Essay Plan: The way in which genes and the environment cause variation
Ref. Section 2.4 syllab. (Genetic and environmental factors affecting variation in individuals).
Introduction:
Definition of variation - every individual unique.
Factors affecting variation i.e environment and genetic factors.
How they affect variation:
Section (1) Genes - Variation occurs at meiosis; 'reshuffling' of genes (independent assortment and crossing over of chromatids).
Random mating/fertilization.
Mutations (in somatic cells)- Substitution (small effect) and deletion (large effect) of bases.
Mutagens.
Mendelian inheritance - interaction between alleles (dominance, codominance, multiple) and gene loci determining the phenotype.
Environment- Diet affects height (e.g more protein leads to growth through cell division, less nitrogen for plants affects height)
Light intensity affecting photosynthesis and therefore height of trees.
Environment increasing probability of mutations in genes (NOT specific mutations).
Environment interacting with genetic factors , eg. height is determined by genes and environment.
Discussion of natural selection affecting the range of phenotypes observed in the population.
Section (2)The difference between modification of phenotype (just in the individual) and variations which are transmitted when the organism reproduces (i.e. mutation in the formation of gametes causes a variation which is inheritable -this is the ultimate source of variation).
Section (3) Different types of variation .
Discontinuous variation: qualitative differences between individuals (e.g. blood group)
Causes of discontinuous variation:
Different alleles at a single gene locus having large effects i.e. dominant/recessive alleles, co-dominant alleles, multiple alleles (gene has more than 2 alleles, e.g.ABO blood grouping where alleles have both dominance and co-dominance).
Different gene loci having quite different effects on characteristics e.g. tomato plants have a no. of genes at different loci coding for different leaf features - shape, presence of chlorophyll, hairs on leaf, etc.
Characteristics determined by 1-2 genes therefore variation in each gene has a large effect on phenotype. The environment has little effect.
Continuous Variation: quantitative differences between individuals (e.g height)
Causes of continuous variation:
Different alleles at a gene locus having a small effect
Different gene loci having the same but often additive effect on a particular phenotype
Polygenes i.e a large no. of genes at different loci involved which have a combined effect on a phenotype.
Continuous variation controlled by several genes and therefore variation in each gene has a small effect on phenotype. Environment often has a larger effect.
Conclusion
Sum up importance of variation, and the relative importance of both genetic and environmental factors.