Transcript Variation

WILF
To be able to explain that
• Variation between individuals may be either
continuous or discontinuous and what this
means
• Similarities and differences between
individuals within a species may be the
result of genetic factors, differences in
environmental factors, or a combination of
both.
Variation
• When organisms reproduce, their offspring
are of the same species and they are all
similar to one another and their parents.
• Variation may be due to:
– Genetic differences
– Environment
– Both
• Why does variation occur?
• Blending theory……
Discontinuous Variation
Sometimes the characteristic has just a few discrete
categories (like blood group). The frequency
histogram has separate bars (or sometimes peaks).
.
This is discontinuous variation. The characteristics:
•have distinct categories into which individuals can
be placed
•tend to be qualitative, with no overlap between
categories
•are controlled by one gene, or a small number of
genes
•are largely unaffected by the environment
Discontinuous characteristics are rare in humans and
other animals, but are more common in plants.
Think of some examples
Shell colouration
in the land snail,
Cepaea nemoralis
Tongue
rolling
Hitchhiker’s
thumb
Attached or
unattached
earlobes
• There are clear cut differences between
some characteristics
• The differences between Mendel’s peas
are good examples of clear cut differences
Number of plants
Height in pea plants is an example of
discontinuous variation – they are
either tall or dwarf
Although there is some variation in both
categories, there is no overlap –
individuals are either tall or dwarf and
fall into one category or another
height
Continuous variation
Much of the variation that occurs within a
species is to with height, mass, size or
shape. Characteristics that do not fall
easily into groups.
This variation is usually controlled by
several genes, each if which has an
effect.
It is difficult to measure the individual
contribution of each gene, especially as
the environment has an influence on
this type of variation
There are no separate values but a
continuous distribution of values
Inheritance and environment
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discontinuous
variation
Discontinuous variation shows that:
• a single gene gives the characteristic
• and the gene is operating with no
environmental effects.
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continuous
variation
Continuous variation shows that:
• either many genes give the characteristic
• or the gene or genes are operating with
environmental effects.
Questions
• Page 128 questions 1a & 3
Answers…
1a i) continuous (1)
ii) many genes or polygenes or more than one gene (1)
3a continuous (1)
b i) difference in mean of parents or significant
difference in length of parents mean cob length of F1
generation halfway between parental lengths, more
variation in F2 than F1 (2)
ii) variation present in parental or F1 generation plants
in parental or F1 generation genetically identical (2)
c mean halfway between means for Tom Thumb and F1
or between 9 and 11 cm cob length (1)
Homework
• Page 120 – 121 questions 1, 2, 3 & 4
Everything I fully understand
Things I still feel unsure
about
Information I must remember
Ideas to help me remember
information