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PAPER 2 BIOLOGY
Unit 7 bio
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Genotype
The
genetic constitution
of an organism
Phenotype
The expression of the genes but also the
interaction
with the environment
Homozygous
When you have a pair of
homologous
chromosomes carrying the
same
alleles for a single gene
Heterozygous
When you have the homologous chromosomes carrying two
different alleles
for a
single
gene
Recessive allele
Only expressed if there's
no dominant
allele present
Dominant allele
Always expressed
Codominant
Both alleles are equally dominant and expressed in the
phenotype
Multiple
alleles
More than
two
alleles for a single
gene
Sex linkage
The gene whose
locus
is on the
X
chromosome
Autosomal linkage
Genes located on the same
chromosome
, not the
sex
chromosomes
Epistasis
One
gene
modifies or masks the expression of a different gene at a
locus
Monohybrid
Inheritance of just
one
gene
Dihybrid
Inheritance of
two
genes at a time
A
genetic
coding table is provided to help with different types of
inheritance
Monohybrid example - Cystic fibrosis
Cystic fibrosis is caused by a
recessive
allele
Parents are
heterozygous
Multiple alleles and codominance example - Blood groups
Blood group
A
has dominant
A
allele or recessive
O
allele
Blood group
B
has dominant
B
allele or recessive
O
allele
Blood group
AB
has both
dominant
A and B alleles
Blood group
O
has two recessive
O
alleles
Sex linkage example - Colour blindness
Colour blindness is caused by a
recessive
allele on the
X
chromosome
Non-colour blind
male
reproduces with
female
carrier
Epistasis example - Labrador coat colour
Gene 1
controls whether pigment will be produced (dominant allele) or not (recessive allele)
Gene 2
controls the colour of the pigment (dominant black, recessive brown)
Dihybrid cross
1. Consider
inheritance
of two genes at the same time
2. Need parental
phenotypes
and
genotypes
3. Work out
gametes
4.
Punnett
square to get offspring genotypes and phenotypes
5. Determine the
ratio
Autosomal linkage
Two genes
located on the same chromosome, not
sex
chromosomes
Crossing over results in new combinations of
alleles
in the
gametes
, affecting the predicted ratio
Crossing over
Results in new combinations of
alleles
in the
gametes
, so the predicted gametes in the Punnett square may differ
Autosomal linkage
Two
genes are located on the same
chromosome
, but not the X or Y chromosome
Autosomal linkage
1.
Alleles
for each gene are linked on the same
chromosome
2. Have to be
inherited
together
3. Whole chromosome pulled to create one
gamete
4. Other chromosome pulled to create other
gamete
Autosomal linkage
Only
two
types of
gametes
possible: dominant alleles or recessive alleles
Autosomal linkage
Results in a
3
:
1
ratio instead of 9:3:3:1
Crossing over
Creates new
combinations
of gametes, resulting in more than
2
phenotypes
Chi-squared
Statistic used to investigate
differences
between expected and observed
frequencies
Using chi-squared
1. State
null hypothesis
2. Convert
ratio to expected frequency
3. Calculate
chi-squared value
4. Compare to
critical value
5. Determine if
significant
difference
Hardy-Weinberg
principle
Mathematical model to predict
allele frequencies
in a population
Gene pool
All the
alleles
of all the
genes
within a population at one time
Population
All the individuals of one
species
in one
area
at one time
Adult frequency
Proportion of an
allele
within a
gene pool
p
Frequency of the
dominant
allele
q
Frequency of the
recessive
allele
p^2
Frequency of the
homozygous dominant
genotype
2pq
Frequency of the
heterozygous
genotype
q^2
Frequency of the
homozygous recessive genotype
Genetic variation is due to
mutations
, random
fertilization
, and meiotic processes
Natural selection
Predation
Disease
Competition
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