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Genetics

Gregor Mendel, 1822 - 1884, is known as 'the Father of Modern Genetics'. He published his work, Versuche ├╝ber Pflanzenhybriden [Experiments on Plant Hybridization], on hereditary traits of peas, in 1866. This great 'gamechanger' came just 7 years after Charles Darwin's revolutionary book Origin of Species. Darwin had demonstrated how species evolve through adaptation. Mendel worked out, through a careful study of plant characteristics, how this adaptation worked: parents pass their traits to children in distinct packets (which have since been named genes).

The insight of his work became apparent later when the science of genetics confirmed his findings, and experiments on insects and animals demonstrated that exactly the same rules apply to all life!

Mendel's Peas

Mendel's peas
Dominant and recessive genes produce offspring with characteristics 75% dominant gene and 25% recessive gene

You might well ask why Mendel chose to study peas, and not animals, or people?

He chose to work with peas because he needed a species which had many easily visible varieties, and plants are much easier to control than insects or animals. He needed to keep careful records of the hierarchies of the plants, which mated with which, and their offspring, and their offspring, for many generations. Peas are small plants, which he could grow quickly and keep in distinct places.

In fact, peas have seven distinct traits: seed shape, seed colour, seed coat colour, pod shape, pod colour, flower position, plant height.

Cross-Fertilization

A basic technique Mendel used is called cross-fertilzation. Plants can fertilize themselves, because they have both stamens and carpel, which are the male and female parts. The stamens are removed from flower A, so it can only be fertilized by pollen from another plant, B. After pollination, the carpel of plant A develops into the peapod, which holds the seeds which can be planted and grown into offspring plants.

A character is a heritable feature, such as flower colour, and a trait is a variation of a character: white, yellow or red flowers.

Hybridization is the process of mating two, contrasting, true-breeding varieties. The parents in this operation are referred to as the P generation.

The result of mating the P generation are hybrid offspring: the $F_1$ generation. When $F_1$ individuals self-pollinate, they produce the $F_2$ generation.

Mendel's first discovery

Using this technique, Mendel cross-fertilized (crossed) white and purple flowered pea plants. All of the offspring ($F_1$) were purple.

Then Mendel crossed the $F_1$ plants: to his surprise he saw that $3/4$ of the plants had purple flowers, and $1/4$ had white flowers! What was going on?

The mathematics

Try an experiment with a friend: you take two red disks, and your friend one red and one white. The white represent a 'recessive gene', which means if it is paired with a dominant red gene, the red colour will be the one that counts.

Now we swap a disk. How many combinations of two are possible?: RR, RW, RR, RW.

Now, you take two of these new combinations ($F_1$ generation) each, and swap a disk again. What are the possible combinations this time?: RR, RW, RR, WW.

You can see that one in four of the $F_2$ generation will have no dominant red gene, and two whites. The trait in 25% of the cases will be white, and 75% red. Exactly what Mendel found!

Meiosis

Genes are located at specific positions on chromosomes. Mendel's genetic studies resulted in two requirements for inheritance: 1. Every organism must inherit a single copy of every gene from each parent, 2. During gamete production, the two sets of genes inherited from the parents must be separated so that each gamete contains just one set of genes.

Chromosomes achieve this separation requirement.

Homologous: each chromosome from one parent has a corresponding chromosome from the other parent.

Diploid: a cell that contains both sets of homologous chromosomes. Humans have 46 chromosomes, so have a diploid number of 2N = 46: i.e. 23 sets of chromosomes and 23 sets of genes from each parent.

Haploid: gametes of sexually reproducing organisms which contain only one set of chromosomes.

Meiosis: reduction division in which the number of chromosomes in a cell is halved through the separation of homologous chromosomes in a diploid cell.

DNA determines how an organism grows and develops. DNA is also passed to offspring in genes. Genes must be passed from cell to cell to ensure growth and reproduction. The study of genes also provides an understanding of the mechanisms behind evolution.

DNA

DNA stands for deoxyribose nucleic acid. It has a helix shape, and is the molecule which passes on the genetic information which make individuals unique.

By the 1940s, a molecule in the nucleus had been identified, but was thought to be of no value to the organism. In fact, researchers nicknamed it 'the stupid molecule'. How wrong they were.

Francis Crick
Francis Crick
1916-2004
DNA crystallography by Rosalind Franklin
DNA crystallography by Rosalind Franklin, allowed the structure of DNA to be deciphered

Led by the American Linus Pauling, who won two Nobel Prizes, a race developed to decipher the structure of the DNA molecule. If it was a race, it was won in 1953 by a collaboration of Francis Crick and James Watson, who used the crystallography photograph of Rosalind Franklin, which revealed the helix shape, to finally work out how the DNA was composed of two strands held together by the pairing of 4 types of bases.

Genes

A gene is a fragment of DNA, and genes contain all the information which characterises an organism, such as its development pattern, appearance and functions. These are achieved by the genes providing instructions for the manufacture of proteins. Genes are also the units of heredity, passing on the characteristics of the parents to the offspring during reproduction.

Chromosomes, DNA and genes

All of the genes of an organism are known collectively as the genome of that organism. Every cell of an organism contains the same genetic information, however since most cells have a specialist function, different parts of the genome are expressed in different cells.

In the 1990s, it became technically possible to sequence entire genomes. The simplest genomes, such as yeast, can have as few as 6000 genes, composed of around 12 million DNA base pairs, while homo sapiens have of the order of 70,000 genes, composed of 3 billion base pairs.

Chromosomes

Genes are carried on chromosomes in the cell nucleus. A chromosome consists of a single DNA molecule and some protein (protein complexes called nucleosomes), and are between 0.2-20 µm long. When a cell nucleus divides, the chromosomes take on the appearance of rods. In prokaryotic organisms, (single-celled) there is no nucleus, so the DNA is attached directly to the cell membrane, and there is no protein.

In eukaryotic cells, chromosomes double up to form homologous pairs.

DNA Scientists

Walter Sutton, Theodor Boveri, Paul Nurse, Francis Crick, Rosalind Franklin, Tomas Lindahl, Paul Modrich, Aziz Sancar, Maurice Wilkins, Sydney Brenner, Linus Pauling, Frederick Sanger, James Watson, Martha Chase, Erwin Chargaff.

Content © Andrew Bone. All rights reserved. Created : August 21, 2015 Last updated :February 15, 2016

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