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| When populations are geographically isolated, there is no interbreeding between members of each population and so there genes are also isolated. In time, the genes within each population will change and this lead to the formation of new species. | Genotype = Genetic make-up of an organism. It describes all the alleles that an organism contains. |
| Genotype sets the limits within which the characteristics of an individual may vary. e.g. if a baby can grow to be 1.8m tall, but the actual height is determined by other factors such as diet. | A lack of an element such as calcium ( for bone growth) at a particular stage of development could mean that the individual never reaches his/her maximum height. |
| Any change to the genotype as a result of a change to the DNA is called a mutation and it may be inherited if it occurs in the formation of gametes. | Phenotype = the observable characteristics of an organism. It is a result of interaction between the expression of the genotype and the environment. |
| The environment can alter an organism's appearance. Any change to the phenotype that does not affect the genotype is not inherited and is called a modification | A gene is a section of DNA, that is, a sequence of nucleotide bases, that usually determines a single characteristic of an organism e.g. eye colour. It does this by coding for particular polypeptides. |
| These polypeptides make up the enzymes that are required in the biochemical pathway that leads to the production of a characteristic e.g. a gene could code for brown pigment in the iris of the eye. | Genes exist in 2, occasionally more, different forms called alleles. |
| The position of a gene on a chromosome is known as the locus. | An allele is a different version of the same gene. |
| All individuals of the same species have the same genes, but not necessarily the same alleles of these genes. | Only one allele of a gene can occur at the locus of any one chromosome. |
| Sexually reproducing organisms have chromosomes that occur in pairs called homologous chromosomes. | There are therefore 2 loci that each carry one allele of a gene. If the allele on each of the chromosomes is the same, then the organism is to be homozygous for the characteristic. |
| If the 2 alleles are different, then the organism is said to be heterozygous for the characteristic. | In most cases where 2 different alleles are present in a genotype, only one of them shows itself in the phenotype. |
| The alleles of the heterozygote that expresses itself in the phenotype is said to be dominant, whilst the one that is not expressed is said to be recessive. | A homozygous organism with 2 dominant alleles is homozygous dominant, whereas one with 2 recessive alleles is called homozygous recessive. |
| The effect of a recessive allele is only apparent in the phenotype of a diploid organism when it occurs in the presence of another identical allele i.e. when it is the homozygous state. | In some cases 2 alleles both contribute to the phenotype , in which case they are referred to as co-dominant. |
| In this situation when both alleles occur together, the phenotype is either a blend of both features ( e.g. snapdragons with pink flowers resulting from an allele for red and white) or both features are represented ( e.g. the presence of both A and B antigens in blood group AB) | In diploid cells, there are 2 copies of each allele. One copy from the mother and one from the father. |
| Sometimes a gene has more than 2 allelic forms. In this case, the organism is said to have multiple alleles for the character. However, as there are only 2 chromosomes in a homologous pair, it follows that only 2 of the 3 or more alleles can be present in a single organism. | Multiple alleles occur in the human ABO blood grouping system. |
| Monohybrid inheritance is the inheritance of a single gene. | If pea plants with green pods are bred repeatedly with plant with green pods, they are said to be pure breeding for the character of green pods. This means that the organisms are homozygous ( they have 2 alleles the same) for that particular gene. |
| When the heterozygous plants of the first filial generation are crossed with one another, the offspring ( the second filial generation) are always in a n approximate ratio of 3 plants with green pods to each 1 plant with yellow pods. | If these pure breeding green pod plants are then crossed with pure breeding yellow pod plants, all 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. |
| In diploid organisms, characteristics are determined by alleles that occur in pairs. Only 1 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 female. The remaining pair are the sex chromosomes. |
| In human females, the 2 sex chromosomes appear the same and are called x chromosomes. In the human male there is a single x chromosome like that in the female, but the second one is the smaller in size and shaped differently. This is the y chromosome. | Unlike other features in organisms, sex is determined by chromosomes rather than genes |
| Females have 2 x chromosomes, all the gametes contain an x chromosome. | Males have 1 x chromosome and 1 y chromosome, they produce different types of gamete. Half have an x chromosome and half 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 larger than the y chromosome. This means for most of the length of the x chromosome, there is no equivalent homologous portion of the y chromosome. |
| Those characteristics that are controlled by a recessive allele on the 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. | x chromosome carries many genes. one example in humans is the condition called haemophilia, in which the blood clots only slowly and there may be a slow and persistent internal bleeding, especially in the joints. |
| There has been selective removal of this gene from the population, so its occurrence is relatively rare. | Although haemophiliac females are known, the condition is almost entirely confined to males. |
| One of a number of causes of haemophilia is a recessive allele with altered DNA nucleotides that therefore does not code for the required protein. This results in the individual being unable to produce a protein that is required in the clotting process. | The extraction of this protein from donated blood means that it can now be given to haemophiliacs, allowing them to lead near normal lives. |
| When showing sex linkage, you show the allele linked to the appropriate chromosome: i.e. XH Xh or XH Y. If the allele is sex linked to the x chromosome, you do not draw an allele present on the y chromosome. There is no equivalent allele on the y chromosome as it does carry the gene for the clotting protein. | As males obtain the y chromosome from their father, it follows that the x chromosome comes from their mother. As the defective allele that does not code for the clotting protein is linked to the x chromosome, males always inherit the disease from their mothers. |
| If their mother does not suffer from the disease, she may be heterozygous for the character ( XH Xh). Such females are called carriers because they carry the allele without showing any signs of the character in their phenotype. | As males pass on the y chromosome to their sons, they cannot pass haemophilia to them. However, they can pass on the allele to their daughters via the x chromosome, who would then become carriers of the disease. |
| Pedigree charts: A useful way to trace inheritance of sex-linked characters, such as haemophilia. | Male is represented by a square female is represented by a circle shading within either shape indicates the presence of a character, such as haemophilia in the phenotype. a dot within the circle signifies a woman with a normal phenotype but who carries the defective allele. |
| co-dominance: in which both alleles are equally dominant. | Multiple alleles: where there are more than 2 alleles, of which only 2 may be present at the loci of an individuals homologous chromosomes. |
| When both alleles are dominant i.e.co-dominant both alleles are expressed in the phenotype. | The inheritance of the human ABO blood group is an example of where there are multiple alleles. |
| There are 3 alleles associated with the gene I ( immunoglobulin gene) which lead to the production of different antigens on the surface membrane of red blood cells. | allele IA leads to the production of antigen A allele IB leads to the production of antigen B allele IO leads to no production of either antigen. |
| Although there are 3 alleles, only 2 can be present in an individual at any one time, as there are only 2 homologous chromosomes and therefore only 2 gene loci. | The alleles IA and IB are co-dominant, whereas the allele IO is recessive to both. |
| Possible crosses: Blood group A = IA IA or IA IO Blood group B = IB IB or IB IO Blood group AB = IA IB Blood group O = IO IO | A cross between an individual of blood group O and on of blood group AB, rather than producing individuals of either of the parental blood groups, produces only individuals of the other 2 groups, A and B. |
| When certain individuals of blood group A are crossed with certain individuals of blood group B, there children could be of any of the 4 blood groups. | In blood groups, alleles IA and IB are co-dominant and IO is recessive to both. Sometimes, however there are more than 3 alleles, each of which is arranged in a hierarchy with each allele being dominant to those below it and recessive to those above it. |
| Gene pool = All the alleles of all the genes of all the individuals in a population at any one time/ All the alleles of one gene of all the individuals in a population at any one time. | Allele frequency = The number of times an allele occurs within a gene pool. |
| If there are 10000 people in a population, there will be twice as many alleles ( 20000) in the gene pool of this gene. | In any population the total number of alleles is taken to be 1.0. |
| If all individuals are homozygous dominant ( GG) the frequency of dominant alleles in the gene pool is 1.0 | If all individuals are homozygous recessive ( gg) the frequency of recessive alleles in the gene pool is 1.0 |
| If all heterozygous ( Gg) the frequency of dominant alleles = 0.5 and recessive alleles = 0.5 | The Hardy Weinberg equation: 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 the same from one generation to the next provided that 5 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 on to the next generation The population is large Mating within the population is random. |
| You can use the hardy-Weinberg equation to: predict allele frequency predict genotype frequency predict % of next generation with each genotype show if external factors affect allele frequency | p + q = 1 p = frequency of dominant allele q = frequency of recessive allele |
| p2 + 2pq + q2 = 1 2 pq = heterozygous frequency | If the frequency of q = 0.018 which is less than previously. Therefore something is affecting it e.g. immigration, emigration, natural selection, migrations etc.... |
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