1. Study of Inheritance

Hereditary traits are characteristics passed down from parents to their offspring. These traits are determined by the DNA found in the gametes produced by male and female parents respectively. DNA that determine these traits are acquired in the offspring when the nucleus of the male gamete fuses with the nucleus in the female gamete.

Gregor Mendel, the father of modern genetics, did extensive studies on the inheritance of traits in pea (Pisum sativum). He did extensive studies of seven traits, shown below:

1. form of the pea seed - round vs deeply wrinkled seed

2. colour of flower- white vs purple

3. colour of pea seed - green vs yellow seed

4. form of the ripe pea pod - inflated vs constricted seed pod

5. difference in appearance of the flower - axial (distributed along the main vein) vs terminal (at the top of the stem).

6. difference in the length of the stem - tall (ie 183 to 213 cm) vs short (30.5 to 46 cm)

7. yellow vs green endosperm

1.1 Gene vs Allele

• For Molecular Genetics, a gene is defined as a segment of DNA that codes for a specific sequence of amino acids on a polypeptide chain.

• A gene is also an unit of inheritance.

• We shall study the inheritance of single gene that controls a specific trait.

• Each gene occupies the same position on a pair of homologous chromosomes.

Above: For this individual, he has inherited brown allele for colour of hair pigment from father and black allele from mother.

• Examples of genes are hair colour pigments, skin colour pigments, number of folds on eye lids and types of ear lobes etc.

• For a single gene, there can be various forms known as alleles.

• For instance, the alleles for hair colour can be yellow, orange, red, black and brown.

1.2 Monohybrid Inheritance

Monohybrid inheritance involves a cross between two individuals with different alleles of a single gene. It usually involves a pair of contrasting alleles of a specific gene.

To illustrate Monohybrid Inheritance, let's look at Mendel's experiment on the inheritance of flower colour of pea plants.

Mendel identified two pure line plants that have purple flowers and white flowers respectively. A purple pure-line plants meant that if the flowers are self-fertilised, fruits bearing seeds that when sown would give rise to purple flowers.

The two plants having purple flowers and white flowers respectively, are considered as parents.

In the first cross, pollen grains from purple flowers of one parent are brushed onto stigma of white flowers of second parent and vice versa. Fruits develop and all the seeds are sown.

The plants that grow from the seeds are offspring of the first filial generation (F₁ generation). All the offspring plants bear purple flowers.

The pollen grains of one F₁ generation offspring are brushed onto stigma of the same flower or different flowers on the same plant. The process is known as selfing. Fruits develop and all the seeds are sown. The plants that grow from these seeds are offspring of the second filial generation (F₁ generation). He counted the number of offspring that bear purple flowers and white flowers respectively. Mendel noted a ratio of purple flowers to white flowers as approximately 3:1.