f5-6.ppt

CHAPTER 6VARIATION

Variation Refers to the differences exist between living organisms as a results of genotypes & environment

Eg of variations in humans ~ height, weigth, blood group, tongue rolling, eye colour & sex

Types of variations2 types :

Continuous variationDiscontinuous variation

Continuous variation

differences in a character are not distinctCharacter are determined by many genes / environment influences

Eg : heights, weight, body mass, hand spanForms a continuous range of intermediate phenotypes between 2 extremesUsually quantitative (character cn be measured)Characters are controlled by a large no. of genesCombines the effects of genes & environment

Discontinuous variationThe differences are discrete (separate)Not influence by environment factorsCharacters are determined by different allelesAn individual either has the characteristic or noQualitative (cannot be measured)Eg : blood group, gender, tongue rolling, ear lobe, thumb hyperextention

Discontinuous characters are rare in humans & animals but more common in plants

Discontinuous characters in plants :

FlowersSeed colours

Comparison between continuous variation & discontinuous variation

Similarities Create varieties

Are caused by environment factors or genetic factors / both

Differences

Continuous Discontinuous Differences in a character are not distinctiveDifferences in a character are distinctiveQuantitative Qualitative With intermediate charactersNo intermediate charactersNormal distribution Discrete distribution

Are controlled by large no. of genes Controlled by 1 gene / small no. of genesInfluenced by environment Not influenced by environmentPhenotype is controlled by many pairs of alleles Phenotype is controlled by a pair of alleles

Causes of variationsResults of :

Genetic factorsEnvironmental factorsInteractions between genetic factors & environmental factors

Phenotype = genetic factors + environmental factors

(observable (complete set characteristics) of genes)

Causes of variationGenetic factors Environmental factorsSexual reproduction Mutation Gene mutation Random fertilisation Independent assortmentCrossing overChromosomal mutation

Genetic variationsSexual reproduction & mutation are 2 sources of variation

There are 3 sources of genetic variations in sexual reproduction:

Crossing over (meiosis prophase I)Independent assortment (meiosis metaphase I)Random fertilisation

a) Crossing over (meiosis)Prophase I, 2 homologous chromosomes are paired up in a bivalent, crossing over occurs between the chromatids

Exchange of genetic materials between chromatids :

New genetic combinations results in variations

b) Independent assortment (methaphase I)Homologous chromosomes arrange themselves randomly at metaphase plate

Produces new combinations of genetic info.

c) Random fertilisationFertilisation between male gamete & female gamete occurs randomly

Each gamete has a unique set of genes. A male gamete can fertilise any of the female gametes

As a results, each zygote is unique, variations occurs

Variations caused by mutationMutation spontaneous change in the DNA of a cell (permanent & irreversible)

Can occur in somatic cells & gamete

Mutation in somatic cells cannot be inherited

Mutation in gametes inherited fr generation to generation

2 types of mutation :

Chromosomal mutationGene mutation

a) Chromosomal mutation4 different types :

DeletionInversionTranslocationDuplication

1. Deletion Occurs when one end of a chromosome is break off

The 2 ends rejoin to form a chromosome deficient in some genes

2. Inversion When a segment of a chromosome breaks loose, rotates 180 & rejoins the chromosome in a reverse direction

No loss of genetic material but a reverse sequence of genes altered gene activity

3. Translocation Occurs when a segment of a chromosome breaks off & joins a non-homologous chromosome

4. Duplication Occurs when a broken segment of a chromosome joins with the other chromosome of the homologous pair

Resultant chromosome therefore contains duplicated genes

Non-disjunction

another type of chromosome abnormalityAddition @ loss of one or more chromosomesOccurs when pairs of chromosomes fail to separate during meiosis

Results : some gametes get both chromosomes while others receive none

Non-disjunction of sex chromosomes results in :

Sperm cell either have X &Y chromosome or have no sex chromsomeEgg cell either have 2 X chromosomes or have no sex chromosomes

Eg of chromosomal mutationDown syndrome

Occurs when a normal sperm fertilises an ovum with extra chromosome 21

Result :zygote with 3 copies of chromosome 21 (total = 47 chromosomes)

Mitosis ~ all the new cells in developing embryo have 47 chromosomes

Risk with the increase in the age of mother

Learning disabilities, have heart defects, short & broad face, slanted eyes, low immunity towards diseases & mental retardation

2. Klinefelters syndromeXXY

47 chromosomes (44 + XXY)

Are men that show mixed secondary sexual characteristics (partial breast development, broadening of hips, small testes)

Infertile (low sperm count)

3. Turners syndrome 45 chromosomes (44 + XO)

Hormones deficiency prevent XO females fr menstruating @ develop secondary sexual characteristics

Infertile, short stature, thick neck, high risk of cardiovascular diseases, hearing loss

Display X-linked recessive disorders (colour blindness)

b) Gene mutation Change in the base sequence of DNA

5 different types :

Duplication AdditionDeletion InversionSubstitution

i) Duplication When a portion of nucleotide chain is repeated

ii) Addition Occurs when an extra nucleotide sequence is inserted into the chain (insertion)

iii) Deletion Occurs when a portion of nucleotide chain is removed from the sequence

iv) Inversion Occurs when a nucleotide sequence separates from the chain, then rejoins its original position but in an inverted manner

v) Substitution Results when one of the nucleotides is replaced by another which has a different nitrogenous base

Eg. of gene mutation Sickle-cell anaemiaMutation of allele responsible for the production of haemoglobin

Abnormal haemoglobin will crystallizes & binds together when O2 levels are low

Haemoglobin crystals causes the blood cells to change into sickle / cresent shapes

2. Albinism Gene mutation of autosomal recessive allele

Albinos have white hair, pink eyes & light-coloured skin

Do not have enzyme for the production of melanin (skin pigment)

3. Cystic fibrosisInherited disease ~ body produce excessively thick & sticky mucus that closes the lungs & pancreas :

Impairing breathingImpairing digestion

Produce salty sweat body loses large amounts of salt & upset mineral balances in blood

Respiratory failure ~ causes death

Mutagen Agents that induce mutationsMutants ~ individuals that undergone mutations

2 types of mutagens :

Radiation ionising mutagens (ultraviolet) & non-ionising mutagens (X-rays, cosmic rays, -, -, )Chemicals (nitrous acid, colchicine, mustard gas & cigarette smoke)

Environmental factors

Includes nutrients, temperature, pH & light

Nutrients

Form of food eaten & types of mineral elements influence growthIdentical twins may differ in size & ability to fight off diseases (different diet)

Temperature

Affect colour forms of Chinese primrose

Light

Darkening of human skin when exposed to light shows a change of phenotype due to environmental factor

pH

Aluminium sulphate & sulphur added to the soil to make it acidicCalcium carbonate, calcium oxide, magnesium oxide & magnesium carbonate added to soil to make it alkaline

Importance of variation in organisms

Variable traits improve an individuals chance of survival & success rate in reproduction

Identical individuals (clones) survives well when conditions are favourable but may be affected when a new disease comes along

Diversity in organisms will not be possible without variations