Chromosomes were not discovered until the end of the nineteenth century, so Mendel was never able to suggest any physical basis for his genetic theories. It was not until the science of cytology (the study of cells) was founded that scientists started to

Image Not Available examine cells and their replication more closely. They discovered that somatic cells (that is, nonrepro-ductive cells) consistently went through a pattern of division in which chromosomes were duplicated and separated between two new daughter cells.

Walter Sutton and Theodore Boveri, working with grasshopper cells, were the first scientists to notice that chromosomes in somatic cells occur in pairs. Sutton and Boveri suggested a connection between the pairs of chromosomes and Mendelian genetics. They believed that chromosomes carried the units of inheritance and that the way chromosomes divided accounted for how Mendel's laws functioned. Their work formed the basis for the chromosomal theory of inheritance.

The chromosomal theory of inheritance suggested that Mendel's genes reside on chromosomes and that when plants and animals reproduce, half their genetic material comes from each parent, forming sets of chromosomes. For example, barley has fourteen chromosomes in each somatic cell. Seven of those chromosomes are contributed from the "mother" plant and seven from the "father" plant, to make a total of fourteen chromosomes in the offspring. Therefore, half of the genes from all organisms come from each parent to determine the progeny's genetic makeup.

Each chromosome is essentially one long, linear strand of deoxyribonucleic acid (DNA), wrapped up and compacted for easy duplication and transport by the cell. There are two copies of each chromosome (called homologous pairs or homologs) in every somatic cell of an organism, each with the same physical appearance.

Such a cell with two copies of each chromosome type is called a diploid cell. Reproductive cells, known as gametes, have half the number of chromosomes and are known as haploid cells. It is these hap-loid cells from each parent that comprise the new diploid cells of the offspring. Half the chromo-

somes in each diploid cell come from each parental haploid cell.

Copies of the same gene on each chromosome pair are found at the same location (called a locus) and control traits of the organism. The copies of the same gene at a locus are called alleles. For example, one copy of chromosome #1 might be from the male parent and have a dominant allele (symbolized by A), and the other copy of chromosome #1 might be from the female parent and have a recessive allele (symbolized by a). These two alleles together (Aa), each on a separate chromosome, would constitute the genotype, and the expression of these alleles produces thephenotype (physical traits of an organism).

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