  As an investigation was done Kapere was un cooperative and his blood group would not be discovered, but both his parents were of blood group A. Work out to find whether kapere would be the likely father of the baby.
  2. A woman of blood group A claims that a man of blood group AB is the father of her child. A blood test reveals that the child’s blood group is O. is it possible that the woman’s claim is correct? Could the father have been of blood group B? Explain your reasoning.

Multiple allele.

Multiple allele is a situation where by more than two alleles are controlling a certain characteristic. For example alleles A, B and O control the ABO blood group system in man.

Other conditions in Man transmitted in mendellian fashion. (Monohybrid inheritance in man)

  1. 1.       Albinism

Is a condition which results when the pigment for normal skin colour fails to form and this due to a recessive gene a.

Characteristics of albinism

White skin, Pink eyes and Golden hair.

To obtain an albino the child must receive recessive genes from both parents. This implies that an albino is homozygous recessive.

Gene for normal skin pigment: A and gene for Albino: a

Homozygous dominant: (AA) normal skin colour

Heterozygous:                  (Aa) Normal skin colour

Homozygous recessive: (aa) Failure of formation of normal skin pigment (albino)

To get a child who is an albino:

I)        Both parents must be carriers

Parents           Mother (carrier)           X                father (carrier)

Genotype                 A a                                             A a


Random fertilization

AA      aa                        Aa                          Aa

AA: normal skin colour

Aa: Normal skin colour but carrier

aa: Albino

II)      One parent is a carrier and the other is an albino

Parents           Mother (albino)           X               father (carrier)

Genotype                 a  a                                            A  a


Random fertilization

Aa       aa                    Aa                       aa

Aa: normal skin colour but carrier

aa: albino

From the above two marriages, the mode of transmission of genes is similar to the mendellian fashion.

  1. 2.       Sickle cell anaemia

In this condition the person doesn’t possess bi concave shaped red blood cells but the shape is like that of a new moon. A person with sickle cells doesn’t have a large surface area so that sufficient oxygen can be transported through haemoglobin found in a normal person. People with sickle cell anaemia have short breath, tend to sleep when tired and have retarded growth.

Gene S is responsible for abnormal Haemoglobin.

Dominant  gene H is responsible for normal haemoglobin.

NB: this is not an example of incomplete dominance.

Homozygous dominant HH: Normal haemoglobin

Heterozygous HS: sickle cell carrier (shows mild signs but never gets attacks)

Homozygous recessive SS: sickler


Sex is determined by a special type of chromosome found in the sperms and the ova and they are termed as sex chromosomes. In man, since one chromosome is X and the second is Y, they may be referred to as heterozomes and those that are similar are autosomes. Ova can only carry the X chromosome; Sperms may either carry the X chromosome or Y chromosome. The sex of the child depends on which sperm fertilizes the egg. If its an X sperm, the off spring is XX (girl) and if it’s the Y sperm, the off spring is XY (boy). Each off spring has characteristics limited to it. These are termed as sex limited characteristics

Man Female
Have a penis Have a clitoris
Have beards No beards
Have narrow tips and nipples Have wide hips and breasts


Parents                     Male           X                        female

Genotype                 X Y                                             X  X


Random fertilization

XX       XX                   XY                      XY

Females                             Males

Sex linkage: sex linked genes

Is a condition where the genes controlling a trait/characteristic have to be transmitted on a sex chromosome. Such traits are referred to as sex linked characters controlled by sex linked genes. Most of the sex linked genes are recessive and commonly found on the X chromosome and in rear cases on the Y chromosome

When the X chromosome has a recessive gene in males, normally the Y chromosome is empty. Since the chance for the Y chromosome to be empty is common, there fore males can inherit a recessive gene from a carrier mother and inherits an empty Y from the father and becomes a sufferer.

There fore sex linked characters are common in males than females. (in most cases females end up as carriers if they have a sex linked gene)


Examples of sex linked genes:

  1. Haemophilia:  Simply means failure of blood to clot such that a person bleeds for a long time.

XHXH-Normal female homozygous dominant:

XHXh-Carrier female (heterozygous)

XhXh – Female Haemophilia sufferer

XHY –normal male

XhY-Male Haemophilia sufferer

  1. Colour blindness:  is the inability to distinguish between primary colours ie red, green and blue.

XCXC-Normal female homozygous dominant:

XCXc-Carrier female (heterozygous)

XcXc –  Colour blind female

XCY –normal male

XcY-Colour male

  1. Premature balding
  2. Browning of teeth
  3. Porcupine man: the growth of thick hair at the entrance of the auditory canal. Its  suspected to be associated with the Y chromosome because its found only in male.


  1. Explain what is meant by sex linked characters
  2. List examples of sex linked characters found in man
  3. In a marriage, a woman who is heterozygous for the sex linked character of colour blindness marries a colour blind man. What would be the result of this marriage?

Show the working if the sons from this marriage got married to carrier females, what would be the result?


37 responses to “Genetics

  1. What hapen to a parents who is a carrier and to a parents who is not a carrier of Albinism?

  2. Micah Wangila

    what’s the difference between allels and genes?

    • Hello Micah
      An allele is an alternative form of a gene that is located on a specific part of a chromosome where as genes are basic unit of inheritance.
      For example the gene for height in pea plants exists in two forms, one form or allele for tall plant (T) and the other for short pea plant (t). Tt are two alternative gene forms and are called alleles which control the trait height.


      An allele is an alternative form of a gene;two alleles makes one gene.WHILE, A gene is a length of DNA which codes for making a particular protein.

  3. what is the difference between Testcross and Backcross?. Define codominance and Monohybrid Inheritance.

  4. what is the difference between Testcross and Backcross?

    • Test cross was utilised fr d detection of genotype of f1 hybrid of mendals cross between pure tall n pure dwarf pea plants ..xo we can term d cross between ressesive parent genotype and any of d f1 generation as test cross ..i.e to test the genotype fr nw generations ……….where as in back cross we can consider any of d parent resembling genotype ..may be homozygous dominant or ressesive…. of mendels pea experiment and cross it with f1 generation ………..
      Xo simply we dont say d cross between homozygous dominant parent genotype with f1 hybrid as test cross …bt we can term it as one back cross …jst cross d parent genotypes separetely with a f1 hybrid …an compare d phenotypic n genotypic character ob d both cross . U will get 2 knw y ??

  5. i have just found this content helpful in my revision.

  6. i like

  7. thanks the article is so helpful

  8. thanx a lot guys :0 this page has cleared all my doubts

  9. jackson msasa

    lovely notes thanks alot

  10. wow…. awesome notes 🙂

  11. Emmanuel bida

    why a test cross can be a back cross while a back can never be a test cross

  12. Good site for revision


  14. Good job

  15. good work

  16. Pingback: Genetics | Biology notes for O level with Questions | Yassirenterprises

  17. I like this

  18. arinaitwe joseph

    good everything clear

  19. thanks nice notes

  20. Truly amazing. Very helpful


    thanks for helping me if its possible you may allow me to ask tutorial questions

  22. Tayebwa marvin

    What is the mendels first law of genetics

  23. very reliable notes..thankyou

  24. A woman who is normal for haemophilia is married to a man who suffers from haemophilia. one of their daughter turns to be haemophilic.What are the genotypes of the offsprings?

  25. Am so so happy for this wonderful note now l will challenge my teacher thanks so much

  26. helpful

  27. xhavick Rogers

    it’s good site for revisiom

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