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Summary AQA A Level Biology (A2) - Genetics
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Inheritance DefinitionsTerm Definition
Chromosome A complete length of DNA (very long) in
eukaryotes, which is packaged with histones.
Gene A section of DNA which encodes the message
for a protein.
Locus The position that a gene occupies on a
chromosome.
Allele Different forms of the same gene.
Diploid Organism/cells which contains two copies of
each chromosome and therefore two copies of
each gene.
Haploid Cells/gametes that contain one copy of each
chromosome and therefore one copy of each
gene.
Phenotype The set of observable characteristics of an
individual resulting from the interaction of its
genotype with the environment.
Genotype The genetic makeup of an individual/the
combination of alleles that they possess.
Homozygous When the allele on each chromosome are the
same.
Heterozygous When the allele on each chromosome are
different.
Dominant allele When the characteristic of an allele is always
seen in the phenotype (even if only one is
present).
Recessive allele When the characteristic of an allele is only seen
in the phenotype if present on both
chromosomes.
Mendalian Inheritance (Monohybrid Inheritance)
Mendalian inheritance is a type of biological inheritance that conforms to the set of principles of
Gregor Mendel regarding the transmission of genetic features from parent organisms to their
offspring through his scientific and cautious breeding experiments on pea plants.
He found that offspring are not a blend or mixture of the parents characteristics. These
characteristics can be hidden in one generation and can reappear in future generations. There are
discrete units that are being passed down to offspring.
He came to this discovery by crossing two different varieties of pea plants together:
Tall x Dwarf
100% Tall
Tall x Tall
3 x Tall 1 x Dwarf
,Parental phenotype: Tall x Dwarf
Parental genotypes: TT x tt
Gametes: T T t t
Possible offspring: t t
T Tt Tt 100% heterozygous
100% tall phenotype
T Tt Tt
F1 parent: Tt x Tt
Gametes: T t T t
Possible offspring: 50% heterozygous
50% homozygous Genotype 1:2:1
75% tall phenotype 3 tall:1 dwarf
25% dwarf phenotype
Test Cross/Back Cross
This is a test to determine whether something has a set of heteozygous alleles or homozygous
dominant alleles.
If we return to Mendel’s pea plant example, a tall pea plant must be either TT (homozygous
dominant) ot Tt (heterozygous). A dwarf plant must be tt so we know the genotype and phenotype
of a dwarf plant because they are recessive alleles.
If the offspring are all tall, then we know that the parent plant was TT (homozygous dominant)
because:
If the offspring are 50% tall and 50% dwarf, then we know that the parent plant must be Tt
(heterozygous) because:
However the expected
proportions do not always
occur due to CHANCE events
at fertilisation.
, Co-dominance (Monohybrid Inheritance)
This is where there is one gene and two alleles for that gene - as with mendalian inheritance.
However both alleles are dominant. This means that both alleles influence the phenotype. Co-
dominance forms a 1:2:1 ratio in the second generation.
For example, snapdragon’s flower colouring is caused by co-dominant alleles. There are two alleles –
R which causes red colouring and W which causes white colouring.
Parental phenotype: Red x White
Parental genotypes: RR x WW
Gametes: R R W W
Possible offspring: R R
When you see 2 parents with
W RW RW offspring that don’t look like
100% Pink either parent suspect co-
W RW RW dominance.
F1 phenotype: Pink x Pink
F1 genotypes: RW x RW
Gametes: R W R W
Possible offspring: R W 50% Pink In the second generation
RR RW 25% Red there will be a reappearance
R
50% White of the original parental
1:2:1 Ratio phenotype.
W RW WW
Multiple Allele Inheritance (Monohybrid Inheritance)
This is where there is one gene but more than two possible alleles. One key thing to remember is
that diploid organisms can only carry two alleles. This allows for different combinations of alleles.
For example, blood group in humans is caused by multiple alleles. There are 3 alleles: IA (blood group
A), IB (blood group B) and IO (blood group O). IA and IB are co-dominant and IO is recessive. This means
that the following combinations make:
Phenotypes Genotypes
A IAIA IAIO
B IBIB IBIO
A B
AB II
O IOIO
Parental phenotype: AB x B
Parental genotypes: IAIB x IBIO
Gametes: IA IB IB IO
Possible offspring: IA IB
50% B
IB IAIB IBIB
25% A
25% O
IO IAIO IBIO
, Hierachy of Dominance (Monohybrid Inheritance)
This is where there is one gene with multiple alleles – as with multiple allele inheritance- but there is
a hierachy of of dominance. This is where each allele has a different level of dominance compared to
the others so one allele may be more dominant than another, but a different allele may be more
dominant than both of them.
For example, fur colour in rabbits is caused by multiple alleles in a hierachy of dominance. The alleles
for the fur colours are represented as:
Most Dominant
FCh - Chinchilla
FAg - Agouti
FH - Himalayan
a
Least Dominant F - Albino
Parental phenotype: Chinchilla x Agouti
Parental genotypes: FCh FH x FAg FH
Gametes: FCh FH FAg FH
Possible offspring: FCh FH
Ch Ag H Ag 50% Chinchilla
FAg F F F F
25% Agouti
Ch H H H 25% Himalayan
FH F F F F
Sex-Linked Inheritance
This is the inheritance of genes on the X and Y chromosomes. The X chromosome is significantly
larger than the Y chromosome which means there is more DNA so there are more genes. This means
that most known sex linked characteristics are carried on the X chromosome. All mammals will be
50% male and 50% female.
For example, the gene for red/green colourblindness is found on the X chromosome. There are two
alleles – XN which causes normal vision (dominant) and Xn which causes the colourblindness
(recessive).
Parental phenotype: Male normal vision x Female normal vision
Parental genotypes: XN Y x X N Xn
Gametes: XN Y X N Xn
Possible offspring: XN Y 25% Normal female
N N
N
X X X X Y
N
25% Carrier female
25% Normal male
N n n
Xn X X X Y 25% Colour blind male
Males only have 1 allele so it is impossible to be carriers!!!
It is very rare for the phenotype to be shown in females as the only way for the to have it is if both
their father has it and their mother either has it or is a carrier.
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