3.1 Human Genetics
6 min read•june 24, 2024
shapes populations over time, favoring traits that enhance survival and reproduction. This process relies on genetic variation, , and differential survival. Understanding how and environment interact is crucial for grasping human evolution and diversity.
Genetics explains how traits are passed down through generations. The interplay between (genetic makeup) and (observable traits) is complex, influenced by factors like dominant and recessive alleles, gene-environment interactions, and . This knowledge helps predict inheritance patterns and disease risks.
Principles of Genetics and Evolution
Principles of natural selection
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- Natural selection process where organisms with advantageous traits survive and reproduce at higher rates than those without these traits
- Advantageous traits increase an organism's chances of survival and reproduction in a given environment (camouflage, resistance to disease)
- Over time, frequency of advantageous traits increases in a population leading to evolutionary change (darker moth coloration in polluted areas)
- Key principles of natural selection:
- Variation: Individuals within a population differ in their traits (height, color, behavior)
- Inheritance: Some traits are heritable and can be passed from parents to offspring (eye color, blood type)
- Differential survival and reproduction: Individuals with advantageous traits have higher chances of survival and reproduction (larger beak size in finches during drought)
- Natural selection played a significant role in human evolution shaping traits such as:
- : Walking upright on two legs freed hands for tool use and improved efficiency in long-distance travel
- Brain size: Increased brain size and complexity enabled advanced cognitive abilities and problem-solving skills
- Digestive adaptations: Changes in digestive enzymes and gut morphology accommodated a more diverse diet (ability to digest lactose in adulthood)
Genotype vs phenotype in genetics
- Genotype refers to an individual's genetic makeup, specific alleles (versions of genes) inherited from their parents
- Alleles can be dominant (expressed when present in one copy) or recessive (expressed only when present in two copies)
- Phenotype is the observable characteristics or traits of an individual resulting from the interaction between their genotype and the environment (eye color, hair color, height, blood type)
- Significance of genotype and phenotype in human genetics:
- Understanding the relationship between genotype and phenotype helps predict the likelihood of inheriting certain traits or genetic disorders
- Some genetic disorders are caused by recessive alleles only expressed in the phenotype when an individual inherits two copies of the (one from each parent)
- caused by a recessive allele; individuals with two copies develop the disorder, while those with one copy are carriers
- Dominant genetic disorders are expressed in the phenotype when an individual inherits one copy of the allele
- caused by a dominant allele; individuals with one copy will develop the disorder
Gene-environment interactions for traits
- Gene-environment interactions occur when the effects of genes on a trait depend on environmental factors or when environmental influences on a trait vary based on an individual's genotype
- Physical traits influenced by gene-environment interactions:
- Height: Determined by multiple genes and influenced by environmental factors such as nutrition and healthcare
- Obesity: Genetic predisposition can interact with environmental factors like diet and physical activity to influence body weight
- Psychological traits influenced by gene-environment interactions:
- Intelligence: Influenced by both genetic factors and environmental stimuli such as education and socioeconomic status
- Personality: Shaped by the interplay between genetic predispositions and environmental experiences such as parenting style and life events
- Epigenetics studies how environmental factors can alter gene expression without changing the underlying sequence
- Exposure to stress or trauma can lead to epigenetic changes that influence mental health outcomes such as an increased risk of depression or anxiety disorders
- Understanding gene-environment interactions is crucial for developing targeted interventions and personalized approaches to physical and mental health
Fundamentals of Genetics
- are structures in the cell nucleus that carry genetic information
- Humans have 23 pairs of chromosomes, including 22 pairs of autosomes and one pair of sex chromosomes
- DNA (Deoxyribonucleic acid) is the molecule that contains the genetic instructions for the development and functioning of all known living organisms
- DNA is organized into genes, which are specific sequences that code for particular proteins
- Genes are the basic units of heredity, determining traits that are passed from parents to offspring
- Inheritance is the process by which genetic information is transmitted from parents to offspring
- , known as the "father of genetics," discovered the fundamental principles of inheritance through his experiments with pea plants
- The is an organism's complete set of genetic material, including all of its genes and DNA sequences
Heredity and Genetic Variation
Principles of natural selection
- Mutations introduce new genetic variation into a population
- Mutations are changes in the DNA sequence that can alter the function or expression of genes
- Some mutations are neutral, while others can be advantageous (resistance to antibiotics) or deleterious (genetic disorders) depending on the environment
- Sexual reproduction and recombination increase genetic variation
- During , genetic material from both parents is shuffled and combined to create unique combinations of alleles in offspring
- This process generates new combinations of traits that can be subject to natural selection (increased diversity in offspring)
- : Random changes in allele frequencies that occur by chance, particularly in small populations
- : A significant reduction in population size that leads to a loss of genetic variation (cheetah population bottleneck)
- : When a small group of individuals establishes a new population, leading to reduced genetic diversity compared to the original population (Amish population in Pennsylvania)
- : The transfer of alleles between populations through migration and interbreeding
- Gene flow can introduce new genetic variation into a population or homogenize allele frequencies between populations (interbreeding between Neanderthals and modern humans)
Genotype vs phenotype in genetics
- are influenced by multiple genes, each contributing a small effect to the overall phenotype
- Examples of polygenic traits in humans include height, skin color, and susceptibility to complex diseases like diabetes or schizophrenia
- occurs when a single gene influences multiple seemingly unrelated phenotypic traits
- The gene responsible for sickle cell anemia also provides protection against malaria
- : When individuals with the same genotype do not always express the associated phenotype
- BRCA1 and BRCA2 gene mutations increase the risk of breast and ovarian cancer, but not all individuals with these mutations develop cancer
- : When individuals with the same genotype exhibit varying degrees of the associated phenotype
- , caused by a dominant allele, can have varying severity of symptoms among affected individuals (some may have mild joint hypermobility while others develop life-threatening cardiovascular complications)
Gene-environment interactions for traits
- : The range of phenotypes that a single genotype can produce across different environments
- Plants with the same genotype may grow to different heights depending on the availability of nutrients and water
- Genotype-by-environment (GxE) interactions can influence the of traits
- Heritability is the proportion of variation in a trait that can be attributed to genetic differences among individuals in a population
- The heritability of a trait can vary depending on the environmental context
- The heritability of IQ is higher in high socioeconomic environments than in low socioeconomic environments
- : The ability of an organism to alter its phenotype in response to environmental cues during development
- Childhood malnutrition can lead to stunted growth and impaired cognitive development, even in individuals with the same genotype as well-nourished individuals
- Genetic predispositions can influence an individual's response to environmental factors
- Individuals with a genetic predisposition to addiction may be more likely to develop substance abuse disorders when exposed to certain environmental triggers (stress, peer pressure)
Key Terms to Review (28)
Allele: An allele is one of two or more alternative forms of a gene that can occupy the same locus on a chromosome. Alleles contribute to the genetic diversity within a population by providing different versions of a particular trait or characteristic.
Bipedalism: Bipedalism is the ability to walk on two legs, which is a defining characteristic of humans and some other primates. It involves the adaptation of the skeletal and muscular systems to support and propel the body using only the lower limbs.
Bottleneck Effect: The bottleneck effect is a loss of genetic diversity that occurs when a population undergoes a severe reduction in size, leading to a small number of individuals contributing to the gene pool of the next generation. This phenomenon is particularly relevant in the context of human genetics, as it can have significant implications for the genetic diversity and evolution of human populations.
Chromosomes: Chromosomes are thread-like structures found in the nucleus of cells that carry the genetic information necessary for growth, development, and inheritance. They are composed of DNA and proteins and are the physical units of heredity, containing the genes that determine an individual's unique characteristics.
Cystic Fibrosis: Cystic fibrosis is a genetic disorder that primarily affects the respiratory and digestive systems. It is caused by a mutation in the CFTR gene, which is responsible for regulating the movement of salt and water in and out of cells. This disruption leads to the production of thick, sticky mucus that can clog the lungs and obstruct the pancreas, causing a range of health complications.
Developmental Plasticity: Developmental plasticity refers to the ability of an organism to adapt its growth, physiology, and behavior in response to changes in its environment during development. It is a fundamental characteristic of living systems that allows them to adjust to varying conditions and optimize their chances of survival and reproduction.
DNA: DNA, or deoxyribonucleic acid, is the fundamental genetic material that carries the instructions for the development and functioning of all living organisms. It is the blueprint that determines an individual's unique characteristics and traits.
Epigenetics: Epigenetics is the study of how environmental and behavioral factors can influence the expression of genes without altering the underlying DNA sequence. It explores how certain genes are turned on or off, affecting an individual's physical traits and susceptibility to various diseases, without changing the genetic code.
Founder Effect: The founder effect is a loss of genetic variation that occurs when a new population is established by a small number of individuals from a larger population. This can lead to the new population having different genetic characteristics than the original population.
Gene Flow: Gene flow is the transfer of genetic material from one population or species to another through interbreeding. It is a key evolutionary process that introduces new genetic variation into a population, allowing it to adapt to changing environmental conditions.
Genes: Genes are the fundamental units of heredity that contain the instructions for the development and functioning of living organisms. They are segments of DNA that encode the information necessary for the production of specific proteins, which ultimately determine an individual's physical and biological characteristics.
Genetic Drift: Genetic drift is a random process that occurs in small populations, where certain alleles may become more or less frequent over generations due to chance events, rather than natural selection. It is a mechanism of evolution that can lead to changes in the genetic composition of a population over time.
Genome: The genome is the complete set of genetic information, including all the genes and DNA sequences, that is present in a living organism. It serves as the fundamental blueprint for an individual's biological characteristics and functions.
Genotype: Genotype refers to the genetic makeup or the specific combination of genes that an individual inherits from their parents. It represents the complete set of genes that determine an organism's unique characteristics and potential for expression.
Gregor Mendel: Gregor Mendel was an Augustinian friar and scientist who is widely regarded as the father of genetics. His pioneering work on pea plants in the mid-19th century laid the foundation for our understanding of heredity and how traits are passed down from parents to offspring.
Heritability: Heritability is a statistical measure that quantifies the proportion of variation in a particular trait or characteristic that can be attributed to genetic factors within a population. It is a fundamental concept in the study of human genetics, intelligence, personality, and psychological disorders.
Huntington's Disease: Huntington's disease is a rare, inherited neurological disorder characterized by uncontrolled movements, emotional problems, and loss of thinking ability. It is caused by a defective gene that damages certain nerve cells in the brain, leading to progressive physical, mental, and emotional deterioration.
Incomplete Penetrance: Incomplete penetrance is a genetic phenomenon where an individual possesses a genetic variant associated with a particular trait or disorder, but the trait or disorder may not be fully expressed or manifested in that individual. In other words, not all individuals with the genetic variant will necessarily develop the associated trait or disorder.
Inheritance: Inheritance refers to the transmission of genetic information from parents to their offspring, determining an individual's physical and behavioral characteristics. It is a fundamental concept in human genetics that explains how traits are passed down through generations.
Marfan Syndrome: Marfan syndrome is a genetic disorder that affects the body's connective tissues, which provide support and structure to various organs and tissues. It is characterized by abnormalities in the development and function of the cardiovascular, skeletal, and ocular systems.
Meiosis: Meiosis is a specialized type of cell division that occurs in sexually reproducing organisms. It is the process by which diploid cells divide to produce haploid gametes, such as sperm and eggs, ensuring genetic diversity in the offspring.
Mutation: Mutation is a permanent change in the DNA sequence of a gene or chromosome that can alter the genetic information and lead to changes in an organism's characteristics or functions. Mutations are a fundamental mechanism of genetic variation and evolution.
Natural Selection: Natural selection is the gradual process by which biological entities adapt to their environment and become better suited to survive and reproduce. It is the core mechanism of evolution, where favorable traits are preserved and passed on to future generations, while unfavorable traits are eliminated.
Norm of Reaction: The norm of reaction is a concept in genetics that describes the range of phenotypes that can result from a single genotype, depending on the environmental conditions. It represents the relationship between an organism's genotype and the set of possible phenotypes it can express.
Phenotype: The phenotype is the observable physical or biochemical characteristics of an organism, determined by its genotype and the influence of environmental factors. It represents the expression of an individual's genetic makeup as manifested in their appearance, behavior, and other traits.
Pleiotropy: Pleiotropy refers to the phenomenon where a single gene can influence multiple, seemingly unrelated phenotypic traits or characteristics in an organism. This concept is particularly important in the context of human genetics, as it helps explain how variations in a single gene can lead to a diverse array of physical, physiological, and behavioral outcomes.
Polygenic Traits: Polygenic traits are characteristics that are influenced by the combined effects of multiple genes, rather than a single gene. These traits typically exhibit a continuous range of variation in the population, as opposed to distinct categories or discrete phenotypes.
Variable Expressivity: Variable expressivity refers to the phenomenon where a genetic trait or disorder can manifest with varying degrees of severity or symptoms in different individuals, even within the same family. This means that the same genetic mutation or variation can lead to a wide range of phenotypic outcomes, ranging from mild to severe, in different people.