Inheritance Patterns Sep 5

Pedigree Analysis

Introduction to Pedigree Analysis

Pedigree analysis is a fundamental tool in genetics for tracking the inheritance of traits across generations. It helps determine the mode of inheritance and predict the likelihood of offspring inheriting specific genetic conditions.

• Pedigree Symbols: Standardized symbols are used to represent individuals and relationships in a pedigree chart.

• Key:

  • Circle: Female

  • Square: Male

  • Filled symbol: Affected individual

  • Horizontal line: Mating

  • Double line: Consanguineous mating (related parents)

  • Diamond: Sex unknown

  • Numbers below: Offspring in birth order

• Application: Used to identify inheritance patterns such as autosomal dominant, autosomal recessive, X-linked, and mitochondrial inheritance.

Inheritance Patterns

Overview of Major Inheritance Patterns

Genetic traits can be inherited in several distinct patterns, each with unique characteristics and implications for disease transmission.

• Autosomal Recessive

• Autosomal Dominant

• X-linked Recessive

• X-linked Dominant

• Mitochondrial Inheritance

Mitochondrial Inheritance

Mitochondria and Mitochondrial DNA

Mitochondria are organelles found in plant and animal cells, responsible for generating ATP through oxidative respiration. They contain their own DNA, which is distinct from nuclear DNA.

• Structure: Includes inner and outer membranes, cristae, and matrix.

• Function: Site of oxidative phosphorylation, generating ATP (energy).

• Mitochondrial DNA (mtDNA):

  • Not located in the nucleus

  • Contains approximately 35 genes

  • Replication, transcription, and translation are independent from chromosomal DNA

  • mtDNA is maternally inherited—only mothers pass it to offspring

Example: In a pedigree, all children of an affected mother inherit her mitochondrial genome, but children of affected fathers do not.

Autosomal Recessive Inheritance

Characteristics and Pedigree Features

Autosomal recessive traits require two copies of the mutant allele for an individual to be affected. These traits often skip generations and affect males and females equally.

• Key Features:

  • Trait often appears in siblings, not parents

  • Parents of affected individuals are often heterozygous carriers

  • Trait can appear in both sexes equally

  • Examples: Cystic fibrosis, Albinism, Tay-Sachs disease

Example: Tay-Sachs disease is caused by mutations in the HEXA gene, leading to a deficiency in the enzyme beta-hexosaminidase A.

Mutations and Disease: Sickle Cell Disease

Molecular Basis of Sickle Cell Disease

Sickle cell disease is caused by a mutation in the beta-globin gene, resulting in abnormal hemoglobin that distorts red blood cells into a sickle shape.

• Hemoglobin Structure: Composed of 2 alpha-globins and 2 beta-globins.

• Normal vs. Mutant Beta-Globin:

  	DNA Sequence	mRNA Sequence	Amino Acid
  Normal	GAG	GAG	Glu
  Mutant	GTG	GUG	Val

• Effect: The substitution of valine for glutamic acid causes hemoglobin molecules to stick together, distorting red blood cells and leading to blockages in blood vessels.

Equation: Probability that neither of two children will develop sickle cell disease (if both parents are heterozygous): $P = (0.75) \times (0.75) = 0.5625$

Example: Sickle cell anemia is prevalent in populations where malaria is common, as carriers have increased resistance to malaria.

Autosomal Dominant Inheritance

Characteristics and Pedigree Features

Autosomal dominant traits require only one copy of the mutant allele for an individual to be affected. These traits typically appear in every generation and affect males and females equally.

• Key Features:

  • Trait appears in every generation

  • Affected individuals have at least one affected parent

  • Trait affects both sexes equally

  • Examples: Huntington disease, Marfan syndrome

Case Study: Huntington Disease

Molecular Basis and Inheritance

Huntington disease is an autosomal dominant neurodegenerative disorder caused by expansion of CAG trinucleotide repeats in the HTT gene.

• Onset: Symptoms typically begin in the 30s or 40s.

• Inheritance: Each child of an affected parent has a 50% chance of inheriting the mutant allele.

• Genetic Basis:

  • Normal alleles: 10–28 CAG repeats

  • Affected alleles: 36–120 CAG repeats

  • Trinucleotide repeat expansion occurs during DNA replication due to polymerase slippage and incorrect repair.

Example: The discovery of the Huntington disease gene in 1983 was a landmark in human genetics, enabling genetic testing and research into neurodegenerative disorders.

Calculating Probabilities of Inheritance

Genetic Probability Calculations

Probability calculations are essential for predicting the likelihood of inheriting genetic traits. For autosomal recessive conditions, the probability that a child of two heterozygous parents will be unaffected is 75%.

• Formula: For two children, the probability that both are unaffected is: $P = 0.75 \times 0.75 = 0.5625$

• Application: Used in genetic counseling to assess risk for inherited diseases.

Summary Table: Inheritance Patterns

Pattern	Key Features	Examples
Autosomal Recessive	Skips generations, affects both sexes equally	Cystic fibrosis, Tay-Sachs disease
Autosomal Dominant	Appears every generation, both sexes equally	Huntington disease, Marfan syndrome
Mitochondrial	Maternally inherited, all children of affected mother affected	Leber's hereditary optic neuropathy

Additional info: Academic context was added to expand on the molecular basis of diseases, probability calculations, and the historical significance of gene discovery.