Classical Genetics

Classical genetics is based on cell division involving chromosomes, which consist of units of inheritance called genes. Somatic cells containing 46 chromosomes (22 pairs of automosomes and two sex chromosomes, either XX or YY) divide by mitosis. Mitosis in a single somatic cell produces two daughter cells each with the diploid number of chromosomes (46). Meiosis occurs only in germ cells and produces two gametes each with the haploid number of chromosomes (23). For a detailed description of mitosis and meiosis see any standard genetics text in Suggested Reading.

The genes on the chromosomes obey mendelian laws as follows:

  1. Inheritance is based on "factors" (genes) that are transmitted to offspring as discrete units. For each trait a person has two factors (alleles), one from each parent.
  2. The two alleles always segregate and pass to different gametes.
  3. Factors for different traits assort to gametes independently, i.e., maternal and paternal genes randomly recombine in the gametes.

Over the years several refinements have been made to Mendel's laws. Because closely linked genes on the same chromosome do not assort independently, they are always inherited together. In contrast, genes whose loci are far apart on a chromosome may experience crossing over in which a new combination of genes (recombinant) is present in an offspring. Also, nondisjunction may occur (a rare event) when two chromosomes fail to separate during meiosis so that both pass to one daughter cell.

The tenets of classical genetics can be applied to the inheritance of blood group genes. To illustrate, for the ABO blood group system parents who are AO and BO can produce offspring of all ABO groups (Figure 1-1).

Instead of normal segregation, nondisjunction may occur, albeit rarely (Figure 1-2).

For closely linked genes on a chromosome the Rh blood group system serves as a model. Because Dd, Cc, and Ee loci are closely linked on a chromosome, crossing over does not occur (Figure 1-3).

To illustrate cross over, the MNSs blood group system is useful (Figure 1-4). In this system M and N are alleles at one locus as are S and s. The MN locus and Ss locus are linked but not as closely as loci in the Rh system.

In transfusion medicine we use these principles when investigating cases of hemolytic disease of the newborn, when researching whether newly discovered blood group genes are independent systems or are part of an existing system, and when doing paternity disputes. The application of mendelian genetics to blood groups will be illustrated again later when some of the common blood group systems are discussed.

Enrichment Activity 1

Today genetics has progressed to where we can now clone sheep, which has led to an ongoing debate over the implications for human cloning. To identify some of the issues, read at least three of the following brief articles from the British Medical Journal (BMJ).

  • Cloning may cause health defects [BMJ 1999; 318: 1230.]

  • Sheep cloned by nuclear transfer [BMJ 1996; 312: 658.]

  • British public opposes human cloning [BMJ 1998; 317: 1613.]

  • UK authorities recommend human cloning for therapeutic research [BMJ 1998; 317: 1613.]

  • Fifty mice cloned by new technique [BMJ 1998; 317: 298.]

  • Bills on human cloning are full of loopholes [BMJ 1998; 316: 571.]

  • Public consultation on human cloning launched [BMJ 1998; 316: 411.]

  • The promise of cloning for human medicine [BMJ 1997;314:913.]

    1. Post a comment or question to the class mailing list about any issue raised in the BMJ articles.

    2. Also post a comment or reply to the class mailing list in response to a comment or question made by another participant.

    Terms to look up

    Historical Perspective Classical Genetics Genetic Symbols