Discontinuous variation and continuous variation can both be measured, but in different formats. Discontinuous variation normally applies to variations that beings either have or do not have. It can be things such as blood type or being able to roll your tongue. They are hard to measure in a complete range and are often placed in bar graphs. This is controlled by alleles of a single gene or a small number of genes. Environment also has a little effect on this discontinuous variation.
Continuous variation can be measured over a complete range of measurements. It is often placed in a line graph or histogram format since it is easier to measure. Continuous variation includes measurements such as height, shoe size, and weight. This is a combined effect of many genes from inheritance. This can be affected by environmental influences and the genetic make-up.
Most of the cells in a human contain two copies of each of 22 different chromosomes. In addition, there is a pair of chromosomes that determines sex: a female contains two X chromosomes and a male contains one X and one Y chromosome. Transmission of genetic information to offspring occurs through egg and sperm cells that contain only one representative from each chromosome pair. An egg and a sperm unite to form a new individual. The fact that the human body is formed from cells that contain two copies of each chromosome explains many features of human heredity, such as how variations that are hidden in one generation can be expressed in the next.
There are different reasons that variations can occur. Mutations are one of those reason. There can be additional chromosomes and single gene mutations from imperfect DNA replication. Crossings can also be done the wrong way. Deletions and substitutions also occur and can cause everything serious diseases or nothing at all. Changes in DNA (mutations) occur spontaneously at low rates. Some of these changes make no difference to the organism, whereas others can change cells and organisms. Only mutations in germ cells can create the variation that changes an organism’s offspring.
Before DNA and chromosomes can be described, an understanding of variations, how these variations are induced, and the discreteness of these variations must be developed. Examination of characteristics, the identification of a trait and how a trait varies, is of prime importance. Measuring the heights of humans provides a common experience for most individuals and lays the groundwork for distinguishing between continuous and discontinuous variations. These experiences lead to the concept of phenotype as an observable characteristic. No mention of DNA, genes, or chromosomes is necessary at this time.
By examining traits, it is easy to observe some that are discrete and others that seem to demonstrate a blending. Analysis of human pedigrees will often reveal that some characteristics skip generations and do not produce individuals with an intermediate version of the trait. Other characteristics seem to exhibit a blending, somewhat as the blending of colored light or pigments. Certain plants offer easy systems to observe this blending and discreteness. It is also important to realize that traits are not always present in a single generation and may or may not be associated with gender. These observations provide evidence for sex-linked traits to be discussed later. Differences in chromosome number in humans and the resulting syndromes are interesting phenomena to students. However, unless students understand how chromosomes are gained or excluded in gamete production these syndromes lose much of their significance. Nondisjunction in fruit flies can be connected to sex-linked traits as the ultimate correlation of genes with chromosomes.
Because chromosomes exist in pairs, genes will exist in pairs. Genes coding for the same trait can exist in different forms. Genes can be expressed in the outward appearance of an organism, or genes may be concealed. If a gene is expressed it is said to be dominant. If the gene is concealed it is said to be recessive. Many other interactions of genes may occur, for example, as incomplete dominance or blended characters and as co-dominance, where both characters are expressed. Many other types of gene expressions result in producing the tremendous variations found in one species.