genetics

Mendel's laws explain the proportion of offspring having various characteristics. When pea plants having smooth yellow peas are crossed with plants with wrinkled green peas, the first-generation offspring all have smooth yellow peas. The second-generation offspring, however, contain smooth yellow, wrinkled green, smooth green, and wrinkled yellow peas. This can be understood by tracing the passage of alleles Y, S, s, y throughout the generations. S and Y are dominant genes.

Branch of biology concerned with the study of heredity and variation - inheritance. It aims to explain how characteristics of living organisms are passed on from one generation to the next. The science of genetics is based on the work of Austrian biologist Gregor Mendel whose experiments with the cross-breeding (hybridization) of peas showed that the inheritance of characteristics and traits takes place by means of discrete ‘particles’, now known as genes. These are present in the cells of all organisms and are the basic units of heredity. All organisms possess genotypes (sets of variable genes) and phenotypes (characteristics produced by certain genes). Modern geneticists investigate the structure, function, and transmission of genes.

Before the publication of Mendel's work in 1865, it had been assumed that the characteristics of both parents were blended during inheritance, but Mendel showed that the genes remain intact, although their combinations change. As a result of his experiments with the cultivation of the common garden pea, Mendel introduced the concept of hybridization (see monohybrid inheritance). Since Mendel, the study of genetics has advanced greatly, first through breeding experiments and light-microscope observations (classical genetics), later by means of biochemical and electron microscope studies (molecular genetics).

In 1909, Danish botanist Wilhelm Johannsen coined the term ‘gene’, from the Greek word genos meaning ‘birth’, to describe these particles. In 1911, he went on to make the distinction between ‘genotype’ and ‘phenotype’. Genotype refers to the sets of genes carried by an organism that are capable of being passed on to the next generation, whereas phenotype refers to the physical traits or characteristics that the genes produce in an organism.

A major discovery in genetics came in 1944, when Canadian-born bacteriologist Oswald Avery, together with his colleagues at the Rockefeller Institute, Colin McLeod and Maclyn McCarty, showed that the carrier of hereditary information was deoxyribonucleic acid (DNA), and not protein or any other material as was previously thought. A further breakthrough was made in 1953 when James Watson and Francis Crick published their molecular model for the structure of DNA, the double helix, based on X-ray diffraction photographs. The following decade saw the cracking of the genetic code. The genetic code is said to be universal since the same code applies to all organisms from bacteria and viruses to higher plants and animals, including humans. Today the deliberate manipulation of genes by biochemical techniques, or genetic engineering, takes place.

Genetics and reproduction Sexual reproduction, meiosis, and mutation tend to promote variation in a species, by producing new combinations of genes. This is the raw material on which natural selection acts and can result in evolutionary change (see evolution). Selective breeding of plants and animals tends to reduce the variety of genes in any one individual but results in the production of varieties of plants and animals that are useful to humans. Inherited information is passed on whenever a cell divides. Mitosis is used to make extra cells for the body during growth and new individuals by asexual reproduction. Meiosis produces gametes which are used in

sexual reproduction. Any change in the nature of a particular gene or chromosome can lead to a change in the organism. This is mutation. This can happen through accidental exposure to chemicals or radiation, some of which is entirely natural.

Genotype and phenotype A description of the genes of an organism is that organism's genotype. This could be the description of the genes for one characteristic of the organism, such as tallness or it could be for the whole organism. The genotype to determine the characteristics of an organism that can be seen or measured is known as the organism's phenotype. However, the phenotype is also partly determined by the environment of the organism. For example, an organism may inherit genes that determine tallness, but if there is little food in the environment the organism will not grow tall.

Genes The nucleus of each cell of every organism contains a number of chromosomes - long threads made of DNA and protein. Each chromosome in the nucleus of a plant or animal has a matching chromosome in the nucleus of each cell and so there will always be two genes for a characteristic in a cell. The number of chromosomes in each cell is constant for and characteristic of a particular species. Within these chromosomes are the genes which determine the characteristics of each organism, or parts of organisms, and allow these characteristics to be transmitted down from generation to generation. The genes carry chemically coded instructions for the production of proteins, which in turn determine the form and function of the organism. Proteins are also essential for the repair and replacement of body tissues, and some, called enzymes, control the chemical reactions that occur within all living things.

Reproduction There are two general types of reproduction, asexual and sexual. Asexual reproduction involves only one parent, the offspring of which have chromosomes identical to those of the parent. Sexual reproduction generally involves two parents, each of which contributes half the chromosomes to the offspring. In the normal body cells of diploid organisms the chromosomes occur in pairs - one set being derived from the male parent and the other from the female parent. The two chromosomes of a pair are called homologous chromosomes. Thus each body cell in humans contains 46 chromosomes - 22 matched pairs from either parent, and one pair of sex chromosomes. In most animals, including humans, there are two kinds of sex chromosomes: X chromosomes, which are similar in size to the other chromosomes, and the smaller Y chromosomes. A combination of two X chromosomes gives a female, while one X and one Y chromosome gives a male.

Located on the 46 chromosomes of the human body are tens of thousands of genes which determine features such as hair and skin colour. The presence or absence of specific genes can be a contributing factor in diseases such as haemophilia and cystic fibrosis. Each chromosome of a homologous pair carries genes for the same characteristics in the same place, or locus. These two kinds of genes defining alternative characteristics are called alleles. If the two alleles match, they are said to be homozygous; if they differ, they are described as heterozygous. Some alleles are dominant, and others are recessive; a dominant allele masks the effects of its recessive partner. In other words, the dominant allele is expressed, and the recessive allele is not. A trait that results from a recessive allele is evident only in an individual that has two recessive alleles for that trait.

Modern genetics There are three major areas of study in modern genetics - molecular genetics, transmission genetics, and population genetics.

Molecular genetics and genetic engineering Studies in these fields examine the structure of genes and the chemical processes associated with them, including replication - the process by which the cell duplicates DNA molecule - and mutation. Any change in the nature of a particular gene can lead to a change in the organism. This can happen through natural mutation or through accidental exposure to chemicals or radiation. Gene mutations may alter the organism's traits in some way and be transmitted to future generations. These studies have led to genetic engineering, whereby organisms can be given new characteristics by manipulation of their genes. A gene removed from a chromosome in one organism is spliced to the chromosome of another to produce a specific change in one or more characteristics in the second organism. These methods have been used to great effect in the agricultural industry; for instance, the alteration of genes to increase crop and livestock production, and the introduction of specific traits in domesticated plants and animals. It is also an established process for the production of antibiotics and hormones.

Transmission genetics In this field, geneticists analyse patterns of inheritance and the way in which genes are transmitted by tracking variations in the patterns of inheritance of a trait over generations. Studies also include gene mapping - the location and description of how and where genes are arranged on chromosomes. From this it is possible to associate certain traits with specific genes, and this information is used for determining the inheritance of particular genes within families. This has many practical applications, especially in the diagnosis of genetic diseases. Identifying the gene responsible for a hereditary disorder helps to pinpoint individual members of a family at risk of developing the condition.

Population genetics This area of research covers the distribution of inherited variation within a population - that is, a group of individuals of the same species living within the same environment or area. Within a population, the potential for change depends upon the sum total of all the different genes available, including alleles; the total hereditary and genetic information of any particular population or species is termed its gene pool. Because of differential reproduction (that is, not all individuals in a species reproduce at the same rate) the overall make-up of the gene pool will change with time. This means that some alleles, or variations, may become more common, while others may be lost to the population. Selective breeding of plants and animals tends to reduce the variety of genes in a gene pool. Thus while this process may, in the short term, be of benefit to mankind, in the longer term, it may be detrimental to the species involved by rendering it less able to respond to change, and therefore less likely to survive any alteration to its environment.

Studies of multiple birth (twins, triplets, and so on) are used to determine the influence of environment as well as heredity on individuals. There are two kinds of multiple births: identical and non-identical (or fraternal). Identical, or monozygotic, twins develop from a single fertilized egg which has divided into two cells. Each cell develops into an independent embryo, resulting in identical individuals - either both male or both female. Non-identical (dizogotic) births occur when more than one egg cell is fertilized at the same time by separate sperms, each or all cells developing into individual embryos which may be all of one sex or of both sexes.

Genetic (DNA) fingerprinting This is a technique in which an individual's DNA is analysed to reveal the pattern of repetition of particular nucleotide sequences (marker sequences) throughout the genome. This pattern is claimed to be unique to the individual concerned. The technique is widely used for identification purposes in forensic medicine and veterinary science; it is also used to settle paternity suits. Very small samples of body tissues, such as blood, semen, or hair can provide sufficient DNA for test purposes.