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Summary Genetics and Inheritance
A full summary of genetics and inheritance, compiled in accordance with the IEB sags document 2021. Very thorough includes textbook definitions and diagrams.
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Topic 5- Genetics and inheritanceUnit 1 Genetics and genes
Genetics is the study of the inheritance of traits or characteristics that are passed down from parent
to offspring (progeny)
Genetic traits in humans
→ Not all genetic traits are visible, example, you cannot determine a person’s blood type by
looking at them.
→ Inherited traits may appear in different generations of the family.
→ Individuals in a family with a particular trait can be shown in a pedigree chart. Each row
represents one generation with the oldest being at the top.
→ An organism's full set of genes on all its chromosomes is called a genome.
→ Hereditary traits: eye colour, hair type, physical aspects such as thumb type and earlobe
type.
Genetics: the study of the hereditary and variation in organisms
Inheritance: the transmission of genetic characteristics from parent to offspring
Offspring (progeny): new organisms produced by a living thing
Genetic trait: a characteristic brought about by the expression of a gene or genes
Haemophilia: an inherited blood clotting disorder
Pedigree chart: a diagram that shows the ancestors of an individual and can be used to analyse
the inheritance of a trait.
Allele
Alternative forms of a gene at particular position (locus) on the homologous chromosomes and
determines a particular genetic characteristic. Therefore, alleles of the same gene are located at the
same locus on homologous chromosomes. A locus is the position of a gene on a chromosome.
If F represents the allele for free hanging earlobes and f represents the allele for attached earlobes,
there are 3 possible combinations of these alleles on the homologous chromosomes.
Homozygous and heterozygous
,When a person or organism has the two same alleles for a particular characteristic, they are said to
be a homozygote. For example, a person with two alleles for free hanging earlobes is homozygous
for earlobe shape.
Homozygous (pure bred organism): an organism that has two of the same alleles for a
particular genetic characteristic.
Heterozygous (hybrid) organism: an organism with two different alleles for a particular
genetic characteristic.
A heterozygous organism is said to be pure bred because it produces offspring with identical genetic
characteristics of itself.
As homologous chromosomes separate during meiosis and form gametes, the alleles on the
homologous chromosomes will end up in different gametes. A homozygous organism can only
produce one type of gamete where heterozygous organisms can produce two types of gametes.
Dominant and recessive alleles
We usually represent the dominant alleles with a capital letter and recessive alleles with a lowercase
letter.
Dominant allele: the allele that is expressed in the heterozygous condition
Recessive trait: the allele that is only expressed if the dominant allele is absent
Genotype: the set of alleles of a gene that an organism has for a particular characteristic
Phenotype: the physical or observable characteristics of an organism which are determined by
its genetic make-up as well as the environment in which it lives.
Genotypes and phenotypes
Eg: Lerato’s genotype for a particular characteristic is FF.
The phenotype is the observable characteristic, for example Lerato’s phenotype is free hanging
earlobes. The Phenotype is determined by the genotype of the organism as well as the environment
in which it lives.
For example, a person may have the genotype to be very tall, but lack of food in his environment
may affect how tall the person grows.
, A dominant allele produces a characteristic that is observable in the phenotype if the dominant
allele occurs in a homozygous or heterozygous condition. For example: the person with the
genotype FF (homozygous) or Ff (heterozygous) has free hanging earlobes (the phenotype)
If a recessive allele occurs in a heterozygous condition, it has no effect and the characteristic is not
observable in the phenotype. A recessive allele will only have an effect and have observable
phenotypes if both alleles are recessive.
Unit 2 – Inheritance and variation
Variation
Variation is the difference in characteristics among organisms of the same species. It is the result of
differences in genes of the organism and/or the influence of the environment in which the
organisms live. For example, variations in blood groups in the human population is a result of the
different genes that people inherit. This type of variation is called discontinuous variation.
Variation in height, weight and skin colour in the human population is usually the result of inherited
genes as well as the influence of the environment. This is an example of continuous variation.
Genetic variation can result from the following:
• Crossing over and random segregation during meiosis
• Mutations
• Random fusions of gametes during fertilization
Gregor Mendel (1822-1884) - the father of genetics
→ Austrian monk who conducted experiments to figure out how genetics work
→ Did most of his research on pea plants because he knew he could control their reproduction
Monohybrid crosses: complete dominance
A plant is pure bred for a particular characteristic, after it has self-pollinated. It produces offspring
with the same characteristics.
A monohybrid cross is a cross that involves the inheritance of one genetic characteristic (alleles of
the same gene) for example cross pollinating tall plants with short plants.
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