3.17 understand the meaning of the terms: dominant, recessive, homozygous, heterozygous, phenotype, genotype and codominance

  • Dominant – A form of a gene that is expressed and masks the recessive gene. It gives the same phenotype in both homozygous and heterozygous conditions as it expresses itself.
  • Recessive – A form of a gene that expresses itself only in the homozygous condition. E.g. Bb, it won’t be expressed because the dominant ‘B’ allele masks it and is expressed instead. But in ‘bb’, this is homozygous recessive, so it will be expressed.
  • Homozygous – Having identical alleles for a particular trait. E.g. BB-homozygous dominant, or bb-homozygous recessive. (two copies of the same allele)
  • Heterozygous A condition where you have different alleles for a particular trait. E.g. if B codes for brown eyes (dominant allele is always upper case) and the recessive allele is b (always the lowercase of the dominant alleles’ letter), then a person with a Bb genotype for eye colour is heterozygous dominant, so will have brown eyes.
  • Phenotype – Describes the physical characteristics of an organism with respect to a particular pair of alleles (the genotype), also the physical appearance resulting from the inherited gene
  • Genotype – Describes the alleles that a cell or organism has for a particular feature.
  • Codominance – occurs when neither the allele is dominant and both contribute to the appearance of the offspring.
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