UNIT 5: Mendelian Genetics Notes (4)
Complex Patterns of Inheritance
LT BI3.a*:
When studying rare disorders, 6 general patterns of inheritance are observed:
 Autosomal recessive
o The disease can appear in male and female children of unaffected parents.
o e.g., cystic fibrosis (A = normal allele, a = disease allele)
AA =
Aa =
aa =
 Autosomal dominant
o Affected males and females appear in each generation of the pedigree.
o Affected mothers and fathers transmit the phenotype to both sons and daughters.
o e.g., Huntington disease (H = disease allele, h = normal allele)
HH =
Hh =
hh =
 Codominant inheritance
o Two different versions (alleles) of a gene can be expressed,
o Both alleles are present seperately in the phenotype
o E.g. Blood Type (A = A allele, B = B allele, o = O allele)
Ao =
Bo =
AB =
oo =
Incomplete dominance inheritance
o Two different versions (alleles) of a gene can be expressed together as a mixture
o The phenotype shows a mixture of both alleles
o E.g. Hair Texture (C = curly hair allele, S = straight hair allele)
CC =
CS =
SS =
 X-linked recessive
o Many more males than females show the disorder.
o All the daughters of an affected male are “carriers”.
o None of the sons of an affected male show the disorder or are carriers.
o e.g., hemophilia (XN = normal allele, Xn = disease allele, Y = no allele)
XNXn =
XnXn =
XNXN =
XNY =
XnY =
 X-linked dominant
o Affected males pass the disorder to all daughters but to none of their sons.
o Affected heterozygous females married to unaffected males pass the condition to half their
sons and daughters
o e.g. fragile X syndrome (XF = disease allele, Xf = normal allele, Y = no allele)
XFXf =
XfXf =
XFXF =
XFY =
XfY =