GENETICS: Biology of Heredity

Genetics can be defined as the scientific study of inheritance or heredity and variations in living things (plants and animals).

Heredity: This can be defined as the transfer of traits(characters) in organisms, from parents to offspring. Living things (plants and animals) offspring usually
look like their parents and one another as a result of the traits or characters inherited by the offspring from their parents. Differences can exist between parents and offspring as a result of variation

VARIATION 

[Photo by Electri]

Variation can be defined as the differences that exist, between parents and offspring, and also among offspring. In other words, variation is the differences that exist between individuals of the same species. There are two types of variation.

Types of variation
1) Morphological variation: Morphological variation is a type of variation which  refers to the noticeable physical differences in appearance of individuals of a species. It helps in the identification of individuals of a population. It  leads to continuous variation. 

Continuous variation are variations that change gradually among a population. It is a type of variation in which a gradual transition is shown within two forms by a given feature. Examples are: height and weight. In a class, if the height of the students are measured, various heights would be recorded. Some students would be short, some will be tall and some will be averagely tall. The students can be classed by their heights.

Examples of morphological variation in man include:

• fingerprint
• weight
• height of the body
• shape of various parts of the body like nose, ears
• color of body parts like skin, eyes
• size of body parts like head, neck

Examples of morphological variation in plants include:

• height of plants
• shape of plants parts like leaves, fruits
• color of plants parts like leaves, root
• size of plant parts like stem, flowers

2) Physiological variation: Physiological variation is a type of variation which relates to the differences in the behavior of  individuals of a species. In other words, physiological variation refers to the behavioral differences among individuals of the same species. It relates to how the body functions. It gives rise to discontinuous variation.

Discontinuous variation is a type of variation by which individuals can be grouped into classes within a population, without any gradation among them.

Examples of physiological variation (discontinuous variation) in man include:

• intelligence
• bravery
• aggressiveness
• ability to roll the tongue
• blood group difference

Examples of physiological variation (discontinuous variation) in plants include:

• color of leave 
• shape of seed
• shape of fruit
• color of fruit

Causes of Variation

The causes of variation can be grouped into two. They are genetic difference and environmental influence.

1) Genetic difference: This is concerned with the characters that are inherited from parents.Each child inherit some characters from father and some from mother. Only two identical twins inherit exactly the same combination of traits from their parents.

2) Environmental influence: Variation may occur as a result of influence of the environment. Housing, food, parental care, health care, educational facilities, e.t.c constitutes of the environment. For instance, a person borne, who is genetically deficient, can improve in physical and mental build-up if raised in a good environment such as good parental care, good health care, good housing, good feeding. And a person, who inherits intelligence from the parents may become dull, if he is raised in a bad environment such as absence of school, lack of medical attention, lack of parental care, e.t.c.

Uses of Variation

Variation can be used in many ways in man's life. Some of the uses include:

• crime detection: some features like height, shape, color, fingerprint, are used in crime detection. No two individuals have exactly the same fingerprints. The fingerprints also never change. They remain throughout life. The use of fingerprint in crime detection is based on the above facts.
• blood transfusion
• determination of paternity: Knowledge of the differences in blood group is used to resolve the dispute of the paternity of  child.
• human race classification: Differences in physical qualities of man are used in the classification of human race into Caucasoid(light skinned people with narrow nose and wavy hair), Negroid(black skinned people with whoolly hair and broad nose), Australoid(brown skinned people with curly hair and moderately broad nose), and Mongoloid(yellow-brown skinned people with straight hair, tiny eyes, flat faces and moderately broad nose)

TRANSFER OF TRAITS IN ORGANISMS

From generation to generation, different traits are transferred from parents to offspring.

Transferable Traits in Man

Some of the traits that are transferable in man include:

• baldness
• intelligence
• height
• bod structure
• sickle cell anemia
• color blindness
• speech or voice

Transferable Traits in Plants

Some of the traits that are transferable in plants include:

• height
• taste
• size
• food content of fruit
• color
• leaf texture
• life span
• habit of growth

HOW TRAITS GET TRANSFERRED IN ORGANISMS

Not all traits are transferable in organisms. Only the traits that are controlled by genes are transferable. Genes determine the inherited traits. A diploid organism possess two sets of chromosomes called homologues. Such typical organism possess two copies of each gene, with the copies occupying identical locations (loci) on the homologous chromosomes.

Meiosis is used to produce gamete by diploid organisms in their reproductive glands. Sperms are produced by male individual while egg cells or ova are produced by female individuals. The number of chromosomes is halved during meiosis. The gametes are haploid, hence contains one set of chromosomes and only one copy of each gene. 

The gametes of a male and female individual fuse during sex reproduction. Every zygote is diploid as it gets one set of chromosomes, and hence once cop one copy of each gene from the gamete of each parent. Traits, which are determined by genes, hence are transferred from parents to offspring through gametes.

The gene inherited by organisms during fertilization, known as genotype, remains constant throughout life. The genotype and environment in which an organism lives determines the phenotype. Phenotype of an organism is the observable features of the organism. Therefore, having the same genotype doesn't mean the organisms have the same phenotype. Organisms with the same genotype may have different phenotype if the organisms live in different environment.

Read also: Mendel's work in Genetics

SOME TERMS USED IN GENETICS

1. Genes: These are basic units of heredity. In other words, they are the simplest units of inheritance.. They are located chromosomes. They are responsible for the transfer of traits from parents to offspring.
2. Chromosomes: These are thread-like bodies, located in the nucleus of a cell. They contain the genes.
3. Gamete: This is a matured sex cell that takes place in sexual reproduction. It is of two types: male gamete and female gamete: spermatozoon(in animals) or pollen grains(in plants), and also egg or ovum(in animals) or ovules(in plants). They are usually haploid.
4. Haploid: This occurs when an organism possess one set of chromosomes in the gamete. It is symbolized by small letter "n".
5. Diploid: This occurs when an organism possess two sets of chromosome in the gamete. It is symbolized by "2n".
6. Hybrid: This is an offspring produced from a cross between individuals of the same species but genetically different.
7. Hybridization: This is the crossing of plants with contrasting traits. It is of two types: monohybridization, which involves the crossing of plants with two pure traits, and dihybridization, which involves the crossing of plants with pairs of contrasting traits.
8. Traits or characters: Theses are features which can be inherited by an organism. 
9. zygote: When a male gamete and a female gamete fuse, a single cell is formed. The cell is called zygote. It is usually diploid.
10. Genotype: This is the genetic make-up of an organism. It includes the dominant and recessive traits which form the genetic make-up.
11. Dominant trait: When two individuals with contrasting traits are crossed, a trait is expressed. The trait is called dominant trait. 
12. Dominant genes: These are genes that control the dominant traits,
13. Recessive traits: This is the trait which does not have an effect in the presence of dominant traits.
14. Allelomorphs: These are pairs of genes that control contrasting traits. They are on the position of a chromosome(locus).
15. Homozygous: When an individual has two identical alleles at the same position on a pair of chromosomes, it is said to be homozygous. It means the organism possess two similar genes of the same character.
16. Heterozygous: When the two members of a pair of genes controlling a pair of contrasting traits are different, the individual is said to be heterozygous.
17. Filial generation: This is made up of offspring of parents.
18. Mutation: This is a change in the genetic make-up of an organism. It results in a new trait that is inheritable.

Read furher: Mendel's woks in Geneicts

Categories: Biology, Biology of Heredity, Genetics