Extensions of the Laws of Inheritance

Mendel studied traits with only one mode of inheritance in pea plants. The inheritance of the traits he studied all followed the relatively simple pattern of dominant and recessive alleles for a single characteristic. There are several important modes of inheritance, discovered after Mendel’s work, that do not follow the dominant and recessive, single-gene model.

MENDEL’S SECOND LAW INDEPENDENT ASSORTMENT

Analyzing two loci in the same cross provides valuable information for genetic mapping and testing gene interactions. This is important for understanding whether heritable traits are controlled by discrete factors that are inherited independently of each other?

MENDEL’S FIRST LAW

In his pioneering studies on inheritance, Gregor Mendel observed that a single trait could manifest in diverse versions, known as alleles, even within an individual plant or animal. Taking the example of seed color, Mendel identified two allelic forms of a gene—one yielding green seed and the other yielding yellow seeds.

Remarkably, he observed that although different alleles could impact a single trait, they remained distinct and could be inherited independently. This fundamental principle, known as Mendel's First Law or The Law of Equal Segregation, underscores that during gamete formation, the two alleles at a gene locus segregate from each other. Each gamete possesses an equal likelihood of containing either allele. This insight lays the groundwork for understanding how genetic traits are inherited in animals, including those of veterinary importance.

CHROMOSOMES MITOSIS AND MEIOSIS

If stretched to its full length, the DNA molecule of the largest human chromosome would be 85mm. Yet during mitosis and meiosis, this DNA molecule is compacted into a chromosome approximately 5µm long. Although this compaction makes it easier to transport DNA within a dividing cell, it also makes DNA less accessible for other cellular functions such as DNA synthesis and transcription. Thus, chromosomes vary in how tightly DNA is packaged, depending on the stage of the cell cycle and also depending on the level of gene activity required in any particular region of the chromosome.

GENES TO GENOMES

Mendel's groundbreaking work in genetics demonstrated that inheritance follows a particulate, non-blended pattern. These distinct hereditary units, known as genes, are crucial in determining an organism's traits. Genes can exist in different versions or forms, which we refer to as alleles. Alleles are variations of a gene that can result in different expressions of a particular trait. For example, in the case of animal’s coat colour, different alleles of a specific gene may lead to variations in an animal's coat appearance.

GENES ENCODE PROTEINS

Genetic information in DNA (genes) is expressed as biological traits, such as the color of eyes and hairs, through a fundamental concept known as the Central Dogma of molecular biology. While not all genes code for proteins, a majority of them do. In this chapter, we will delve into the Central Dogma and explore key experiments that have supported and refined this concept

DNA STRUCTURE AND REPLICATION

One of the fundamental things to know when studying genetics is the basic structure of DNA and how it is replicated. DNA is the “blueprint” that contains all the instructions for making the proteins that each cell needs, whether it is a single celled bacterium or a multicellular organism like humans. J. Watson, F. Crick, and M. Wilkins received the Nobel Prize (1962) for discovering the structure of DNA. (R. Franklin might have also received the prize for this discovery, but she died in 1958.)

DNA IS THE GENETIC MATERIAL

Genetics is the scientific study of heredity and the variation of inherited characteristics. It includes the study of genes, how they function, and how they produce the visible and measurable characteristics we see in individuals and populations of species as they change from one generation to the next, over time, and in different environments.

Heredity is the concept that the characteristics of an individual plant or animal in a population could be passed down through the generations. Offspring look more like their parents (Figure 1). People learned that some heritable characteristics (such as the size or colour of fruit) varied between individuals, and that they could select or breed crops and animals for the most favourable traits.

In the past, humans could only manipulate and select from naturally existing combinations of genes. More recently, with the discovery of the substance and nature of genetic material, DNA, we can now identify, clone, and create novel, better combinations of genes that will serve our goals. Understanding the mechanisms of genetics is fundamental to using it wisely and for the betterment of all.

Prior to Mendel (1865) heredity was considered to be of a “blended inheritance” but his work demonstrated that inheritance was particulate in nature (particulate inheritance). We now call these “particles” genes and their different forms, alleles. By the early 1900’s, biochemists had isolated hundreds of different chemicals from living cells, but which of these was the genetic material? Proteins seemed like promising candidates, since they were abundant, diverse, and complex molecules. However, a few key experiments demonstrated that DNA, rather than protein, is the genetic material.