#131 Terms for human genetics

Studies of human genetic conditions have revealed the links between genes, enzymes and the phenotypes.

• gene: length of DNA that codes for a particular protein/polypeptide
• locus: position at which a particular gene is found on a particular chromosome; same gene on same locus
• allele: particular variety of a gene

• dominant: the allele whose effect on the phenotype of a heterozygote is identical to its effect on a homozygote
• recessive: the allele that is only expressed when no dominant allele is present
• codominant: alleles that both have an effect on the phenotype of a heterozygous organism
• linkage: the presence of 2 genes on the same chromosome so that they tend to be inherited together and do not assort independently

• test cross: a genetic cross in which an organsim showing a characteristic caused by a dominant allele is corssed with an organism that is homozygous recessive --> phenotype of offspring is a guide to whether the 1st organism is homozygous or heterozygous
• F1: generation of offspring produced from homozygous dominant x homozygous recessive genotype
• F2: generation of offspring produced from cross between 2 F1 organisms

• phenotype: organisms' characteristics; often resulting from an interaction between its genotype and the environment
• genotype: alleles possessed by an organism

• homozygous: having 2 identical alleles of a gene
• heterozygous: having 2 different alleles of a gene

Syllabus 2016-2018 16.2 The roles of genes in determining the phenotype  Patterns of inheritance are explained by using genetic diagrams. The results of genetic crosses are analysed statistically using the chisquared test.  Studies of human genetic conditions have revealed the links between genes, enzymes and the phenotype.  a) explain the terms gene, locus, allele, dominant, recessive, codominant, linkage, test cross, F1 and F2, phenotype, genotype, homozygous and heterozygous  b) use genetic diagrams to solve problems involving monohybrid and dihybrid crosses, including those involving autosomal linkage, sex linkage, codominance, multiple alleles and gene interactions (the term epistasis does not need to be used; knowledge of the expected ratio for various types of epistasis is not required. The focus is on problem solving)  c) use genetic diagrams to solve problems involving test crosses  d) use the chi-squared test to test the significance of differences between observed and expected results (the formula for the chi-squared test will be provided) (see Mathematical requirements)  e) explain that gene mutation occurs by substitution, deletion and insertion of base pairs in DNA and outline how such mutations may affect the phenotype  f) outline the effects of mutant alleles on the phenotype in the following human conditions: albinism, sickle cell anaemia, haemophilia and Huntington’s disease  g) explain the relationship between genes, enzymes and phenotype with respect to the gene for tyrosinase that is involved with the production of melanin