Pedigree Genetics

What is a pedigree?

A pictorial family history tracing inheritance of one or more characteristics.

Why are pedigrees useful in human genetics?

Controlled human crosses cannot be performed, families are small, and generation times are long.

What is a proband?

The individual from whom a pedigree investigation begins.

What does a square represent?

Male.

What does a circle represent?

Female.

What does a shaded symbol represent?

An affected individual.

What does an open symbol represent?

An unaffected individual.

What does a double mating line represent?

Mating between relatives.

What do Roman numerals label?

Generations.

What does II-3 mean?

Third individual in generation II.

What does the horizontal line between partners mean?

A mating or parental relationship.

What does a vertical line descending from parents indicate?

Offspring.

What genotype expresses a standard autosomal recessive trait?

aa.

What is the genotype of an unaffected carrier for autosomal recessive inheritance?

Aa.

Can an autosomal recessive trait skip generations?

Yes.

Does autosomal recessive inheritance usually affect both sexes equally?

Yes.

What fraction of offspring from Aa × Aa is affected?

1/4, or 25%.

What fraction of offspring from Aa × Aa are heterozygous carriers?

1/2, or 50%.

What genotype results from aa × AA?

All offspring are Aa carriers.

Why can related-parent matings increase autosomal recessive traits?

Relatives are more likely to share the same recessive allele.

What rare-disorder assumption is made for unrelated spouses in lecture pedigrees?

Individuals marrying into the family are assumed not to carry the rare disease allele.

What genotypes express autosomal dominant inheritance in the standard model?

AA or Aa.

What is the unaffected autosomal dominant genotype?

aa.

Can autosomal dominant traits show father-to-son transmission?

Yes.

What proportion of Aa × aa offspring are expected to be affected?

1/2, or 50%.

Do autosomal dominant traits usually skip generations?

No.

What parental pattern suggests autosomal dominant inheritance?

Affected individuals usually have an affected parent.

Which chromosome carries X-linked traits?

The X chromosome.

Why are males often more frequently affected by X-linked recessive traits?

They usually have one X chromosome, so one recessive allele can produce the trait.

What is the genotype of an X-linked recessive affected male?

XᵃY.

What is the genotype of an X-linked recessive carrier female?

XᴬXᵃ.

What is the genotype of an X-linked recessive affected female?

XᵃXᵃ.

Can an X-linked father transmit his X chromosome to his son?

No; sons inherit the father's Y chromosome.

What fraction of a carrier mother's sons are affected in XᴬXᵃ × XᴬY?

1/2 of sons.

What fraction of daughters are carriers in XᴬXᵃ × XᴬY?

1/2 of daughters.

For XᵃY × XᴬXᴬ, what are the daughters?

All XᴬXᵃ carriers.

Does X-linked recessive inheritance commonly skip generations?

Yes.

How many dominant X-linked trait alleles can cause expression?

One.

What fraction of daughters inherit an affected father's X-linked dominant allele?

All daughters.

What fraction of sons inherit an affected father's X-linked dominant allele?

No sons.

What fraction of sons and daughters of a heterozygous X-linked dominant mother inherit her disease allele?

1/2 of each sex.

Do X-linked dominant traits typically skip generations?

No.

What distinguishes an affected father in X-linked dominant versus recessive inheritance?

For dominant inheritance, all daughters are affected; in the lecture's recessive cross with a noncarrier mother, all daughters are carriers.

Which sex expresses Y-linked inheritance?

Males.

What fraction of sons inherit a father's Y-linked allele?

All sons.

Do daughters inherit the paternal Y chromosome?

No.

Can a Y-linked trait skip a generation in a father-to-son line in the lecture model?

No.

What pattern strongly suggests Y-linked inheritance?

Every son of an affected father is affected, while daughters are not.

What inheritance pattern best explains the pedigree in slides 11-14?

X-linked recessive.

Why is X-linked dominant inheritance ruled out in the lecture example?

Affected II-6 has an unaffected daughter III-9.

Why is Y-linked inheritance ruled out in the lecture example?

Affected II-6 has four unaffected sons.

Who are the likely carrier females in the chapter example?

II-3 and III-9.

Why is autosomal dominant inheritance ruled out in the lecture example?

Affected individuals occur without an affected parent.

Does lack of father-to-son transmission alone prove X-linked inheritance?

No; it is one clue, not proof.

What does an unaffected mother with an affected son suggest?

A recessive mode, potentially X-linked recessive if the broader pedigree fits.

Which inheritance patterns allow father-to-son transmission?

Autosomal recessive, autosomal dominant, and Y-linked.

Which inheritance patterns do not show father-to-son transmission?

X-linked recessive and X-linked dominant.

What is the first step in interpreting a pedigree?

Identify affected individuals by sex and generation.

Why should you examine the entire pedigree instead of one family?

Several inheritance patterns may explain a single family, but the larger pattern can rule them out.