polygenic inheritance and genes in populations
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Inheritance
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Contents
Variation
Chromosomes and DNA
Monohybrid inheritance
Selection
Genetic engineering
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Variation
Living organisms vary in many ways.
E.g. humans all have the same general shapeand the same set of body organs, but somefeatures differ from one person to the next,such as:
height, weight, eye and hair colour, shape ofnose, language, knowledge and skills.
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Within a species there is usually a great deal ofvariation between individuals
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These snails are allthe same species
(Cepaea nemoralis)but they varyconsiderably
What variationscan you see ?
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There are variations in skin colour, hair colour,
hair curliness, eye colour and sex
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Variation
Variations may be inherited or acquired
Inherited variations result from the activity ofgenes.
They are genetically controlled
Genetically controlled variations cannot bealtered
For example, hair colour, skin colour, bloodgroup, finger prints and sex cannot bechanged naturally
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Variation
Acquired characteristics result from an individualsactivities or nutrition or from environmental conditionsduring a lifetime
Examples of acquired conditions in humans are*
language obesity
athletic skills mental skills
body building sun tan
Acquired characteristics cannot be inherited
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Two types of variations
Continuous and
Discontinuous
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Continuous variation
Characteristics that show continuous variation havedifferent features:
1. Every organism within one species shows the
characteristic, but to a different extent. Thecharacteristic can have any value within a range.
2. These characteristics are usually quantitative theycan be measured.
3. They result from several genes acting together, or from
genes and the environment.
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Examples Height
Weight
Foot length
Intelligence
Chest circumference
Body mass
Hand span
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The figures can
be made to fit a
smooth curve
because there aremany intermediates
Normal distribution curve
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Discontinuous variation
Characteristics that show discontinuous variation haveseveral features:
An organism either has the characteristic or it doesnt
have it. There is no range of these characteristicsbetween extremes.
These characteristics are usually qualitative theycannot be measured.
They are the result of genes only they are not affected
by environment.
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Examples
Your ABO blood group is either A, B, AB or O
Presence or absence of ear lobe
Gender
Eye colour
Genetic diseases such as colour blindness, albinism,sickle cell anaemia
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Blood group
The figures cannot
be made to fit asmooth curve
because there are
no intermediates
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Chromosome structure
Chromosomes are thread likestructures found inside thenucleus of a cell.
A human cell nucleus
contains 46 chromosomes. Each chromosomes contains
a long molecules of DNA
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DNA stands fordeoxyribonucleic
acid
DNA issubdivided intosegments calledgenes
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Genes
A gene is a unit of inheritance.
Genes may be copied and passed on to the next generation
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Location of genes
The position of a gene on achromosome is known asthe locus.
In sexually reproducing
organisms, most cells have ahomologous pair ofchromosomes (one fromeach parent)
Chromosomes from a
homologous pair have genesthat control the same traitat the same locus.
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Alleles Genes occupying the
same position (locus) onhomologouschromosomes are calledalleles.
Alleles are alternativeforms of a gene
Any one individual canonly have a maximum oftwo alleles for a givengene.
There may be more thantwo alleles in apopulation, e.g bloodgroups A, B, O
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Homozygous and
heterozygous Alleles are always represented by letters.
Since there are two alleles for a given gene, a gene isrepresented by a pair of letters.
E.g. AA, Aa and aa When the two letters are same, they are termed
homozygous (AA, aa)
When they are different they are termed heterozygous(Aa)
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Dominant and recessive
When an allele expresses itself both in homozygous andheterozygous conditions, it is said to be dominant.
A dominant allele is always represented by a capital
letter. When an allele is always expressed when present in
homozygous state and suppressed in the heterozygousstate it si said to be recessive.
A recessive allele is always represented by a small
letter.
E.g A is dominant, a is recessive
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Homologous
chromosomes
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Phenotype and genotype
The genetic constitution of an organism iscalled its genotype.
The external appearance of an organism isknown as its phenotype.
E.g. For a plant producing purple flowers
genotype = PP and
Phenotype = purple.
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