Mendelism

• Gamete = sperm or egg, a single set of genetic instructions, a haploid cell
• Zygote = the fertilised egg and all body cells, a double set of genetic instructions in a diploid cell
• Locus = the chromosomal location of a genetic instruction
• Allele = alternative forms at a particular locus
• Homozygote = two identical copies of an allele
• Heterozygote = non-identical copies of an allele
• Phenotype = physical appearance of an individual with a particular genotype
• Genotype = genetic constitution that underlies the phenotype
• Dominant = an allele that in a heterozygote masks the effects of the other allele
• Recessive = an allele than in a heterozygote has its effects masked by the other allele and is only expressed in the homozygote
• Mendel's first law = the two members of a gene pair segregate into separate gametes, half carrying one member of the pair and the other half the other member
• Mendel's second law = during the formation of gametes, the segregation of alleles at one locus is independent of that of the segregation of alleles at any other
• Incomplete dominance = the expression of the heterozygote is a mixture of the dominant homozygotes (e.g. red + white --> pink)
• Codominance = condition in which the phenotypic effects of a gene's allele are fully and simultaneously expressed in the heterozygote (red + white --> red + white)
• Multiple alleles = when three or more alleles can occupy a given locus, e.g. ABO blood group
• Lethal alleles = homozygotes may be lethal, e.g. achondroplasia, manx cat tails
• Pleiotropy = one gene affects many phenotypes, e.g. all blue-eyed white cats are deaf
• Incomplete dominance = the expression of the heterozygote is an intermediate (mixture) between the different homozygotes
• Codominance = the phenotypic effects of a gene's allele are fully and simultaneously expressed in the heterozygote
• Sex limitation = the expression of a phenotype is dependent on sex, e.g. schizophrenia more common in males due to testosterone, BRCA1 more common in females because of oestrogen
• Gene interaction = several genes affect one character, e.g. mouse coat colours:

  A locus: distribution of colour in hair shaft

  B locus: colour of hair

  C locus: presence or absence of pigment

  S locus: pigment distribution on body
• Complementation = when you cross two strains of an organism with the same phenotype but different homozygous to produce a new phenotype, e.g. foxglove petals
• Atavism = phenotypic traits disappearing then reappearing later in the phylogenetic tree