#FocusOnExams: Biology Form 5 – 9th April 2020

GENETICS, MUTATION, VARIATION AND BIOTECHNOLOGY

 OVERVIEW

 INTRODUCTION TO GENETICS:

Genetics is the study of genes, heredity and variation.

Heredity is the transfer of inherited parental characteristics to offspring from generation to generation.

Gene: A gene is the basic unit of inheritance for a given characteristic OR

It is a length of DNA found at a specific position on a chromosome

For centuries, humans used the inheritance of characteristics to breed new varieties of plants and animals such as maize and rice plants that produce healthy and more grains, dairy cows that produce more milk etc, but these principles could only be explained in 1855 by Gregor Mendel, an Australian monk who was a mathematician, known as ‘the father of genetics’. He carried out breeding experiments with the garden pea plant (Pisum sativum), and gave the first explanations for the mechanisms of heredity.

SOME TERMS RELATED TO GENETICS

1. Alleles: Alleles are different forms of the same gene occupying the same locus on a chromosome OR
   • One of a group of genes that occur alternatively at a given locus
2. Gamete: Mature sex (reproductive) cells, which take part in sexual reproduction and carry half the chromosome number as the parent cell. e.g. sperms, eggs, pollen grains.
3. Zygote: A single cell formed from the fusion of a haploid male gamete and a haploid female gamete. Each zygote is said to be diploid because it contains a diploid chromosome number. Half from the male gamete and half from the female.
4. Loci (singular = Locus): The exact position of a gene along a chromosome.
5. Hybrid: An offspring with a pair of different alleles for a particular gene. They are often produced from two pure lines.
6. Pure Lines: These are parents of the same characteristics or trait which when crossed will produce offspring of the same trait or characters over many generations.

e.g.                  TT       X         TT

7. Homozygous (Pure breeding) individual: An individual having 2 identical alleles for the same gene e.g. TT or tt controlling a contrasting character.
8. Heterozygous individual: An individual having 2 different alleles for the same gene e.g Tt OR

A pair of un-identical alleles controlling a contrasting character e.g. Rr.

9. Genotype: The genetic constitution of an individual. That is all the genes with observable and non-observable characteristics present in the cells of an individual. E.g. TT, Tt and tt.
10. Phenotype: The physical outlook of a characteristic in an organism based on the genotype. e.g. Tall, short, blond hair, blue eyes, wrinkle seeds etc.
11. Dominant Gene: A gene which can express itself outwardly (in the phenotype) in the presence of another gene said to be recessive e.g. ‘T’ is dominant over ‘t’ for height.
12. Recessive Gene: A gene that cannot express itself in the phenotype (outwardly) in the presence of the dominant gene, except the gene is in the homozygous state. e.g. aa, tt, bb etc.
13. Filial Generation: Offsprings of parents made up of the first and second generations.

• F₁ Generation: Offspring of the 1^st cross between individuals.
• F₂ Generation: Offspring from a cross between F₁ progeny (offspring)

14. Heredity: The passing on of characteristics through successive generations
15. Carrier: An apparently normal heterozygote which ‘carries’ a recessive abnormal gene.
16. Haploid: A single set of chromosomes ‘n’ consisting of one chromosome from each pair of homologous chromosomes
17. Diploid: The number of chromosomes ‘2n’ found in body cells, made up of homologous pairs.