The Modern Study of Human Genetics Chapter 3

Polygenic Inheritance Polygenic traits, or continuous traits, are governed by alleles at two or more loci, and each locus has some influence on the phenotype Hair, eye and skin color, stature, shape of face, and fingerprint patterns are all examples of polygenic traits

Polygenetic Traits and Racial Categorization Polygenic traits account for most of the readily observable phenotypic variation seen in humans Traditionally serve as a basis for racial classification Skin coloration is determined by pigment produced by specialized cells called melanocytes The amount of melanin produced determines how dark or light skin will be Melanin production is influenced by interactions between several different loci that, until recently had not been identified

Mitochondrial Inheritance All cells contain mitochondria that convert energy into a form that can be used by the cell Animals of both sexes inherit their mtDNA, and all mitochondrial traits, from their mothers All the variation in mtDNA is caused by mutation, which makes it very useful for studying genetic change over time

Sex There are multiple ways of determining an individual’s gender including phenotypical sex, chromosomal sex, and genetic sex Y-Linkage Sry - gene that is responsible for the initiation of male sex determination in humans A person with two X chromosomes can be male if the Sry gene breaks off a Y chromosome in the father’s sperm and attaches to an X chromosome. An XY individual can be female if the Sry gene is not operating or if receptor sites on the cells do not respond to the presence of the hormone.

Antigens Large molecules found on the surface of cells Several different loci govern various antigens on red and white blood cells Foreign antigens provoke an immune response The ABO blood-type system consists of two basic antigens: A and B

Blood Type Inheritance Over 19,000 human trains are known to be inherited according to Mendelian principles Human ABO blood system The A and B alleles are dominant to the O allele. Neither the A or B allele are dominant to one another; they are codominant. The expression of two alleles in heterozygotes Neither allele is dominant or recessive Both influence the phenotype

Inherited Abnormalities Many inherited abnormalities involve errors in metabolism Another type of abnormality is one that leads to an anatomical problem. An example would be achondroplastic dwarfism Chromosomal Abnormalities Nondisjunction - A common error of meiosis which leads to abnormal chromosome numbers Down syndrome – the individual has three copies of chromosome 21 Extra or missing sex chromosomes More common than nondisjunction of autosomes are extra or missing sex chromosomes. Turner syndrome – female with a missing X Webbed neck, low-set ears and short stature Klinefelter syndrome - two or more X chromosomes in males Sterility, weak muscles, discoordination

Structural Aberrations of Chromosomes Several types of structural abnormalities can occur: Deletion: the chromosome breaks and a segment fails to be included in the second-generation cell Duplication: a section of chromosome is repeated Inversion: parts of a chromosome break and reunite in a reversed order Translocation: segments of chromosomes detach and then reattach to other non-homologous chromosomes

Genetics and Human Affairs Perhaps no other area of scientific discovery affects human affairs more profoundly than genetics. Current issues include: Eugenics Cloning Gene therapy Genetic engineering Genetic counseling Stem cell research Genomics Epigenetics