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Recall that a diploid organism has a karyotype made up of a number of pairs of chromosomes (homologous) eg human has 23 pairs. These chromosomes are each made up of one molecule of DNA along which are sections called genes. A pair of chromosomes hold the genes for specific features at specific loci on the DNA length. Each half of the pair, may hold slightly different version of this gene (alleles). This contributes to the notion of dominance and recessive characteristics eg as for eye colour or tongue rolling. Both halves of the pair of chromosomes must contribute their allele to make up the gene - whether the alleles are the same or not. Hope this helps and request tutor session if further assistance is needed! Thanks!

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