Variation and Inheritance
Variation is differences between members of the same species (such as humans) It is caused
by the information contained in genes on chromosomes. The study of these traits through
generations is called genetics.
There are 2 main types of variation: Discrete
Continuous
Discrete Variation
Discrete variation describes differences in characteristics that allow individuals to be placed
in two or more distinct groups.
Examples include, blood groups, gender and ability to roll the tongue in humans
Data obtained on such characteristics can be presented in the form of a bar graph as shown
by the blood groups chart below.
Continuous Variation
Characteristics showing continuous variation are those where a population of individuals
show slight differences between and upper and lower extreme.
Examples of continuous variation include, height, mass, width, length etc. These
characteristics can be applied to humans and plants alike.
This means that any data which is given for continuous variation often includes a category
range (1-5kg) and is often presented for easy identification in the form of a bell shaped
histogram as shown below. This allows us to readily identify the most common height and
still take into consideration the extremes.
Genetics terms
Word / Term
Meaning
The characteristics that an organism has.
Phenotype
The genes an organism has for a particular characteristic
Genotype
Dominant
In a heterozygous individual, the gene that is expressed
in the phenotype
Recessive
In a heterozygous individual, the gene that is not
expressed in the phenotype
Alleles
Alternative forms of a gene, e.g. the gen R for tongue
roller and r for non-roller are alleles
Homozygous
Having two identical alleles in the genotype
Heterozygous
Having two different alleles in the genotype
Symbols for genes
A gene is usually represented by a letter, usually the first letter of the characteristic controlled
by the dominant gene
The dominant allele is given a capital letter (e.g. Brown eyes = B)
The RECESSIVE form of the gene, which is always masked in heterozygous individuals and
only shows in individuals who are homozygous, is given the lower case letter (Blue eyes =
b).
This means that 2 different genotypes (BB or Bb) can still provide the same phenotype
(Brown eyes).
We can predict the expected ratio for phenotypes using a punnet square however since
fertilisation is a random process, the ratio obtained in an actual genetics experiment is often
different from the expected ratio.
An example of a punnet square is shown below:
R – dominant gene for round seeds
r – recessive gene for wrinkled seed
Heterozygous round seeds X Heterozygous round seeds
Genotypes
Gametes
Rr
Rr
R and r
R and r
R
r
R
RR
Rr
r
Rr
rr
The expected ratio is 3 round seeds : 1 wrinkled seeds