6[1].2 form 5
DESCRIPTION
TRANSCRIPT
BIOLOGY FORM 5
LEARNING OUTCOMES• To state the factors causing variation;• To explain the effects of genetic factors
on variation;• To explain the effects of environmental
factors on variation;• To explain the effects of interaction
between genetic factors & environmental factors on variation;
• To explain mutation;• To explain the importance of variation
in the survival of a species.
CAUSES OF VARIATION
• Both genetic & environmental factors.
• Phenotype = genetic factors + environmental factors
CAUSES OF VARIATION
Genetic factors
Environmental factors
Sexual reproducti
onMutation
Independent
assortment
Crossing
over
Random fertilisat
ionGene
mutation
Chromosomal
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.
• Refer to every factor which influences the organism from the outside.
• Examples : nutrients, temperature, pH, water, sunlight, & other stimuli.
The effects of environmental factors
Nutrients• 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 mutationb)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.
DELETION
• 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 (-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
MUTAGENS•Agents that induce mutations.
•Individuals who have undergone mutation MUTANTS
•Divided into 2 = radiation & chemicals
RADIATION•Ionising mutagens : X-rays,
cosmic rays, & -, -, -radiation from radioactive isotopes.
•Non-ionosing mutagens : Ultraviolet radiation
CHEMICALS• Nitrous acid, colchicine, mustard
gas & cigarette smoke.
• Nitrous acid alters a nitrogenous base in the DNA molecule.
• Colchicine prevents spindle formation in mitosis doubles the chromosome number in a daughter cell.
IMPORTANCE OF VARIATION
•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).