PUNNETT SQUARES & MONOHYBRID CROSSES

=Punnett Squares= The punnett square below shows a '''monohybrid cross''' between two heterozygous (Aa) Individuals = Aa x Aa The term 'monohybrid' simply means that we are only investigating a single gene and its alleles. In this example we are investigating the gene for feather colour and its two alleles ('''A''' which codes for red feathers and '''a''' which codes for blue feathers). The term 'cross' simply tells us the two individuals have been deliberately bred as part of the investigation (it's actually an abbreviation for 'cross-breeding') A punnett square is used to show the possible genotypes of any potential offspring and their likelihood. Parent 1 (the father) could produce sperm cells that carry either a dominant allele (A) or a recessive allele (a). This is shown by placing both an (A) and an (a) on top of the punnett square grid. Likewise parent 2 (the mother) can produce egg cells that carry either a dominant allele (A) or a recessive allele (a). This is shown by placing both an (A) and an (a) left of the punnett square grid. The grid itself contains the genotypes of any possible offspring. For instance '''AA''' is the genotype of any offspring formed when a sperm carrying an '''A''' fertilised an egg carrying an '''A'''. It's worth noting that the Dominant allele is always written first (e.g. Aa) irrespective of which parent contributed the dominant allele (A) [image:http://i.imgur.com/aI4EuRd.png?1] [image:http://i.imgur.com/sTl3Cje.png?1] In this cross we would expect approximately 1/4 (25%) of the offspring to be blue and 3/4 (75%) to be red. The study of these patterns of inheritance is often referred to as '''Mendelian Genetics''' -named after an Austrian monk who first realised that these patterns could only be explained if sexually reproducing organisms had two "factors" (which later came to be called alleles) and that they could be dominant and recessive.