Blueprint of Life Topic 7: Modern Genetics. Biology in Focus, HSC Course Glenda Childrawi , Margaret Robson and Stephanie Hollis. DOT POINT(s). distinguish between the terms allele and gene, using examples
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Biology in Focus, HSC Course
Glenda Childrawi, Margaret Robson and Stephanie Hollis
On the basis of his discoveries, Mendel proposed that each kind of hereditary characteristic is determined by two ‘factors’ in every individual. Mendel’s factors are equivalent to what we call genes today, but, to make this advance, chromosomes and their behaviour in mitosis and meiosis had to be discovered.
This, combined with the discovery of DNA (the chemical of which chromosomes are made) led to the field of study known as genetics—the study of heredity and variation.
In our studies of modern-day genetics, we know that cells contain units of heredity known as genes on chromosomes. Different genes influence different characteristics. For example in pea plants, one gene may determine seed colour, while another determines stem length. In humans, genes determine characteristics such as height, eye colour, hair colour and freckles.
Each cell contains two copies of every gene, one inherited from each parent. Different variations (contrasting versions) of the same gene are termed alleles of that gene.
For example, the gene for height has two alleles—tall (T ) and short (t). These versions of the same gene are found in identical positions (loci) on a pair of similar chromosomes (homologous chromosomes) within cells. Diploid individuals have two alleles for each gene and haploid cells (gametes) have only one allele of each gene.
Sometimes within a population there are more than two alleles for a particular gene, e.g.:
Mendel’s terms pure breeding and hybrid are known in modern genetics as homozygous and heterozygous respectively. These terms are used to describe the combination of alleles present in a cell.
If both copies of the gene in a cell are the same the pure-breeding individuals for this trait are, in modern terminology, said to be homozygous, meaning that the organism has identical alleles for a particular genetic trait (e.g. TT or tt). (The term ‘homozygous’ is derived from two words: zygote = a fertilised egg, having half its material from each parent; and homo = the same).
If the alleles of a gene are contrasting or differ for that trait (e.g. Tt), the hybrid individuals (as described by Mendel) are now said to be heterozygous (hetero = different).
The genetic make-up or genotype of organisms, described by the terms homozygous and heterozygous, determines the physical appearance or phenotype (that is, the genotype is the ‘cause’ of the way it looks and the phenotype is the ‘effect’—its actual physical appearance).
When a pair of differing alleles (e.g. Gg) occurs in an individual (a heterozygote) and only one of the alleles is expressed (visibly appears in the organism), this allele is known as the dominant allele.
The allele which is not expressed (i.e. hidden or masked) is called the recessiveallele. For example, the allele for the short characteristic (t) in pea plants. These alleles are different variations of the same gene. The phenotype (appearance) of an organism is determined by the dominant (expressed) genes.
Recessive alleleslikered hair in humans, are often carried for several generations without being expressed (without appearing in the phenotype), but may appear in a later generation. This later appearance of recessive traits is sometimes referred to as ‘skipping’ a generation (when masked in the hybrid form) and only reappears if it occurs in the homozygous recessive form (as a result of a monohybrid cross).
In current genetic studies, phenotype is recognised as not only the physical appearance of an organism, but may also include its physiology (functioning) and aspects of its behaviour.
Phenotype is predominantly determined by genotype, but may also be influenced or modified due to interaction with the environment. For example the final height of a human adult depends on a combination of that person’s genotype as well as their nutrition—if they are underfed or lacking protein while growing, they may never attain their full potential height as determined by their genotype. (This will be dealt with in more detail in Chapter 3.)
Genes are considered to be the units of selection in the neo-Darwinian theory of evolution by natural selection. The concept of alleles being different forms of the same gene can account for the source of variation that was lacking in Darwin’s proposal of the theory.
The term variation applies to the differences in the characteristics (appearance or genetic make-up) of individuals within a population. Organisms in a population may vary in appearance, physiology (functioning) and behaviour—that is, they vary in phenotype.
At the time when Darwin and Wallace proposed their theory of evolution by natural selection, there was no knowledge of what was responsible for the differences in individuals within a population or of how such characteristics could be passed on from one generation to the next.
Today, with our knowledge from studies of genetics, we know that the genotype of an individual (which alleles are present for particular genes on chromosomes) determine characteristics inherited.
Furthermore, characteristics are carried from parents to offspring on chromosomes by gametes (sex cells). When gametes combine, they may bring together a different combination of alleles for the same gene. For example,the gene for eye colour from each parent.
Since some genes have more than two varieties (alleles) within a population (e.g. eye colour and hair colour in humans), these genes further increase the variation that can be inherited in individuals, an essential ingredient in the process of evolution.
Mendelian principles of inheritance, such as dominance and segregation, apply to all eucaryotes, including humans. Although there are some instances where there are deviations from exact Mendelian inheritance (such as sex-linkage and co-dominance, which will be dealt with later), in general Mendel’s laws apply when:
■ characteristics are inherited as individual units
■ characteristics are controlled by a single pair of genes showing dominance.
-Students to complete Genetics Terminology Worksheet