DNA, Genes, Cells, and Mendelian Genetics

Abstract

Molecular genetics in conjunction with cytology provides the necessary tools for the understanding of hereditary processes. Hereditary material in prokaryotes is made up of DNA that is not contained in any cellular compartment, whereas in eukaryotes, the cell nucleus contains several molecules of DNA organized into chromosomes. Chromosomes are the collection of DNA and some of its associated proteins. Normally, in eukaryotes there are several types of chromosomes that can be differentiated by their shape. Of each type or shape of chromosome, there are at least two types of chromosomes (when there are two, organisms which contain them are named diploid organisms; when there are three, organisms which contain them are named triploid, etc.), which are morphologically the same but may differ in the nucleotide sequence of their DNA. These sets of chromosomes grouped by their shape are called homologous chromosomes. Each set of homologous chromosomes encodes for the same traits. The physical space in which a gene is located on the chromosome that determines a certain trait is called the locus, and the information that exists at the locus on each homologous chromosome is called the allele. In diploid organisms, the presence of the same allele at the same locus in both homologous chromosomes gives rise to homozygous individuals, and originates heterozygous individuals when the alleles of the same locus are different. When studying a trait, its genotype is the allelic set that exists at the locus, at a given locus, and the phenotype is the translation of that set. The understanding of these concepts allows the explanation of Mendel’s first and second laws using meiosis, specifically during anaphase I.

Knowledge Integration Question

Explain the Relationship Between Cell Division and Mendelian Laws

Mendel’s first law, or the law of independent segregation, states that in a diploid individual the two alleles making up the genotype of a given trait will form different gametes. If the individual is homozygous it will obviously not be possible to observe segregation. This is evident in heterozygous individuals. In anaphase I of meiosis, it can be observed that each of the homologous chromosomes of each pair of chromosomes migrate to a different pole. Later in telophase I, homologous chromosomes are completely separated in different cells, i.e., the alleles segregate in different cells, independently.

Mendel’s second law, or the law of independent distribution, states that in a diploid individual, the segregation of a gene is done independently compared to the segregation of another gene. This is absolutely true if one considers that genes are located in loci that correspond to different chromosomes, or if they are located on the same chromosome, they must not be linked (see Chap. 8). In metaphase I, the chromosomes are randomly located at the equator of the cell; therefore, the fact that a homologous chromosome of a pair is directed during anaphase I toward one of the poles is absolutely independent of which of the two homologous chromosomes of the other pair are toward the same pole. During telophase I, when considering two genes at the same time and to observe which homologous chromosome of a pair is located in a given cell, it is seen that this is completely independent of what happened with another pair of homologous chromosomes.