3.1 Genetics and Heredity

Genetics and heredity shape who we are from the moment we're conceived. Our genes, passed down from our parents, determine everything from our eye color to our risk for certain diseases. But it's not just about DNA—our environment plays a huge role too.

Understanding genetics helps us grasp how traits are inherited and expressed. We'll explore how DNA carries genetic information, how mutations can lead to disorders, and how genes interact with our environment to influence our development and behavior.

Principles of Genetics and Heredity

Principles of genetic inheritance

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• Genes serve as the fundamental units of heredity that are passed from parents to offspring
  • Composed of DNA segments that code for specific traits (eye color, hair texture)
  • Located on chromosomes within the nucleus of cells
• Alleles are variant forms of a gene that can result in different expressions of a trait
  • Dominant alleles are expressed when present, even if only one copy is inherited (brown eyes)
  • Recessive alleles are only expressed when two copies are inherited (blue eyes)
• Mendelian inheritance patterns describe how traits are passed from parents to offspring
  • Law of segregation: Alleles separate during gamete formation (sperm, egg), and offspring inherit one allele from each parent
  • Law of independent assortment: Alleles for different genes are inherited independently of each other (eye color and hair color)
• Punnett squares are diagrams used to predict the probability of offspring inheriting specific traits based on the genotypes of the parents

DNA's role in trait determination

• DNA (deoxyribonucleic acid) serves as the genetic material that carries hereditary information from parents to offspring
• DNA structure consists of a double helix composed of nucleotide base pairs
  • Base pairs: Adenine (A) with Thymine (T), and Guanine (G) with Cytosine (C)
• DNA replication ensures that genetic information is accurately passed from parent cells to daughter cells during cell division
• Genes are specific sequences of DNA that code for proteins, which ultimately determine an individual's characteristics and traits
  • Proteins play crucial roles in cell structure, function, and regulation (enzymes, hormones)
• Mutations in DNA can lead to changes in traits or the development of genetic disorders
  • Point mutations involve changes in a single nucleotide base pair (sickle cell anemia)
  • Chromosomal mutations involve changes in the structure or number of chromosomes (Down syndrome)

Genetic Disorders and Gene-Environment Interactions

Genetic disorders and conditions

• Genetic disorders are caused by mutations or changes in an individual's DNA that disrupt normal gene function
• Monogenic disorders are caused by mutations in a single gene and follow clear inheritance patterns
  • Sickle cell anemia: Caused by a mutation in the hemoglobin gene, leading to abnormal red blood cells
  • Cystic fibrosis: Caused by a mutation in the CFTR gene, affecting the respiratory and digestive systems
  • Huntington's disease: Caused by a mutation in the HTT gene, resulting in progressive neurological degeneration
• Chromosomal disorders are caused by changes in the number or structure of chromosomes
  • Down syndrome (trisomy 21): Caused by an extra copy of chromosome 21, leading to intellectual disability and distinctive facial features
  • Turner syndrome (45,X): Caused by the absence of one X chromosome in females, resulting in short stature and infertility
  • Klinefelter syndrome (47,XXY): Caused by an extra X chromosome in males, leading to tall stature, infertility, and learning difficulties
• Multifactorial disorders are caused by a complex interaction of multiple genes and environmental factors
  • Diabetes: Influenced by genetic predisposition and lifestyle factors (diet, exercise)
  • Heart disease: Affected by genetic risk factors and environmental influences (smoking, high cholesterol)
  • Some forms of cancer: Result from a combination of inherited genetic mutations and environmental exposures (UV radiation, carcinogens)

Gene-environment interactions in development

• Gene-environment interactions describe how genes and environment work together to influence an individual's traits, behaviors, and development
• Epigenetics studies how environmental factors can modify gene expression without changing the DNA sequence
  • DNA methylation: Addition of methyl groups to DNA, which can silence gene expression (imprinted genes)
  • Histone modifications: Changes to proteins that DNA wraps around, affecting gene accessibility (acetylation, methylation)
• Behavioral genetics investigates how genes and environment contribute to individual differences in behavior and psychological traits
  • Twin studies compare the similarities between identical (monozygotic) and fraternal (dizygotic) twins to estimate the heritability of traits
  • Adoption studies compare the similarities between adopted children and their biological and adoptive parents to distinguish genetic and environmental influences
• Genetic predispositions can be influenced by environmental factors, leading to the development of certain conditions or behaviors
  • Obesity: Individuals with genetic susceptibility may be more likely to develop obesity when exposed to an unhealthy diet and sedentary lifestyle
  • Addiction: Genetic factors can increase the risk of developing substance abuse disorders, but environmental influences (peer pressure, stress) also play a role
• Environmental factors can also influence gene expression and development, particularly during sensitive periods
  • Prenatal exposure to toxins (alcohol, drugs) or stress can affect gene expression and lead to developmental problems (fetal alcohol syndrome)
  • Early life experiences (nurturing or neglectful caregiving) can impact brain development and shape behavioral patterns through epigenetic mechanisms