Variation and Inheritance
Notes
Variation
You studied this
in 3rd year
 A characteristic shows discrete variation if it can be used to divide up the
members of a species into two or more distinct groups
 Humans can be split into two groups depending on their ability to roll
their tongue and into four groups based on blood group types A, B, AB
and O
 Data obtained from a survey of a characteristic that shows discrete
variation is represented by a bar chart
National 5
work
 Some characteristics are controlled by the alleles of a single gene – they
are expressed as clear-cut phenotypic groups showing discrete variation
 In humans, the ability or inability to roll the tongue is an example of
single gene inheritance
 In pea plants, the possession of lilac or white flowers is an example of
single gene inheritance
You studied this
in 3rd year
 A characteristic shows continuous variation when it varies amongst the
members of a species in a smooth, continuous way from one extreme to
another and does not fall into distinct groups
 Continuous variation can be represented by a normal distribution curve
 Some characteristics are controlled by the alleles of several genes
National 5
work
 This results in the characteristic being expressed as a range of
phenotypes e.g. Human height
 A characteristic controlled in this way by more than one gene is said to
show polygenic inheritance
Phenotypes and Dominant Genes
You studied
this in 3rd year
 For every characteristic we have 2 genes- one from our mother the
other from our father.
 Genes are part of chromosomes
 Each characteristic is controlled by two form of a gene
 Each parent contributes one form of the gene.
 Each gamete (sex cell) carries one of these two form of the gene.
 Differing forms of a gene are called ALLELES.
 Example - The alleles for the gene for eye colour are blue, green ,brown
etc
 PHENOTYPE - is the physical appearance results from the inherited
information
 Example - Someone with blue eyes has the phenotype blue eyes
 Genes or alleles can be said to be DOMINANT (shows up in the
phenotype) or RECESSIVE (hidden when it is present along with the
dominant gene).
 GENOTYPE is the combination of genes in a gene pair
 Genotype is represented by 2 letters (one letter for each gene)
National
4/5 work
 BB has the phenotype black it is said to have a HOMOZYGOUS genotype
 (Homozygous is often called ‘pure breed’ or true breeding)
 Bb has the phenotype black but is said to have a HETEROZYGOUS
genotype
 bb has the phenotype white and is said to be HOMOZYGOUS recessive.
Genetic Crosses
A genetic cross is laid out as follows:


1.
2.
3.
4.
National
4/5 work
Example
A pea plant which produces round pea seed is crossed with a pea plant
which produces wrinkled peas seeds.
All the offspring are round.
Which is the dominant allele of the gene?
What is the genotype and phenotype of the parents and F1.
What is the genotype and phenotype of the F2?
What is the ratio of phenotype in F2?
 The phenotype ratio when 2 heterozygous individuals cross is always 3:1
 The actual ratio may differ from the expected ratio since fertilisation is a
random process. An element of chance is involved.
Family Trees
You studied
this in 3rd year
A family tree can be used to show the links between all the members of a
family
Example
 It is possible to work out the genotypes of the individuals in a family
tree. Always start with the individuals who show the recessive
phenotype.