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Gene.
A section of DNA that codes for a specific protein or polypeptide.
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Locus.
A specific position of a gene on a chromosome.
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Allele.
One of the different forms of a particular gene, which arises as a result of gene mutation.
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Gene point mutation.
- A change in the nucleotide sequence of a gene.
- Therefore a gene mutation may result in a change in the amine acid sequence of a protein.
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Phenotype.
The physical, observable, measurable characteristics of an organism.
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Genotype.
The genetic make-up of an organism.
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Homozygous.
- Genotype where the alleles of a particular gene are identical to each other.
- eg. GG
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Heterozygous.
- Genotype with contrasting alleles.
- eg. Gg
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Dominant.
An allele is said to be dominant if it is always expressed in the phenotype of an organism.
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Recessive.
- An allele is said to be recessive if it is only expressed in the phenotype of an organism if the other allele paired is identical.
- If an organism shows the recessive characteristics in the phenotype then the genotype must be homozygous recessive.
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Monohybrid Cross.
A genetic cross between individuals in which a single gene is being considered.
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Dihybrid cross.
- A genetic cross between individuals in which genes at two different loci are considered.
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Multiple Alleles.
A single gene that has more that two forms.
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Codominance.
Two alleles are said to be codominant if both are expressed fully and equally in the phenotype of the heterozygote.
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Pure breeding.
Organisms which, when crossed, produce offspring identical to their parents.
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Test cross/backcross
A cross performed to determine an unknown genotype.
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Autosomal Linkage.
Refers to two or more genes found on the same chromosome.
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Sex linkage.
- A characteristic is said to be sex linked if it is controlled by a gene carried on one of the sex chromosomes.
- Y chromosome is very small and carries few genes, therefore sex linked genes are more likely to be found on the X chromosome.
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Polygenic inheritance.
Where a number of genes are responsible for controlling a characteristic.
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Mendel's First Law.
The characteristics of an organism are determined by factors ( alleles) which occur in pairs.
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Mendel's Second Law.
- Either one of a pair of contrasted characters may combine with either of another pair.
- Each member of an allele pair may combine randomly with either of another pair.
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Overproduction
All organisms have the reproductive potential to greatly increase their populations but rarely do so due to limiting factors.
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Intraspecific competition
- Competition between individuals of the same species. tra - intranetThe basis of origin on species by natural selection.
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Population
Individuals of the same species occupying a particular area.
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Genotype frequency.
The number of times a particular genotype occurs in a population.
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Allele Frequency
The number of times a particular allele occurs in a population.
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Selection pressure
- Environmental pressure that results in the survival of the best adapted organisms in a population.
- Thus produces changes in the allele frequency in the gene pool.
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Genetic drift.
Random variations in allele frequencies which occur in populations by chance.
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