BIO 101 Genetics and Epigenetics PDF

Genetics and Epigenetics Introduction to Biology Chapter: 9 Have you ever wondered why children have similarities to both their parents? We acquire these similar characteristics (traits) through a process called inheritance (heredity). Inheritance occurs through transfer of g...

Genetics and Epigenetics Introduction to Biology Chapter: 9 Have you ever wondered why children have similarities to both their parents? We acquire these similar characteristics (traits) through a process called inheritance (heredity). Inheritance occurs through transfer of genetic material from parents to their offspring.  Chromosomes They are present in the nucleus of the cell (eukaryotes) or present in the cytoplasm (prokaryotes). Different organisms have different numbers of chromosomes. Humans have 23 pairs of chromosomes (46 in total): 1 from each pair comes Karyotyping from father and one from mother.  Chromosomes Thread like structures that contain an organism’s DNA. The DNA are coiled in proteins called histones (nucleosomes).  Genes Units of information about specific traits A sequence of DNA Passed from parents to offspring The complete set of DNA of an organism is called its genome. The study of genes, genetic variation and heredity is called Genetics.  Alleles Different molecular forms of the same gene Arise by mutation Each allele is passed from each parent to the offspring  Locus Each gene has a specific location (locus) on a chromosome Alleles are located in the same locus of a chromosome  Allele combinations Homozygous having two identical alleles at a locus AA or aa Heterozygous having two different alleles at a locus Aa Dominant & Recessive Alleles A dominant allele is always expressed and hence masks a recessive allele that is paired with it. Recessive alleles are only expressed when both alleles are recessive. Dominant alleles are written in capital letters (B) and recessive alleles are written in small letters (b).  Examples Allele Term Result 1 dominant + 1 Homozygous dominant Dominant trait is dominant (BB) expressed 1 recessive + 1 Homozygous recessive Recessive trait is recessive (bb) expressed 1 dominant + 1 Heterozygous Dominant trait is recessive (Bb) expressed Genotype & Phenotype Genotype refers to particular genes an individual carries Phenotype refers to an individual’s observable traits We cannot always determine genotype by observing phenotype: Homozygous dominant or Heterozygous? Gregor Mendel Strong background in plant breeding and mathematics. Using pea plants, found indirect but observable evidence of how parents transmit genes to offspring. Father of genetics! Particulate theory: physical traits are inherited as ‘particles’ The Garden Pea Plant Self-pollinating True breeding (different alleles not normally introduced) Can be experimentally cross-pollinated Tracking Generations P Parental generation mates to produce: First-generation offspring mate to produce: F1 Second-generation offspring F2 Monohybrid cross Female gametes a a A mix of genetics between two homozygous genotypes: - Either completely dominant or A Aa Aa completely recessive Male gametes Mono: one A Aa Aa Hybrid: combination of two organisms For garden pea plants, Mendel observed the colour of pea plants Purple: AA White: aa  F1 Results of One Monohybrid Cross Result All offspring were heterozygous dominant (Aa)  F2 Results of Monohybrid Cross Result 2 offspring were heterozygous dominant (Aa) 1 offspring was homozygous dominant (AA) 1 offspring was homozygous recessive (aa) Phenotypic ratio – 3 dominant : 1 recessive  Mendel’s Theory of Segregation An individual inherits a unit of information (allele) about a trait from each parent. During gamete formation, the alleles segregate from each other. Dihybrid Cross Experimental cross between individuals that are homozygous for different versions of two traits. Di = two Hybrid = combination of two organisms +  F1 Results of Mendel’s Dihybrid Crosses All plants displays the dominant form of both traits (Purple and tall) We now know: All plants inherit one allele for each trait from each parent All plants were heterozygous (AaBb) Phenotypic Ratios in F2 If the two traits are coded for by genes on separate chromosomes, sixteen allele combinations are possible: 1/4 1/4 1/4 1/4 AB Ab aB ab Four Phenotypes: 1/4 1/16 1/16 1/16 1/16 AB Tall, purple-flowered (9/16) AABB AABb AaBB AaBb 1/4 1/16 1/16 1/16 1/16 Tall, white-flowered (3/16) Ab AABb AAbb AaBb Aabb 1/4 Dwarf, purple-flowered (3/16) 1/16 1/16 1/16 1/16 aB AaBB AaBb aaBB aaBb Dwarf, white-flowered (1/16) 1/4 1/16 1/16 1/16 1/16 ab AaBb Aabb aaBb aabb Phenotypic ratio: 9:3:3:1  Mendel’s Law of Independent Assortment Mendel concluded that the two “units” for the first trait were to be sorted into gametes independently of the two “units” for the other trait. Members of each pair of homologous chromosomes are sorted into gametes at random during meiosis. Dominance Relations Dominance Complete Incomplete Codominance dominance dominance Complete dominance Dominance in heterozygous condition where the dominant allele completely masks the effects of the recessive allele.  Flower colour in Snapdragons Incomplete dominance Dominant allele does not completely mask the effect of the recessive allele. Heterozygous phenotype is somewhere between that of two homozygotes. Blend of both traits Codominance Both alleles of a particular trait are fully expressed. Non-identical alleles specify two phenotypes that are both expressed in heterozygotes. Offspring has a phenotype that shows combination of both traits. Epigenetics Epigenetics is the genetic control of a living organism through factors other than the DNA sequence During development, DNA accumulates chemical marks that affect how much of a gene is expressed. These chemical marks are called epigenomes Epigenetic changes can also switch certain genes ‘on’ or ‘off’! How Epigenetics Works? All our cells have the same DNA, so why do we even have different cells? Why are genotypically identical twins not phenotypically identical? What are the factors that cause people to develop differently? Factors Affecting Epigenomes Four key factors: 1. Genetics 2. Environment 3. Biological Networks 4. Development Genetics and Environment The protein structure and arrangement of genes themselves can sometimes influence how the chemical marks are distributed across the DNA Environmental factors consist of ‘outside’ factors that can have an impact on genes. These can be: Pollutants Allergens Heavy metals Chemical toxins Electromagnetic radiation Developmental Factors Development can include all stages of life, starting from embryonic, to early childhood, all the way through to adulthood Examples of developmental factors are: Nutrient intake (balance of diet) Stress and psychology Smoking and alcohol Social behavior patterns Case study – Identical Twins, Different behaviour? Suppose there is a pair of identical twins – they share the same genome, and thus have the same genotype However, one twin likes to read books and spend time on the computer growing up, while the other likes to play musical instruments and prefers to spend more time outdoors. How is this difference explained through epigenetics?