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Genetic Imprinting, Environmental Influences, and Trait Inheritance

  • Genetic Imprinting and Offspring Traits

    • The expression of some genes and their effect on offspring traits depend on which parent they were inherited from (maternal or paternal).

    • Example: IGF2 Gene

    • Paternal Inheritance (from sperm): Results in larger offspring size. This is theorized as an evolutionary strategy, as for a male, this may be their only chance to mate, and they aim for the biggest, most fit offspring.

    • Maternal Inheritance (from egg): Results in smaller offspring. This is theorized as an evolutionary strategy for females, who may be pregnant with multiple offspring and hope to get pregnant again; smaller offspring represent less energy investment for her.

  • Unstable Repeats and Anticipation

    • Anticipation in genetic disorders can occur due to unstable nucleotide repeats.

    • Example: CCG Repeats

    • Having 5 to 10 CCG repeats typically results in a phenotypically normal individual.

    • However, having 50 to 100 CCG repeats can lead to a defect.

  • Temperature-Sensitive Enzyme Activity

    • Environmental factors, such as temperature, can significantly influence gene expression and phenotypic traits.

    • Example: Himalayan Rabbits and Siamese Cats

    • The darker coat coloring around the nose, feet, ears, and tail is due to a temperature-sensitive color-depositing enzyme.

    • This enzyme only functions at cooler temperatures.

    • At the core of the body, where body heat is higher, the enzyme is non-functional, resulting in white fur.

    • Any increase in temperature, such as inflammation during development, can make areas like the ears lighter-colored because the enzyme becomes non-functional.

  • Types of Characteristics

    • Discontinuous Characteristics:

    • Involve relatively few distinct phenotypes (e.g., a flower is either purple or white, a plant is either tall or short).

    • Do not exist on a continuum or spectrum.

    • Continuous Characteristics:

    • Exhibited by traits like human height, where individuals show a range of slight variations, making it difficult to establish clear-cut categories (e.g., distinguishing between 'tall' and 'short' in a group).

    • Typically arise in two main ways:

      • Polygenic Characteristics: Traits influenced by multiple genes. Examples include eye color and hair color, which, despite attempts to categorize, exist in numerous shades and variations.

      • Pleiotropy: A phenomenon where a single gene affects multiple distinct characteristics. This is commonly observed in many disease alleles where a minor mutation can disrupt an enzyme, leading to a cascade of problems affecting multiple organs or systems (e.g., circulation problems).

  • Environmental Factors Affecting Phenotype

    • Chemicals and Fetal Development:

    • Phenylketonuria (PKU): A condition leading to mental defects at an early age caused by the buildup of phenylalanine due to an inability to break it down. However, the defects are preventable by restricting phenylalanine in the diet. All newborns are now tested for PKU, allowing for diet alteration to prevent the disorder's effects. Products often carry warnings for phenylketonurics if they contain phenylalanine.

    • Phenocopies (Chemically-Induced): Environmental changes in the fetal environment can cause phenotypic modifications that mimic genetic defects.

      • Example: Thalidomide Babies: Thalidomide, a sedative, caused abnormal limb development in fetuses if taken by pregnant mothers because the chemical had a different effect during the critical limb-development stage. Adults taking thalidomide would not experience limb loss as their limbs are already developed.

    • Gene-Environment Interactions in Phenotype Expression:

    • The ultimate phenotype is a combination of an individual's genes and their environment.

    • Seasonal Changes in Trees: Leaves on a single tree, despite having the same genome, gradually change color, size, and shape due to varying exposure to wind and sun.

    • Human Traits:

      • Nutrition influences height.

      • Exercise alters build.

      • Sun tanning darkens skin.

      • Educational experiences can improve performance on intelligence tests.

      • Identical Twin Study: Two identical twins, despite having the same genes, can exhibit significant phenotypic differences. For example, one twin being 6 inches taller due to the other having been sick as an infant and experiencing nutritional deficiencies. Similarly, identical twins may score differently on an exam based on study effort.

    • Nature vs. Nurture and Epigenetics: The ongoing debate of nature versus nurture is increasingly being explained by epigenetics, which involves changes to the genome based on environmental factors and life experiences.

      • The environmental contribution to gene expression varies for different genes.

      • Human Height: Has increased by approximately 1 inch per generation over the past 100 years due to improved nutrition and healthcare, demonstrating environmental influence.

  • Norm of Reaction

    • Genes have a range within which they can be expressed, known as the norm of reaction.

    • Phenotypes are not rigidly defined but rather a range of possibilities based on this norm of reaction.

    • Wide Norm of Reaction: Human height can vary significantly (e.g., from 4 feet to 8 feet).

    • Narrow Norm of Reaction: Hydrangea flower color (pink or purple depending on soil acidity) is within a narrow range; it will not produce yellow or green flowers.

    • No Breadth (Fixed Phenotype): Blood type (A, B, AB, or O) has no variation or range.

    • Multifactorial Characters: Traits like behavior and IQ are influenced by complex interactions between genes and the environment.

    • IQ in Identical Twins: Identical twins, especially if separated at birth and raised in different intellectual environments (e.g., one read to frequently, the other not), can have significantly different IQ scores.

    • Implication for Human Potential: This highlights that while genes are fixed, environmental changes (e.g., diet, study habits, exercise, exposure to literature) can positively affect phenotypic traits such as height, muscle build, and intelligence.

  • Further Pleiotropic Effects and Modifier Genes

    • Pleiotropy: Changes in one gene can lead to multiple changes in different characteristics due to intricate molecular and cellular interactions during development.

    • Example: Individuals with lighter eye colors (including light brown) tend to have increased sun sensitivity, indicating a pleiotropic effect of the gene(s) affecting eye color.

    • Modifier Genes: These genes alter or intensify specific traits. Unlike epistasis, which masks gene expression, modifier genes impact the degree or manner in which other traits are expressed.

    • Example: Sickle Cell Anemia (Detailed Pleiotropy Example):

    • It is caused by a single point mutation in individuals homozygous for the sickle cell allele, meaning a change in just one base pair.

    • This single change results in abnormally shaped,