Heredity

Heredity refers to the transmission of characters from parents to off-springs. An inherited trait is a particular genetically determined feature that distinguishes a person from others.

The most obvious outcome of the reproductive process remains the generation of individuals of similar designs. The rules of heredity determine the process by which traits and characteristics are reliably inherited.

• Gregor Johann Mendel started his experiments on plant breeding and hybridisation. He proposed the laws of inheritance in living organisms.
• Both the father and the mother contribute practically equal amounts of genetic material to the child. This means that both paternal and maternal DNA can influence each trait.
• For each trait, there will be two versions in each child.
• Pisum Sativum (garden pea) was the plant selected by Mendel for his experiments.
• Gregor Johann Mendel uses a number of contrasting characters for garden peas.

Monohybrid Cross:-

Mendel used a number of contrasting visible characters of garden peas – round/wrinkled seeds, tall/short plants, white/violet flowers and so on. He took pea plants with different characteristics – a tall plant and a short plant and produced progeny by crossing them and calculating the percentages of tall or short progeny.

In the first place, there were no halfway characteristics in this first generation, or F1 progeny – no ‘medium-height’ plants. All plants were tall. This meant that only one of the parental traits was seen, not some mixture of the two. Mendelian experiments test this by getting both the parental plants and these tall F1 plants to reproduce by self-pollination. The progeny of the parental plants are, of course, all tall. However, the second-generation, or F2, progeny of the F1 tall plants are not all tall. Instead, one-quarter of them are short. This indicates that both the tallness and shortness traits were inherited in the F1 plants, but only the tallness trait was expressed. This led Mendel to propose that two copies of factor (now called genes) controlling traits are present in sexually reproducing organisms. Depending on the parentage, these two may be identical or different.

Pure or homozygous condition:-

(TT, tt): Both are dominant, Both are recessive.

Heterozygous condition (Hybrid):-

Tt: One is dominant, one is recessive

Phenotype means physical appearance, either they are Tall or Short.

Genotype means genetic makeup that is TT, Tt or tt.

Phenotypic ratio: - 3:1 (three tall and one short)

Genotypic ratio: - 1:2:1 (TT-one, Tt-two, tt-one)

Conclusions:-

• ’TT’ & ‘Tt’ are both tall plants, while ‘tt’ is short.
• A single copy of ‘T’ is enough to make the plant tall, while both copies of ‘t’ have to be present for the plant to be short.
• Traits like ‘T’ are called dominant traits because it expresses, and ‘t’ are recessive traits because it remains suppressed.

Dihybrid cross:-

It is the cross between two plants which have two pairs of contrasting characters. This takes into consideration the alternative traits of two different characters. For example, a cross between one pea plant with round and green seeds and the other pea plant having wrinkled and yellow seeds.

Phenotypic ratio:-

Round, Yellow: 9

Round, Green: 3

Wrinkled, yellow: 3

Wrinkled, green: 1

Observations:-

1. When RRyy was crossed with rrYY in the F1 generation, all was Rr Yy round and yellow seeds.
2. Self-pollination of F1 plants gave parental phenotype and two mixture seeds plant in the ratio of 9:3:3:1
3. Round and yellow seeds are dominant characters.
4. Occurrence of new phenotype combinations shows that genes for round and yellow seeds are inherited independently of each other.

HOW THESE TRAITS GET EXPRESSED:-

• Cellular DNA is the information source for making proteins in the cell.
• A section of DNA that provides information for one protein is called the gene for that protein.
• Plant height can thus depend on the amount of a particular plant hormone. The amount of the plant hormone made will depend on the efficiency of the process for making it.
• If this enzyme works efficiently, a lot of hormones will be made, and the plant will be tall. If the gene for that enzyme has an alteration that makes the enzyme less efficient, the amount of hormone will be less, and the plant will be short. Thus, genes control characteristics or traits.