🔬General Biology I Unit 13 – Modern Understandings of Inheritance

Key Concepts and Terminology

• Alleles are different versions of a gene that can result in variations in inherited characteristics
  • For example, the gene for flower color in pea plants has two alleles, one for purple and one for white
• Genotype refers to the genetic makeup of an organism, while phenotype is the observable physical or biochemical characteristics
• Homozygous individuals have two identical alleles for a particular gene, while heterozygous individuals have two different alleles
• Dominant alleles mask the expression of recessive alleles when present in a heterozygous individual
• Punnett squares are diagrams used to predict the probability of offspring having particular genotypes and phenotypes
• Incomplete dominance occurs when neither allele is completely dominant, resulting in a blended phenotype (red and white flowers producing pink offspring)
• Codominance happens when both alleles are expressed equally in the phenotype (human ABO blood types)

Historical Context of Inheritance

• Before the 19th century, the blending theory of inheritance proposed that offspring traits were a mixture of parental traits
• Preformationism suggested that fully formed miniature organisms existed in the sperm or egg and simply grew larger during development
• Lamarckism, proposed by Jean-Baptiste Lamarck, hypothesized that acquired characteristics could be passed on to offspring
  • For example, giraffes stretching their necks to reach leaves would produce offspring with longer necks
• Charles Darwin's theory of evolution by natural selection, published in 1859, explained how populations change over time but lacked a mechanism for inheritance
• In the late 19th century, Gregor Mendel's experiments with pea plants provided the foundation for the modern understanding of inheritance
• Mendel's work was largely ignored until the early 20th century when it was rediscovered and combined with Darwin's theory to form the modern synthesis of evolution

Mendel's Laws and Experiments

• Gregor Mendel, an Austrian monk, conducted experiments with pea plants to study inheritance patterns
• Mendel's first law, the law of segregation, states that each individual possesses two alleles for each trait, which segregate during gamete formation
  • As a result, an offspring receives one allele from each parent for each trait
• Mendel's second law, the law of independent assortment, proposes that alleles for different traits are inherited independently of one another
  • For example, the inheritance of seed color is independent of the inheritance of seed shape
• Mendel crossed true-breeding pea plants with contrasting traits and observed the offspring over multiple generations
  • He focused on seven traits, including seed shape, flower color, and plant height
• Mendel's experiments demonstrated that traits are inherited in discrete units (genes) rather than blending together
• Mendel's laws laid the foundation for the field of genetics and the understanding of inheritance patterns

DNA Structure and Function

• Deoxyribonucleic acid (DNA) is the genetic material that carries hereditary information in most organisms
• DNA is composed of four nucleotide bases: adenine (A), thymine (T), guanine (G), and cytosine (C)
  • A always pairs with T, and G always pairs with C through hydrogen bonds
• The structure of DNA is a double helix, with two complementary strands wound around each other
• The sugar-phosphate backbone provides structural support and connects the nucleotide bases
• Genes are specific sequences of DNA that encode instructions for making proteins or functional RNA molecules
• The central dogma of molecular biology describes the flow of genetic information from DNA to RNA to proteins
  • DNA is transcribed into RNA, which is then translated into proteins
• DNA replication is the process by which DNA makes an exact copy of itself during cell division, ensuring that genetic information is passed on to daughter cells

Chromosomes and Genetic Material

• Chromosomes are highly condensed, thread-like structures that contain DNA and associated proteins called histones
• In eukaryotic cells, DNA is packaged into chromosomes located in the nucleus
• The number of chromosomes varies among species; humans have 46 chromosomes (23 pairs)
• Autosomes are non-sex chromosomes, while sex chromosomes (X and Y in mammals) determine an individual's sex
  • Females have two X chromosomes, while males have one X and one Y chromosome
• Chromatin is the combination of DNA and proteins that make up chromosomes
  • Chromatin can be loosely packed (euchromatin) or tightly packed (heterochromatin)
• During cell division, chromosomes condense and become visible under a microscope
• Sister chromatids are identical copies of a chromosome attached at the centromere, which separate during cell division
• Telomeres are repetitive DNA sequences at the ends of chromosomes that protect them from degradation and fusion with other chromosomes

Patterns of Inheritance

• Mendelian inheritance patterns include autosomal dominant, autosomal recessive, and sex-linked inheritance
• Autosomal dominant traits are expressed when an individual has at least one dominant allele (Huntington's disease)
• Autosomal recessive traits are expressed only when an individual has two recessive alleles (cystic fibrosis)
• Sex-linked traits are determined by genes located on the sex chromosomes, primarily the X chromosome (color blindness, hemophilia)
• Non-Mendelian inheritance patterns include incomplete dominance, codominance, and polygenic traits
• Polygenic traits are influenced by multiple genes and often result in a continuous range of phenotypes (height, skin color)
• Epistasis occurs when the expression of one gene is influenced by the presence or absence of another gene
• Pleiotropy is when a single gene influences multiple seemingly unrelated phenotypic traits (sickle cell anemia affecting red blood cells and resistance to malaria)

Genetic Mutations and Variations

• Mutations are changes in the DNA sequence that can alter gene function and lead to variations in phenotype
• Point mutations involve the substitution, insertion, or deletion of a single nucleotide (sickle cell anemia)
• Frameshift mutations occur when the number of nucleotides inserted or deleted is not divisible by three, altering the reading frame and often resulting in a nonfunctional protein
• Chromosomal mutations involve changes in the structure or number of chromosomes
  • Examples include deletions, duplications, inversions, and translocations
• Aneuploidy is an abnormal number of chromosomes, such as trisomy 21 (Down syndrome) or monosomy X (Turner syndrome)
• Polyploidy is the presence of more than two complete sets of chromosomes, which can occur naturally in some plants and can be induced artificially
• Mutations can be spontaneous or induced by environmental factors like radiation or chemicals (mutagens)
• While many mutations are harmful or neutral, some can be beneficial and contribute to genetic variation and evolution

Modern Applications and Technologies

• Genetic engineering involves the direct manipulation of an organism's DNA to modify its characteristics
  • Examples include genetically modified crops with increased yield or resistance to pests and diseases
• Gene therapy is the introduction of functional genes into cells to replace or correct defective genes (treatment for genetic disorders)
• CRISPR-Cas9 is a powerful gene-editing tool that allows for precise modifications to DNA sequences
  • Potential applications include treating genetic diseases, developing disease-resistant crops, and creating animal models for research
• DNA sequencing technologies have enabled the rapid and cost-effective determination of complete genome sequences
  • Applications include personalized medicine, evolutionary studies, and forensic science
• Genetic testing can identify individuals with a predisposition to certain genetic disorders or inform treatment decisions based on a patient's genetic profile
• Pharmacogenomics studies how an individual's genetic makeup influences their response to medications, allowing for personalized drug therapies
• Synthetic biology aims to design and construct new biological systems or organisms with novel functions (biofuels, pharmaceuticals)
• Ethical considerations surrounding genetic technologies include privacy, informed consent, equitable access, and the potential for unintended consequences or misuse