evolution and genetics

Chapter Overview

This chapter delves into genetic inheritance, particularly examining problems related to carrier status in genetic disorders like phenylketonuria (PKU), the impact of allele frequencies on genotype distributions, and the interpretation of heritability in twin and adoption studies. It underscores the impact of both genetic and environmental factors on human characteristics.

Understanding Carrier Status in Genetic Disorders

Phenylketonuria (PKU)

• PKU is a genetic disorder caused by the mutation in the PAH gene, leading to an inability to metabolize phenylalanine.

• The prevalence of PKU is approximately 1 in 10,000, which indicates that the aa genotype (mutant homozygous) is rare in the population.

• To demonstrate how every 50th person might be a carrier:

  • Calculate carriers using the Hardy-Weinberg Principle: if aa (affected individuals) frequency is q^2 = 0.0001, then the frequency of allele q is sqrt(0.0001) = 0.01.

  • Consequently, p = 1 - q = 0.99, and calculating carriers using 2pq = 2(0.99)(0.01) = 0.0198, approximating to about 2% or 1 in 50.

Genotypes and Allele Frequencies

Definitions

• Allele Definitions:

  • A = normal allele (wild type)

  • a = mutant allele

• Genotypes:

  • AA (homozygous wild), aa (homozygous mutant), Aa (heterozygous)

Frequencies of Genotypes

When examining allele frequencies:

• Given equal allele frequency of A (50%) and a (50%):

  • Frequency calculation results in:

    • AA = 0.5 x 0.5 = 0.25

    • aa = 0.5 x 0.5 = 0.25

    • Aa + aA = 2(0.5 x 0.5) = 0.5

• Different frequency settings mirror different genetic distributions in populations.

The Hardy-Weinberg Equilibrium

Key Concepts

• The Hardy-Weinberg equation: (p + q)^2 = p^2 + 2pq + q^2 = 1 helps in calculating allele, genotype, and carrier frequencies, asserting that frequencies remain constant across generations barring outside influences such as migration or natural selection.

• Equilibrium conditions include:

  • The absence of selection, mutation, migration, and ensuring random mating.

  • p + q = 1 reflects the total allele frequency.

Behavioral Genetics: Nature vs. Nurture

Family and Adoption Studies

• Family Studies assess the resemblance between parents and offspring, reflecting both genetic and environmental influences.

• Adoption Studies provide a comparison of genetic influences (bio parents) vs. environmental influences (adoptive parents).

  • Genetic siblings show a resemblance due to genetics, while environmental siblings reflect similarities due to shared upbringing.

Insights from Adoption Studies

Historical Perspectives

• Early studies (1920s-1940s) showed adoptees correlated more highly with biological parents than with adoptive parents, suggesting a significant genetic influence.

• The concept of psychopathology in children raised apart often led to misconceptions about environmental factors, as early beliefs linked mental conditions to adverse parenting influences.

Significant Adoption Studies

• Leonard Heston's 1966 study found that adopted children of schizophrenic mothers had a higher incidence of schizophrenia than control groups, indicating a likely genetic component in developing mental health conditions.

Insights from Twin Studies

Types of Twins Explained

• Monozygotic (MZ) Twins: Identical twins sharing 100% genetic material.

• Dizygotic (DZ) Twins: Fraternal twins sharing about 50% of their genes, formed from separate eggs fertilized independently.

Heritability in Twin Studies

• The Equal Environment Assumption (EEA) postulates that both MZ and DZ twins experience similar environmental influences, making them suitable for genetic studies to demonstrate heritability.

• Heritability (h^2) indicates the proportion of phenotypic variance attributable to genetic factors. Calculated using the correlation coefficient of twins:

  • If rMZ > rDZ, genetic factors are essential, and the formula for heritability becomes h^2 = 2(rMZ - rDZ).

Understanding Variance in Genetics

Phenotypic Variance Components

• The equation for total phenotypic variance is represented as P = G + E, where

  • G is the genetic component,

  • E represents environmental influences.

• Interpretations of heritability and studies on height illustrate that significant individual differences arise primarily due to genetic variations, not necessarily implying that every individual’s height is dictated by genes alone.

Conclusion and Further Readings

• Understanding how genetic and environmental factors influence traits and disorders can have profound implications in genetics and behavioral studies. The Hardy-Weinberg equilibrium and adoption and twin studies provide frameworks for disentangling these influences.

• For further reading, consult chapters 2 and 5, focusing on the implications of Hardy-Weinberg calculations, and explore contemporary research connecting genetics and behavior.