Genetics: Heredity and Variation

• Genetics is the study of heredity and variation of inherited characteristics
• It explores how traits are passed from parents to offspring and the mechanisms responsible for similarities and differences among individuals

Basic Genetic Terminology

• Gene: A unit of heredity that codes for a specific trait
• Allele: Different forms of a gene (e.g., alleles for eye color might be blue, brown, or green)
• Genotype: The genetic makeup of an organism (e.g., BB, Bb, bb)
• Phenotype: The observable characteristics of an organism, resulting from the interaction of its genotype with the environment (e.g., brown eyes)
• Homozygous: Having two identical alleles for a gene (e.g., BB or bb)
• Heterozygous: Having two different alleles for a gene (e.g., Bb)
• Dominant allele: An allele that expresses its phenotype even when heterozygous with a recessive allele (e.g., in Bb, if B is dominant, the phenotype will show the B trait)
• Recessive allele: An allele that only expresses its phenotype when homozygous (e.g., in bb, the phenotype will show the b trait)

Mendelian Genetics

• Gregor Mendel, the "father of genetics," established basic principles of heredity through experiments with pea plants
• Mendel's experiments demonstrated that traits are inherited as discrete units (genes)
• Mendel's Laws:
  • Law of Segregation: During gamete formation, the two alleles for each gene segregate, so that each gamete carries only one allele for each gene
  • Law of Independent Assortment: Genes for different traits assort independently of each other during gamete formation (this law holds true for genes located on different chromosomes or far apart on the same chromosome)
  • Law of Dominance: In a heterozygote, one allele (the dominant allele) may mask the expression of another allele (the recessive allele)

Monohybrid Cross

• A cross between individuals that involves one pair of contrasting traits (e.g., tall vs. short)
• The phenotypic ratio in the F2 generation of a monohybrid cross is typically 3:1 (dominant:recessive)
• The genotypic ratio in the F2 generation is typically 1:2:1 (homozygous dominant: heterozygous: homozygous recessive)

Dihybrid Cross

• A cross between individuals that involves two pairs of contrasting traits (e.g., seed color and seed shape)
• The phenotypic ratio in the F2 generation of a dihybrid cross is typically 9:3:3:1
• This ratio assumes independent assortment of the genes involved

Test Cross

• A cross of an individual with an unknown genotype to a homozygous recessive individual
• Used to determine whether the individual with the unknown genotype is homozygous dominant or heterozygous

Extensions to Mendelian Genetics

• Incomplete dominance: A condition in which neither allele is dominant, and the heterozygote phenotype is intermediate between the two homozygous phenotypes (e.g., a red flower crossed with a white flower produces pink flowers)
• Codominance: A condition in which both alleles are equally expressed in the heterozygote phenotype (e.g., human blood type AB, where both A and B alleles are expressed)
• Multiple alleles: A gene with more than two alleles in the population (e.g., human ABO blood groups, where alleles A, B, and O exist)
• Pleiotropy: A single gene affects multiple phenotypic traits (e.g., sickle cell anemia, where a single gene mutation affects red blood cell shape, oxygen transport, and other traits)
• Epistasis: One gene affects the expression of another gene (e.g., coat color in Labrador retrievers, where one gene determines whether pigment is produced, and another gene determines the color of the pigment)
• Polygenic inheritance: Multiple genes affect a single phenotypic trait, resulting in continuous variation (e.g., human height, which is influenced by many genes)

Sex-linked Inheritance

• Genes located on the sex chromosomes (X and Y chromosomes) exhibit sex-linked inheritance
• In mammals, females have two X chromosomes (XX), and males have one X and one Y chromosome (XY)
• X-linked traits are more commonly expressed in males because they have only one X chromosome
• Examples of X-linked traits include hemophilia and color blindness

Chromosomal Basis of Inheritance

• Genes are located on chromosomes
• Chromosomes undergo segregation and independent assortment during meiosis, which explains Mendel's laws
• Linked genes: Genes located close together on the same chromosome tend to be inherited together and do not assort independently
• Crossing over: The exchange of genetic material between homologous chromosomes during meiosis
• Crossing over results in the recombination of linked genes
• The frequency of recombination between two genes is proportional to the distance between them on the chromosome
• Genetic maps: Maps of the relative locations of genes on a chromosome, based on recombination frequencies

Chromosomal Abnormalities

• Nondisjunction: The failure of chromosomes to separate properly during meiosis, resulting in gametes with an abnormal number of chromosomes
• Aneuploidy: A condition in which there is an abnormal number of chromosomes in a cell (e.g., trisomy, monosomy)
• Down syndrome (trisomy 21): A genetic disorder caused by an extra copy of chromosome 21
• Turner syndrome (monosomy X): A genetic disorder in females caused by the absence of one X chromosome
• Klinefelter syndrome (XXY): A genetic disorder in males caused by the presence of an extra X chromosome
• Chromosomal mutations: Changes in the structure of chromosomes
• Deletion: The loss of a segment of a chromosome
• Duplication: The repetition of a segment of a chromosome
• Inversion: The reversal of a segment of a chromosome
• Translocation: The movement of a segment of a chromosome to a nonhomologous chromosome

DNA Structure and Function

• DNA (deoxyribonucleic acid) is the molecule that carries genetic information
• DNA has a double helix structure, consisting of two strands of nucleotides
• Each nucleotide consists of a deoxyribose sugar, a phosphate group, and a nitrogenous base
• The four nitrogenous bases in DNA are adenine (A), guanine (G), cytosine (C), and thymine (T)
• A pairs with T, and G pairs with C (complementary base pairing)
• DNA replication: The process of copying DNA
• DNA polymerase: An enzyme that adds nucleotides to the growing DNA strand during replication
• DNA serves as a template for its own replication, ensuring accurate transmission of genetic information from one generation to the next

Gene Expression

• Gene expression: The process by which the information encoded in a gene is used to synthesize a functional gene product (protein or RNA)
• Transcription: The synthesis of RNA from a DNA template
• RNA polymerase: An enzyme that catalyzes the synthesis of RNA
• Translation: The synthesis of a polypeptide (protein) from an mRNA template
• Ribosomes: The cellular structures where translation takes place
• Codon: A sequence of three nucleotides in mRNA that specifies a particular amino acid or a stop signal
• The genetic code: The set of rules by which information encoded in genetic material (DNA or RNA) is translated into proteins by living cells

Mutations

• Mutation: A change in the nucleotide sequence of DNA
• Mutations can be spontaneous or induced by mutagens (e.g., radiation, chemicals)
• Point mutations: Changes in a single nucleotide base
• Base substitutions: The replacement of one nucleotide base with another
• Insertions: The addition of one or more nucleotide bases
• Deletions: The removal of one or more nucleotide bases
• Frameshift mutations: Insertions or deletions that alter the reading frame of the mRNA, leading to a completely different amino acid sequence downstream of the mutation
• Mutations can be harmful, beneficial, or neutral, depending on their effect on the phenotype

Biotechnology

• Biotechnology: The use of living organisms or their products to develop useful products or processes
• Genetic engineering: The manipulation of genes for practical purposes
• Recombinant DNA technology: The process of combining DNA from different sources
• Cloning: The production of identical copies of genes, cells, or organisms
• Polymerase chain reaction (PCR): A technique for amplifying DNA in vitro
• Gel electrophoresis: A technique for separating DNA fragments based on size
• DNA sequencing: The process of determining the nucleotide sequence of DNA
• Applications of biotechnology:
  • Medicine: Development of new drugs, gene therapy
  • Agriculture: Development of genetically modified crops
  • Forensics: DNA fingerprinting for identification
  • Environmental science: Bioremediation