PEDIGREESChapter 14

Pedigree• A pedigree is a chart for tracing genes in a family• They can be used to study the transmission of a

hereditary condition• Phenotypes are used to infer genotypes on a pedigree

Symbols Used in a Pedigree Normal male Affected male Normal female Affected female

Vertical Lines- offspring Marriage/Mating

A marriage with five children, two daughters and three sons. The middle-aged son is affected by the condition.

Oldest child Youngest child

Organizing the Pedigree

• Generations are identified by Roman numerals

I

II

III

IV

Organizing the Pedigree• Individuals in each generation are identified by numbers. • Therefore the affected individuals are II3, IV2 and IV3

I

II

III

IV

Patterns of Inheritance Basic patterns of inheritance

autosomal, recessiveautosomal, dominantX-linked, recessiveX-linked, dominant (very rare)

Autosomal Recessive Traits

• Trait is rare in pedigree

• Trait often skips generations • hidden in heterozygous carriers

(sometimes shown half shaded)

• Trait affects males and females equally

Autosomal Recessive Traits Most common ones • Cystic fibrosis • Sickle cell anemia• Phenylketonuria (PKU)• Tay-Sachs disease

For each of these, over dominance (heterozygote superiority) has been suggested as a factor in maintaining the disease alleles at high frequency in some populations

Autosomal Recessive Trait• If you ever see this situation

in the pedigree (two unaffected parents have an affected child) then the trait MUST be recessive!

• The parents are heterozygous.

Autosomal Dominant Trait• Trait is common in the

pedigree

• Trait is found in every generation

• Affected individuals transmit the trait to ~1/2 of their children (regardless of sex)

X-linked Recessive Trait

• Trait is rare in pedigree

• Trait skips generations

• Affected fathers DO NOT pass to their sons,

• Males are more often affected than females

X-linked Recessive Trait• ex. Hemophilia in European royalty

Steps to Determine Inheritance Pattern of a Trait

Step 1: Determine whether it’s dominant or recessive.

• Does it skip generations? YES= recessive

NO= dominant.

Step 2. Determine whether is is autosomal or x-linked.

• Does it affect one gender more than the other?

YES= X-linked

NO= autosomal

What Inheritance Pattern Does this Trait Follow?

• What is the inheritance pattern?• What is the genotype of III-1, III-2, and II-3?• What are the odds that IV-5 would have an affected son?

Pedigree Analysis in Real Life

• Remember: • dominant traits may be rare in population

• recessive traits may be common in population

• alleles may come into the pedigree from 2 sources

• mutation happens

• often traits are more complex

• affected by environment & other genes