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).