Pedigree Analysis and Patterns of Inheritance

Pedigree Analysis

Introduction to Pedigree Analysis

Pedigree analysis is a fundamental tool in genetics used to study the inheritance patterns of traits within families. By examining family trees and the occurrence of specific phenotypes, geneticists can infer the mode of inheritance for genetic conditions.

• Pedigree: A diagram that depicts the biological relationships between family members and shows which individuals express or transmit a particular trait.

• Generations are identified by Roman numerals (I, II, III, etc.).

• Family members within a generation are identified by Arabic numerals (1, 2, 3, etc.).

• Squares represent males; circles represent females.

• Filled symbols indicate individuals affected by the trait; open symbols indicate unaffected individuals.

• Children are listed left-to-right in order of birth within a family.

Pedigree Symbols and Their Meanings

	Male	Female	Sex Unknown/Unspecified
Unaffected person	□	○	◇
Person affected with trait	■	●	◆
Obligate carrier	□ (dot)	○ (dot)	◇ (dot)
Asymptomatic carrier	□ (half-filled)	○ (half-filled)	◇ (half-filled)
Multiple persons (e.g., 5)	5 □	5 ○	5 ◇
Deceased person	□ (slash)	○ (slash)	◇ (slash)
Proband (first affected family member studied)	■ (arrow)	● (arrow)	◆ (arrow)
Family history unknown	□ (question mark)	○ (question mark)	◇ (question mark)

Special Pedigree Relationships

• Family: Parents and children are shown with lines connecting them; children are listed in birth order.

• Adoption: Brackets enclose adopted individuals; dashed lines indicate adoptive parents, solid lines indicate biological parents.

• Twins: Identical twins are connected by a horizontal line and a vertical line; nonidentical twins have only a vertical line.

• Consanguinity: Mating between related persons is indicated by a double horizontal line.

Patterns of Inheritance

Overview

Patterns of inheritance observed in pedigrees help determine the genetic basis of a trait. The main types include autosomal recessive, autosomal dominant, X-linked recessive, X-linked dominant, and Y-linked inheritance.

Autosomal Recessive Inheritance

• Occurs with equal frequency in both sexes.

• Usually, parents are heterozygous carriers (unaffected but carry one copy of the mutant allele).

• One-quarter ($\frac{1}{4}$) of offspring of two carriers will be affected.

• Tends to skip generations.

• Consanguinity (mating within family) increases the probability of the trait appearing.

• Example: Cystic fibrosis.

Genotype ratio for offspring of two heterozygous carriers:

• 1 AA : 2 Aa : 1 aa (where 'aa' is affected)

Punnett Square:

• Parents: Aa x Aa

• Offspring: 1 AA (unaffected), 2 Aa (carriers), 1 aa (affected)

Autosomal Dominant Inheritance

• Occurs with equal frequency in both sexes.

• Does not skip generations (vertical transmission).

• Unaffected individuals do not transmit the trait.

• If one parent is affected (heterozygous) and the other is unaffected, about half of the offspring will inherit the trait.

• Example: Huntington's Disease.

Genotype ratio for offspring of affected (Aa) x unaffected (aa):

• 1 Aa (affected) : 1 aa (unaffected)

X-linked Recessive Inheritance

• Appears more frequently in males (who have only one X chromosome).

• Tends to skip generations.

• Half of sons born to carrier mothers will be affected; half of daughters will be carriers.

• Trait is not transmitted from father to son.

• Example: Red/Green colorblindness.

Genotype example:

• Carrier mother (XAXa) x normal father (XAY):

• 1/4 XAXA (unaffected daughter), 1/4 XAXa (carrier daughter), 1/4 XAY (unaffected son), 1/4 XaY (affected son)

X-linked Dominant Inheritance

• Trait is passed to females from both father and mother, but to males only from mother.

• Affected mothers pass the trait to half of their sons and half of their daughters.

• Does not skip generations.

• Example: Fragile X syndrome.

Y-linked Inheritance

• Only males are affected.

• Trait is passed from father to all sons.

• Does not skip generations.

• Example: Y chromosome infertility.

Summary Table: Patterns of Inheritance

Pattern	Sex Ratio	Generational Pattern	Transmission	Example
Autosomal Recessive	Equal	Skips generations	Carriers can transmit	Cystic fibrosis
Autosomal Dominant	Equal	Does not skip	Unaffected do not transmit	Huntington's Disease
X-linked Recessive	More males	Skips generations	Carrier mothers to sons	Red/Green colorblindness
X-linked Dominant	More females	Does not skip	Fathers to daughters, mothers to both	Fragile X syndrome
Y-linked	Only males	Does not skip	Father to all sons	Y chromosome infertility

Example Applications

• Genetic Counseling: Pedigree analysis is used to assess the risk of inherited disorders in families.

• Research: Helps identify the mode of inheritance for newly discovered genetic traits.

Additional info: Pedigree analysis is a key component of Chapter 6 (Pedigree) in college genetics courses and is foundational for understanding human genetic disorders and inheritance patterns.