Genotype - the genetic constitution (make-up) of an organism. it describes all the alleles that an organism contains and sets the limit within which the characteristics of an individual may vary.
Allele - one form of a gene. For example, the gene for the shape of pea seeds has two alleles: one for 'round' and one for 'wrinkled.' Only one allele of a gene can occur at the locus of any one chromosome.
Gene - lengths of DNA on a chromosome normally coding for a particular polypeptide.
Phenotype - the observable characteristics of an organism. It is the result of the interaction between the expression of the genotype and the environment.
Homologous chromosomes - a pair of chromosomes that have the same gene loci and therefore determine the same features. They are not neccessarily identical as individual alleles of the same gene may vary. Homologous chromosomes are capable of pairing during meiosis.
Homozygous - when an organism has the same alleles on each chormosome.
Hetrozygous - when an organism has two different alleles on each chromosome.
Dominant - the allele of the hetrozygote that expresses itself as the phenotype.
Recessive - the allele of the hetrozygote that is not expressed.
Homozygous dominant - a homozygous organism with two dominant alleles.
Homozygous recessive - a homozygous organism with two recessive alleles.
Diploid - a term applied to cells in which the nucleus contains two sets of chromosomes.
Haploid - a term referring to cells that contain only a single copy of each chromosome e.g. the sex cells or gametes.
Co-dominant - when two alleles both contribute to the phenotype, e.g. in snapdragons where pink flowers result from an allele for red and an allele for pink both being expressed.
Multiple alleles - when a gene has more than two allelic forms.
Monohybrid inheritance is the inheritance of a single gene. For example, the colour of the pods of pea plants. Pea pods come in two basic colours: green and yellow.
If pea plants with green pods are bred repeatedly with each other so that they constantly give rise to plants with green pods, they are said to be pure breeding for the characteristic of green pods. Pure breeding strains can be bred for almost any character. This means that the organisms are homozygous for that particular gene.
If these pure breeding green-pod plants are then crossed with pure breeding yellow-pod plants, all of the offspring, known as the first filial generation, produce green pods. This means that the allele for green pods is dominant to the allele for yellow pods, which is therefore recessive.
When the hetrozygous plants of the first generation are crossed with one another, the offspring are always in an approximate ratio of three plants with green pods to each one plant with yellow pods.
In diploid organisms, characteristics are determined by alleles that occur in pairs. Only one of each pair of alleles can be present in a single gamete.
Humans have 23 pairs of chromosomes. 22 of these have partners that are identical in appearance, whether in a male or in a female. The remaining pair are the sex chromosomes. In human females, the two sex chromosomes appear the same and are called the X chromosomes. In the human male there is a single X chromosome like that in the female, but the second one of the pair is smaller in sized and shaped differently. This is the Y chromosome.
Unlike other features of an organism, sex is determined by chromosomes rather than genes. In humans:
- as females have two X chromosomes, all the gametes are the same in that they contain a single X chromosome.
- as males have one X chromosome and one Y chromosome, they produce two different types of gametes - half have an X chromosome and half have a Y chromosome.
Any gene that is carried on either the X or Y chromosome is said to be sex linked. However, the X chromosome is much longer than the Y chromosome. This means that, for most of the length of the X chromosome, there is no equivilant homologous portion of the Y chromosome. Those characteristics that are controlled by recessive alleles on this non-homologous portion of the X chromosome will appear more frequently in the male.This is because there is no homologous portion on the Y chromosome that might have the dominant allele, in the presence of which the recessive allele does not express itself.
By contrast, the X chromosome carries many genes. One example in humans is the condition called haemophilia, in which the blood clots slowly. As such it is potentially lethal if not treated. This has resulted in some selective removal of the gene from the population, making its occurance relatively rare. Although haemophiliac females are known, the condition is almost entirely confined to males.
As the defective allele is linked to the X chromosome, males always inherit the disease from their mother. If their mother does not suffer from the disease, she may be hetrozygous for the character. Such females are called carriers because they carry the allele without showing any signs of the character in their phenotype.
Co-dominance occurs where, instead of one allele being dominant and the other recessive, both alleles are dominant to some extend. This means that both alleles of a gene are expressed in the phenotye.
One example is in snapdragon flowers where one allele codes for a and enzyme that catalyses information of a red pigment. Another allele codes for an altered enzyme that lacks this catalytic activity and so doesn't produce a pigment, resulting in the flower being white. In plants that are hetroygous, with their single allele for the functional enzyme, produces just sufficient red pigment to produce flowers.
Sometimes a gene has more than two alleles. The inheritance of the human ABO blood groups is an example. There are three alleles associated with the gene I, which lead to the production of different antigens on the surface membrane of red blood cells:
- Allele I(A), which leads to the production of antigen A
- Allele I(B), which leads to the production of antigen B
- Allele I(O), which does not lead to the of either antigen
Although there are three alleles, only two can be present at any one time in an individual as there are only two homologous chromosomes and therefore only two gene loci. The alleles Ia and Ib are co-dominant, whereas the allele Io is recessive to both.
Sometimes there may be more than three alleles, each of which is arranged in a heirarchy with each allele being dominant to those below it and recessive to those above it. One example is coat colour in rabbits.
The Hardy-Weinburg Principle
The Hardy-Weinburg principle provides a mathematical equation that can be used to calculate the frequencies of the alleles of a particular gene in a population. The principle predicts that the proportion of dominant and recessive alleles of any gene in a population remains in the same from one generation to the next provided that five conditions are met:
- No mutations arise
- The population is isolated, that is, there is no flow of alleles into or out of the population
- There is no selection, that is, all alleles are equally likely to be passed onto the next generation
- The population is large
- Mating within the population is random
The frequency of allele A = p and the allele of a = q, therefore; p+q=1.0
p(2) + 2pq + q(2) = 1.0
Reproductive Success and Allele Frequency
- All organisms produce more offspring than can be supported by the environment, which means that there is competition between members of a species to survive.
- Within any population of a species there will be a gene pool containing a wide variety of alleles.
- Some individuals will possess combinations of alleles that make them better able (fitter) to survive in their competition with others.
- These individuals are more likely to obtain the available resources and so grow more rapidly and live longer. As a result, they will have a better chance of breeding and producing more offspring.
- Only those individuals that successfully reproduce will pass on their alleles to the next generation.
- As these new individuals have 'advantageous' alleles, they in turn are more likely to survive, and so reproduce successfully.
- Over many generations, the number of individuals with the 'advantageous' alleles will increase at the expense of the individuals with the 'less advantageous' alleles.
- Over time, the frequency of the 'advantageous' alleles in the population increases while that of the 'non-advantageous' ones decreases.
When selection favours individuals that vary in one direction from the mean of the population, this is called directional selection. If the environmental conditions change, so will the phenotypes needed for survival. Some individuals, which fall either to the left or the right of the mean in a normal distribution curve, will possess a phenotype more suited to the new conditions. These individuals will be more likely to survive and breed. They will therefore contribute more offspring (and the alleles these offspring possess) to the next generation than the other individuals. Over time, the mean will then move in the direction of these individuals.
Directional selection therefore results in phenotypes at one extreme of the population being selected for and those at the other extreme being selected against.
When selection favours average individuals, it is called stabilising selection and preserves the characteristics of a population. If environmental conditions remain stable, it is the individuals with the phenotypes closest to the mean that are favoured. These individuals are more likely to pass on their alleles to the next generation. Those individuals with phenotypes at the extremes are less likely to pass on their alleles. Stabilising selection therefore tends to climinate the phenotypes at the extremes.
Stabilising selection results in phenotypes around the mean of the population being selected for and those at both extremes being selected against.
Speciation is the evolution of new species from existing species. A species is a group of individuals that share similar genes and are capable of breeding with one another to produce fertile offspring. In other words, they belong to the same gene pool.
Any species consists of one or more populations. Within each population of a species, individuals breed with one another. Although it is possible for them to breed with individuals from other populations, they breed with each other most of the time. Therefore a single gene pool still exists.
If two populations become seperated in some way, the flow of alleles between them may cease. The environmental factors that each group encounters may differ. Selection will affect the two populations in different ways and so the type and frequency of the alleles in each will change.Each population will evolve along seperate lines. In time, the gene pools of the two populations may become so different, that even if reunited, they would be incapable of successfully breeding with each other. They would have become seperate species, each with its own gene pool. Speciation would have taken place. Speciation depends on groups within a population becoming isolated in some way. One way is geographical isolation, which is when a physical barrier prevents two populations from breeding with one another. Such barriers include oceans, rivers, mountain ranges and deserts.