Surprising New Twists in the Study of Inheritance

Tamsen K. Meyer and Cheryl H. Powers
1994 Woodrow Wilson Collection

Just when you thought you had Mendelian inheritance and molecular genetics "wired"...

  • How much of your DNA codes for proteins?

  • Why do eukaryotic cells have two types of DNA?

  • How do we explain the worsening of symptoms of a human genetic disease as it passes from one generation to the next?

  • Does the environment alter the structure or function of genes?

  • How can human genetic traits differ in expression if the disease is maternal or paternal in origin?

  • How stable is the sequence of information on the DNA molecule?

  • Is the genetic code really universal?

  • How can some traits be passed only from a mother to her offspring?
  • These are intriguing questions you and your students may be asked. Recent revisions to the basic mechanisms of modern genetics violate many long held ideas of inheritance. You may wish to have students investigate how an understanding of mitochondrial DNA, jumping genes, junk DNA, introns, genomic imprinting, anticipation, the "not-so-universal" genetic code, and directed mutations help answer these questions. The teacher may wish to develop a study guide that would accompany reference articles on these anomalies. Background information is summarized for the teacher, since older textbooks will not have references for these genetic surprises.

  • Form vs. function: Prokaryotic chromosomes are circular and have no associated proteins in their structure. Virtually all of the prokaryotic DNA is transcribed. Eukaryotic chromosomes are made of proteins called histones as well as DNA. Only 1%-10% of the information on the DNA molecule actually codes for functional genes. There are long stretches of DNA that code for no protein; these stretches (introns) are snipped out in mRNA processing before it leaves the nucleus. The coding regions that remain are called exons. There are transposable elements or jumping genes on eukaryotic and prokaryotic chromosomes that can disrupt the function of other genes; in addition to jumping from one chromosome to another, they sometimes jump from one organism to another.

  • Mitochondrial DNA: This DNA is closely related to bacterial DNA and its presence supports the endosymbiotic theory of origin of mitochondria and chloroplasts. Violations in the universality of the genetic code occur in mitochondrial DNA. In mammalian mitochondria, AGA, AGG = stop, not arginine; AUA = methionine, not isoleucine; in yeast mitochondria, AUA = methionine, CUU, CUA, CUC, CUG = threonine, not leucine.

    Some genes show only maternal inheritance because they are found on mitochondria which only come from the mother. Since mitochondrial replication continues throughout the life of the cell, if there is a mtDNA mutation over time, the proportion of mutant mitochondria may increase and cause clinical disease. Some human genetic diseases related to mistakes on the mitochondrial DNA affect the nervous, muscular, and cardiovascular systems as well as vision.

  • Genomic imprinting. Chromosomes seem to carry chemical tags that identify whether they originated in an organism's mother or its father. The paternal and maternal genomes are not equivalent and both are required for mammalian development. Uniparental disomy (an exception to Mendel's Law of Segregation) which implies that an offspring inherits two copies of a gene from one parent, yet none from the other, may underlie a whole range of inherited disorders. Some cases of cystic fibrosis have shown this type of inheritance, and this condition can also occur in somatic tissue, causing localized problems such as kidney and adrenal tumors. If a baby has two copies of chromosome 15 from its mother, it will develop Prader-Willi syndrome (short, fat, compulsive eater) and if the two number 15 chromosomes come from the father, Angelman syndrome, characterized by a small head, widely spaced teeth, clumsy jerky movements and inappropriate laughter, develops.

    Errors in chromosomal segregation in meiosis, result in egg or sperm with two copies of a particular chromosome or at least two copies of a part of that chromosome. DNA methylation most likely is the mechanism for turning off the alleles of one of the parents. Some scientists believe that introns may be involved in the process of repressing alleles from one parent.

  • Anticipation or amplification: the worsening of a phenotype in subsequent generations in a family; evidence has been found that some genes "expand." Triplet repeat diseases get worse as they are passed through generations. These are gene level changes and they involve expanding 3-base nucleotide sequences that are involved in gene regulation. Fragile X syndrome, which causes a wide range of symptoms of mental impairment, was the first triplet disease to be identified. Huntington's disease, Myotonic Dystrophy, and Spinal Bulbar Atrophy all have varying degrees of expansion in the certain sequences.

  • Directed mutations: Scientists have found indications that some organisms (yeast and bacteria) may respond to stressful changes in their environments increasing their mutation rate. This controversial idea resurrects Lamarck's inheritance of acquired characteristics at the molecular level.
  • Bibliography:

    Freedenberg, Debra. June 1994. New Genetics: A Clinical Update (lecture). Texas A & M Health Science Center.

    Lewis, Ricki. 1994. Human Genetics. Dubuque, IA: Wm. C. Brown.

    Lowenstein, Jerold M. December 1992. "Genetic Surprises" Discover, pp. 85-88.

    Rennie, John. March 1993. "DNA's New Twists," Scientific American, pp. 122-132.

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