HOW TO WORK PEDIGREE CHARTS

Pedigree charts are graphic representations of inheritance in a family. There are certain features common to all charts. Males are represented by squares and females by circles. If an individual exhibits a trait, the circle or square is filled in. If they lack a trait, the circle or square remains open. Marraiges (or matings) are indicated by lines connecting a circle to a square. Offspring are shown as connected lines under the parents.

Test questions often ask whether the trait indicated on a pedigree chart is

  1. sex-linked dominant
  2. sex-linked recessive
  3. autosomal dominant
  4. autosomal recessive
  5. holandric.

Holandric genes are often easy to eliminate because any female on the chart exhibiting the trait cannot be holandric (remember, only males are holandric).

Below is a pedigree chart tracing a trait in a family. Try to figure which of the five possiblilites listed above fit the pedigree chart.



The best way to work out the anwer is to simply choose one of five choices; i.e., sex-linked dominant. If the trait is sex-linked dominant, then for a female, XPXP or XPXp would indicate a female with the trait. The representation of XpXp would indicate a normal female that does not exhibit the trait.

For example, XPY would indicate a male with the trait and XpY would indicate a normal male, not exhibiting the trait.

To find out if the trait is sex-linked dominant, simply trace the pedigree chart out by substituting representations shown above for each figure. For example a female with a filled circle would have to have at least one gene for the trait. An open circle would indicate a female homozygous recessive for the trait.





The pedigree chart indicates it is not a sex linked dominant trait. What is the pattern of inheritance?



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