SS3 First Term Biology Lesson Note – Biology of Heredity

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WEEK 7

TOPIC: BIOLOGY OF HEREDITY

CONTENT:

1. Transmission and expression of characters in organisms
2. Chromosomes the basis of heredity.
3. Probability in genetics.
4. Applications of principles of heredity
5. Explanation on cross and self-fertilization

BIOLOGY OF HEREDITY: GENETICS

Genetics is the scientific study of heredity and variation in living organism. Scientists who study genetics are known as geneticists.

The laws of genetics were laid down by Gregor Mendel, an Austrian monk, in 1866 although the work was not credited to him until 1900.

Johannsen, a Danish botanist called the ‘factors’ that transmitted Mendel’s characters, genes in 1909.

Thomas Morgan, an American geneticist showed that genes were on chromosomes, in 1912.

SUB-TOPIC 1: TRANSMISSION AND EXPRESSION OF CHARACTERS IN ORGANISMS

Character is a distinct structural or functional feature of an organism. Heredity is the transmission of inherited characters from parent to offspring through genes.

Common experience has shown that plants and animals produce offspring which look like them but are still not exactly like the parents.

Every member of a species shares in common a set of traits. These traits or characters distinguish one species from the other.

Hereditary Variations

Hereditary variations refer to differences among individuals which can be passed from parents to their offspring (progeny). Variations are due to a new combination of genes. If variation makes an offspring more suited to the environment more suited to the environment, it stands a better chance of surviving and reproducing to pass on its genes to the next generation.

Characters that can be Transmitted

Only characters controlled by genes can be transmitted. A gene (or genes) controlling a character direct the development of one or more proteins. These proteins lead to the visible expression of the character/trait.

These characters include:

1. Colour of the skin,
2. Colour Eye
3. Shape and colour of teeth
4. Hair texture,
5. Length of neck,
6. Voice,
7. Intelligence,
8. Composure and
9. sickle cell anaemia in animals.

While in plants variations is characterized by

1. Height of plants,
2. Colour of leaves and flowers,
3. Size of seed and fruits and
4. Pigmentation may be observed.

The sum total of genes that an offspring inherits from its parents is referred to as its genotype or genetic make-up. The actual physical expression of the character is the organism’s phenotype. The phenotype is due to the interaction between an organism’s genotype and its environment. For instance, a person may inherit genes for growing tall, but malnutrition may result in the individual becoming stunted. This is a modification of the inherited character brought about by the environment and cannot be transmitted by the individual to its offspring. Such traits are called acquired traits and they do not change the structure of genes. A change in the structure of genes is called a mutation and can only be inherited if it occurs in the gamete, gamete- producing cells or in the zygote (germline mutations).

These variations may be described as:

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