BIO 201 — Genetics I: Lecture Note

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BIO la-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS

BIO la-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS Course code: BIO 201 Course title: Genetics I Course credit load: 2 Units Series 1: Heritable and Non-Heritable Characteristics • Part 1.1: Fundamentals of Heredity o Sub Part 1.1.1: Definition and scope of genetics o Sub Part 1.1.2: Historical background of genetics o Sub Part 1.1.3: Chromosomes and the basis of heredity • Part 1.2: Heritable Characteristics o Sub Part 1.2.1: Definition and nature of heritable characteristics o Sub Part 1.2.2: Mendelian inheritance: monohybrid crosses o Sub Part 1.2.3: Mendelian inheritance: dihybrid crosses • Part 1.3: Non-Heritable Characteristics o Sub Part 1.3.1: Definition and nature of non-heritable characteristics o Sub Part 1.3.2: Environmental influences on phenotype o Sub Part 1.3.3: Distinguishing heritable from non-heritable traits • Part 1.4: Extensions of Mendelian Genetics o Sub Part 1.4.1: Incomplete dominance and codominance o Sub Part 1.4.2: Multiple alleles and the ABO blood group system o Sub Part 1.4.3: Pleiotropy and epistasis

BIO la-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS Introduction Genetics is the branch of biology concerned with heredity and variation, the mechanisms by which traits are transmitted from parents to offspring, and the causes of differences and similarities among related individuals. At the heart of genetics lies the distinction between characteristics that are inherited through the genetic material and those that arise through the influence of the environment on gene expression. This distinction is fundamental to understanding both normal biological variation and the genetic basis of disease. This Series establishes the conceptual and historical foundations of genetics, introduces chromosomes as the physical basis of heredity, distinguishes heritable from non-heritable characteristics with supporting evidence, presents the classical Mendelian model of inheritance through monohybrid and dihybrid crosses, examines the role of environment in shaping phenotypes, and extends the Mendelian framework to cover incomplete dominance, codominance, multiple alleles, pleiotropy, and epistasis. These topics provide the essential framework for the quantitative and population genetics topics that follow in later Series. Series Learning Outcomes Upon completing this Series, you should be able to: • SLO 1.1 define genetics and describe its scope, outline the key historical milestones in the development of genetics, and explain the chromosomal basis of heredity (Linked to Part 1.1), • SLO 1.2 define heritable characteristics and apply Mendel’s laws to predict offspring ratios in monohybrid and dihybrid crosses (Linked to Part 1.2), • SLO 1.3 define non-heritable characteristics, describe the environmental influences that modify phenotype, and explain how heritable and non-heritable traits are distinguished experimentally (Linked to Part 1.3), and • SLO 1.4 explain incomplete dominance, codominance, and multiple alleles with examples, and describe how pleiotropy and epistasis extend the Mendelian model (Linked to Part 1.4). 1.1 Fundamentals of Heredity

BIO la-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS 1.1.1 Definition and scope of genetics Genetics is the scientific study of genes, heredity, and genetic variation in living organisms. The word genetics was coined by William Bateson in 1905 from the Greek ‘genesis’ (origin). A gene is the fundamental unit of heredity: a segment of DNA that encodes information used to produce a functional product (usually a protein or structural RNA) that contributes to the phenotype of the organism. Heredity is the process by which traits are passed from parents to offspring through the transmission of genes. Variation refers to the differences in traits observed among individuals of the same species. The scope of genetics is broad and interdisciplinary: classical (Mendelian) genetics studies the patterns of trait transmission from parent to offspring using crosses and statistical analysis; molecular genetics examines the structure, function, and regulation of genes and their products at the molecular level (DNA, RNA, protein); cytogenetics studies the structure and behaviour of chromosomes during cell division and their relationship to heredity; population genetics studies allele frequencies and genetic variation in populations over time; genomics is the large-scale study of the complete genetic content of organisms; epigenetics studies heritable changes in gene expression that do not involve changes in the DNA sequence itself. Genetics underlies all of biology: it explains evolution (through heritable variation and natural selection), development (through differential gene expression), disease (through mutation and genetic disorders), and crop and livestock improvement (through selective breeding and genetic engineering). Fill in the blank: Genetics is the scientific study of genes, ________, and genetic variation; the word was coined by William Bateson in 1905; a gene is the fundamental unit of heredity and encodes information to produce a functional product that contributes to the phenotype. 1.1.2 Historical background of genetics The history of genetics spans more than 150 years and includes several landmark discoveries that transformed biology. Gregor Mendel (1822 to 1884): an Augustinian monk working in Brünn (now Brno, Czech Republic), Mendel conducted carefully controlled cross-pollination experiments on garden peas (Pisum sativum) between 1856 and 1863, analysing the inheritance of seven pairs of contrasting characters; his two laws (the Law of Segregation and the Law of

BIO la-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS Independent Assortment) were published in 1866 but were largely ignored until 1900. The Rediscovery of Mendel (1900): three botanists working independently, Hugo de Vries (Netherlands), Carl Correns (Germany), and Erich von Tschermak (Austria), each arrived at results similar to Mendel’s and searched the earlier literature to find Mendel’s pre-existing work; this rediscovery in 1900 launched the modern science of genetics. Walter Sutton and Theodor Boveri (1902 to 1903): independently proposed the chromosomal theory of inheritance, noting that the behaviour of chromosomes during meiosis parallels the segregation and independent assortment of Mendelian factors; this identified chromosomes as the physical carriers of Mendel’s hereditary units. Thomas Hunt Morgan and colleagues (1908 to 1920s): working with Drosophila melanogaster (fruit fly), Morgan demonstrated that genes are located on chromosomes and discovered sex-linkage, gene linkage, and genetic recombination; his group produced the first genetic maps; Morgan received the Nobel Prize in Physiology or Medicine in 1933. Frederick Griffith (1928): demonstrated the transforming principle in bacteria; heat-killed virulent Streptococcus pneumoniae could transform non-virulent bacteria into virulent forms, suggesting a heritable transforming substance. Oswald Avery, Colin MacLeod, and Maclyn McCarty (1944): identified DNA (not protein) as Griffith’s transforming principle, providing the first evidence that DNA is the genetic material. Alfred Hershey and Martha Chase (1952): confirmed DNA as the genetic material using bacteriophage T2 and radioactive tracers. James Watson and Francis Crick (1953): proposed the double-helix structure of DNA based on X-ray crystallography data from Rosalind Franklin and Maurice Wilkins; this opened the molecular era of genetics. Nirenberg, Khorana, and colleagues (1961 to 1966): deciphered the genetic code. The Human Genome Project (1990 to 2003): sequenced the complete human genome (approximately 3.2 billion base pairs; approximately 20,000 to 25,000 protein-coding genes). Fill in the blank: Gregor Mendel published his laws of inheritance in ________; his work was ignored until 1900 when it was independently rediscovered by Hugo de Vries, Carl Correns, and Erich von Tschermak; Watson and Crick proposed the double-helix structure of DNA in 1953. 1.1.3 Chromosomes and the basis of heredity

BIO la-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS The chromosomal theory of heredity, proposed by Sutton and Boveri in 1902 to 1903, states that genes are located on chromosomes and that the behaviour of chromosomes during meiosis explains Mendel’s laws. Evidence supporting the chromosomal theory: (1) Parallel behaviour: chromosomes segregate during meiosis (Meiosis I) in exactly the way that Mendel’s hereditary factors segregate; homologous chromosomes from different pairs assort independently during Meiosis I, exactly as Mendel’s law of independent assortment predicts. (2) Sex determination: the presence or absence of specific sex chromosomes (X and Y in humans and Drosophila) correlates precisely with sex-linked traits. (3) Linkage and recombination: genes on the same chromosome are linked (tend to be inherited together) unless separated by crossing over (recombination); the frequency of crossing over between two loci is proportional to the physical distance between them on the chromosome, enabling the construction of genetic maps. Chromosome structure: each eukaryotic chromosome is a single, linear, double-stranded DNA molecule (complexed with histone proteins to form chromatin; further coiled into the visible chromosome structure during cell division). Humans have 46 chromosomes: 22 pairs of autosomes (identical in both sexes) and one pair of sex chromosomes (XX in females; XY in males). The karyotype is the complete set of chromosomes of an organism or cell, typically displayed in a standardised arrangement of homologous pairs ordered by size. Changes in chromosome number (aneuploidy: trisomy, monosomy) or structure (deletion, inversion, translocation, duplication) have profound effects on phenotype. Mitosis: cell division in somatic (body) cells that produces two genetically identical daughter cells (each with the same number of chromosomes as the parent cell; 2n); maintains the chromosome number. Meiosis: the specialised cell division that produces haploid gametes (1n) with half the chromosome number of the parent cell; consists of two successive divisions (Meiosis I: reductive division; separates

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