Biology Exam Lessons


Chromosomes are thread like structure that occur in the nuclear of plants and animals and carry heredity information from generation to generation.

There are seen only during nuclear division and occur in identical pair called homologous pair. The body cell (somatic) of man has 46 chromosomes (23pairs). The sex cell (sperm, ovum) have 23 chromosomes. The 23pairs of chromosomes are called diploid number (2N) and 23 chromosomes are referred is as haploid number (N).chromosome are made up of two threads ‘chromatids’. Chromosome carry genes that are hereditary unit for inherited traits

Structure of chromosome:

During the early cell division chromosome become condensed and visible, each, then ,duplicate or replicate to produce a compact twins connected by a centromere. Each member of a bond pair is called a chromatid which tread like in appearance

Chemical analysis: of chromosomes show that they are composed of proteins combined with nucleic acids which are of two  kinds.

i.          De oxyribonucleic acid (DNA) Which is confined to the nuclear material of the cell which the primary heredity material making up the gene.

ii.         Ribonucleic acid (RNA) in some viruses.

The DNA consists of a double chains formed by repeating small chemical units known as Nucleotides. Each nucleotide is composed of

i.          deoxyribo sugar (s)

ii.         Phosphate group (p)

iii.        Nitrogeneous bases which may be purine (Adenine and Guanine)or

pyrimidine (cytocine and Thymine).

The two chain of nucleotides of DNA are coiled like a spring to give a structure called a double helix. The nitrogenous base of the nucleotide pairs along the DNA chain.

Adenine (A) pairs with thymine (T) A-T while Guanine (G) pairs with cytocine

(C) G-C

Role of chromosome in transmitting hereditary characters.

1.         Formation of gametes – gametes are formed by meiotic divisions of cells. Each sperm or egg producing cell in reproductive organ is diploid that is having two sets of chromosome.

During meiosis the number of pair homologous separate first, then the twin chromatids separate The outcome of the cell division result into four daughter cells.

2          It allows for reshufflement of gene by allowing crossing over. It happens during the prophase of meiosis. It is important because it leads to new combination of alleles on a chromosome which leads to more types of allele combination in gametes.

a.         Homozygous chromosome pair up, each chromosome has already replicated

b.         Exchange of genetic material takes place between chromatids of homologous pair of chromosome

 c.        crossing over result in re arrangement of alleles A and

Probability in Genetics

This explains the predicted ratio or chances of having a particular trait or character in a cross or transmission from generation to another. This explains the predicted ratio of 3:1 for monohybrid cross and 9:3:3:1 for dihybrid cross.


An albino man marries a normal woman (homozygous) for skin pigmentation). What is the probability the couple could have an albino child?

Gene for albinism = aa

Gene for the Normal pigmentation = AA

 (1) In a cross involving a heterozygous red flowered plant (Rr) and a white flowered plant (rr), what is the probability that the offspring will be (Rr)?

2.         What is the probability that two consecutive children of the same parent will be males?

3.         In human beings, the albino trait is recessive is the probability of parents who are heterozygous for albinism having an albino child?

Application of Genetics in Agriculture

  1. It improves crop yield.
  2. It improves the quality of product of plant and animal.
  3. It leads to development of  early maturing varieties of plant and animals
  4. It leads to development of disease resistant varieties of plant and animal.
  5. It leads to production of crops and animal that can adapt to climatic condition.

           Application of Genetics in in field of Medicine

            It help in determination of the paternity of a child.

      The knowledge helps in blood transfusion who to donate and who should receive.

       It enlightens the marriage partner about genotype through marriage counseling 

  1. The method is used in diagnosis of diseases
  2. The knowledge is used in crime detection
  3. The method is development of test tube baby
  4. The knowledge can be used to choose the sex of a baby
  5. It can be used to know the sex of a baby.
  6. NB: Do crosses for the students.

Question on Genetics

1.         Two varieties of maize, one with yellow seeds and the other with pink seeds were crossed with the parent variety with pink seeds, half of the resulting offspring had yellow seeds white the other half had pink seeds.By Means of labeled diagram only

a.         Deduce the genotype of the seeds of Fi generation

b.         How the results of the cross  between the F generation and the parent pink variety.

2.         How many X chromosome will the girl have in each body cell?

3.         If the parents already have a family of one boy and two girls. What are the chances of their next baby being a boy? Give a reason.