Test cross

1. Test cross • Individuals that are show the dominant phenotype and are homozygous are called pure breeding • Individuals that show the dominant phenotype and are heterozygous are called carriers (the recessive allele is not expressed and is masked by the dominant allele which is expressed) • In order to find out whether an individual with dominant phenotype is heterozygous or homozygous a test cross is carried out. This involves crossing the individual with unknown phenotype with the an individual who shows recessive phenotype and therefore their genotype is homozygous recessive . • If any offspring show the recessive phenotype then the individual with the unknown genotype is heterozygous as it has passed a recessive allele to its offspring. • If all offspring show the dominant phenotype then the then the individual with the unknown genotype is homozygous (pure breeding) SEE TEXT / NOTES FOR PUNNET SQUARES - GIVE EXPECTED PHENOTYPE RATIOS / PERCENTAGES • It is important to do a large number of crossings to ensure the results are reliable as the actual outcome from the crossings is due to chance. If only a few crossings are done, due to chance, it is possible for no offspring to show the recessive phenotype even though the parent is heterozygous.

2. pedigrees • Pedigrees show relationships within families • Males are represented by squares and females by circles • Vertical lines show parent / offspring • Horizontal lines show matings • Shaded squares or circles indicate the individual has a particular trait or characteristic From a pedigree chart we can • Determine phenotypes and genotypes of individuals • Whether an allele is dominant or recessive • Probability of a child inheriting an allele or an inherited disorder