Similarities or Differences?
Norfolk Marino
Cheviot Dorset
1
Note: Any page numbers
refer to Hodder 2016
OCR endorsed Biology A
level Textbook
Tuesday, 12 February 2019
Chapter 10
Patterns of inheritance
17.1a Phenotypic and Genotypic Variation
Objectives:
• Describe the differences between continuous and discontinuous
variation
• Describe how environment affects the expression of the genotype
• Explain how genetic variation is a consequence of sexual
reproduction
2
Note: Pages 160-161 of
Hodder 2016 OCR
endorsed A level
BiologyTextbook
•
Explain the difference between discontinuous and continuous variation
3
Phenotypic variation
• Phenotypic variation can
be either continuous or
discontinuous
• The difference between
the two is a result of the
number of genes
involved in determining
a characteristic and the
expression of the alleles
of those genes.
4
Continuous variation
Heights of pupils in class
• A range of variables
between two
extremes is possible
• Examples in nature
include: height,
weight, length, size
• Data is normally
presented in a Normal
Distribution bell• In continuous variation every
shaped curve
person differs slightly, most
people falling around a mean
value
5
Discontinuous variation
Number of Tongue rollers
• Variables are assigned
to clearly defined
discrete categories or
distinct groups
• Examples include:
gender, blood group,
shoe size
• Data usually presented • In discontinuous or categoric
variation the characteristic in
as a bar graph
question falls into a clear
category eg. ability to roll
tongue
6
Complete the table using the
statements on the next slide
Feature
Discontinous variation
Continuous variation
Definition
Gene locus
Number of alleles
Effect on phenotype
Environmental
influence
Example
7
Use these statements to complete
the table
Usually only one locus
Environmental factors
have a significant
effect
Environment has little
influence
Many genes
contribute to
inheritance of
characteristic, each
with its own alleles
Human blood groups,
ability to roll tongue
Features cannot be
measured across a
complete range but
rather form
categories
Features can be
measured across a
complete range
between two
extremes
Height in humans,
milk yield in cows
The characteristic is
either present or
absent
The characteristic has
many intermediates
between the two
extremes
Many loci and these
may be on different
chromosomes
Often just a single
pair of alleles
8
Answers…
Feature
Definition
Gene locus
Number of alleles
Effect on phenotype
Discontinous variation
Features cannot be measured
across a complete range but
rather form categories
Usually only one locus
Often just a single pair of alleles
The characteristic is either
present or absent
Continuous variation
Features can be measured
across a complete range
between two extremes
Many loci and these may be on
different chromosomes
Many genes contribute to
inheritance of characteristic,
each with its own alleles
The characteristic has many
intermediates between the two
extremes
Environmental
influence
Environment has little influence
Environmental factors have a
significant effect
Example
Human blood groups, ability to
roll tongue
Height in humans, milk yield in
cows
9
Note: Pages 160-162 of
Hodder 2016 OCR
endorsed A level
BiologyTextbook
•
Explain how the environment affects the expression of genotype
10
Environmental Factors
•
•
•
A number of factors affect
expression of the genotype &
therefore the phenotype
For example, a healthy balanced
diet will result in a child growing
to their full potential
A poor diet will limit growth due
to a lack of essential nutrients
Mary and Alice are twins. At the age of nine years Mary weighs 14
pounds more than her sister because Alice omitted milk and cereal from
her diet; Mary has also outstripped Alice in height.
11
Environmental Factors
•
•
Plants kept in the dark
are deficient in
magnesium and
become yellow as they
are unable to synthesise
chlorophyll, this is
known as chlorosis
Seedlings grown in the
dark are chlorotic and
also have long stems
with small curved
yellow leaves, this is
known as etiolation
12
Note: Pages 160-162 of
Hodder 2016 OCR
endorsed A level
BiologyTextbook
• Explain how genetic variation is a consequence of sexual reproduction
13
Genetic variation
•
•
•
Genetic variation exists due to the presence of different forms of
a gene, that is, the presence of different alleles
Sexual reproduction increases variation when DNA from two
organisms are combined
Only parents of the same species can combine their DNA during
fertilisation to produce viable offspring that are fertile
14
Meiosis generates variation
Meiosis creates variation by:
•
Crossing over between maternal
and paternal chromosomes
(meiosis I) results in an
exchange of alleles between
the chromatids of
homologous
chromosomes
•
Independent assortment of
maternal and paternal
chromosomes in a
homologous pair (meiosis I)
15
Meiosis generates variation
Meiosis creates variation by:
• Independent segregation between sister chromatids (meiosis II)
•
•
Haploid cell restored to diploid upon fertilisation with another
gamete carrying different alleles
Chromosome mutations due to incorrect copying of DNA
16
Independent assortment & segregation
occur twice during meiosis
Metaphase I
• During meiosis I homologous
chromosomes align themselves
along the equator in a random
manner during metaphase I,
this is independent assortment
• Later on, there is
independent segregation
when homologous
chromosomes unpair during
anaphase I
Maternal
chromosomes
Paternal
chromosomes
Anaphase I
17
Independent assortment & segregation
occur twice during meiosis
Metaphase II
• During meiosis II the chromatids line
up randomly along the equator
during metaphase II for another
round of independent assortment
• Later on, the chromatids separate
to opposite poles in another round
of independent segregation during
anaphase II
Anaphase II
• The final result is four haploid cells that are different from
each other
18
Can you tell the difference between
independent assortment & segregation?
Metaphase I
1
Independent
assortment
Anaphase I
2
Independent
segregation
Metaphase II
3
Independent
assortment
Anaphase II
4
Independent
segregation
19
Cut out and stick down linking each diagram to
the correct label
Metaphase I
1
2
3
Independent
segregation
Anaphase I
2
Metaphase II
3
Independent
assortment
Independent
assortment
Anaphase II
1
Independent
segregation
Independent
segregation
Metaphase II
Metaphase I
Independent
assortment
Independent
assortment
Anaphase I
Cut out and stick down linking each diagram to
the correct label
4
Anaphase II
Independent
segregation
4
Phenotypic and Genotypic Variation
I can:
Level:
• Describe the differences between continuous and
discontinuous variation
C
• Describe how environment affects the expression of
the genotype
B
• Explain how genetic variation is a consequence of
sexual reproduction
A
Keywords: continuous variation, discontinuous variation, chlorosis,
etiolation, crossing over, random assortment, random segregation,
chiasma, chromatids
21
Answers…
Metaphase I
3
Independent
assortment
Anaphase I
1
Independent
segregation
Metaphase II
2
Independent
assortment
Anaphase II
4
Independent
segregation
22