The pattern of those affected and unaffected in a family with a genetic disorder can help identify the likely manner of inheritance. This can aid the genetic counselor in evaluating and advising the family members.
Common patterns of inheritance:
(1) autosomal dominant
(2) autosomal recessive
(3) X-linked dominant
(4) X-linked recessive
|
Features |
autosomal dominant |
autosomal recessive |
|---|---|---|
|
usual recurrence risk |
50% |
25% |
|
usual sex ratio |
males = females |
males = females |
|
transmission pattern |
generation after generation; an affected person has an affected parent |
multiple siblings may be affected in one generation, with previous generations often unaffected |
|
those affected |
homozygous (severe but rare) or heterozygous (usual) |
homozygous; heterozygous may be mildly affected |
|
one parent heterozygous and one parent normal |
50% of children heterozygous (affected) |
50% heterozygous (unaffected) |
|
one parent homozygous and one parent normal |
100% of children heterozygous (affected) |
100% of children heterozygous (unaffected) |
|
one parent homozygous and one parent heterozygous |
100% of children affected (50% homozygous, 50% heterozygous) |
50% homozygous (affected), 50% heterozygous (unaffected) |
|
both parents homozygous |
100% of children homozygous (affected) |
100% of children homozygous (affected) |
|
both parents heterozygotes |
25% homozygous, 50% heterozygous (75% of children affected), 25% normal |
25% children homozygous, 50% children heterozygous, 25% children normal |
|
unaffected children of an unaffected parent |
will have unaffected children and grandchildren |
may have affected children or grandchildren if carrier |
|
other |
father-to-son transmission may occur |
consanguinity may be noted; father-to-son transmission may occur with homozygous father and heterozygous mother (rare) |
|
Features |
X-linked dominant |
X-linked recessive |
|---|---|---|
|
usual sex ratio |
females > males |
males >> females |
|
transmission pattern |
|
from unaffected or carrier females to sons |
|
heterozygous father (inherited X involved) |
father affected; father-to-son transmission never occurs; all daughters affected |
father affected; father-to-son transmission never occurs; all daughters will be carriers |
|
heterozygous mother |
50% of daughters will be affected; 50% of sons will be affected |
usually phenotypically normal; 50% of sons affected; usually 50% of daughters will be carriers |
|
unaffected children of unaffected parent |
unaffected sons usually do not transmit disease to children |
unaffected sons usually do not transmit disease to children |
|
other |
|
affected women are homozygous and have an affected father and a heterozygous or homozygous mother |
Limitations:
• Not all genetic conditions will follow one of the these 4 patterns.
• Clinical expression may be affected by other factors.
Specialty: Genetics
