Key Area 2.5 – Variation and Inheritance
Variation is the differences that occur between individual members of a species. Variation can occur
between individuals of the same animal species or within plant species. Examples include height or
hair colour in humans or carrot length in plants.
During sexual reproduction genetic information from two different parents joins together creating a
new organism that is genetically different from its parents. Sexual reproduction contributes to
variation within a species.
Two types of variation in a species:
1. Discreet or Discontinuous variation – where a characteristic falls into distinct groups.
E.g. Human blood group: Every human can be placed in one of the following blood groups A,
B, AB or O; there is no in between. Other examples include ability to tongue roll, unattached
or attached ear lobes or the presence/absence of a widow’s peak
2. Continuous variation – where a characteristic can have any value in a range. E.g. Height in
humans can have any value in a range. This value can be as accurate as the apparatus you
have available to measure it.
Presenting data
Discrete variation data is normally
presented as a bar graph. Each group is
represented by a single bar with a space
between
Continuous variation is usually displayed
as a histogram. The range is divided into
subsets. There are no gaps between the
bars.
Inheritance
Polygenic inheritance – this is an inherited characteristic that is controlled by many different genes
acting together. This shows continuous variation which include things like height, weight, skin colour
and hand span in humans.
Genes – a single piece of genetic information made from a short section of DNA. A gene carries the
information for a particular characteristic. The gene for one characteristic can exist in different
forms known as alleles. Alleles are given symbols usually in the form of letters from the alphabet.
Phenotype – describes the outward appearance of the organism in words such as brown hair,
blonde hair, blue eyes or green eyes. **PHenotype is the PHysical appearance of the organism**
Genotype – describe the set of genes that the organism possesses. This is written as letters with the
dominant alleles given a capital letter (e.g. H, G, T) and the recessive alleles given a lowercase letter
(e.g. h, g, t). The same letter is used for one characteristic.
Diploid organisms have two copies (alleles) of every gene, one from each parent, and these alleles
can be dominant or recessive. Dominant alleles will always show up in the phenotype of the
organism even if just one copy is present. Recessive alleles will only show up in the phenotype of an
organism if two copies are present in the genotype.
Homozygous
Heterozygous
The two alleles for one characteristic are the
same
e.g. HH or hh
(both dominant or both recessive)
The two alleles for the one characteristic are
different
e.g. Hh
(one dominant and one recessive)
Example
Pea plants – One gene controls the shape of seeds in pea plants. Seeds can be round or wrinkled.
The allele for round seeds is dominant (R) to the recessive allele for
wrinkled seeds (r).
A plant with two round alleles (RR) is described as homozygous and will have round seeds.
A plant with two wrinkled alleles (rr) is also described as homozygous but will have wrinkled seeds.
A plant with one round allele and one wrinkled allele (Rr) is described as heterozygous but its seeds
will be round (because it possesses the dominant round allele).
Monohybrid cross
To investigate patterns of inheritance and find out which alleles are dominant or recessive we can
carry out a cross between two true-breeding organisms which differ in only one way (meaning we
cross one homozygous dominant parent with one homozygous recessive parent) and observe the
phenotypes of the offspring produced. E.g. smooth and wrinkled pea seeds.
Worked example of a monohybrid cross:
Parents
Phenotype
Round seeds
x
Wrinkled seeds
Genotype
RR
x
rr
Gametes
All R
x
All
(<- True breeding)
r
Top tip:
Draw circles
around the
gametes
F1 Generation Punnett square
Male (round)
gametes
Female (wrinkled)
gametes
X
r
r
R
Rr
Rr
R
Rr
Rr
Conclusion: All of the F1 offspring have the
genotype Rr and therefore they all have round
seeds phenotype.
The next stage on the monohybrid cross is to cross two individuals from the F1 Generation:
Parents
Phenotype
Round seeds
x
Round seeds
Genotype
Rr
x
Rr
Gametes
R
x
R
or
r
or
r
F2 Generation Punnett square
Male (round)
gametes
Female (round)
gametes
X
R
r
Conclusion:
R
RR
Rr
r
Rr
rr
There is a genotype ratio of 1RR:2Rr:1rr
There is a phenotype ratio of 3 round : 1 wrinkled
In a monohybrid cross the expected ratio of the F2 generation is 3:1. In reality the expected numbers
are different from the actual numbers. This is because fertilisation is a random process and there
is a considerable element of chance involved against the theory of expected outcomes.
Human Inheritance
Some organisms, such as humans, produce far too offspring and at a much slower rate for us to draw
conclusions about phenotypic ratios and patterns of inheritance. We can instead use a family tree
diagram to trace back particular traits through many generations to observe the laws and patterns
of inheritance.
Family tree
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Males and females are given different symbols. eg. males may be represented by a square
and females by a circle.
Symbols are then shaded or left unshaded depending on the phenotype of the individual.
Individuals that produce offspring together are joined with a horizontal line.
A vertical line joins offspring with their parents.
It is possible to work out the genotypes of certain individuals in a family tree by analysing
those of their parents and siblings.
In this family tree the dominate form of the gene determines brown eye colour; blue eyes is a
recessive gene trait. The different levels of the tree represent each generation (Parent, F1 and F2
generations) and the numbers represent certain individuals (e.g. Individuals 1 and 2 are the original
parents).
Person 1 is a brown eyed male. Person 2 is a blue eyed female.
Because blue eyes is a recessive trait we know that any member with blue eyes has the genotype bb.
Members of this family with brown eyes may have the genotype Bb or BB. To determine which
genotype they have we must look at the genotypes of their parents and/or their offspring. E.g.
Person 4 has brown eyes but one of their children has blue eyes (a recessive trait) and so must have
received a ‘b’ allele from both parents. Also Person 4’s mother has blue eyes and so she must have
receives a recessive allele from her and a dominant allele from her father and so we know that her
genotype is Bb.
Can you determine the phenotypes of any other members of this family?