Variation.pptx

CHAPTER 6 : VARIATION

CHAPTER 6 : VARIATIONVARIATIONPhenotypic differences among the individuals of a species

Types of VariationsThere are 2 types of variations :a)continuous variationb)discontinuous variationContinuous variation is a type of variation in which the differences in a character are not distinct, but forms a continuous range of intermediate phenotype between two extremes.Discontinuous variation is a type of variation in which the differences in a character can be divided into two or a few types of distinct phenotypes, with no intermediate characteristics.

DimplesEar LobeHairComparison between Continuous Variation and Discontinuous VariationSimilaritiesCreate varieties in the population of species.Caused by environmental factors or genetic factors or both. Differences

Continuous variationAspectDiscontinuous variationA type of variation in which the differences in a character are not distinctiveDefinitionA type of variation in which the differences in character are distinctiveHeight and weightExamplesA, B, AB & O blood groupsHave intermediateIntermediate characteristicsNo intermediate Quantitative Can be measured Quantitative or qualitative Qualitative Cannot be measuredInfluenced by environmental factorsEnvironmental factorsIs not influenced by environmental factors2 or more genes control the same characterGenesA single gene determines the differences in the traits of a characterUsually controlled by many pairs of allelesPhenotypeIs controlled by a pair of allelesGraph Distribution

Continuous variationDiscontinuous variation Shows a normal distribution(bell shaped) Shows a discrete distribution7Causes of VariationFactors causing variationa)Genetic factorsb)Environmental factorsc)Both genetic and environmental factorsDiscontinuous variation can be inherited and is caused by genetic factors whereas continuous variation is caused by either genetic or environmental factors or both.

Genetic VariationsIn genetic variation, sexual reproduction and mutation are two sources of variation.Sexual reproductiona)Involves the production and fertilisation of gametes, results in genetic variations in the offspring.b)There are 3 sources of genetic variations in sexual reproduction :Crossing over during meiosis IIndependent assortment of chromosomesRandom fertilisationCrossing over during meiosisa)During prophase I of meiosis, when two homologous chromosomes are paired up in a bivalent, crossing over occurs between the chromatidsb)The exchange of genetic materials between the chromatids result in new, different genetic combinations result in variations.

Independent assortment of chromosomesa)At metaphase I of meiosis, the homologous pairs of chromosomes, each consisting of one maternal and one paternal chromosome, are arranged on the metaphase plate at random.b)Each homologous pair of chromosome is positioned relative to the poles of the cell independent of the other pairs.c)Hence, at the first meiotic division, there is an independent assortment of maternal and paternal chromosomes into daughter cells. This results in a variety of gametes.Random fertilisationa)The random nature of fertilisation of gametes from different parents is another cause for genetic variations.b)Gametes with diverse combination of homologous chromosomes are fused together to form a zygote with an even greater variety of gene combinations.MutationMutation is a sudden random change in the genetic composition of a cell.Can involve a single nucleotide pair or a large region of a chromosomeRarely occurs but if and it does, it is random and spontaneous , and can lead to changes in human characteristicsPermanent and irreversibleCan occur in somatic cells or in gametes

Can be caused by chemical or physical agents (mutagens), mistakes that occur during DNA replication and uneven distribution of chromosomes during cell division

DrugsCaffeinePesticidesThere are 2 types of mutation :a)Gene mutationb)Chromosomal mutation Gene mutation a)Involves a chemical change in the structure of geneb)Can be due to the substitution, deletion or insertion of a nucleotide basec)It causes genetic diseases such as sickle-cell anaemia, albinism and haemophilia

AlbinismChromosomal mutation a)Involves changes in the structure of chromosomes or number of chromosomes during meiosis.b)Can be due to deletion, duplication, inversion and translocation during crossing over in meiosisDeletion : A fragment of chromosome breaks off and is lostDuplication : A fragment of chromosome is doubled or repeated . The new chromosome has extra genetic material. Inversion : A fragment of chromosome is removed. The fragment rotates through 180oTranslocation : A fragment of chromosome breaks off and then attaches to another chromosome

c)Changes in the number of chromosomes occur when homologous chromosome fail to separate during meiosisd)There is either an addition or loss of chromosomes in the gametese)Chromosomal mutation due to changes in the number of chromosome causes genetic diseases such as Downs syndrome, Turners syndrome and Klinefelters syndrome

Downs syndromeEnvironmetal VariationsContinuous variation is greatly influenced by the environment.Environmental factors that cause variations includes temperature, light intensity, humidity, nutrition and soil fertility.Different environmental factors give rise to different variations in plants. For example, the flowering plant Hydrangea sp. produces blue flowers when the soil is acidic but pink flowers when the soil is alkaline.

Both Genetic and Environmental FactorsThe traits of an organism are controlled by alleles. Some traits are controlled by a single pair of alleles and some by several pairs of alleles.The traits controlled by several pair of alleles are influenced by the environment resulting in continuous variation.For example, a person may inherit a mixture of tall and short genes, leading to average height. However, this height is also affected by the environmental factors like nutrition, causing further change to height.Importance of Variations in the Survival of a SpeciesProvide the necessary adaptations for them to survive in an ever changing environment.Essential to the survival of species

Respecting Others in Spite of VariationsEach individual is unique because he or she has a unique set of genesWhatever differences may be, we still belong to the same speciesWe have to accept these variations as part of Gods creation and thank God that each one of us is uniquely and differently made