quick pedigree

Autosomal dominant: giveaway clue

every single affected child has at least one affected parent; the trait never skips a generation

Autosomal recessive: giveaway clue

two completely unaffected parents produce an affected child

X linked recessive: giveaway clue

only or mostly males are affected; unaffected carrier mothers have affected sons

X linked dominant: giveaway clue

an affected father passes the trait to ALL daughters and NO sons

Mitochondrial: giveaway clue

ALL children of an affected mother are affected; an affected father passes it to NO children

The one rule that eliminates all X linked patterns

if a father passes the trait to a son, it cannot be X linked because fathers give sons the Y chromosome not the X

How to tell autosomal recessive from X linked recessive

in autosomal recessive both males and females are affected equally; in X linked recessive almost only males are affected

How to tell autosomal dominant from mitochondrial

in autosomal dominant an affected father CAN pass it to children; in mitochondrial an affected father NEVER passes it to anyone

Two affected parents having an unaffected child

this only happens in autosomal dominant where affected parents are usually heterozygous (Aa) and can produce an aa unaffected child; impossible in autosomal recessive

Why an unaffected woman can have an affected son in X linked recessive

she is a carrier (XA Xa); she does not show the disease because her normal XA masks the recessive Xa, but she can still pass Xa to her sons who have no second X to mask it

Why daughters of X linked recessive carriers are not automatically affected

they receive the father's normal XA as well as possibly the mother's Xa; because the disease allele is recessive, having one normal XA is enough to mask it so they are unaffected carriers at most

Skipping a generation

a strong clue for recessive inheritance (autosomal or X linked recessive) because carriers look completely normal but pass the hidden allele to the next generation