Table 6.1 Pedigree characteristics of autosomal recessive, autosomal dominant, X-linked recessive, X-linked dominant, and Y-linked traits

Autosomal recessive trait

1. Appears in both sexes with equal frequency.

2. Trait tends to skip generations.

3. Affected offspring are usually born to unaffected parents.

4. When both parents are heterozygous, approximately 1/4 of the offspring will be affected.

5. Appears more frequently among the children of consanguine marriages.

Autosomal dominant trait

1. Appears in both sexes with equal frequency.

2. Both sexes transmit the trait to their offspring.

3. Does not skip generations.

4. Affected offspring must have an affected parent, unless they possess a new mutation.

5. When one parent is affected (heterozygous) and the other parent is unaffected, approximately 1/2 of the offspring will be affected.

6. Unaffected parents do not transmit the trait.

X-linked recessive trait

1. More males than females are affected.

2. Affected sons are usually born to unaffected mothers; thus, the trait skips generations.

3. A carrier (heterozygous) mother produces approximately

1/2 affected sons.

4. Is never passed from father to son.

5. All daughters of affected fathers are carriers.

X-linked dominant trait

1. Both males and females are affected; often more females than males are affected.

2. Does not skip generations. Affected sons must have an affected mother; affected daughters must have either an affected mother or an affected father.

3. Affected fathers will pass the trait on to all their daughters.

4. Affected mothers

(if heterozygous) will pass the trait on to 1/2 of their sons and 1/2 of their daughters.

Y-linked trait

1. Only males are affected.

2. Is passed from father to all sons.

3. Does not skip generations.

eliminate. Because the trait appears only in males, autosomal dominant and autosomal recessive modes of inheritance are unlikely, because these occur equally in males and females. Additionally, autosomal dominance can be eliminated because some affected persons do not have an affected parent.

The trait is observed only among males in this pedigree, which might suggest Y-linked inheritance. However, with a Y-linked trait, affected men should pass the trait to all their sons, but here this is not the case; II-6 is an affected man who has four unaffected male offspring. We can eliminate Y-linked inheritance.

X-linked dominance can be eliminated because affected men should pass an X-linked dominant trait to all of their female offspring, and II-6 has an unaffected daughter (III-9).

X-linked recessive traits often appear more commonly in males, and affected males are usually born to unaffected female carriers; the pedigree shows this pattern of inheritance. With an X-linked trait, about half the sons of a heterozygous carrier mother should be affected. II-3 and III-9 are suspected carriers, and about /2 of their male children (three of five) are affected. Another important characteristic of an X-linked recessive trait is that it is not passed from father-to-son. We observe no father-to-son transmission in this pedigree.

X-linked recessive is therefore the most likely mode of inheritance.

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