Genetics, Meiosis, and Cells - Why do you have a particular blood type or hair color? Why do some people have the same skin color as their parents and others have a skin color different from that of their parents? Why do flowers show such a wide variety of colors? Why is it that generation after generation of plants, animals, and microbes look so much like members of their own kind? These questions and many others can be better answered if you have an understanding of genetics.
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A gene is a portion of DNA that determines a characteristic. Through meiosis and reproduction, genes can be transmitted from one generation to another. The study of genes, how genes produce characteristics, and how the characteristics are inherited is the field of biology called genetics. The first person to systematically study inheritance and formulate laws about how characteristics are passed from one generation to the next was an Augustinian monk named Gregor Mendel (1822–1884). Mendel’s work was not generally accepted until 1900, when three men, working independently, rediscovered some of the ideas that Mendel had formulated more than 30 years earlier. Because of his early work, the study of the pattern of inheritance that follows the laws formulated by Gregor Mendel is often called Mendelian genetics.
To understand this chapter, you need to know some basic terminology. One term that you have already encountered is gene. Mendel thought of a gene as a particle that could be passed from the parents to the offspring (children, descendants, or progeny). Today we know that genes are actually composed of specific sequences of DNA nucleotides.
The particle concept is not entirely inaccurate, because a particular gene is located at a specific place on a chromosome called its locus (locus = location; plural, loci). Another important idea to remember is that all sexually reproducing organisms have a diploid (2n) stage. Because gametes are haploid (n) and most organisms are diploid, the conversion of diploid to haploid cells during meiosis is an important process.
2(n) → meiosis → (n) gametes
The diploid cells have two sets of chromosomes—one set inherited from each parent.
n + n gametes → fertilization → 2n
Therefore, they have two chromosomes of each kind and have two genes for each characteristic. When sex cells are produced by meiosis, reduction division occurs, and the diploid number is reduced to haploid. Therefore, the sex cells produced by meiosis have only one chromosome of each of the homologous pairs that were in the diploid cell that began meiosis. Diploid organisms usually result from the fertilization of a haploid egg by a haploid sperm. Thus they inherit one gene of each type from each parent. For example, each of us has two genes for earlobe shape: one came with our father’s sperm, the other with our mother’s egg.
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