The cell
The cell is the most fundamental unit of life. It is too small to be seen by a naked eye. It can be seen clearly with a microscope.
Organelles: parts of a cell carrying out particular functions
A cell contains several organelles for example nucleus ,chromosomes. Cytoplasm, mitochondria, ribosomes etc. Thousands of chemical processes take place in the cell and at the same time.
Some organelles;
Nucleus;
Contains chromosomes carrying genetic factors.
Controls all activities in the cell.
Mitochondria; Sites for respiration produce respiratory enzymes for production of energy
Ribosomes: Make up of RNA and protein. Sites for protein synthesis.
Endoplasmic Reticulum: of series of membranes of channels, on it are ribosomes, has enzymes, which are responsible for synthesis of fats.
Centrioles -Occurs in pairs and arranged at right angles to one another.
-during cell division ,Centrioles migrate to opposite poles of the spindle .
Lysosomes -contains lytic enzymes which destroy useless/worm -out organelles that is carry out autolysis for example if mitochondria is worn out ,digestive enzymes are poured on it ,digested and contents made use of to make new organelles.
Nucleus - manufactures ribosomes.

CELL DIVISION
A cell has the power to duplicate itself that is make a copy of itself. Cells are capable of growing. Cells arise only from other already existing cells(parent) by cell division. There are two types of cell divisions: -(I) Mitosis (II) Meiosis.
MITOSIS: -is the process of cell division whereby new cells of the body (somatic) are made. The number of chromosomes remains constant. They are diploid cells.
MEIOSIS: The process of cell division giving rise to sex cells( gametes). During this process the number of chromosomes is halved .The products are haploid cells.(having a single set of unpaired chromosomes in each nucleus).
RESEMBLANCE: - The fact that we originate from mother and father, that is why we resemble our parents. There are certain factors(genes) or characters, which are passed (transmitted ) on from one generation to another , that is from parent to offspring.

CHROMOSOMES,DNA & GENES;
Chromosomes contain the hereditary material or genes that are transmitted from parent to offspring. A particular plant or animal cell has a definite constant number of chromosomes for example in man each somatic (body) cell contains 46 chromosomes while a gamete contains 23 chromosomes after meiosis.
Genes are chemical in nature; a gene is in a form of a nucleic acid molecule called DNA (deoxyribo nucleic acid).
DNA is composed of three different components. These are nitrogen base; a 5-carbon sugar and a phosphate molecule .A combination of the three above form a nucleotide. The building block of DNA is a nucleotide.

In 1953 Francis Crick and James Watson worked out the DNA structure. DNA consists of several nucleotides joined together to form long chains or DNA strands. DNA molecule is of two strands twisted onto one another hence forming a double helix.

What are the nitrogen bases? They are four:

Adenine(A), Guanine(G), Thymine(T) and Cytosine(C). It was observed that the quantities of Adenine and Thymine were always equal while those of cytosine and Guanine were also equal. This suggested that Adenine always pairs with Thyamine while Guanine pairs with Cytosine when forming the double helix structure of the DNA.
A=T
C=G
DNA looks like a twisted ladder where the nitrogen bases form the steps or tungs as shown below:

The double helix structure of the DNA molecule.

A small section of DNA molecule.

DNA Replication
The DNA double helix can unwind or unzip forming two separate strands. After replication the information(base sequence ) is copied out onto a new DNA structure using the parent as the template. The replication makes it possible to pass on the DNA molecule together with its exact genetic information to daughter cells during cell division in the organism.
Role of DNA in protein Synthesis:
DNA molecule is basically responsible for protein synthesis in the cells. This is important because a cell is virtually protein in nature,enzymes and hormones are all proteins. Every one of the three bases on DNA strand is responsible for a particular amino acid. The set of a base triplet is known as a codon and hence codes for an amino - acid for example AAA codes for phenylalamine TTT for lysine
A protein molecule composed of 1000 amino acids will require 1000 codons for its synthesis. A gene consists of several DNA bases. Thus genes are made of long chains of bases along DNA strand. The type and sequence of these bases determines the nature of protein synthesized by the cell. Protein synthesis occurs in cytoplasm sites are the ribosomes. Instructions are received from DNA in the nucleus. The message is taken by another nucleic acid called RNA (ribo nucleic acid ) as to what type of protein should be made.

MUTATION
A mistake on the DNA base sequence if transferred to RNA strand may result in a wrong order of amino acids on the polypeptide chain. A wrong and defective protein molecule may be formed. A mistake in the DNA base sequence is responsible for the many disorders in both the structural and functional protein molecules in organisms.
The genes have under gone some unusual change. The phenomena of gene distortion is described as Mutation. Mutations are therefore genetic errors or mistakes and can be inherited.
Mutations may be due to miscopying of DNA or several mistakes during stages of cell division. Mutagens like ultra-violet light,X-rays, certain drugs,
Severe temperatures,mustad gas etc, can cause mutations.
Mutations may be due to deletion,inversion,duplication,polyploidy.
MITOSIS

MEIOSIS
Meiosis takes place only when the sex cells or gametes are being formed.(Spermatogenesis and Oogenesis). During this process the number of Chromosomes is halved.
EVENTS IN MEIOSIS
There are two types of cell division referred to as first meiotic division and second meiotic division. The end product are four cells each having half the number of chromosomes as were present in the parent cell. During first meiotic division the centromeres do not divide, unlike in Mitosis.
In the first meiotic division the chromosomes duplicate. There is crossing over that is exchange of genetic material.
N.B At the end of this cell division cytoplasm divide to form four gametes ( haploid cells)
In the second meiotic cell division centromeres divide and the new chromosomes move to opposite ends of each spindle. At the end four cells have been formed. During fertilization each new individual receives one set of chromosomes from his father and one set from his mother. These chromosomes are similar in size and shape to one another and once they are together in a cell they pair up. The pairs are referred to as homologous chromosomes.

GREGOR MENDEL (1822-1884)
AND THE STUDY OF HEREDITY
We owe most of our genetic knowledge to Gregor Mendel . He was an Austrian Monk and thus spent most of his time in a monastery. He was a naturalist, a mathematician and a successful science teacher. He carried out breeding experiments in 1856 using the garden pea -pisum sativa . unfortunately his work was not recognised until his death.
Why Mendel used Peas?
They have number of sharply contrasting characteristics. The characteristics were easily recognisable for example tall and short. They are self-pollinating/self-fertilising and so he could obtain pure strains. They have a short life cycle and are easily cultivated.

Non True breeding characteristics
When a red flowered plant is mated with white flowered plant, all the offspring are PINK, different from either parent.

Modern Genetics
The fruit fly (Drosophila melanogaster) has been used in such experiments. They are discussed else where.

Glossary Terms;
-illustrated with Mendel's Experiments
Mendel crossed yellow seeds with green seed plants. All the F1 offspring produced were yellow seeds only. The yellow characteristic is said to be DOMINANT over the Green, and the green is RECESSIVE. However, if plants from the first generation are crossed, that is the yellow, the next generation shows offspring of two types. Some yellow and others green in the ratio 3:1 that is the f2 ratio (yellow:green).
He concluded that two plants could look alike (phenotypically) but genetically are different (that is their genetic constitution differs). He suggested that the "impure" yellow must contain two 'factors' a yellow and a green one that is Heterozygous. Therefore if two factors in this case genes are the same, then it breeds true, if different it produces a mixture.

Homozygous - having two identical alleles for any one gene (both paternal and maternal genes are identical)

Heterozygous -having two different alleles for any one gene.

Allele - different forms of gene that occupy the same locus (relative position) on homologous chromosomes and produce contrasting characteristics. Each pair of genes is called an allelic pair or each gene is referred to as an allele of the other. In case of the gene for flower color in peas the allele for white is recessive.

Phenotype -The outward appearance of an organism determined by both the genotype and the environment.
Genotype - the genetic make-up of an organism; it is expressed in symbols and not in words for example Tt,tt,Rr.
F1 -(first filial generation) the offspring produced by crossing the parental generation.
F2 -(second filial generation) the offspring produced by crossing the F1 generation.
Progeny - All the offspring produced.
Hybrid -an organism produced from a cross between parents that are not genetically identical.
P-generation -parental generation

N.B -The genes in an organism are present in pairs, but each gamete carries only one gene from each pair. We receive two genes for every characteristic, one from the father(paternal) and one from the mother (maternal).

Modern interpretations of Mendel's Work
We designate the genes with symbols/letters. Symbols or letter of the alphabet represents the Genotype.
Example

Example of Monohybrid inheritance

Punnett Square
Another method we can use to figure out possible Genotype of the F2 generation. Construct a checkerboard or Punnett Square

Therefore the F2 phenotype ratio is 3:1
From his monohybrid crosses,Mendel put his conclusions into a general statement called Mendel's first Law. He called it the "Law of Segregation" that is during gamete formation genes separate where each passes intact and unchanged into a gamete

Try it out , we shall get some red, some pink, and some white flowers, in the ratio 1:2:1 respectively.Significance;
The reappearance of the phenotype for red flowers or white flowers in the F2 generation indicates that recessive genes are neither modified nor lost in the hybrid generation. They may not express themselves in the final phenotype but they remain intact in any case.
To establish an organism's genotype.
1:To establish whether a given tall plant is homozygous or heterozygous ,is to self-pollinated it. If the resulting offspring are all tall, therefore the parent is TT. If however we get a mixture of tall and short plants therefore the parent if Tt. This can be applied to any given characteristics chosen for observation.
2:The Test cross:
Done on organisms, which cannot be self-pollinated, for example animals such as Drosophila melanogaster. The organisms is crossed with a corresponding double recessive. If the unknown is homozygous, all the offspring will show the dominant phenotype ; if heterozygous half the offspring will show dominant and half- recessive phenotypes.

For example using heights,say T for the 'tall' allelle and 't' for the dwarf allelle, T being dominant, if the unknown plant is homozygous, say TT, then all the f1 offspring will be heterozygous tall, Tt...and if the plant was tt, the off spring will all be dwarf...tt.

But if the unknown tall plant is heterozygous, 50% of the progeny will be (Tt) and will be tall: 50% will be homozygous recessive(tt) and dwarf.

Backcross and Testcross;
A backcross involves the crossing of F1 back to any of the parents while a Testcross involves crossing the F1 back to a recessive parents. When the F1 offspring are testcrossed to the recessive parent, a ratio of 1:1 is obtained.
Probability and chance in Mendelian inheritance
The presentation of a breeding experiments such as those shown above is prediction of the likely outcomes. It represents the probable result provided that the following conditions are true. One is that the fertilization should be random. Secondly there is an equal opportunity of survival among the offspring and lastly large number of offspring are produced.
What is actually observed in a breeding experiment may not necessary agree precisely with prediction. For example there is chance that more pollen grains one genetic constitution may fuse with egg cells than another. If the cross produces very few progeny, the progeny may not appear in the expected proportions.
In Mendel's tall and dwarf experiment the probability is it that of in every 4 of the F2 generation will be dwarf, and that 3 in every 4 of the F2 generation will be tall. For every individual offspring in the F2 generation, the chance that it will be tall or dwarf is the same. The chance of a 3:1 ratio increases, however, as more progeny are produced.
If the actual result is not the same as the prediction, is the difference due to chance or is the prediction wrong? The larger the difference between the observed an predicted outcomes, the more likely it is that the prediction is wrong. The smaller the difference, the more likely that it is that the result is due to chance. The chi-squared test is used to estimate the probability that differences between the observed and expected results are due to chance.This chance and probability are discussed on the next page along with other interesting principles like the Hardy-Weinberg principle..................