Independent assortment and dihybrid crosses
  Linked genes and recombinant chromosomes
  Analysis of human pedigrees
  Chromosomal abnormalities


Analysis of human pedigrees

This is a good time to introduce another common way to examine inheritance across generations in humans: a pedigree analysis.

Humans are difficult subjects to deal with regarding the analysis of inheritance patterns. They have a small number of offspring (usually one) for every mix of gametes, so examining phenotypic and genotypic ratios of offspring is not an option. Also, humans cannot be subjected to test-crosses or other controlled breeding systems to obtain quantifiable data on inheritance of a trait. One way to circumvent these difficulties is to follow a trait over successive generations, charting its "behavior."  Gene behavior often offers clues to its mode of inheritance. For example, does the trait skip generations?  Such behavior indicates that the trait is recessive. (Can you explain why?)
Several family histories (pedigrees) can be examined to determine whether a trait is dominant vs. recessive and whether it is sex linked vs. autosomal.  Pedigrees are typically used to assess the probability of a particular family member passing on a trait whose inheritance pattern is known. In this part of the laboratory assignment, we will use pedigrees to determine the inheritance pattern of a trait. 

Use this interactive pedigree simulation to test your ability to analyze pedigrees. If the program does not work on your computer, use the help sheet diagrams instead (see diagrams 2).

A Guide for the Symbols used in the Pedigree Simulation:
The legend below describes the symbols used in pedigrees. 

male Denotes a male female Denotes a female
female-dark A darkened symbol denotes an individual who exhibits the trait, in this case a female.
line A line joining a male and female symbol denotes a union producing children.
vertical line A vertical line coming down from the parent union line joins a horizontal line where children are shown (sibling relationship) line. 
children The number and sex of children produced from a particular union are indicated by vertical lines joining the appropriate sex symbols to the horizontal line.

Remember that a black symbol indicates that the individual shows the trait in his/her phenotype.

Study the pedigrees until you understnd them. The help sheet gives advice for solving pedigree problems and provides hints on the types of reasoning that you should use to justify your answers (associating a particular mode of inheritance with a particular pedigree).

When you are proficient in analyzing pedigrees, answer questions 13 and 14. There will be a question on exam 3 that requires you to interpret a human pedigree.

Chromosomal abnormalities

You should be familiar with the consequences of abnormal chromosome number from the topic Linkage and Genetic Disorders. There are several well-known human disorders caused by mistakes during meiosis that produce gametes with extra or missing chromosomes. These conditions can be diagnosed by examining the karyotype of the affected individual. Before birth, tests of fetal membranes can determine whether the newborn will have one of these genetic disorders.

To review the construction of karyotypes and confirm your understanding of human disorders caused by aneuploidy, go to the Karyotyping Activity, a biology project on the University of Arizona website. Read the introduction, then click on "Patient Histories". You will find the histories of patients A, B, and C. For each patient, complete the karyotype. Then list the correct notation to characterize the patient's karyotype and make a diagnosis. (The directions are on the web site). When you have finished the exercise, answer question 15.

Chromosomal abnormalities are also found in cancerous cells. Because these cells divide in an unregulated fashion, mitosis is prone to mistakes. Aneuploidy is common in cancerous cells and many extra copies of chromosomes may be present. Because they divide rapidly and are often "immortal", cancer cell lines have been maintained in culture systems and utilized for biological and medical research. The first human cell line put to such use was established in 1951. These cells are still in use today and are named HeLa cells after the patient who supplied them.

hela cells This micrograph shows HeLa cells growing in a culture system.
chromosomes The DNA in these HeLa cells has been stained blue. Note the mitotic figure in the cell on the left.
karyotype These chromosomes have been isolated from a HeLa cell during metaphase and sister chromatids are clearly visible. How many chromosomes are present in this diploid cell?

While HeLa cells have been very useful as a research tool, their chromosome number is abnormal as can be seen in the above image. Different lines of HeLa cells have different numbers of chromosomes, as many as 100 in some cases. Read the first topic in the Wikipedia article, George Otto Gey and Henrietta Lacks, to learn more about the origin and uses of HeLa cells. Then answer question 16. If you are interested in HeLa cells, more information can be found in the magazine article listed in news of the Linkage and Genetic Disorders topic.

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