level 2 bio genetics 2010

Level 2 Bio Genetics 2010Feedback to students

OVERALL• Very poor attempt by many students. • LOTS OF REVISION NEEDED!• Often you must answer all parts of a question

correctly to gain the achieved grade for that question.

OVERALL• Q1 generally completed well but without

getting an achieved or higher in a question relating to gene pool you will not pass the external exam. You MUST show knowledge across all areas of the standard to pass (i.e. mendelian genetics AND population genetics)

YOU MUST LEARN DEFINITIONS!!!

• Writing just the names of the terms should be done only when the question states IDENTIFY or NAME.

• DESCRIBE or DEFINE: means basically “give the definition for the key terms”.

• N.B. If in doubt give the definition of the key words in the question. This is often all that is required for the Achieved grade.

DEFINITIONS YOU MUST KNOW

The following terms were often misused or incorrectly defined in student exam answers:• Allele• Gene• Organism• Species• Population• Community

• Gene pool• Evolutionary process• Genetic Drift• Natural Selection• Test cross• Inbreeding

N.B. There are more definitions that you should know for this genetics topic.

However these are just the ones that were in the particular set of questions used

in this school exam.

QUESTION ONEQuestion Achievement Merit Excellence

ONE

4A or better 2M and 2A or better

1E, 2M and 2A or better

N.B. Each question was made up of parts. Each part was assigned a grade and these grades were then totalled for your overall grade for each question.

QUESTION ONE a)

• Most answered correctly• Required genotype of parents

– NOT their gametes therefore needed 2 B/b alleles AND 2 W/w alleles

Achievement Merit Excellence

Duroc : bbww

Hampshire : BBWW

F1 : BbWw

QUESTION ONE b)

• Again learn your definitions! • Many on the right track but what

was written was not correct.• N.B. pure breeding is a term used to

describe an individual – it is not the mating process!


Pure breeding means that the individual has a homozygous geneotype / can produce only one type of allele

Homozygous parent can pass on only one type of allele (for that trait) to its offspring (there is NO variation in the alleles) so offspring have same alleles and phenotype as parent.

QUESTION ONE c)

• Most did this well• If you did not get an M then you

must get some help to get this right in the future.

• There will definitely be a dihybrid cross to complete in the external.


Gametes correct Whole Punnet Square correct

QUESTION ONE d)

• Many repeated bbWw


One of bbWW; bbWw Both bbWW and bbWw and no other genotype given.

QUESTION ONE e)

• Most did this correctly


3/16 or 3 : 15

or correct decimal (0.1875) or 18.75%

QUESTION ONE f)

• Many failed to identify that a TEST CROSS was required


Gives genotype as bbWW and describes breeding suspect pigs with recessive genotype (bbww) pigs i.e. test cross

AND check whether offspring have white bands or are solid red only.

OR Good attempt at Punnet Squares with probabilities and linked in answer

As for Achieved and explains that

•Solid red offspring indicates suspect pig is bbWw; all white banded offspring indicates suspect pig (may be) bbWW.

•if any offspring do not have white band / are solid red (bbww) then adult is heterozygous (bbWw) and not pure breeding for white banded offspring.

As for Merit and includes explanation of the need for several breedings eg if no solid red (bbww) pigs occur in the large numbers of offspring, then the suspect pig is (almost certainly) bbWW so pure breeding.

Answer supported with correct Punnet Squares of crosses (bbWw x bbww and bbWW x bbww) to show the probability of getting white banded and solid red offspring.

QUESTION ONE f) cont • Red is recessive therefore only appears

when there is no B allele present. Punnet squares can therefore be simplified to monohybrid crosses of W/w alleles.

• Many did not read the information provided to recognise the desired pure breeding phenotype for this question was red with a white band.

QUESTION ONE f) cont • Many failed to mention that lots of

crosses would be needed to make “almost certain” that the unknown individual was bbWW

• Many did not say that if only one offspring of the test cross had recessive “no band” phenotype then the unknown parent was heterozygous so should be kulled as not pure breeding.

QUESTION TWOQuestion Achievement Merit Excellence

TWO

2A or better 1M for (c) and 1A or better

1E, 1M and 1A

QUESTION TWO a)

• DESCRIBE does NOT mean “only name”• A Mutation does NOT make a new GENE – it

makes a new allele• Source = where it comes from


Describes TWO of eg

Mutation(s) in DNA creates new alleles.

Independent assortment of homologous chromosomes produces genetically varied gametes.

Crossing over and recombination between non sister chromatids in meiosis produces new allele combinations in gametes.

OR both correctly identified and 1½ reasonable descriptions

QUESTION TWO b)

• Question was flawed as a) was regarding a species and b) was regarding a gene pool hence alteration in italics above.


Mutation with brief description of how it shows in individual or in gene pool

OR

Migration with correct description (as question states gene pool – not species)

Mutation must occur in gamete producing cells [not somatic cells] for it to enter gene pool. If the gamete with the mutation is fertilized and the offspring produced is viable, then the mutation will enter the gene pool of the population.

QUESTION TWO c)

• NS acts on PHENOTYPE (NOT genotype/allele/gene). This leads to increase/decrease of the ALLELES for that phenotype in the next generation


Describes Natural selection as causing :

•mutations that are helpful / favourable to become established in the gene pool / produce more offspring

OR

•mutations that are helpful / unfavourable to not become established in the gene pool

OR

•Correct definitions of NS and links to gene pool e.g. increase in allele frequency of favourable alleles (or vise versa for unfavourable alleles)

Explains ONE of ;

•Selection pressures will cause individuals with favourable alleles to survive and reproduce (increased fitness) so establishing and increasing the frequency of these alleles in the gene pool.

•Selection pressures will cause individuals with unfavourable alleles to not survive / have reduced chance of surviving (reduced fitness) so these alleles will not get established in the gene pool or be present in low frequencies in the gene pool

Discussion explains the role of NS in determining the presence of both favourable and unfavourable alleles in the gene pool linked into a comprehensive answer

QUESTION TWO c) cont • Most answers lacked link to gene

pool• NS is NOT mate selection• NS still occurs even in a stable (i.e.

non-changing) environment

QUESTION THREEQuestion Achievement Merit Excellence

THREE

2A or 1M or better

1M and 1A or better

1E, and 1A

QUESTION THREE a)

• Majority of students did not know what an evolutionary process was.

• Many did not read the question so did not give three processes or only named them and did not give a description.


Any three with correct (or reasonable attempts at) descriptions for each of :

Bottleneck effect

Founder effect

Genetic drift

Mutation

Natural selection

QUESTION THREE a) cont Descriptions for this example include:• Bottleneck effect – eg numbers of

robins were greatly reduced so likely loss / reduction in allele frequency with likely a reduction in genetic diversity.

QUESTION THREE a) cont • Founder effect – eg only 6 breeding

pairs introduced to Ulva so alleles present in their gene pool not liely to be representative of the mainland / ancestral population so likely reduced genetic diversity.

QUESTION THREE a) cont • Genetic drift – loss / reduction in

specific alleles due to chance which is likely to be more pronounced in the small robin population so corresponding [increased] loss of genetic diversity.

• Mutation – changes to genetic code creating new alleles. If occurring in gamete producing cells will enter gene pool as inherited.

QUESTION THREE a) cont • Natural selection – changes in the

environment cause change in selection pressures on robins so favourable alleles will be selected for and increase in frequency in gene pool.

QUESTION THREE b)Achievement Merit Excellence

Describes inbreeding eg

• the breeding of closely related individuals eg siblings, children to parents; grand children to grandparents in the robin population.

and its effect on one evolutionary process eg

•Inbreeding in the small robin population is likely to further reduce the genetic diversity of the population that has occurred in bottleneck effect / founder effect / genetic drift

•Mutations that are favourable will become established / increase in frequency in the gene pool more rapidly with inbreeding.

•Favourable alleles selected for in NS will increase in frequency more rapidly with inbreeding.

Or gives a comprehensive explanation of inbreeding (as per merit column)

Explains inbreeding eg

•The small size of the population increases the occurrence of inbreeding occurring by chance so that the degree of relatedness between all members of the population is high. This increases the chances of harmful recessive alleles coming together in any individual so reducing its fitness.

and explains its effect on one evolutionary process eg

•Inbreeding will reduce the genetic diversity of the small population that has already occurred in bottleneck effect / founder effect / genetic drift as the breeding of closely related individuals increases the chances of the loss of alleles from the gene pool.

•Closely related individuals have alleles in common. A mutation that creates a new allele will be more common in related individuals. Inbreeding will therefore increase the chances of a mutation spreading through the population and so becoming established / increasing in frequency in small gene pool.

•Favourable alleles are selected for so increases in frequency in the gene pool. Inbreeding will increase the spread of these favourable alleles through the small gene pool so further increasing their frequency.

Explains inbreeding and its effect on three evolutionary processes linked to the decrease of diversity in the gene pool and the rapid increase in frequency of favourable alleles in the gene pool.

QUESTION THREE b) cont • Definition of inbreeding often too basic

or poorly worded. Many did not recognise that the main problem with inbreeding is that it increases the chances of (harmful) recessive alleles coming together in offspring and being expressed thus reducing the fitness of the individual.

• Others, because of lack of recall on evolutionary processes, failed to achieve second part of question.

QUESTION ONE

• Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Vivamus et magna. Fusce sed sem sed magna suscipit egestas.

• Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Vivamus et magna. Fusce sed sem sed magna suscipit egestas.

JUDGEMENT STATEMENTAchievement Achievement with

MeritAchievement with

Excellence

Two questions answered correctly. Minimum of 2 x A.

All questions answered correctly including TWO at Merit level.Minimum 2 x M + 1 x A

All questions answered correctly including TWO at Excellence level.Minimum 2 x E + 1 x A

REMEMBER:• READ THE INFORMATION PROVIDED• READ THE QUESTION – underline the key

terms• ATTEMPT ALL PARTS OF ALL QUESTIONS! If

you are not sure then simply put the definitions of the key terms.

QUESTION ONE

Achievement Achievement with Merit Achievement with Excellence

Two questions answered correctly. Minimum of 2 x A.

All questions answered correctly including TWO at Merit level.Minimum 2 x M + 1 x A

All questions answered correctly including TWO at Excellence level.Minimum 2 x E + 1 x A

level 2 bio genetics 2010

Documents