Propositus: in a human pedigree, the person which first came to the attention of the geneticists; usually the phenotype of the propositus exceptional in some way. To see single-gene inheritance the patterns in the pedigree have to be interpreted according to mendel"s law of segregation but humans -> few kids -> 3:1 ratios & 1:1 ratios not usually seen until many similar pedigrees are combined. The affected phenotype of an autosomal recessive disorder is inherited as a recessive allele unaffected allele = dominant allele. Disorder appears in the progeny of unaffected parents. Affected progeny include both males and females-> dealing with simple mendelian inheritance of a gene on an autosome rather than a gene on a sex chromosome. Shows up in groups of affected siblings; ppl earlier and later in the generation tend not to be affected. Inbreeding increases the chances that two heterozygotes will mate -> higher chance of producing recessive disorders.