L18 - Pedigrees

Introduction to Genetics

  • Overview of genetic techniques and concepts discussed throughout the course.

  • Content spans Chapters 5 and 6 from Pierce, focusing on inheritance patterns and methodologies.

Learning Objectives

  • At the end of the lecture, students should be able to:

    • 5.2 Methods

    • Explain how to determine if two recessive mutants with similar phenotypes have mutations in the same gene or different genes.

    • Use a complementation test and interpret outcomes.

    • 5.4 Polygenic Inheritance

    • Explain why continuous characteristics arise from polygenic inheritance.

    • Discuss the interaction of genes and environment in phenotype expression.

    • Chapter 6

    • Explain the use of pedigrees in studying human genetics.

    • Detail twin and adoption studies in understanding genetic and environmental influences on phenotype.

    • Identify the genetic basis of traits in pedigrees, including locus encoding (sex chromosome, autosome, mitochondrial DNA, dominance or recessiveness).

    • Describe pre-implantation, prenatal, and post-natal genetic testing methods and discuss ethical issues.

Understanding Complementation Testing

  • Purpose of Complementation Testing: Determine if two mutations causing similar recessive phenotypes reside in the same gene or different genes.

Steps in Complementation Testing

  • Crossing: Parents homozygous for different mutations are crossed.

  • Mutation Type: Must use recessive mutations to analyze for complementary effects.

  • Outcomes:

    • If dihybrid heterozygous offspring exhibit a mutant phenotype → conclude mutations are in the same gene (different alleles).

    • If dihybrid heterozygous offspring exhibit wild-type phenotype → conclude mutations are in different genes (can complement each other).

Example of Complementation Testing

  • F2 Ratio Analysis:

    • A 9:7 F2 ratio indicates complementary action of genes in the same pathway.

    • Example phenotypes:

    • 9 individuals exhibit blue phenotype: combinations of alleles yield blue.

    • 7 individuals exhibit white phenotype: specific allele pairings lead to white phenotype.

  • Conducting Tests in Yeast Mitochondria:

    • Homozygous mutant crosses result in tubular rather than fragmented mitochondria, concluding that the mutations are likely at different loci.

Polygenic Inheritance

  • Definition: Many genes contribute to a single characteristic (polygenic traits).

  • Key Characteristics:

    • Complexity: Continuous distribution of phenotypes, such as height and weight.

    • Continuous traits often display a bell-shaped curve in frequency.

Gene-Environment Interaction

  • Influence of Environment:

    • Systematic changes in environment (temperature, nutrition) affect gene expression.

    • Environment's impact can lead to different phenotypic outcomes for the same genotype.

  • Examples:

    • The Himalayan allele: Temperature impacts the enzyme function necessary for pigment production.

    • Cancers and blood pressure levels as phenotypes influenced by environmental factors.

Pedigrees in Human Genetics

  • Utilization: Historical information combined with visual diagrams (pedigrees) allows geneticists to interpret inheritance patterns over generations.

Pedigree Symbols and Interpretation

  • Significant Symbols:

    • Square: Male

    • Circle: Female

    • Shaded: Affected individual

    • Unshaded: Unaffected individual

    • Diamond: Sex unknown

    • Brackets: Adoption relationships

Key Inheritance Patterns

  • Monogenic Traits (traits determined by single genes):

    • Examples: Autosomal recessive, autosomal dominant, X-linked recessive, X-linked dominant, and Y-linked inheritance.

  • Pedigree Analysis:

    • Identify inheritance patterns based on observed traits, such as tracking affected versus unaffected individuals through generations.

Identifying Genetic Disorders

Autosomal Recessive Disorders

  • Characteristics:

    • Phenotype expressed only in homozygotes.

    • Can appear to “skip” generations—parents are often carriers (unaffected).

    • Examples: Cystic fibrosis, phenylketonuria.

Autosomal Dominant Disorders

  • Characteristics:

    • Phenotype expressed in heterozygotes or homozygotes.

    • Usually present in every generation.

    • Examples: Huntington disease, polydactyly.

X-Linked Recessive Disorders

  • Characteristics:

    • Usually more common in males; can manifest in females as homozygous recessive.

    • Skips generations.

    • Examples: Hemophilia, color blindness.

Y-Linked Disorders

  • Characteristics:

    • Present only in males and passed from father to son.

    • No known heritable disorders; all lead to sterility.

    • Limited gene count, approx. 50-60 genes.

Mitochondrial Disorders

  • Characteristics:

    • Typically inherited from the mother and affect both males and females equally.

    • New mutations may occur, complicating inheritance.

Conclusion & Study Preparation

  • Recognize identifiable traits in pedigrees.

  • Questions to consider for midterm evaluation:

    • Dominant vs recessive classification.

    • Autosomal, sex-linked, or mitochondrial inheritance.

  • Develop key references for analyzing genetic information during examinations.