MODERN BOTANY - P. RAVEN - 1990

SECTION III. GENETICS

CHAPTER 9. EUKARYOTIC GENETICS

Conclusion

Sexual reproduction involves two main stages: the reduction of the diploid chromosome number through Meiosis and its restoration via syngamy.

Meiosis results in The formation of Gametes or spores. A gamete is a haploid Cell that fuses with another gamete to form a diploid zygote. A spore is a cell that develops into a mature Organism without fusing with another cell.

Meiosis consists of two successive nuclear divisions that yield four nuclei (or Cells), each containing the haploid chromosome number.

During the first meiotic division, homologous Chromosomes pair up along their entire length. The chromosomes are duplicated, each consisting of two chromatids. Chiasmata form between the chromatids of homologous chromosomes. Chiasmata are the visible manifestation of Crossing-over—the exchange of chromatid segments between homologous chromosomes. Bivalents align randomly at the equatorial plate (with the centromeres of paired chromosomes pointing in opposite directions), resulting in the complete reassortment of maternal and paternal chromosomes during anaphase I. This chromosome shuffling and crossing-over ensure that the products of meiosis possess novel combinations of chromosomes, distinct from both the parental types and one another. Thus, meiosis serves as a mechanism for generating genetic variation within the diploid genotype.

In the second meiotic division, chromosomes segregate much as they do in mitosis.

The entire Complement of an organism's genes is termed its genotype, whereas the set of its observable traits is called the phenotype. In diploid organisms—which include the vast majority of known PLANT AND ANIMAL species—all genes are represented by at least two copies. Each Gene in a pair is called an allele. Simple phenotypic traits in diploid organisms, such as those studied by Mendel, are determined by the interaction of two alleles located at corresponding loci on homologous chromosomes. Both alleles may be identical (homozygous) or different (heterozygous). Consequently, Mutations are more difficult to detect in diploid organisms than in haploid ones, although mutations in haploid organisms can hold significant evolutionary importance. Several Selection/21.html">Types of mutations exist: point mutations, deletions, position effects, inversions, translocations, and changes in chromosome number.

Despite the presence of both alleles in the genotype, only one is expressed in the phenotype. The allele that manifests in the phenotype is called dominant, while the one that remains phenotypically hidden is termed recessive. When two organisms, each heterozygous for a given pair of alleles, are crossed, the phenotypic ratio of dominants to recessives in the progeny is 3:1. If the action of one allele is insufficient to mask The Effect of the other (incomplete dominance), the phenotypic ratio becomes 1:2:1.

In eukaryotes, genes are typically represented by multiple copies—sometimes hundreds or even thousands—which may be scattered across the chromosome or clustered together. Major Groups of duplicated genes include satellite sequences, tandem clusters, Transposons, and multigene families. Tandem gene clusters are responsible for synthesizing substances (such as Histones) required by The Cell in large quantities. Transposons are gene sequences repeated thousands of times that can spontaneously change their position on the chromosome, occasionally mobilizing other genetic material along the way, which can profoundly affect Gene Expression.

Such traits typically result from the interaction of many genes, which largely explains why scientists prior to the twentieth century found it so difficult to uncover the laws of genetics.

The segregation pattern for most traits is continuous and controlled by multiple genes. Some of these genes influence the expression of others (a phenomenon known as epistasis). Furthermore, a single gene can control multiple phenotypic traits in an organism (pleiotropy).