1. The Modern Study of Human Genetics
Chapter 3

2. 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

3. 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

4. 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

5. 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.

6. 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

7. 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

8. 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

9. 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

10. 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