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Pedigrees and Autosomal Inheritance
Pedigree – a chart that shows the genetic relationships between individuals in a
family
-using a pedigree chart and Mendelian genetics, you can determine whether the
allele for a given trait is dominant, recessive, autosomal or sex-linked
female – unaffected
female – affected
male – unaffected
male – affected
Autosomal Inheritance = the inheritance of traits determined by genes found on
autosomal chromosomes (chromosomes 1-22 in humans).
Autosomal genes are responsible for many inherited genetic disorders, such as
Huntington disease and cystic fibrosis. These disorders may be classified as
either autosomal dominant or autosomal recessive.
Autosomal dominant = the inheritance of a dominant phenotype whose gene is
on an autosomal chromosome. Therefore this type of disorder occurs when the
disease-causing allel is dominant and the individual has one or both copies of the
allele. An example is Huntington disease.
Here is an example of a pedigree with an autosomal dominant disorder:
Characteristics of pedigrees showing an autosomal dominant disorder:
1) The disorder is present in every generation (children with trait have at least one
parent with trait).
2) Affects males and females equally.
3) Once a trait is no longer in the family, it will not reappear.
Autosomal recessive = the inheritance of a recessive phenotype whose gene is
on an autosomal chromosome. Therefore this type of disorder occurs when the
disease-causing allele is recessive and an individual has both copies of the allele.
An example is cystic fibrosis.
Here is an example of a pedigree with an autosomal recessive disorder:
Characteristics of pedigrees showing an autosomal recessive disorder:
1) The disorder can (but not always) skip a generation, therefore children with the
trait have parents without trait.
2) Affects males and females equally.
3) Two unaffected parents can have an affected child.
Check your understanding:
The following is a pedigree chart illustrating the occurrence of an autosomal trait
in family.
Q: Is the trait dominant or recessive?
Task – Identify any heterozygous individuals. Do so by writing Ho or He under
each circle or square.
Now answer questions 1 – 4. Write your answers here.
1) For each pedigree below, tell if the trait can be autosomal dominant or
autosomal recessive. If the pedigree cannot fit a mode of inheritance, tell why.
Pedigree A
Pedigree B
Pedigree C
2) Albinism is inherited as an autosomal recessive. In the figure below, assuming
that persons from the general population are not heterozygous for albinism (Aa),
what are the genotypes of all persons whose genotypes are known? (i.e., indicate
the genotypes on the figure for all known AA, Aa, and aa individuals)
3) Which type of inheritance pattern, autosomal domiant or autosomal recessive,
is likely to remain hidden in a family, only to appear in a future generation?
Explain your answer.
4) Construct a pedigree below using the following information. Albinism, a lack of
skin pigmentation, is a recessive phenotype. An individual with this disorder is
referred to as albino. Two parents with normal pigmentation have four children,
three girls and a boy. The eldest girl and the boy are both albino. Determine the
genotypes of all the members of the family. For those whose genotype cannot be
determined, what is the probability that they are heterozygous? (Hint: Since you
know they are not albino, do not count that probability in your calculation).