AQA A2 Biology - Chapter 8 Inheritance and Selection

Charlotte Lloyd
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AQA A2 Biology Unit 4 - Chapter 8 Inheritance and Selection

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Define: a) Genotype b) Phenotype a) Genetic constitution or make up of an organism, describes the alleles an organism possesses b) Observable characteristics of an organism, as a result of the interaction between the expression of the genotype and the environment
Define: a) Gene b) Allele a) A section of DNA, sequence of nucleotide bases, that determines a single characteristic by coding for a particular polypeptide b) One of the different forms of a gene, only one allele (of a gene) occurs at a particular locus of a chromosome
What is co-dominance? When both alleles, ie. the two alleles at the same locus on each homologous chromosome, contribute to the phenotype. The resulting phenotype is either a blend of both features or both features are represented
What is a sex-linked trait? Any characteristic that is determined by a gene which lies on the XY chromosome
Why are sex-linked diseases more common in men? For much of the length of the X chromosome there is no equivalent homologous portion of the Y chromosome. Characteristics controlled by recessive alleles on the non-homologous portion of the X chromosome appear more frequently in males as there is no equivalent portion of the Y chromosome which may carry a dominant allele, therefore only one recessive allele (from the mother) has to be present for the characteristic to display itself
What is haemophilia? Sex-linked condition where the blood doesn't clot, or does so very slowly, it is almost entirely confined to the male population
What is meant by the term 'multiple alleles'? There are more than two alleles, of which only two may be present at the loci of an individuals homologous chromosomes
Define: a) gene pool b) Allelic frequency a) All the alleles of all the genes of all the individuals in a population b) Number of times a particular allele occurs within a gene pool
What does the Hardy-Weinberg principle predict? The proportion of dominant and recessive alleles of any gene in a population remains the same from one generation to the next
What five conditions must be met for the Hardy-Weinberg principle to be true? 1. No mutations arise 2. Population is isolate, ie. no flow alleles into and out of the population, no immigration or emigration 3. So selection, alleles are equally likely to be passed on 4. Population is large 5. Mating is random
Explain how 'advantageous allele' frequency increases over time: Intra-specific competition. Some individuals possess alleles which make them better adapted to environment these individuals more likely to survive and pass on 'advantageous' alleles. Offspring likely to also possess advantageous alleles, more likely to survive and reproduce. Over time no. individuals with advantageous alleles increase at the expense of those with less advantageous alleles
What are two types of selection? Directional & Stabilising
What causes directional selection? Change in environment resulting in a change in the phenotype (and therefore genotype) needed for survival.
What occurs during directional selection? Individual to either the left or right of the mean will possess this phenotype/genotype. These are more likely to survive and reproduce, passing on advantageous alleles. Over time mean will move in the direction of these individuals.
What occurs as a result of directional selection? Phenotypes at one extreme being selected for and those at the other extreme being selected against
What causes stabilising selection? Environment remaining stable, individuals closest to the mean are favoured
What occurs during stabilising selection? Those with phenotypes nearer to the mean are favoured and so are more likely to survive and reproduce passing on there advantageous alleles. Those with phenotypes at the extremes are less likely to survive and reproduce and are eliminated
What happens as a result of stabilising selection? Phenotypes near mean are selected for and those at both extremes are selected against
What is speciation? Evolution of new species from an existing species, Occurs when populations of a species become isolated, Over time gene pools of each population become so different they can't breed to produce fertile young
When does speciation occur? - Give an example When populations of a species become geographically isolated, Forest fires separate populations of forest dwelling species, Development of a mountain ranges or rivers, Climatic changes
Describe how speciation occurs: Two separate populations produced in different environments/habitats, Different environmental conditions in each habitat, Continued adaptation and selection leads to the evolution of two separate species which cannot breed to produce fertile young
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