Nursing Preparation Study Guide

Nursing Preparation Study Guide Mendel’s work with peas revolutionized the scientific understanding of heredity and resulted in two significant laws – the law of segregation and the law of independent assortment. To better understand these laws, however, we first need to look at the work of another geneticist, Reginald Punnett. While Punnett was doing his graduate work in 1900 at the University of Cambridge in England, Gregor Mendel’s work on genetics, which did not receive much attention during his lifetime, was being rediscovered. Punnett became an early follower of Mendelian genetics and developed the Punnett square as a means to organize the assortment of inherited alleles as Mendel described them. A Punnett square is simply a box with several squares drawn inside it and with the allele for a particular gene from each parent listed on either the top or the side. Each square shows a possible genotype (or set of alleles that define the gene) that can be inherited by the offspring of those parents. We will see Punnett squares as we explain Mendel’s laws. Law of Segregation Mendel’s law of segregation says that only half of the alleles of each parent’s genes are transferred to their offspring, with the other half coming from the other parent. Each gene contains two alleles. For instance, a gene for trait ‘A’ could contain the alleles AA, Aa, or aa, with the ‘A’ being the dominant form of the allele and ‘a’ being the recessive form. Remember, offspring with one or more dominant alleles exhibit the trait; offspring with only recessive forms do not. The law of segregation states that one allele will come from one parent, and one will come from the other, and it is the parent’s combined genetic makeup (rather than one parent’s particular genotype) that will determine the genes of their offspring. Mendel also showed that the probability a certain trait would spread from parents to children was 3:1 if both parents had one dominant and one recessive form of the gene, also known as having heterozygous alleles. Having two of the same alleles – AA or aa – is homozygous. 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.

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