Download Patterns of Inheritance Humans cannot be studied using planned

Patterns of Inheritance
Humans cannot be studied using planned crosses, so human geneticists rely on pedigree
charts, which show phenotype segregation in several generations of related individuals.
Pedigree facts:
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Since humans have such small numbers of offspring, human pedigrees do not
always show clear proportions.
The actual number of affected versus unaffected offspring is impossible to predict
for a certain couple because outcomes for small samples fail to (closely) follow
the expected outcomes.
The probability of a carrier (heterozygote) for a rare allele unknowingly marrying
another unrelated carrier is quite low.
Marriage between close relatives results in a higher likelihood that both parents
will be carriers of a rare allele and produce affected children.
The major use of pedigree analysis is in clinical evaluation and counseling of
patients with inherited abnormalities.
Instructions for Analyzing a Pedigree Chart:
1.
First, determine whether the trait is sex linked or autosomal.
If the trait is sex linked, it will be expressed more frequently in males, whereas if
it is autosomal it will appear in both sexes equally.
2.
Second, determine if the trait is dominant or recessive.
If the trait is dominant, every child that expresses the trait will have a parent that
expresses the trait; if it is recessive, both parents can appear normal as both
parents may be heterozygous for the trait.
Autosomal – Dominant Pedigree
Clues:
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condition appears in every generation
allele is usually present in heterozygous genotype
Autosomal – Recessive Pedigree
Clues:
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condition is typically rare
allele is usually present in heterozygous genotype
condition often skips generation
condition often occurs in matings of close relatives
X linked - Recessive Pedigree
Clues:
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condition occurs predominantly in males
affected males do not have affected offspring
X linked – Dominant
Clues:
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affected males always pass the condition to
daughters
affected females are usually heterozygous
Y linked Pedigree
Clues:
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condition occurs in all male descendants and
only males