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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: • • • • • 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: • • condition appears in every generation allele is usually present in heterozygous genotype Autosomal – Recessive Pedigree Clues: • • • • 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: • • condition occurs predominantly in males affected males do not have affected offspring X linked – Dominant Clues: • • affected males always pass the condition to daughters affected females are usually heterozygous Y linked Pedigree Clues: • condition occurs in all male descendants and only males