Download 2.3 Genetic Variation Assessment Schedule 07

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