Differences between members of a species Result of inheritance and environment Eg: Exposure to UV light, Diet, Fitness
1. Random Assortment of Chromosomes during meiosis Chromosomes exist in homologous pairs Homologous: genes on one member of the pair control the same characteristics as the genes on the other member of the pair These homologous pairs assort themselves randomly during the first meiotic division. The genes on a single chromosome will generally be inherited together (gene linkage)
2. Random Fertilisation The variation which is a consequence of the number of possible sperm/egg combinations. 3. Crossing Over At times the chromosomes of a homologous pair may become tangled with each other during the 1st meiotic division (called crossing over) Chiasma (plural: chiasmata): point where two chromatids cross over On some occasions the chromatids break at the chiasma and reconnect to the other chromatid (called recombination) This results in a new set of alleles on the recombinant chromosome
4. Non-Disjunction At times, during the 1st meiotic division, homologous pairs fail to separate OR During the 2nd meiotic division the pairs of chromatids fail to separate. Results in one daughter cell having an extra chromosome and another having one less. (often causes severe and distinctive birth defects or miscarriages) Trisomy: Individual inherits an extra copy of a chromosomes (three rather than two) Monosomy: individual is missing a chromosome (one instead of two) Partial Trisomy/Monosomy: Part of an extra/missing chromosome
E.g. Trisomy 21 (Down Syndrome): characteristic facial appearance, variable degrees of mental retardation, & physical abnormalities E.g. Trisomy 13 (Patau syndrome): mental retardation, small head, extra finger on each hand, cleft lip/palate, and malformation of ears and eyes. (80% die within 1st month) E.g. Trisomy 16: spontaneous miscarriage in first 3 months of pregnancy E.g.Trisomy XXY (Klinefelter’s syndrome): small testes incapable of producing sperm, enlarged breasts, sparse body hair, possible mental retardation
Monosomy X (Turner’s syndrome): females are short in stature, lack secondary sexual characteristics and are often infertile. E.g. Partial Monosomy 5 (Cri du chat syndrome): Missing section of chromosome 5 – cat like cry due to problems with larynx and nervous system, difficulty swallowing / suckling, poor growth, low birth weight and unusual facial features.
Mutations Permanent change in the DNA Results in sudden occurrence of new and different characteristics in offspring, not evident in parents. Accepting Human Variations Species: group of organisms, with common characteristics, are able to interbreed producing fertile offspring under natural conditions