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L2 BIO: GENETIC VARIATION AND CHANGE…… DEFINITIONS Note: All definitions and information has come from http://www.biology-online.org/ and BIOZONE Homozygous (1) an individual (or a condition in a cell or an organism) containing two copies of the same allele for a particular trait located at similar positions (loci) on paired chromosomes (see homologous chromosomes). (2) Having two identical alleles that code for the same trait. Supplement A ‘homozygous’ organism for a particular trait is described to possess either a pair of dominant alleles (e.g. AA) or a pair of recessive alleles (e.g. aa). Heterozygous (1) an individual (or a condition in a cell or an organism) containing two different alleles for a particular trait. (2) Having dissimilar alleles that code for the same gene or trait. Supplement Example: a zygote having one dominant allele and one recessive allele, i.e. Aa, for a particular trait. Linked genes Genes that are inherited together with the other gene(s) as they are located on the same chromosome. Supplement When a pair or set of genes are on the same chromosome, they are usually inherited together or as a single unit. For example, in fruit flies the genes for eye color and the genes for wing length are on the same chromosome, thus are inherited together. Codominance A condition in which the alleles of a gene pair in a heterozygote are fully expressed thereby resulting in offspring with a phenotype that is neitherdominant nor recessive. Supplement A typical example showing codominance is the ABO blood group system. For instance, a person having A allele and B allele will have a blood type AB because both the A and B alleles are codominant with each other. Incomplete dominance A kind of dominance occurring in heterozygotes in which the dominant allele is only partially expressed, and usually resulting in an offspring with an intermediate phenotype. Supplement In incomplete dominance, a heterozygous organism carrying two alleles wherein one is dominant and the other one is recessive, (e.g. Aa), the dominant allele will only be partially expressed. Hence, the heterozygote (Aa) will have an intermediate phenotype. A typical example of incomplete dominance is the colour of the flower in which R symbolizes the dominant allele for red pigment and r is the recessive allele for no pigment. In incomplete dominance, the heterozygous plant carrying both alleles, Rr, will not be able to produce enough red pigment (since the dominant allele is only partially expressed) and therefore will appear pink. Meiosis A form of cell division happening in sexually reproducing organisms by which two consecutive nuclear divisions (meiosis I and meiosis II) occur without the chromosomal replication in between, leading to the production of four haploid gametes (sex cells), each containing one of every pair of homologous chromosomes (that is, with the maternal and paternal chromosomes being distributed randomly between the cells). Supplement Meiosis encompasses interphase, meiosis I and meiosis II (both of these consist of four major subphases - prophase, metaphase, anaphase and telophase). The process of meiosis is briefly described as follows. During the interphase, the chromosomes in a cell are duplicated. This is followed by meiosis I wherein the chromosomes condense along the center of the nucleus, and pair with their homologues during crossing over. Next, the pairs of chromosomes separate and move to opposite ends of the cell. The cell divides for the first time producing two cells. The two cells will undergo meiosis II wherein both of them divides further into two cells, each containing one of every decoupled chromosome’s sister strands (chromatids), thus, producing four genetically different, haploid cells. Meiosis is a vital process because it reduces the original number of chromosomes to half, and allows genetic variability by genetic recombination and independent assortment. Meiosis produces four haploid cells that may develop into potential gametes so that when fertilization occurs, a new individual with the full number of genes results, thereby maintaining the integrity of chromosomal number across generations while promoting genetic diversity and variability in forms in the population. Independent assortment The process of random segregation and assortment ofchromosomes during anaphase I of meiosis resulting in the production of genetically unique gametes. Supplement The Law of Independent Assortment speaks of alleles of a gene separating independently from alleles of another gene (unless they are linked). Hence, the inheritance pattern of one trait will not affect the inheritance pattern of another. For instance, the gene for the eye colour is inherited independently from the gene for hair colour. That is, not all individuals with brown eyes will always have a black hair colour; others may still have a different hair colour. It is because the gene coding for the eye colour separates independently (and randomly) from the gene coding for the hair colour during formation of gametes (meiosis). Independent assortment of genes is important to produce new genetic combinations that increase genetic variations within a population. Law of Segregation Paired alleles separate during gamete formation. Consequently, each gamete would contain only one copy of every paired gene. Supplement During anaphase II of meiosis, the genes on the homologous chromosomes separate as the homologous chromosomes move apart from each other toward the opposite sides of the dividing cell. Crossing over A process occurring during meiosis wherein two chromosomes pair up and exchange segments of their genetic material. Supplement This occurs at the stage when chromatids of homologous chromosomes pair up during synapsis, forming X-structure (chiasma). The chromatids break into segments (of matching regions), which are then exchanged with one another. Crossing over is important because it results in new combinations of genes that are different from either parent, contributing to genetic diversity. Recombination The process or act of exchanges of genes between chromosomes during meiosis when chromosomes exchange genes. This process results in a different genetic combination and ultimately to the formation of unique gametes with chromosomes that are different from those in parents. Thus, recombination is one of the important ways to promote and increase genetic diversity between generations. Mutation A permanent, heritable change in the nucleotide (base) sequence in a gene or a chromosome. Supplement Mutation may be small scale (affecting the nucleotide (gene) sequence of a gene) or large scale (involving a change in the chromosome). It may arise from exposure to ultraviolet or ionizing radiation, chemical mutagens, viruses, etc. Such a change may result in the creation of a new character or trait. Mutations can be SILENT (if they do not change the amino acid sequence or the final protein produced); HARMFUL (because they stop or alter the production of an essential protein such as an enzyme); BENEFICIAL (because they help the survival of an organism). Genetic variation 1. Variations of genomes (all the genetic material) between members of species, or between groups of species thriving in different parts of the world as a result of genetic mutation. 2. Genetic diversity in a population or species as a result of new gene combinations (e.g. crossing over of chromosomes), genetic mutations, genetic drift, etc. 3. Important in maintaining biodiversity among species. Gene pool The total number of genes of every individual in an interbreeding population. Supplement A large gene pool indicates high genetic diversity, increased chances of biological fitness, and survival. A small gene pool indicates low genetic diversity, reduced chances of acquiring biological fitness, and increased possibility of extinction. Gene pool increases when mutation occurs and survives. Gene pool decreases when the population size is significantly reduced (e.g.famine, genetic disease, etc.). Some of the consequences when gene pool is small are low fertility, and increased probability of acquiring genetic diseases and deformities. Gene pool gives an idea of the number of genes, the variety of genes and the type of genes existing in a population. It can be used to help determine gene frequencies or the ratio between different types of genes in a population. Evolution (1) The change in genetic composition of a population over successive generations, which may be caused by natural selection, inbreeding, hybridization, or mutation. (2) The sequence of events depicting the evolutionary development of a species or of a group of related organisms. Supplement In order for evolution to occur, there must be genetic variation. Genetic variation brings about evolution. Without it there will be no evolution. There are two major mechanisms that drive evolution. First is natural selection. Individuals with advantageous traits are more likely to reproducesuccessfully, passing these traits to the next generation. This kind of evolution driven by natural selection is called adaptive evolution. Another mechanism involves genetic drift, which produces random changes in the frequency of traits in a population. Genetic drift The process of change in the genetic composition of a population due to chance or random events rather than by natural selection, resulting in changes in allele frequencies over time. Supplement The effect of genetic drift in large populations is usually negligible whereas in small populations, it predominates. In a small population, genetic drift results in some alleles becoming more common while others become less common or lost over time. Natural selection A process in nature in which organisms possessing certain genotypic characteristics that make them better adjusted to an environment tend to survive, reproduce, increase in number or frequency, and therefore, are able to transmit and perpetuate their essential genotypic qualities to succeeding generations. Supplement It is the process by which heritable traits that increase an organism’s chances of survival and reproduction are more favoured than less beneficial traits. Originally proposed by Charles Darwin, natural selection is the process that results in the evolution of a species.