The Study Of Life – Biology Of Heredity (I)

GENETICS

Genetics  is defined as the scientific  study of heredity and variation in living things.

Heredity: Heredity or Inheritance is defined as the transmission and expression of characters or traits in organisms from parents to offspring. It is observed that the offspring of man and other animals and plants usually resemble one another. This is because the offspring inherit characters or traits from their parents. 

Variation: This is defined as the differences which exist between parents and offspring as well as among offspring. Even though offspring may resemble their parents, they can differ also from them and from one another. For example, all human beings have human features, but each differs from the other, hence each person can be recognised. These differences between individuals of the same species are called Variation.

TRANSMISSION AND EXPRESSION OF CHARACTERS IN ORGANISMS

Different characters or traits are transmitted from parents to offspring or progeny and from generation to generation.

Hereditary Variation

Hereditary Variation refers to the differences among individuals which can be passed from parents to their offspring (progeny). Heredity variation arises because, apart from the case of identical twins, no two offspring inherit exactly the same set of characteristics from their parents. Each offspring inherits a different combination of characteristics from parents. Plants and animals have transmittable characters.

Transmittable Characters In Human Beings

Transmittable characters in Human Beings include:

1. Body stature or shape
2. Shape of head, nose and ear
3. Size of nos, head and ear
4. Colour of Skin, hair and Eye
5. Characteristic of voice and speech
6. Intelligence
7. Height of Human
8. Blood grouping
9. Baldness
10. Finger prints
11. Tongue rolling
12. Sickle Cell Anaemia
13. Haemophilia
14. Colour Blindness
15. Ability to Taste (BTC)

Transmittable Characters In Plants

These include:

1. Height of plant
2. Size or weight of Fruit
3. Size of Leaf
4. Taste of Fruit
5. Food content of Fruit
6. Colour of Leaf, Flower, Fruits or Seeds
7. Resistance to environmental factors like diseases, pests and drought
8. Shape of Leaf, Fruit and Flower
9. Leaf Texture
10. Life span or Habit of Growth

How Characters Get Transmitted And Behave From Generation To Generation

Only characters controlled by genes can be transmitted. In other words, Inherited characters can only be determined by the genes. A diploid organism has two sets of chromosomes referred to as homologues. Such an organism has two copies of each gene, with the copies occupying identical locations or loci on the homologous chromosomes.

Diploid organisms produce gametes by meiosis din their reproductive organs. A  male individual produces Sperm and a Female releases eggs or ova. During meiosis, the number of chromosomes in  a cell is halved. The gametes are therefore Haploid, containing one set of Chromosomes, and hence only one copy of each gene.

During sexual reproduction, the gametes of a male and female individual  or parent fuse to form Zygotes. Each Zygote is diploid as it gets one set of chromosomes, and hence one copy of each gene from the gamete of each parent. Characters determined by genes are thus transmitted from parents to offspring through gametes. The gene an offspring inherits during fertilisation called genotype, remains constant through out life. The phenotype which is the physical appearance or features of an organism is determined by its genotype and the environment in which it lives. Hence, organisms with the same genotype may possess different phenotypes if they live in different environment.

Some Term Used In Genetics

1. Genes: Genes are hereditary units or basic units of inheritance. They are located in the chromosomes and they are responsible for the transmission of characters from parents to offspring.
2. Chromosomes: Chromosomes are rod or thread like bodies found in the nucleus of a cell. The chromosomes house or contain the genes.
3. Characters or Traits: These are the inheritable attributes or features possessed by an organism. e.g seed colour, seed size, plant height e.t.c. in plants
4. Gamete: This is a matured sex cell that takes part in reproduction. There are two types: the male gamete or spermatozoon (in animals) and pollen grains (in plants)  and the female gamete (egg or ovum in animals) and ovules (in plants). Gametes are usually diploid.
5. Zygote: Zygote is a single cell formed as a result of the union of a male gamete with a female gamete. Gamete is usually diploid.
6. Allelomorphs: These are pairs of on the position of a chromosome (i.e. locus) that control contrasting characters. A pair of allelomorphs are called Allelic Pair while  each member of the pair is the allele of the other.
7. Phenotype: Phenotype is the sum total of all observable features of an organism i.e. the physical, physiological and behavioural traits e.g. height, weight, skin colour.
8. Genotype: This term is used to describe those traits or sum total of genes inherited from both parents. In other words, the genotype of an individual is his genetic make-up or constitution. Genotype ncludes both the dominant and the recessive traits that form the genetic make-up of an individual.
9. Dominant Character: This is a trait or character that is expressed in an offspring when two individuals with contrasting characters or traits are crossed. Dominant genes are genes which control dominant characters. For instance, in a very tall plant, there may be the gene for shrtness but the gene has no influence on the gene for tallness.
10. Recessive Character: This is the character or trait from one parent which is masked or does not produce the effect in the presence of dominant character. From the illustration above, shortness  is the recessive character while tallness s the dominant  character. Recessive genes control recessive characters.
11. Homozygous: An individual is said to be homozygous if it has two similar genes for the same character, i.e. it has two identical alleles at the same position on a pair of chromosomes; the pair of genes controlling a given pair of contrasting characteristics are identical e.g. TT for tallness, tt for shortness.
12. Heterozygous: An individual is said to be heterozygous f the two members of a pair pf gene controlling a pair of contrasting characters are different i.e. it has two different or contrasting alleles located on the same position on a pair f chromosomes e.g. Tt for tallness or a plant with Rr genetic composition has a heterozygous red flower plant.
13. Filial Generation: The offspring of parents make up the filial generation. The first, second and third generations of offspring are known as the first, second and third filial generations respectively and are denoted by the symbols F, f, and F respectively. too. F generation gives rise to F generation.
14. Hybrid: Hybrid is an offspring from a cross between parents that are genetically different but of same species
15. Hybridization: This is the crossing of plants with contrasting characters. Mono hybridization involves the crossing of two pure traits while Di-hybridization involves the crossing of plants with two pairs of contrasting characters.
16. Locus: Locus is the site for location of a gene in a chromosome.
17. Haploid: Haploid is when an organism has one set of chromosomes in the gamete. Gametes at certain stages in the life cycle of plants are haploid. It is represented by small letter (n).
18. Diploid: Is when an organism has two sets of chromosomes in the body cell. The bodies of animals and plants are diploids. Diploid number is double the haploid number of chromosomes and is represented by (2n).
19. Mutation: Mutation is the change in the genetic make-up of an organism resulting in a new characteristic that is inheritable.