Pedigree -- Isn't that a brand of dog food?
Well, there may be a brand of dog food named Pedigree, but pedigrees are also an important tool in analyzing genetic inheritance in a family. Below is an example of a pedigree:
Looks confusing, eh? Don't worry, it's all really quite simple: just think of a pedigree as a puzzle.
But what's it all mean?
A pedigree is just a family tree that shows the phenotypes of each member of the family for a particular trait. Here's what all the symbols mean:
So, all the squares are male, the circles are female, and if filled that person expresses the recessive phenotype. Please note that even though a male or female may be marked as dominant, that means that he or she expresses the dominant phenotype; therefore, someone who is dominant may still be heterozygous and carry the recessive allele. A pedigree does not show genotypes!
What's the point of pedigrees?
Pedigrees are an extremely useful tools used to help deduce a person's genotype for a particular trait (usually for a genetic disease or undesirable trait, like colorblindness), and to determine what his or her offspring may have. For example, if a man is colorblind and we can deduce from his wife's family tree's pedigree that she is a carrier for colorblindness, we can deduce that half of his children (regardless of sex) will be colorblind.
Pedigrees for Dummies
Analyzing pedigrees can be tough, but we're here to make it easy. So, we've broken things down and we are about to provide yoy with a list of rules to help you determine the genotype of an individual on a pedigree. If you just follow these rules, you have mastered the world of pedigrees! But first, we must tell you about the two types of pedigrees: autosomal pedigrees and sex linked pedigrees.
An autosomal pedigree is probably one of the most common pedigrees that you will find. If a trait is inherited on any chromosome except for the sex chromosomes (X or Y), then you are studying an autosomal pedigree.
Here are the rules to follow if you are trying to determine a proband's genotype:
Sex Linked Pedigrees
A sex linked pedigree deals with a trait that is found on the X chromosome (one of the sex chromosomes). These can be a little tricky, because the thing you have to remember is that the allele is only carried on the X chromosome. Since males are XY, they will only carry one allele; therefore the allele that they possess will directly be reflected in the genotype. Females, on the other hand, are XX, so they carry two alleles per trait. A female who is heterozygous for a sex-linked trait is said to be acarrier. A carrier expresses the dominant phenotype, but can still pass on her recessive allele to her male sons.
For practice using pedigrees, please take our pedigree quiz.
Learn more about another important tool used in genetic analysis: karyotypes!
1. The University of Utah School of Medicine, The Eccles Institute of Human Genetics, and The Utah Museum of Natural History. "Karyotype Analysis." 1996-1997. <http://raven.umnh.utah.edu/units/karyotyping/index.html >
2. Magloire, Kim. Cracking the AP Biology. New York: Random House, Inc., 1997.
3. Curtis, Helena and Barnes, N. Sue. Biology. New York: Worth Publishers, Inc., 1989.