Key terms

• Genotype = The underlying genetic constitution of an organism
• Phenotype = the physical expression of the genotype taking into account the environment
• Gene = a sequence of nucleotide bases that code for a polypeptide
• Locus = the position of a gene on a particular DNA molecule
• Allele = an alternative form of a gene
• Haploid cell = a cell containing a single set of chromosomes
• Diploid cell = a cell containing two sets of chromosomes
• Homologous pair = two pieces of DNA within a diploid organism which carry the same genes, one from each parent
• Homozygous = the allele on each of the chromosome is the same
• Heterozygous = the alleles on each chromosome are different
• Dominant allele = the allele that expresses itself in the phenotype regardless of any other alleles
• Recessive allele = an allele that only expresses itself when two copies are present
• Homozygous dominant = 2 dominant alleles
• Homozygous recessive = 2 recessive alleles
• Co-dominance = 2 alleles are equally dominant and equally recessive, so are both expressed in the phenotype
• Multiple alleles = when a gene has more than 2 forms
• Recombinant offspring = offspring with a different combination of characteristics to their parents
• How are recombinant offspring produced?

  Crossing over during meiosis
• Sexosome = the 23rd chromosome, containing either XX for females or XY for males
• Autosomal linkage = when 2 or more genes are carried on the same autosome
• Autosomes = non-sex chromosomes
• Epistasis = when one gene effects the expression of another gene - 2 genes on different chromosomes affect the same feature
• Dominant Epistasis = a dominant allele at one locus masks the expression of alleles at a second locus
• Recessive Epistasis = recessive alleles at one locus mask the expression of alleles at a second locus
• Complementary epistasis = the presence of a particular allele at each locus is required for an expression of a particular phenotype