Lesson 1. What is a gene? How are genes passed on from parent to offspring?

In animal breeding, individuals with desirable traits are selected as parents for the next generation. It is hoped that these traits will be passed on to the offspring. When traits are selected, genes that favourably influence those traits are also selected. At the same time, undesirable traits and the genes that cause them are avoided.

But what is a gene? How are genes passed on from one generation to the next? Let’s see. Click to start the first video!

What is a gene?

All living things are made up of cells. For example, there are skin cells, muscle cells and bone cells. Cells form tissues. When an individual develops, there is only one cell at the beginning, a fertilised egg. All the cells in the individual are made by the division of this cell.

The fertilised egg cell first divides into two, then into four, eight, sixteen, and so on. Eventually there are trillions of cells.

Trillion, million… what???

As cells divide, they differentiate and form tissues, which in turn form organs. Organs make up the body. All this needs precise instructions. These instructions are called genes. Genes are the coded instructions, the recipes, for the development of the organism and the subsequent functioning of the cells. Animals, like humans, have tens of thousands of different genes for different purposes.

The instructions from the genes are used to make proteins. Proteins are the building blocks of cells. They also control growth, cell specialisation and function, and thus the development of traits. Examples of proteins are haemoglobin, antibodies, and hormones.

Genes are passed from parent to offspring. A fertilised egg contains all the genes that an individual receives from their mother and father. Each new cell that divides from the egg also contains the same genes.

How are genes passed on from parents to offspring?

Genes are located in DNA, which is a chain-like molecule. A gene is a piece of this DNA chain.

DNA is packaged into chromosomes in the cell nucleus. Each chromosome contains countless different genes. A dog has about 19,000 genes, a human and a cat 20,000, and a horse 22,000.

All chromosomes come in pairs. One chromosome comes from the sire and the other from the dam.

Each animal species has its own specific number of chromosomes. A dog has 78 chromosomes, a cat 38 and a horse 64. These animals therefore have 39, 19 and 32 pairs of chromosomes respectively.

Dog chromosomes
(Photo: Junko Maeda, Charles R. Yurkon, Hiroshi Fujisawa, Masami Kaneko, Stefan C. Genet, Erica J. Roybal, Garrett W. Rota, Ethan R. Saffer, Barbara J. Rose, William H. Hanneman, Douglas H. Thamm, Takamitsu A. Kato, CC BY 2.5, via Wikimedia Commons)

All mammals have two sex chromosomes, either two X chromosomes (XX) or both X and Y chromosomes (XY).

Chromosomes that are not sex chromosomes are called autosomes. A dog has 76 autosomes, a cat 36 and a horse 62.

Depending on its’ location, a gene is said to be either sex chromosomal or autosomal, i.e., located on a chromosome other than a sex chromosome.

A locus is the exact position on the chromosome where the gene is located – so the locus is in fact the address of the gene. An example of this is a locus called FGF5 on cat chromosome B1. This locus contains a gene that affects the coat length.

(Unlike dogs and horses, cat chromosome pairs are labelled with the letters A to E: A1-A3, B1-B4, C1-C2, D1-D4, E1-E3, F1-F2, XX/XY).

When an animal has offspring, one chromosome from each chromosome pair is passed on to the offspring. So, the offspring get half their chromosomes from each parent. Since genes are on chromosomes, half the genes from the parent are also passed on to the offspring.

Let’s look at how chromosomes are passed from parents to offspring. Let’s take the example of sex chromosomes, which determine the sex of an individual. An individual is female if she has two X chromosomes (XX). If an individual has both X and Y chromosomes (XY), he is male.

Because a female has only X chromosomes and no Y chromosomes, all her offspring will always get an X chromosome from her.

A male can pass either X or Y to his offspring, so it is the male’s chromosome that determines the sex of the resulting offspring.

Sex determination

However, the sex is not always clear. Sometimes individuals develop with reproductive organs or glandular tissue of both sexes. These individuals are known as hermaphrodites. In humans, this is called intersex. In many groups of organisms, hermaphroditism is the dominant reproductive system. It is most common in invertebrates, but also occurs in fish and other vertebrates.

What have we learned?

  • Each individual comes from a fertilised egg.
  • The fertilised egg and all the cells in the body that come from it contain genes from both the sire and the dam.
  • Genes are an individual’s recipes. They are used to make proteins.
  • Proteins are the building blocks of cells. They also regulate ontogenesis and body function.
  • A gene is a piece of DNA of a certain length.
  • Genes are passed on from parent to offspring on chromosomes. For each chromosome, an individual has a version from the father and a version from the mother.
  • Only one of each pair of chromosomes is passed on to the offspring. This means that the offspring inherits half of the individual’s genes.

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