Heredity of phenotypic traits: Father and son with prominent ears and crowns. Bases are in the centre, surrounded by phosphate—sugar chains in a double helix.
The frequencies of recombination are approximately proportional more Genes and Enzymes Early genetic studies focused on the identification and chromosomal localization of genes that control readily observable characteristics, such as the eye color of Drosophila. How these genes lead to the observed phenotypes, however, was unclear.
The first insight into the relationship between genes and enzymes came inwhen it was realized that the inherited human disease phenylketonuria see Molecular Medicine in Chapter 2 results from a genetic defect in metabolism of the amino acid phenylalanine.
This defect was hypothesized to result from a deficiency in the enzyme needed to catalyze the relevant metabolic reaction, leading to the general suggestion that genes specify the synthesis of enzymes.
Clearer evidence linking genes with the synthesis of enzymes came from experiments of George Beadle and Edward Tatum, performed in with the fungus Neurospora crassa. In the laboratory, Neurospora can be grown on minimal or rich media similar to those discussed in Chapter 1 for the growth of E.
For Neurospora, minimal media consist only of salts, glucose, and biotin; rich media are supplemented with amino acids, vitamins, purines, and pyrimidines. Beadle and Tatum isolated mutants of Neurospora that grew normally on rich media but could not grow on minimal media. Each mutant was found to require a specific nutritional supplement, such as a particular amino acidfor growth.
Furthermore, the requirement for a specific nutritional supplement correlated with the failure of the mutant to synthesize that particular compound. Thus, each mutation resulted in a deficiency in a specific metabolic pathway. Since such metabolic pathways were known to be governed by enzymes, the conclusion from these experiments was that each gene specified the structure of a single enzyme—the one gene-one enzyme hypothesis.
Many en-zymes are now known to consist of multiple polypeptides, so the currently accepted statement of this hypothesis is that each gene specifies the structure of a single polypeptide chain. Identification of DNA as the Genetic Material Understanding the chromosomal basis of heredity and the relationship between genes and enzymes did not in itself provide a molecular explanation of the gene.
Chromosomes contain proteins as well as DNAand it was initially thought that genes were proteins. The first evidence leading to the identification of DNA as the genetic material came from studies in bacteria.
These experiments represent a prototype for current approaches to defining the function of genes by introducing new DNA sequences into cells, as discussed later in this chapter.
The experiments that defined the role of DNA were derived from studies of the bacterium that causes pneumonia Pneumococcus. Virulent strains of Pneumococcus are surrounded by a polysaccharide capsule that protects the bacteria from attack by the immune system of the host. Because the capsule gives bacterial colonies a smooth appearance in culture, encapsulated strains are denoted S.
Mutant strains that have lost the ability to make a capsule denoted R form rough-edged colonies in culture and are no longer lethal when inoculated into mice. In it was observed that mice inoculated with nonencapsulated R bacteria plus heat-killed encapsulated S bacteria developed pneumonia and died.
Importantly, the bacteria that were then isolated from these mice were of the S type. Subsequent experiments showed that a cell-free extract of S bacteria was similarly capable of converting or transforming R bacteria to the S state.
Thus, a substance in the S extract called the transforming principle was responsible for inducing the genetic transformation of R to S bacteria. In Oswald Avery, Colin MacLeod, and Maclyn McCarty established that the transforming principle was DNAboth by purifying it from bacterial extracts and by demonstrating that the activity of the transforming principle is abolished by enzymatic digestion of DNA but not by digestion of proteins Figure 3.
Although these studies did not immediately lead to the acceptance of DNA as the genetic material, they were extended within a few years by experiments with bacterial viruses.
In particular, it was shown that, when a bacterial virus infects a cell, the viral DNA rather than the viral protein must enter the cell in order for the virus to replicate.
Moreover, the parental viral DNA but not the protein is transmitted to progeny virus particles. The concurrence of these results with continuing studies of the activity of DNA in bacterial transformation led to acceptance of the idea that DNA is the genetic material.
DNA is extracted from a pathogenic strain of Pneumococcus, which is surrounded by a capsule and forms smooth colonies S.Jul 20, · Science has made mice look good by reversing age-related wrinkles and hair loss at the genetic level.
Humanity could get a similar make-over in the future. “Wrinkled skin and hair loss are hallmarks of aging. Environmental factors also can influence human behavior.
Behavioral geneticists work to discover how much of people's behavior is determined by the genetic information they inherited from their parents and how much is caused by their living conditions, learning .
Mice are often used as a model for human genetic behavior since mice and humans have homologous genes coding for homologous proteins that are used for similar functions at some biological levels. Mice aggression studies have led to some interesting insight in human aggression. Understanding the role of genetic variation in addiction genes can also help inform treatments.
The effectiveness of medications vary from person to person, depending on their genetic make-up. In the future, genetic tests could be used to determine which medications are likely to be most effective based on an individual’s genetic profile.
Selective breeding is the process of breeding plants and animals for particular traits.
Typically, strains that are selectively bred are domesticated, and the breeding is sometimes done by a professional breeder. Heredity - Epistatic genes: Some genes mask the expression of other genes just as a fully dominant allele masks the expression of its recessive counterpart.
A gene that masks the phenotypic effect of another gene is called an epistatic gene; the gene it subordinates is the hypostatic gene.
The gene for albinism (lack of pigment) in humans is an epistatic gene.