Nursing Entrance Exam

also duplicate itself, or shift, triggering the half of the base pair on one side of the DNA strand to link to the wrong half on the other side. Heredity: Mendelian Inheritance The father of genetics was a 19 th century Austrian monk named Gregor Johann Mendel, who became famous for his work crossbreeding peas in the garden of his monastery. Aside from his life as a monk, Mendel was a highly educated physicist, studying first at the University of Olomouc (in the modern-day Czech Republic) and later at the University of Vienna. 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.

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