Theme 5: Sex Chromosomes and Linkage
16.5 Pa(erns of Inheritance Observed in Family
Histories
• Segrega'on of alleles occurs in human meiosis BUT, the results aren't
easily observed
◦ Humans don't choose their ma'ng pa@erns for the convenience of
biologists
◦ Number of children in human families is small and so, Mendelian
ra'os are obscured by random fluctua'on
◦ BUT, pa@erns are easiest to spot for inherited traits that are rare
because the mutant alleles occur only in affected individuals and
their close rela'ves
• Pedigree- Record of the ancestral rela'onships among individuals
summarized in a diagram of family history
16.5.1 Dominant traits appear in every genera@on
• Brachydactyly: Middle long bone in the fingers fails to grow and so, the
finger remains very short; The below pedigree was the first
demonstra'on of dominant Mendelian inheritance in humans
• Results from muta'on in gene whose normal product is a
protein involved in car'lage forma'on which is needed for
bone growth
• Suggests dominant inheritance:
▪ Affected individuals are equally likely to be females or
males
▪ Most ma'ngs that produces affected offspring have
only one affect parent; This occurs because the
brachydactyly trait is rare, and therefore a ma'ng
between two affected individuals is extremely unlikely
▪ Among ma'ngs in which one parent is affected, ~1/2
the offspring are affected
, • If dominant trait is rare, affected individuals will almost always be
heterozygous
◦ In a ma'ng in which one parent is heterozygous for the dominant
gene and the other is homozygous recessive, 1/2 the offspring are
expected to be heterozygous and the other 1/2 homozygous
recessive
16.5.2 Recessive traits skip genera@ons
• Albinism: The amount of melanin pigment in the skin, hair and eyes is
reduced; Very high frequency in Hopi and other Na've American tribes
of the Southwest (~1/200)
• Due to muta'on in the gene OCA2 which encodes a
membrane transporter protein
▪ Recall: Another type of muta'on affec'ng expression
of this gene is associated with blue eyes
• Suggests recessive inheritance:
▪ Trait may skip 1/+ genera'ons
▪ Affected individuals are equally likely to be females or
males
▪ Affected individuals may have unaffected parents
• For a recessive trait that is quite rare, virtually
all affected individuals have unaffected parents
▪ Affected individuals o]en result from ma'ng between
rela'ves (usually first cousins)
▪ Above traits occur because recessive genes can be
transmi@ed from genera'on to genera'on without
manifes'ng the recessive phenotype
• Ma'ng between rela'ves o]en allows rare
recessive alleles to become homozygous
because an ancestor that is shared between the
rela'ves may carry the gene (Aa)
• Recessive gene in the common ancestor can be
transmi@ed to both parents, making them each
a carrier of the gene as well
16.5 Pa(erns of Inheritance Observed in Family
Histories
• Segrega'on of alleles occurs in human meiosis BUT, the results aren't
easily observed
◦ Humans don't choose their ma'ng pa@erns for the convenience of
biologists
◦ Number of children in human families is small and so, Mendelian
ra'os are obscured by random fluctua'on
◦ BUT, pa@erns are easiest to spot for inherited traits that are rare
because the mutant alleles occur only in affected individuals and
their close rela'ves
• Pedigree- Record of the ancestral rela'onships among individuals
summarized in a diagram of family history
16.5.1 Dominant traits appear in every genera@on
• Brachydactyly: Middle long bone in the fingers fails to grow and so, the
finger remains very short; The below pedigree was the first
demonstra'on of dominant Mendelian inheritance in humans
• Results from muta'on in gene whose normal product is a
protein involved in car'lage forma'on which is needed for
bone growth
• Suggests dominant inheritance:
▪ Affected individuals are equally likely to be females or
males
▪ Most ma'ngs that produces affected offspring have
only one affect parent; This occurs because the
brachydactyly trait is rare, and therefore a ma'ng
between two affected individuals is extremely unlikely
▪ Among ma'ngs in which one parent is affected, ~1/2
the offspring are affected
, • If dominant trait is rare, affected individuals will almost always be
heterozygous
◦ In a ma'ng in which one parent is heterozygous for the dominant
gene and the other is homozygous recessive, 1/2 the offspring are
expected to be heterozygous and the other 1/2 homozygous
recessive
16.5.2 Recessive traits skip genera@ons
• Albinism: The amount of melanin pigment in the skin, hair and eyes is
reduced; Very high frequency in Hopi and other Na've American tribes
of the Southwest (~1/200)
• Due to muta'on in the gene OCA2 which encodes a
membrane transporter protein
▪ Recall: Another type of muta'on affec'ng expression
of this gene is associated with blue eyes
• Suggests recessive inheritance:
▪ Trait may skip 1/+ genera'ons
▪ Affected individuals are equally likely to be females or
males
▪ Affected individuals may have unaffected parents
• For a recessive trait that is quite rare, virtually
all affected individuals have unaffected parents
▪ Affected individuals o]en result from ma'ng between
rela'ves (usually first cousins)
▪ Above traits occur because recessive genes can be
transmi@ed from genera'on to genera'on without
manifes'ng the recessive phenotype
• Ma'ng between rela'ves o]en allows rare
recessive alleles to become homozygous
because an ancestor that is shared between the
rela'ves may carry the gene (Aa)
• Recessive gene in the common ancestor can be
transmi@ed to both parents, making them each
a carrier of the gene as well
