Learning Objectives
17.1 Gene Interactions
- Characterize polygenic inheritance, and specify how non-genetic factors can influence observed phenotypes.
- Explain how the expression of one gene could mask the expression of another.
17.2 Genes on the Same Chromosome: Linkage, Recombination, and Mapping
- Explain why testcrosses might yield recombinants instead of parental types.
- Describe how to calculate and interpret the map distance between two genes.
- Explain how recombination frequency is related to genetic distance.
17.3 Extranuclear Inheritance: Organelle Genomes
- Define extranuclear inheritance
- Explain why the presence of DNA in organelles leads to non-Mendelian inheritance.
17.4 X Inactivation, Genomic Imprinting, and Maternal Effect
- Compare and contrast X inactivation, genomic imprinting, and the maternal effect.
- Recognize how genomic imprinting can lead to non-Mendelian inheritance.
Chapter Summary
Mendel was fortunate that he chose straight forward traits. The inheritable characteristics he studied made it simple to calculate the predictable probabilities of gene expression in offspring. However, there are more complex genetic patterns associated with continuous variation, pleiotropic genes, lack of complete dominance, environmental modifications of genes, and epistasis. Many human genetics disorders follow Mendelian principles. Most are recessive like Tay-Sachs disease. Hunington’s disease is an example of a dominant allele that remains in populations because its effect is not expressed until after children are born. Human blood groups are an example of traits stemming from multiple alleles. In the ABO system, four phenotypes arise from the combination of three alleles coding for red cell surface antigens. The transmission of a genetic disorder can often be tracked through pedigree analysis, shown in example by Royal hemophilia in the lineages of the British monarchy. Disorders like sickle-cell anemia, are a result of nucleotide changes that alter the linear and three-dimensional structure of critical proteins. Current genetic research uses molecular techniques to try to cure disorders like muscular dystrophy by inserting new genes into disabled cells.
Modern geneticists have modified Mendel’s laws to be consistent with discovery of meiosis and crossing over, identification of chromosomes as hereditary material, and the structure of genes and DNA. Genetic crosses in which recombination is evident can be used to construct gene maps, identifying the location of alleles on chromosomes and specific positions within chromosomes. The Human Genome Project has produced vast amounts of data elucidating the genetic sequence of our own genome. A normal human cell possesses twenty-two pairs of autosomal and one pair of sex chromosomes for a total of forty-six chromosomes. Any variance from that number is detrimental and often lethal. Down syndrome, one of the few non-lethal trisomies, results from primary nondisjunction during meiosis. Abnormal separation of the sex chromosomes can result in individuals with extra or absent X or Y chromosomes. The minimal amount of sex chromatin needed for survival is a single X chromosome. A YO zygote fails to develop as the Y lacks the necessary information present on the X. Genetic counseling attempts to prevent the production of children with genetic disorders by identifying parents at risk. Prenatal diagnosis is valuable and uses amniocentesis, ultrasound, and/or chorionic villi sampling.
Mendel did not have an understanding of epigenetic factors that influence an organism’s characteristics. Eukaryotic cells are now known to be influenced by the genetic information carried in chloroplasts and mitochondria. These organelles can contribute to or modify gene expression of the cell’s genomic DNA. They are also subject to genetic variation that produces genetic disorders inherited by transfer of the organelle during gamete formation.
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