Comprehensive and in-depth notes on the biological aspects of this chapter, using specification points as headings to ensure that all required material is included- and no irrelevant content (like many of the textbooks). Created and used by an A-Level Biology student for the NEW SPECIFICATION from ...
Biology Revision
Chapter 25 - Patterns of Inheritance
VARIATION:
• There is big differences between the smallest and Variation: The RANGE of differences in
largest organisms
characteristics between organisms
• Looking into genetic and environmental factors, as
well as roles played by meiotic cell division and
sexual reproduction Intraspecific Variation: The differences
• These processes are significant as they help create WITHIN the SAME species
GENETIC VARIATION and DIFFERENCES within a
species
• Which make it possible for ADAPTATIONS and Interspecific Variation: The differences
CHANGES to occur in RESPONSE to an
ACROSS DIFFERENT species
environmental change.
PHENOTYPIC VARIATION:
• All features of an organism comprise the PHENOTYPE
• This includes all features except its GENOME
• PHENOTYPIC VARIATION is the variation of these features and the phenotype is the expression
of an organisms GENOTYPE
• The effects may be VISUAL, for example hair colour or eye colour, or can be detected by
chemical testing such as blood group or enzyme presence
• Some features are only DETERMINED by the genotype, for example BLOOD TYPE
• Some are determined by the INTERACTION between GENOTYPE and the organisms
ENVIRONMENT
• And others the organisms environment ONLY, such as scars from past injuries
• ENVIRONMENT may influence the FINAL EXPRESSION of the genotype to such an extent that
the 2 individuals with the same genotype raised in different environments may show phenotypes
which are CONSIDERABLY different.
HOW THE ENVIRONMENT AFFECTS VARIATION:
• Both the GENOTYPE and the ENVIRONMENT contribute to the phenotype
• The genotype DETERMINES the characteristics of the organism in terms of the alleles for a
particular gene that it contains
• In DIPLOID CELLS which carry 2 copies of every chromosome, the pair of alleles present at a
given locus may be IDENTICAL, which is called HOMOZYGOUS
• When the alleles at the locus are DIFFERENT then it is called HETEROZYGOUS
• Each allele in a pair of alleles may be DOMINANT or RECESSIVE
PHENOCOPY:
• Sometimes the environment affects the phenotype in the same way as effects that would be seen
by genotypic change
• This effect is called PHENOCOPY and can be observed in multiple organisms
• Some varieties of tomato have a purple stem, which is only seen in COLD ENVIRONMENT
wheres those which grow in normal temperatures have green stems, which are indistinguishable
from the other varieties of tomato.
The difference between the 2 types of variation is a result of the number of genes involved in
determining a characteristic and the expression of the alleles of those genes
Definition Features cannot be measured across a Features can be measured across a
complete range so they form distinct complete range from one extreme to the
classes or categories: discrete or other, data collected is quantitative data
categorical data (qualitative)
Gene Locus Usually only one but there may be a very Many loci may be on different chromosomes
small number
Number of Often just one pair of alleles (monogenic) Many goes contribute to the inheritance
alleles but there may be a very small number (polygenic) each has its own alleles
Effect on The feature is either present or not. There are many intermediates between the
Phenotype Differences are discrete categories extremes, i.e. tallest and shortest
Environment Environment has little influence Environment has significant influence
al Influence
Example Ability to role tongue, or blood type height in humans, milk yells in cattle
• However there are a number of different factors that affect expression of genotype, will in turn
affect the phenotype
• Although the GENETIC POTENTIAL may be present, it may be modified or altered completely
by ENVIRONMENTAL FACTORS
EXAMPLE:
• Take a growing child, with a well balanced diet they should grow to their FULL POTENTIAL
• However if they have an un-balanced unhealthy diet, they WILL NOT grow to their FULL
POTENTIAL
• CHLOROSIS in plants occurs when they are kept in the dark, the plants turn yellow, as their
chlorophyll synthesis is HALTED and slowed down
• Seedlings grown in the dark are chlorotic, but also have long stems with small, curved leaves
yellow in colour which is known as ETIOLATION
GENETIC VARIATION AS A RESULT OF SEXUAL REPRODUCTION
• Genetic variation exists because there are differences in the GENES due to the presence of
DIFFERENT ALLELES
• It is increased by SEXUAL REPRODUCTION when DNA from one individual is combined with
that from another organism during FERTILISATION
• For any selective advantage or disadvantage to be passed on the host of such genome must be
FERTILE
• For this to happen, the parents must be from the same SPECIES
, MEIOSIS during sexual reproduction generates variation by:
• Crossing over between maternal and parental chromosomes in MEIOSIS I
• INDEPENDENT ASSORTMENT in meiosis I of maternal and paternal chromosomes in a
homologous pair, which later separate
• SEGREGATION between sister chromatids in meiosis II, which is also RANDOM
• The whole process halving chromosomes number to produce HAPLOID GAMETES that are then
restored to a diploid cell by fertilisation with another gamete carrying different alleles
• The occurrence of chromosome MUTATIONS
• During PROPHASE I, crossing over occurs
• This is an exchange of some alleles between chromatids of the homologous chromosomes;
essentially redistributed
• This happens because chromatids from opposing HOMOLOGOUS chromosomes form
attachments called CHIASMATA
• These attachments act as BREAKING POINTS when the chromosomes separate
• The bokken section rejoin, the non-sister chromatid from the other chronometer of the
homologous pair, creating new combinations of alleles on the chromatids
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