Variation refers to differences in characteristics within the same species, even among identical twins. (p.256)
Variation plays an important role in evolution and forms the basis of natural selection, which allows species to adapt and survive. (p.256)
Variation allows species to survive environmental change, allows cross breeding to form new species, and lets natural selection keep beneficial characteristics. (p.257)
Continuous variation shows gradual differences from one extreme to the other, with most individuals in intermediate phenotypes, giving a bell-shaped curve. (p.258)
Continuous variation is quantitative, influenced by environmental factors, and examples include height, body weight and skin colour. (p.258)
Discontinuous variation shows distinct differences with no intermediate phenotypes, giving a bar chart. (p.259)
Discontinuous variation is qualitative, controlled by genetic factors and usually a single gene, and is not influenced by the environment. (p.259)
Examples of discontinuous variation are ability to roll tongue, eye colour and fingerprint pattern (loop, arch, whorl). (p.259)
About 8 to 10% of the human population has blue eyes due to low melanin in the iris. (p.259)
Genetic factors causing variation: crossing over, independent assortment of chromosomes, random fertilisation and mutation. (p.261)
Crossing over occurs between non-sister chromatids during prophase I of meiosis and produces new gene combinations. (p.261)
Independent assortment: during metaphase I, homologous chromosome pairs are arranged randomly, producing gametes with different combinations. (p.262)
Random fertilisation between sperm and egg produces zygotes with new genetic combinations. (p.262)
Mutation is a permanent, spontaneous change in genes or chromosomes; mutation in germ cells can be inherited, while mutation in somatic cells cannot. (p.263)
Environmental factors include temperature, light and soil pH; they produce environmental variation that cannot be inherited. (p.263)
Siamese cat fur is dark at parts with lower temperature because its pigment enzyme works only below body temperature. (p.264)
Hydrangea sp. produces blue flowers in acidic soil (pH below 5.5) and pink flowers in alkaline soil. (p.264)
Genetic and environmental factors interact; identical twins raised apart can show different phenotypes due to diet and lifestyle. (p.264)
12.2 Variation in Humans
Human traits are controlled by pairs of alleles; a dominant trait shows when one or two dominant alleles are present, and a recessive trait only when both alleles are recessive. (p.266)
Examples of dominant and recessive traits: tall/short, curly/straight hair, right/left-handed, presence/absence of dimples, free/attached earlobes. (p.266)
12.3 Mutation
Mutation is a spontaneous and random change in DNA. (p.269)
A mutagen is a substance that causes mutation or increases its rate to a dangerous level. (p.269)
Mutagens are physical (ultraviolet, ionising rays), chemical (carcinogens in cigarette smoke, formaldehyde, benzene) or biological (viruses, bacteria). (p.269)
A mutant is new genetic material produced by mutation. (p.269)
Gene mutation is a change in nucleotide base sequence of a gene and is also called point mutation. (p.269)
Gene mutation occurs by base substitution, base insertion or base deletion; it changes the amino acid sequence and the protein structure. (p.270)
Examples of gene mutation: sickle cell anaemia, albinism (lack of skin, hair and eye pigment), cystic fibrosis, thalassemia. (p.270)
Sickle cell anaemia causes red blood cells to become crescent-shaped due to abnormal haemoglobin. (p.270)
Chromosomal mutation changes chromosome structure or chromosome number. (p.271)
Types of chromosomal structural change are deletion, duplication, inversion and translocation. (p.271)
Nondisjunction occurs when homologous chromosomes fail to separate in anaphase I or sister chromatids fail to separate in anaphase II, producing gametes with extra or missing chromosomes. (p.272)
Cri du chat syndrome results from a partial deletion on the short arm of chromosome 5; affected infants have a cry similar to a meowing kitten. (p.273)
Jacob syndrome (44 + XYY) produces tall males with learning difficulties. (p.273)
Mutation in somatic cells cannot be inherited; mutation in gametes can be inherited by descendants. (p.274)