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, Variation & definitions
Variation arises as a result of mutations (changes in the genetic code) . It is
essential for natural selection & therefore evolution.
• Genotype- combination of alleles an organism inherits for a characteristic
• Phenotype- The observable characteristics of an organism
• Dominant allele-version of the gene that is always expressed if present.
• Recessive allele- only expressed if 2 copies of this allele are present.
• Homozygous- have identical alleles for a characteristic
• Homozygous dominant- contains 2 alleles for the dominant phenotype
• Homozygous recessive- contains 2 alleles for the recessive phenotype
• Heterozygous- have 2 different alleles for a characteristic. In this case
the alleles for the dominant phenotype will be expressed.
Continuous variation Discontinuous variation
A characteristic that can take any value A characteristic that can only appear in
within a range specific (discrete) values
Genetic & environmental causes of
Mostly genetic causes of variation
variation
Polygenes- controlled by a number of
Controlled by 1 or 2 genes
genes
e.g. leaf surface area, animal mass, skin e.g. blood group, albinism, round &
colour wrinkle pea shape
, Inheritance
Genetic variation is created
by the versions of genes
you inherit from your
parents. For most genes in
your body, 2 alleles are
inherited. These alleles may
be the same or different
versions of the gene.
Genes are sections of DNA found on
chromosomes. In sexually reproducing
organism, most chromosomes occur in pairs
known as homologous chromosomes (one
comes from each parent) the allele of a
particular gene occurs in the same position
(locus) on each of the homologous
chromosomes.
, Variation examples
The actual characteristics an organism displays are also often influenced by the
environment. Changes that the environment makes to a persons phenotype are
referred to as modifications & they cannot be inherited.
Chlorosis
Some plants suffer from chlorosis which is a condition that
causes the leaves to look pale or yellow. This occurs because
the cells are not producing the normal amount of chlorophyll.
This lack of chlorophyll reduces the ability of the plant to
make food by photosynthesis.
Most plants which show chlorosis have normal genes coding
for chlorophyll production so the change in their phenotype
is the result of environmental factors:
Lack of light- plants turn off their chlorophyll production in the areas where little
light is reaching, to conserve resources.
Mineral deficiencies- e.g. lack of iron & magnesium. Iron is needed as a cofactor by
some enzymes that make chlorophyll & chlorophyll contains magnesium. If either of
these elements are lacking in the soil, the plant cannot make chlorophyll.
Virus infections- viruses interfere with the metabolism of cells. The infected plant
tissue may no longer be able to support the chlorophyll.
Animal body mass
Within a species the body mass of individual animals varies. An organisms body mass is
determined by a combination of both genetic & environmental factors.
, Monogenic inheritance- genetic cross
Monogenic inheritance is the inheritance of a single gene. Gregor
Mendel established the basic laws by which characteristics are
inherited.
Homozygous genetic cross
Organisms that contain homozygous alleles for a particular gene are
known as true breeding or pure breeding individuals. When 2 true
breeding individuals are crossed all the offspring, known as the F1
generation, will be heterozygous & show the phenotype of the dominant
allele.
G=allele for green pods g= allele for yellow pods
Parental phenotypes Green pods Yellow pods
Parental genotypes GG (Randomly assumed as male) gg (Randomly assumed as female)
Meiosis Meiosis
Gametes G G g g
Gametes ♂
Offspring (F1) Gametes ♀ G G
genotypes g Gg Gg
g Gg Gg
(F1) phenotypes All plants have green pods
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