VARIATION

6.2 THE CAUSES OF VARIATION

Crossing OverIndependent/ Random assortment

Random FertilisationTakes place when a male gamete fertilizes another female gamete from the same species by chanceAllows alleles from the male and female alleles to combine in new waysProduces new genotypes which in turn are expressed as new phenotypes MUTATIONMutationAny change in an organisms genetic material (DNA & chromosome)Random & spontaneousChanges in the genetic material include changes in the:Amount of chromosomeStructure of the chromosomeStructure of the DNAMutant :Refers to the gene, chromosome / organism that has undergo mutation

a) Chromosomal MutationsDowns SyndromeCaused by:Improper separation of the chromosomes non disjunction

Downs Syndrome

Non Disjunction in Sex Chromosome

Turners Syndrome

44 autosomes & 1 sex chromosomes 44 + XOCharacteristics:Shows growth retardation short stature & webbed neckSterile & lacks of normal female secondary sex characteristics

Klinefelters Syndrome

44 autosomes, 2 X & 1Y44 +XXYCharacteristics:Undeveloped testesSterileFemale characteristics : enlarged breast, female high-pitched voice, reduced facial & body hair, long hands & legsChromosomal MutationsCrossing over takes place during prophase I of meiosisDuring crossing over, mistakes that happen lead to a major changes in the structure of the structure of the chromosomeMistakes happen through:DeletionInversionDuplicationTranslocation

A segment of the chromosome breaks off & lostThe 2 ends join up to give shorter chromosomeLethalCri-du-chat SyndromeDeletion of part of 5th chromosome

A segment of the chromosome breaks offAnd reattaches itself in inverted positionLead for phenotypic changes (variation)

A segment of chromosome breaks offRejoin with another non-homologous chromosomes

A segment of chromosome copied twice, resulting in an extra length of chromosomeNot as harmful as deletionGENE MUTATIONb)Gene MutationsSickle-cell anaemiaSickle-shaped RBC are brittleThey stick together easily & block flow of bloodInherited two recessive alleles from parentsIf heterozygous : mild anaemia

HaemophiliaDue to genes mutations, instructions for these clotting factor are altered / missingSo clotting factors produce by these mutated genes do not work properly

Albinismmutation in the genes that are responsible for the production of melaninreduced amount /absence of melanin in eyes, skin & hairInherited mutant double recessive gene that do not produce the normal amount of melanin

ENVIRONMENTAL FACTORS OF VARIATIONPhenotype = Genetic Factors +ENVIRONMENTAL FACTORSEnvironmental factors refer to the abiotic factors, such as:SunlightTemperatureNutritionSoil conditions WaterHumidity

Effect of sunlight on plantsAffects the expression of gene for chlorophyll production in plantsUnder strong sunlight, leaves turn green as more chlorophyll is producedIn dark, leaves turn yellow

Effect of sunlight on humansAffects the expression of the gene for melanin productionMelanin : protects body cells from damage by UV radiationMore sunlight more melanin darker

Effect of temperature on fruit fliesAffects the expression of gene for curly wings in fruit fliesThe egg of homozygous curly winged fruit fly hatched at:16C produces an offspring with normal wings25 C produces an offspring with curly wings

Survival of the fittest6.3 IMPORTANCE OF VARIATION IN THE SURVIVAL OF A SPECIES

Industrial Melanism