1. It is possible to ascertain the mode of inheritance of genes in organisms
where designed crosses and the production of large
numbers of offspring are not practical.
2. Pedigree: A family tree that shows the phenotype of the trait
in question for each member. Such a family tree is called a
3. By analyzing the pedigree, we may be able to predict how the gene
controlling the trait is inherited. If many similar
pedigrees for the same trait are found, the prediction is strengthened.
4. Conventions used to construct pedigrees:
Circles: represent females.
Squares: designate males.
Diamond: represent sex unknown.
Shaded: phenotype is expressed.
Half shaded: a recessive, heterozygous phenotype, a trait allele carrier.
Horizontal line: connecting parents.
Vertical line: Leading to their offspring. All such offspring are called sibs
Horizontal sibship line: Connecting sibs; Sibs are placed from
left to right according to birth order and are labeled with Arabic
Each generation is indicated by a roman numeral. I, II, III…
Twins are indicated by diagonal lines from the vertical line connected to the sibship line.
For identical twins, the diagonal lines are linked by a horizontal line,
fraternal twins lack this connecting line.