Nursing Entrance Exam

Punnett Squares The Punnett square below represents the possible children born to two parents with Aa alleles expressing the ‘A’ gene. A a A AA Aa a Aa aa The three genes in bold, with at least one capital letter (AA, Aa, and the other Aa), represent cases in which the presence of at least one dominant allele will cause the trait to manifest in the offspring. The remaining one (aa) represents the one case where the child does not manifest the trait, even though both do. For example, this could be where there is one brunette in a family of redheads. Provided both parents have one dominant and one recessive allele, the distribution will always be 3:1. Law of Independent Assortment Mendel’s second law, the law of independent assortment, shows that the alleles of multiple genes will mix independently with one another. When two separate genotypes are tracked, the genes will produce 16 separate possible combinations spread out in a 9:3:3:1 ratio. This is also known as a dihybrid cross; when dealing with a single set of alleles, it is referred to as a monohybrid cross. We can demonstrate this by assuming that we have a male and a female each with heterozygous alleles making them blond and tall. We can represent this with the genotypes BbTt in each. We should also assume that a ‘bb’ genotype would give someone brown hair and ‘tt’ would make them short. Since the law of independent assortment purports that each allele will mix independently, we end up with four combinations of a genotype that each parent can pass on: BT, Bt, bT, and bt. These can then be mapped in a slightly larger Punnett square, which looks like this: BT Bt bT bt BT BBTT BBTt BbTT BbTt Bt BBTt BBtt BbTt Bbtt bT BbTT BbTt bbTT bbTt bt BbTt Bbtt bbTt bbtt This is the distribution of the tall, blond couple’s possible children. Nine would also be tall and blond, three would be short and blond, three would be tall and brunette, and one would be short and brunette. This follows the 9:3:3:1 ratio set out byMendel.

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