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Download 2.3 Genetic Variation Assessment Schedule 07
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1 Assessment Schedule AS 90459 Describe genetic variation and change [Bio 2.3] 2007 Question Achievement Achievement with Merit 1a Describes two correct processes [not mutation] Eg Independent assortment Segregation Fertilisation Crossing over / recombination b Describes: Mother as having the recessive trait (red hair) while the father must have the dominant trait (brown hair). Or Children are heterozygous so don’t show the recessive trait Explains two processes correctly: Eg Independent assortment – homologous pairs of chromosomes ‘line up’ in random manner /independently during Meiosis I. This produces, by chance, many different combinations of alleles in the gametes. Segregation – in Meiosis II the separation of the chromatids that have been produced in independent assortment produces gametes that will differ in their combinations of allele. Fetilisation – the joining /fusion of two gametes each with their differing combination of alleles, produces an offspring that has its own unique combination of alleles. Crossing over / recombination – parts of homologous chromosomes may cross over during prophase I of meiosis and recombine. This produces new allele combinations in the gametes. Explains the occurrence of the trait eg Mother has recessive trait and is homozygous recessive ; the father has the dominant trait and is (most likely) homozygous dominant. Children are heterozygous so do not express the trait/red hair [it has ‘jumped’ them] The grandchildren (eg the first grandchild) may have the trait/red hair if the children marry and produce children with a person who carries the recessive allele/ is red haired and the two recessive alleles combine so the trait/red hair is expressed in the phenotype. Achievement with Excellence 2 2a b c Gives both genotypes Bull ppRR Cow PPWW Gives both genotype PpRW and phenotype polled (hornless) and roan. Gives correct cross PpRW x PpRW and gametes PR PW pR pW Cross, gametes, punnet all correct. PR PR PW pR pW PW PPRR PPRW PPRW PPWW PpRR PpRW PpRW PpWW pR pW PpRR PpRW ppRR ppRW PpRW PpWW ppRW ppWW d Gives correct phenotypes and ratio mainly correct. Both phenotypes and ratio correct. 6 hornless roan : 3 hornless red : 3 hornless white : 1 horned red : 2 horned roan : 1 horned white. e Describes a test cross eg use a horned / recessive animal to breed with the suspect individual presence of horned / recessive offspring shows that the parent cannot be used for breeding Explains a test cross : as for Discusses a test cross : Achieved but includes Punnets to as for Merit but includes show the difference in offspring from an explanation of the a parent that is homozygous compared need to do multiple to one that is heterozgous for horns breedings if none of eg. the offspring have Parent homozygous : PP x pp the recessive trait as the expected 50% p p of the offspring with P Pp Pp horns may not P Pp Pp happen as it is a Offspring all have dominant /hornless chance event. Large trait. Parent suitable for breeding. numbers of offspring are needed Parent heterozygous : Pp x pp to be certain that the parent is homozygous. p p Recognises that the P Pp Pp colour is not p pp pp important in the cross ie the Offspring expected in the ratio of 1 : 1 individual tested dominant / hornless : recessive with needs only to /horned trait. be recessive for Parent not suitable for breeding. horns [not white as well]. 3 f 3a b Describes /defines inbreeding eg Inbreeding involves the breeding together of closely related individuals and an advantage eg Inbreeding increases the chances of getting favourable alleles / traits in the offspring Or a disadvantage eg Inbreeding also increases the chances of getting unfavourable traits /alleles in the offspring Description of affects on gene pool due to geographical isolation eg Genetic differences between populations accumulate/gene pools differ over time as a result of random mutations/genetic drift/natural selection/sexual reproduction. Description of process and consequence eg Bottleneck/ a significant reduction in the size of the gene population that may decrease the size of the gene pool. AND Reduced genetic diversity/population more likely to go extinct (even if numbers increase). As for Achieved with both an advantage and a disadvantage explained eg The more closely related individuals are, the more alleles they have in common. This means that their offspring have a greater chance of inheriting these favourable alleles and so having desirable phenotypes. However, individuals tend to have harmful recessive alleles in their genotype too. By breeding closely related individuals the chances of these recessive alleles coming together in the offspring and so being expressed is increased. The offspring therefore have both increased chances of getting not only favourable but unfavourable phenotypes /traits. Explanation relates changes to gene pools to changes in allele frequencies within each population as a consequence of the different environmental pressures in each area or no gene flow during ice age. As for Merit but includes the effect on genetic diversity eg Inbreeding reduces genetic biodiversity as the frequency of favourable [and unfavourable] alleles increases / dominates /becomes fixed in the gene pool and suggests breeding with unrelated individuals /outbreeding as a means of reducing the chances of harmful recessives coming together and increasing genetic diversity as new combinations of alleles are produced [so increasing variation in the gene pool]. Explanation of why genetic diversity is important to a population eg Discussion of why programs need to increase genetic diversity of Maui population or how conservation measures could achieve this goal eg Genetic variation is important for the survival of the population if the environment changes. OR Genetically similar individuals who reproduce have a higher probability of producing weaker offspring because they carry similar recessive/unfavourable alleles. 2 Reasons how/why given from either Merit OR Cross breeding dolphins from different gene pools/ screening for genetic diversity/ transporting dolphins to different protected locations around the country/ introduce new alleles 4 Sufficiency Statement Achievement 10 Achievement opportunities Achievement with Merit 7 Merit opportunities. Achievement with Excellence 3 Excellence opportunities. 5A including Q3 Achievement plus Merit in Q3 and 2 other merits Merit plus 1 Excellence Describe Genetic Variation Mock Exam Feedback A: General Handwriting, some of you need to take care with your handwriting as it was very hard to read. Spelling , especially the year 12 bio vocabulary, learn to spell them Don’t write “etc” write in full Some planned their answers and were not full of repetition B: Questions Feedback Q1 a] To get merit these definitions needed to include HOW. Q 1b] This is a cross so use/draw punnet squares! Q 2 a, b, c .d Generally well answered. The people that can’t do a dihybrid cross need to ask how to do one. A dihybrid cross is important Q 2e No merits were given if you did not show punnet squares or similar. Test crosses will always take time more than one generation to cancel out chance. Generally well answered. Q 3a For merit you needed to mention allele frequencies. Traits / alleles are not the same thing. Understanding of natural selection, mutations, genetic drift needs to improve. Changes in the environment and no gene alleles migration/emigration flow are key points. Q3b Bottleneck definition was generally well understood by some. Small population = small gene pool= limited alleles= less variation = more at risk to changes in environment and higher probability of unfavourable alleles being expressed. Just because increase population doesn’t mean more varied alleles= need to bring in new alleles