How to work a pedigree chart?
A brief note: while we give you the phenotypic ratios in this review, it is probably not an efficient use of your time to memorize these ratios. Instead, use the tables to test your understanding of the inheritance mechanisms. If you understand the mechanisms well, you should be able to generate the ratios quickly on test day.
Autosomal recessive traits require two copies of the recessive allele to be expressed.
Autosomal recessive pedigrees can look differently based on the genotype of the parents. Here is an example pedigree:
One trick for identifying a recessive trait is that if a trait skips a generation in a pedigree, it is often an autosomal recessive trait (although a trait can be autosomal recessive and not skip generations).
These traits appear with equal frequency in both sexes.
An autosomal dominant trait will result in the dominant phenotype if one or more copies of the dominant allele are present.
An autosomal dominant trait will result in the same ratios of dominant to recessive phenotype as seen above in the autosomal recessive chart!
In a pedigree this phenotype will appear with equal frequency in both sexes but it will not skip generations.
Here is an example of an autosomal dominant recessive pedigree:
X-linked recessive traits are carried on the X chromosome. Because male offspring receive only one copy of the X chromosome, the trait is expressed phenotypically in all men with the X-linked recessive allele. Female offspring can also express an X-linked recessive trait although only if they inherit two X-linked chromosomes (one from each parent) containing the recessive allele.
As you can see above, this trait is more likely to appear in males than females so the pedigree may contain more affected males than females
This trait is never passed from father to son, because a father carries the allele on his X chromosome, but always passes his Y chromosome on to any sons.
Here is a sample X-linked recessive pedigree:
Above are the three most common inheritance patterns that will appear in an introduction to inheritance patterns. Below is a shorter overview of two, more unusual, inheritance patterns that you may come across in your studies:
This kind of inheritance is less common than X-linked recessive. It occurs when a dominant gene is carried on the X chromosome. Unlike X-linked recessive traits, only one copy of the dominant X-linked allele is required to result in the phenotype in both male and female offspring.
This is a rare type of inheritance, where the gene of interest is located on the Y-chromosome. Because, at most, one copy of the Y chromosome is inherited, dominant and recessive don’t really apply here.
Because only males have a Y chromosome, female offspring of an affected father cannot express the trait
A pedigree chart is a diagram that displays the frequency and development of phenotypes from one generation to the next of a single gene or organism and its descendants, most notably humans, show dogs, and racehorses.
Pedigree is a heritage record, which is used in human genetics to map the origin of a single trait, abnormality, or disease. A male is represented by a square or symbol, a female is represented by a circle or symbol.
Pedigree charts can be used to determine genotypes by examining history, classify phenotypes and forecast whether a trait will be passed on in the future.
First of all, determine whether the condition is recessive or dominant. If the trait is dominant, the trait must be one of the parents'. If the phenotype is recessive, because they may be heterozygous, no parent is expected to have the phenotype.
Then, determine whether an autosomal or sex-linked (usually X-linked) trait appears on the chart. For example, males are often more frequently affected than females in the X-linked recessive traits.
A common misconception is that the involvement of multiple individuals affected within a family does not necessarily mean that the trait is dominant. The dominant and recessive words refer to the way in which a trait is displayed, not how often it occurs within a population.
You will not often be able to determine an individual's genotype based on a pedigree chart. For a trait, a person can often be either homozygous dominant or heterozygous. Also, in evaluating genotypes, we will use the relationships between a person and their mother, relatives, and offspring.
Draw any siblings in birth order from left (oldest) to right (youngest). Siblings are connected by a horizontal line above the symbols, with vertical lines connecting the symbols to the horizontal line. Leave space to add any partners and children. Add aunts, uncles, grandparents in the same manner.
Write the person's first name, or initials below the symbol.
Write the person's current age below the symbol.
Indicate the disease or disorder the individual has along with the age of onset below the symbol.
Next, draw the person’s parents. To indicate partners/marriage draw a horizontal line connecting the two symbols (see below). If the individuals are consanguineous (i.e. related) indicate consanguinity with a double horizontal line. If the degree of consanguinity is not clear on the pedigree, please write above the relationship line, i.e. “2nd cousins.”
Add the parents current age, or age at death (d. age or year) with cause of death. Also, indicate any diagnoses (dx. Disease X) the individuals may have along with the age at diagnosis (dx. Disease X 50y.o.).
Draw any siblings in birth order from left (oldest) to right (youngest). Siblings are connected by a horizontal line above the symbols, with vertical lines connecting the symbols to the horizontal line. Leave space to add any partners and children.
Add aunts, uncles, grandparents in the same manner. All affected individuals should be included in the pedigree and as many unaffected individuals as possible (parents, grandparents, and siblings of any affected individual).
For each individual add the following below their symbol
** The current place of residence (City, State) for each individual willing to participate in the study can be recorded on the pedigree, or must be provided to the IIHG as a separate list.
At the top of the pedigree write the ethnicity of each grandparent. Record the date the pedigree was obtained.
- Autosomal dominant pedigree.
- Autosomal recessive pedigree.
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