What is Punnett Square

Charles Darwin proposed a mechanism that explained how organisms evolved, but nothing was known on the form those heritable changes took, what they were, and how they were passed from one generation to another. Gregor Mendel started experiments using the common pea studying easily observable characteristics. Genes are units of DNA carrying information about specific traits. Any given trait has two alleles that may be the same (homozygous) or different (heterozygous). Mendel's first theory studied one trait (monohybrid) in peas. Mendel's second theory studied two traits (dihybrid). Two examples of inheritance do not conform to Mendel's theories. In incomplete dominance, the expression of both alleles results in an intermediate phenotype. In codominance, both alleles are expressed resulting in a new phenotype comprised of both alleles. Mothers who are Rh- but bearing a Rh+ baby will need a Rh immunoglobulin shot to counteract the formation of antibodies against a future foetus. This chapter explores genes.