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Chromosomes and Genes
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Within each human somatic cell there are 46 chromosomes (23 pairs)
Karyotype of metaphase chromosomes homologous pair sex chromosomes
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Each chromosome is made of DNA
Long strands of chromatin are packed tightly together to make chromosomes A segment of DNA is called a gene
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Each chromosome contains many genes
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A pair of homologous chromosomes have the same genes but the specific information may differ slightly For example: this homologous pair contains the gene for eye colour; but one chromosome has the gene for brown eyes, while the other has the gene for blue eyes
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A = brown eyes; a = blue eyes
These are called alleles (the different forms of expression of a gene) Some alleles are dominant, while others are recessive
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If a person has the dominant and recessive alleles (heterozygous), only the dominant will be observed in their phenotype If a person has 2 dominant or 2 recessive alleles, their genotype is homozygous Genotype = the specific alleles a person has (e.g. Aa) Phenotype = the observable traits a person has (e.g. brown eyes)
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E.g. Aa = brown = heterozygous (hybrid) AA = brown = homozygous (pure dominant) aa = blue = homozygous (pure recessive)
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However, eye colour inheritance is not actually this simple…
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