Genetics

All living organisms reproduce, creating offspring of the same kind. For example, dogs produce dogs, and humans produce humans. However, the offspring may not look exactly like the parents. There can be differences between individuals of the same species. These similarities and differences are inherited from their parents.

Heredity is the passing of traits from parents to offspring through genes. This process is especially noticeable in sexual reproduction, where a variety of traits are inherited.

Genetics is the study of heredity and variation. It looks at how traits are passed down through genes and how they result in the characteristics of living things. Scientists who study this are called geneticists. The word "genetics" was first used by William Bateson in 1905. Genes are parts of DNA that determine traits, and they are passed down from one generation to the next through reproduction.

Important Terms in Genetics

1. Genes: Genes are the basic units of inheritance. They are located in chromosomes and control traits passed from parents to offspring.
2. Chromosomes: Rod-shaped structures found in the cell nucleus that contain genes.
3. Traits/Characters: Inherited features of an organism, such as seed color, plant height, etc.
4. Gamete: A mature sex cell (sperm in animals, pollen in plants) that takes part in reproduction. Gametes are haploid (contain half the number of chromosomes).
5. Zygote: A single cell formed when a male gamete combines with a female gamete. Zygotes are diploid (contain a full set of chromosomes).
6. Allelomorphs (Alleles): Pairs of genes that control contrasting traits. Each member of the pair is called an allele.
7. Phenotype: The physical appearance or observable traits of an organism, like height or eye color.
8. Genotype: The genetic makeup of an organism, including both dominant and recessive genes inherited from both parents.
9. Dominant Character: A trait that is expressed in the offspring when two different traits are crossed.
10. Recessive Character: A trait that is hidden or not expressed when a dominant trait is present.
11. Homozygous: An individual with two identical alleles for a specific trait.
12. Heterozygous: An individual with two different alleles for a specific trait.
13. Hybrid: An offspring from a cross between parents that are genetically different but of the same species.
14. Hybridization: The process of crossing plants with contrasting traits. It can be:
    • Mono hybridization: Crossing two plants with one contrasting trait.
    • Di hybridization: Crossing two plants with two pairs of contrasting traits.
15. Haploid (n): When an organism has one set of chromosomes in the gamete (e.g., 23 in humans), represented by n.
16. Diploid (2n): When an organism has two sets of chromosomes in the body cells (e.g., 46 in humans), represented by 2n.
17. Filial Generation (F1, F2): The offspring produced by parents. F1 is the first generation, F2 is the second generation, and so on.

Variation and its causes

Hereditary variation refers to the differences among individuals that can be passed from the parents to their offspring (progenies). No two offspring inherit exactly the same set of characteristics from parents, except in identical twins. Hereditary variation arises because of:

• Genetic reshuffling: During meiosis, independent assortment and segregation lead to the formation of a new combination of genes, resulting in a totally unique individual.
• Crossing over: During the prophase stage of meiosis, chromatids come in contact, and homologous chromosomes break and rejoin at a point called chiasma. This crossing over of genetic material leads to variation in the offspring.

Transmittable characters in animals include:

• Body stature
• Shape or size of the head, nose, and ear
• Height
• Characteristic voice of speech
• Colour of skin
• Hair colour
• Eye colour
• Intelligence
• Blood group
• Sickle cell anaemia
• Colour blindness
• Baldness
• Tongue rolling

Transmittable characters in plants include:

• Colour of fruit or seed
• Weight or shape of plants
• Food content
• Fruit shape
• Leaf shape
• Fruit taste
• Colour of leaf or flower
• Resistance to environmental factors like disease, pest, and wind
• Leaf texture
• Height of plants
• Life span

How characters get transmitted:

Only characters controlled by genes are inheritable. In diploid organisms, gametes are produced through meiosis in their reproductive organs. This process results in the formation of haploid gametes, meaning the male gamete (sperm cell) and the female gamete (ovum) each contain one set of chromosomes, with one copy of each gene from their homologous pairs.

During sexual reproduction, when fertilization occurs, the male and female gametes (sperm and ovum) fuse to form a zygote. The resulting zygote is diploid, having two sets of chromosomes, with two copies of each gene—one from each parent.

Mendelian genetics

Gregor Mendel was a monk in Austria. He is referred to as the father of genetics because of his work, which formed the foundation for the scientific study of heredity and variation.

Gregor Mendel carried out several experiments on how hereditary characters were transmitted from generation to generation. He worked with a garden pea called Pisum sativum.His major aim was to find out the pattern of inheritance of difference characteristics of the pea plant. He worked with the peas because:

• Peas are usually self-pollinating, and Mendel could manually pollinate them.
• They have a very short life span compared to animals and some other plants.
• They have several unique genetic characteristics e.g round or wrinkled seeds, tallness or shortness, seeds /pods/ flowers colouration, pod texture etc

Mendel's approach to studying genetics involved two methods:

1. Monohybrid Inheritance: The inheritance of a single trait or characteristic.
2. Dihybrid Inheritance: The inheritance of two different traits or characteristics at the same time.

Monohybrid Inheritance

Mendel studied the inheritance of a single trait by crossing two plants that differed in one contrasting characteristic, such as tall and short plants. The steps he followed are:

1. He grew tall plants for several generations and observed that all the offspring were tall.
2. Similarly, he grew short plants for several generations and observed that all the offspring were short.
3. He then cross-pollinated tall plants with short plants by transferring the pollen grains (male gamete) from the tall plant to the stigma (female gamete) of the short plant.
4. He planted the seeds produced from the cross and found that all the resulting plants were tall. This generation was called the first filial generation (F1).
5. He then allowed the F1 plants to self-pollinate, collected the seeds, and grew them. The resulting plants were both tall and short, in a 3:1 ratio. This generation was called the second filial generation (F2).

This experiment resulted into Mendel’s first law of inheritance which is based on the principle of complete dominance.

Example of Monohybrid Inheritance

Mendel's First Law of Inheritance

Mendel's first law, known as the Law of Segregation, states that genes are responsible for determining individual traits and are passed down from one generation to the next without being altered. During reproduction, these genes segregate, ensuring that each parent contributes one gene to their offspring independently.

Dihybrid Inheritance

In 1865, Gregor Mendel performed experiments on dihybrid crosses with pea plants and discovered an essential law of genetics called the Law of Independent Assortment. Mendel started his work by crossing two homozygous parental organisms that differed in two traits. Homozygous organisms carry two identical alleles for a specific trait at a genetic locus.

Mendel chose to cross a pea plant that was homozygous and dominant for round seeds (RR) and yellow seeds (YY) with a pea plant that was homozygous and recessive for wrinkled seeds (rr) and green seeds (yy). This is represented by the following notation:

RRYY x rryy

These organisms are known as the parental or P generation. The offspring produced from the initial cross, called the F1 generation, were all heterozygous plants with round, yellow seeds and the genotype RrYy.

Next, Mendel crossed two plants from the F1 generation. This cross, known as the dihybrid cross, is represented as:

RrYy x RrYy

Mendel observed that the F₂ progeny of his dihybrid cross exhibited a 9:3:3:1 phenotypic ratio:

• Nine (9) plants with round, yellow seeds
• Three (3) plants with round, green seeds
• Three (3) plants with wrinkled, yellow seeds
• One (1) plant with wrinkled, green seeds

From his experiment, Mendel concluded that the pairs of traits in the parental generation sorted independently of one another from one generation to the next. This principle became known as the Law of Independent Assortment.

Mendel’s Second Law

Mendel’s second law of inheritance, also known as the Law of Independent Assortment, states that alleles of genes on different chromosomes assort independently during meiosis. Each character behaves as a separate unit and is inherited independently of any other character.

Illustration

Parent's genotypes: RYRY (Round yellow seed) x ryry (Wrinkled green seed)

Gametes:

F1 generation: All round yellow

Self pollinating: RYry x RYry

Gametes:

Phenotypic Ratios

• Round and yellow seeds = (1), (2), (3), (4), (5), (7), (9), (10), (13) = 9
• Wrinkled and yellow seeds = (6), (8), (14) = 3
• Round and green seeds = (11), (12), (15) = 3
• Wrinkled and green seeds = (16) = 1

Principle of Incomplete Dominance

In Mendel’s experiments, he studied traits controlled by a single gene, where one allele was always dominant over the other. However, not all inheritance patterns follow Mendel’s rules. One such exception is incomplete dominance.

Incomplete dominance occurs when two contrasting alleles interact to produce a heterozygous phenotype that is distinct from both of the homozygous phenotypes. This differs from Mendel’s principle of complete dominance, where one allele completely masks the other.

In incomplete dominance, the heterozygote expresses an intermediate phenotype relative to the parental phenotypes. For example, if a red-flowered plant is crossed with a white-flowered plant, the offspring will have pink flowers.

When two pink-flowered plants are crossed, the resulting offspring will follow a ratio of:

• 1 red-flowered plant
• 2 pink-flowered plants
• 1 white-flowered plant

Examples of organisms exhibiting incomplete dominance include the Mirabilis jalapa (4 o’clock plant) and Andalusian fowl.

Co-Dominance

In co-dominance, both alleles in a heterozygous individual are fully expressed, meaning neither allele can hide or modify the effects of the other. This results in three distinct phenotypes. An example of co-dominance is the inheritance of human "ABO" blood groups.

The gene responsible for human ABO blood groups has three alleles: IA, IB, and IO. In this system, IA and IB are co-dominant, meaning neither one is dominant over the other, but both are dominant over the recessive IO allele.

Possible Genotypes and Phenotypes

Explanation of Alleles

• Allele IA: Causes the addition of antigen A to the surface of red blood cells, resulting in blood group A.
• Allele IB: Causes the addition of antigen B to the surface of red blood cells, resulting in blood group B.

In heterozygous individuals with the IA IB genotype, both antigens A and B are present on the surface of red blood cells, resulting in blood group AB.

Application of Genetics

Application of Genetics in Agriculture

• Enhances the yield of crops.
• Improves the quality of both plant and animal products.
• Enables the development of faster-growing varieties of plants and animals.
• Contributes to the creation of disease-resistant plant and animal varieties.
• Supports the production of crops and livestock that can thrive in different climatic conditions.

Application of Genetics in Medicine

Genetics has contributed immensely to various fields of medicine. These include:

• Marriage counselling to avoid cases of genetic disorders
• Determination of the paternity of a child
• Development of test-tube babies
• Crime detection
• Blood transfusion
• Diagnosis of diseases
• Sex determination
• Knowing and choosing the sex of a baby