LE AR N IN G O U TC O M E S• T st t t fa or ca v r t o ae he ct s using aiaion;• T expl in t effect ofgenet fa or on v r t o a he s ic ct s aiaion;• T expl in t effect ofenv onment lfa or on v r t o a he s ir a ct s aiaion;• T expl in t effect ofint a ion bet een genet fa or & o a he s er ct w ic ct s env onment lfa or on v r t ir a ct s aiaion;• T expl in mut t o a aion;• T expl in t impora ofv r t in t sur iv lofaspecies. o a he t nce aiaion he v a
C A US E S OF V A R I A T I ON• Both genetic & environmental factors.• Phenotype = genetic factors + environmental factors
C A US E S OF V A R I A T I ON Genetic Environmenta factors l factors S exual MutationreproductionIndependent Crossing Randomassortment over fertilisation Gene Chromosomal mutation mutation
The effects of genetic factors• CROSSING OVER – An exchange of portion of chromatids between homologous chromosomes. – During Prophase I. – Point of crossover are called chiasmata. – Enables maternal & paternal alleles to mix.
• INDEPENDENT ASSORTMENT – During Metaphase I the bivalents of homologous chromosomes can line up in any orientation on the equator of the spindle. – The number of possible orientations = the number of pairs of chromosomes. – Human, 23 pairs = 223 (8,388,608 combination) even in the absence of crossing over. – The maternal & paternal chromosomes are mixed up in the gamete.
• RANDOM FERTILISATION –Two gametes fuse to form a zygote –Any of the numerous male gametes can fertilise any of the female gametes every zygote is genetically unique.
The effects of environmental factors• R t ev yfa orw infl efer o er ct hich uences t or nism fr t out he ga om he side.• E mpl : nutient t xa es r s, emper t e, pH, w t , sunl , &ot st i. aur aer ight her imul
N u t r ie n t s• Influence their growth.• Identical twins may differ in size & ability to fight off diseases due to different diets.• Lack of certain nutrients can lead to various diseases.
Light• In a few varieties of corn, a red pigment develops only in parts of plant that is exposed to sunlight.• The darkening of human skin when exposed to light shows a change of phenotype due to an environmental factors.
pH• The plant Hydrangea macrophylla has either pink / blue flowers.• Flowers colour is determined by soil pH. (acidic = blue, alkaline = pink)
MUTATION• A spontaneous change in the genetic material (DNA) of a cell or organism.• Two types : a) Chromosomal mutation b) Gene mutation
• May occur in somatic cell or gametes.• Gametes inherited from generation to generation• Somatic cells not passed on to the next generation.
CHROMOSOMAL MUTATION• Occurs when the structure of a whole chromosome / set of chromosomes is altered in some way.• DELETION : when a chromosome breaks in two places & the section in the middle is lost as the part rejoin.
• INVERSION : when a chromosome breaks in two places & the middle piece is reinserted with the sequence of genes reversed.
• TRANSLOCATION : when a section of one chromosome breaks off & becomes attached to another chromosome.• DUPLICATION : when a section of chromosome is copied twice.
• Another kind of chromosome abnormality is caused by the addition / loss of one @ more whole chromosomes non-disjunction.• When pairs of chromosomes fail to separate during meiotic division. some gametes get both chromosomes while others receive none.• Examples of chromosomal mutation : Down’s syndrome, Klinefelter’s syndrome & Turner’s syndrome.
GENE MUTATION• A change in the base sequence of DNA.• ADDITION – When an extra nucleotide sequence is inserted into the chain.• DUPLICATION – When a portion of a nucleotide chain is repeated.
• DELETION (β-thalassaemia) – When a portion of the nucleotide chain is removed from the sequence.• INVERSION – When a nucleotide sequence separates from the chain, then rejoins its original position but in an inverted manner.• SUBSTITUTION (sickle-cell anaemia) – When one of the nucleotides is replaced by another which has a different nitrogenous base.
EXAMPLES OF GENE MUTATION• Sickle-cell anaemia – Incorrect a.a. in haemoglobin. – RBC to become sickle-shape low binding capacity for oxygen die• Cystic fibrosis – Produce excessively thick & sticky mucus closes the lungs & pancreas impaired breathing & digestion – Production of salty sweat upsets the balance of minerals
M U TA G E N S• Ag e n t s t h at i n d u c e mu t at i o n s .• I n d i vi d u al s wh o h ave u n d e r g o n e mu t at i o n MUTANTS• D i vi d e d i n t o 2 = r ad i at i o n & c h e mi c al s
RADIATION• Ionising m utagens : X -rays, cosm ic rays, & α-, β-, γ-rad iation from rad ioactive isotopes.• N on-ionosing m utagens : U ltraviolet rad iation
C H E M IC A L S• N itrous acid , colchicine, m ustard gas & cigarette sm oke.• N itrous acid alters a nitrogenous base in the D N A m olecule.• C olchicine prevents spind le form ation in m itosis d oubles the chrom osom e num ber in a d aughter cell.
IM P O R T A N C E O F V A R IA T IO N• The variable traits improve an individual’s chance of survival & success rate in reproduction.• A better chance that some of the individuals could be resistant to the disease & survive (not like the clone).