Heredity and Evolution

Heredity

• Heredity is the passing of traits from parents to offspring; variation is the difference between individuals.
• Gregor Mendel (Father of Genetics) worked on pea plants (Pisum sativum) and published in 1866. He studied seven pairs of contrasting traits (height, seed shape, seed colour, flower colour and others).
• Mendel's laws: the law of dominance, the law of segregation, and the law of independent assortment.
• Terms: dominant (expressed), recessive (hidden unless both alleles are recessive), allele, genotype (genetic make-up), phenotype (visible character), homozygous (identical alleles), heterozygous (different alleles).
• Monohybrid cross (tall x dwarf pea): F1 all tall; F2 ratio 3 : 1 (phenotype) and 1 : 2 : 1 (genotype). Dihybrid ratio: 9 : 3 : 3 : 1.
• Chromosomes carry genes; DNA has the genetic code. Watson and Crick (1953) gave the double-helix model of DNA (with Rosalind Franklin and Maurice Wilkins contributing the X-ray data). DNA bases: A pairs with T; G pairs with C. RNA has uracil (U) in place of thymine.
• Genes and proteins: DNA, mRNA, protein; RNA types: mRNA, tRNA, rRNA.
• Sex chromosomes: XX female, XY male; autosomes are the other 22 pairs. Human chromosome number: 46.
• Genetic disorders: haemophilia (blood does not clot; X-linked), colour blindness (X-linked), thalassemia, sickle-cell anaemia, Down syndrome (trisomy 21, an extra chromosome 21), Turner syndrome (XO), Klinefelter (XXY).
• Mutation: sudden change in DNA. Genetic engineering and biotechnology: Bt cotton (resistant to bollworm), recombinant insulin, DNA fingerprinting (Alec Jeffreys), GM crops, gene therapy, cloning (Dolly the sheep, 1996, first cloned mammal), stem cells.
• Human Genome Project (completed in 2003) mapped human DNA (about 20,000 to 25,000 genes).

Evolution

• Evolution is the gradual change of organisms over time.
• Lamarck: theory of inheritance of acquired characters (giraffe's neck); now rejected.
• Charles Darwin: book "On the Origin of Species" (1859); natural selection ("survival of the fittest"); voyage on HMS Beagle; visited the Galapagos Islands (finches).
• Evidences: fossils (dating by carbon-14), homologous organs (same structure, different function: human arm, whale flipper, bat wing) and analogous organs (different structure, same function: wings of birds and insects), vestigial organs (appendix, wisdom teeth, tail bone), embryology, missing links (Archaeopteryx: reptile and bird).
• Speciation forms new species through geographical isolation and variation.
• Human evolution: Dryopithecus, Ramapithecus, Australopithecus, Homo habilis, Homo erectus, Homo neanderthalensis, Homo sapiens (modern humans, appeared in Africa about 300,000 years ago).
• Artificial selection (breeding): wild cabbage gave cabbage, cauliflower, broccoli, kale and kohlrabi.