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NEL The Basis of Heredity 605

Pedigree Charts 18. 318. 3

Pedigree analysis is another tool for solving genetic problems. This approach is especially

useful when it is not possible to perform crosses using specific individuals or to generate

large numbers of progeny, such as for humans. A pedigree chartis like a family tree

in which the inheritance of a trait can be traced from parents to offspring.

A pedigree chart shows the family relationship among individuals. Symbols identify the

gender of each individual and whether an individual had the trait of interest. Pedigree charts

may also show when an individual is known to be homozygous or heterozygous for a trait.

The top ofFigure 1shows some commonly used symbols. The pedigree chart in the

lower part ofFigure 1shows the transmission of an inherited disease among members

of a family. Genetic counsellors may use pedigree charts in their work.

I

II

male

female

mating

Roman numerals

symbolize

generations.

Arabic numbers symbolize

individuals within a given

generation.

Birth order within each group of offspring

is drawn left to right, oldest to youngest.

identical

twins

fraternal

twins

affected

individuals

known

heterozygotes

for autosomal

recessive

123

Pedigree Symbols

Sample Pedigree

I

II

III

2

12 3

1

4 5

1 23

Figure 1
Squares represent males and circles represent females. A slash through a symbol indicates
that person is deceased. Vertical lines connect parents to offspring, horizontal lines connect
mates and connect siblings. Individuals affected by the inherited disease are identified by the
darker-coloured symbols. Symbols having two different colours identify individuals
heterozygous for the disease.

pedigree charta chart used to

record the transmission of a

particular trait or traits over several

generations