Heredity explores how traits are passed from parents to offspring through genes. This unit covers Mendelian genetics, chromosomes, DNA structure, and inheritance patterns. It also delves into gene expression, mutations, and genetic variation's role in evolution.
Modern applications of genetics are examined, including genetic engineering, gene therapy, and personalized medicine. The unit highlights how understanding heredity impacts fields like agriculture, medicine, and forensics, showcasing genetics' broad relevance in science and society.
Alleles are different versions of a gene that can result in variations in the characteristics they control
Alleles can be dominant, recessive, or codominant
The combination of alleles an individual possesses is their genotype
Phenotype refers to the physical expression of a trait determined by the genotype and environmental factors
Homozygous individuals possess two identical alleles for a specific gene (homozygous dominant or homozygous recessive)
Heterozygous individuals have two different alleles for a specific gene
Punnett squares are diagrams used to predict the probability of offspring inheriting specific allele combinations
Pedigree charts illustrate the inheritance of traits within a family across multiple generations
Incomplete dominance occurs when a heterozygous genotype results in a phenotype intermediate between the two homozygous phenotypes
Codominance is when both alleles in a heterozygous genotype are expressed equally in the phenotype (blood types)
Mendelian Genetics
Gregor Mendel, known as the "father of modern genetics," discovered the fundamental principles of inheritance through experiments with pea plants
The law of segregation states that allele pairs separate during gamete formation, with each gamete receiving one allele
This explains the 3:1 phenotypic ratio observed in the F2 generation of monohybrid crosses
The law of independent assortment posits that alleles for different genes assort independently during gamete formation
This results in the 9:3:3:1 phenotypic ratio seen in dihybrid crosses
Mendel's laws form the basis for predicting inheritance patterns using Punnett squares
Testcrosses involve crossing an individual with an unknown genotype with a homozygous recessive individual to determine the unknown genotype
Mendel's work laid the foundation for the field of genetics, although the molecular basis of inheritance was not yet known
Beyond Mendelian Inheritance
Polygenic traits are controlled by multiple genes, each with a small additive effect on the phenotype (height, skin color)
Polygenic traits often exhibit a continuous range of phenotypes rather than distinct categories
Epistasis occurs when the expression of one gene is influenced by the presence of one or more modifier genes
Pleiotropy is when a single gene influences multiple seemingly unrelated phenotypic traits (sickle cell anemia)
Environmental factors can interact with genetic factors to influence the expression of a phenotype
Epigenetic modifications, such as DNA methylation and histone modifications, can alter gene expression without changing the underlying DNA sequence
Genomic imprinting is an epigenetic phenomenon where genes are expressed differently depending on whether they are inherited from the mother or father
Gene linkage occurs when genes located close together on the same chromosome tend to be inherited together
Linked genes violate the law of independent assortment and can alter expected phenotypic ratios
Chromosomes and DNA
Chromosomes are highly condensed structures composed of DNA and proteins that carry genetic information
Humans have 23 pairs of chromosomes (22 autosomal pairs and one pair of sex chromosomes)
Autosomes are chromosomes not involved in determining sex, while sex chromosomes (X and Y) determine an individual's biological sex
DNA (deoxyribonucleic acid) is the genetic material that stores hereditary information in the form of genes
DNA is composed of four nucleotide bases: adenine (A), thymine (T), guanine (G), and cytosine (C)
Complementary base pairing (A with T, G with C) allows for the formation of the double helix structure and enables DNA replication and transcription
Genes are segments of DNA that code for specific proteins or functional RNA molecules
The central dogma of molecular biology describes the flow of genetic information from DNA to RNA to proteins
Transcription is the synthesis of RNA from a DNA template
Translation is the process of synthesizing proteins using the genetic code carried by messenger RNA (mRNA)
Mitochondrial DNA (mtDNA) is a small circular chromosome found in mitochondria that is inherited exclusively from the mother
Gene Expression and Regulation
Gene expression is the process by which the information encoded in a gene is used to synthesize functional gene products (proteins or RNA)
Transcription factors are proteins that bind to specific DNA sequences to regulate the transcription of genes
Enhancers are regulatory sequences that increase transcription when bound by activator proteins
Silencers are regulatory sequences that decrease transcription when bound by repressor proteins
Promoters are DNA sequences located upstream of a gene that serve as binding sites for RNA polymerase and transcription factors
Alternative splicing allows for the production of multiple protein isoforms from a single gene by selectively including or excluding exons during mRNA processing
DNA methylation and histone modifications (epigenetic marks) can alter chromatin structure and influence gene expression
DNA methylation typically represses gene expression by preventing transcription factor binding
Histone acetylation generally increases gene expression by promoting an open chromatin structure
RNA interference (RNAi) is a post-transcriptional gene silencing mechanism that uses small RNA molecules to degrade complementary mRNA
Genetic Mutations and Variation
Mutations are changes in the DNA sequence that can alter gene function and phenotype
Point mutations involve the substitution, insertion, or deletion of a single nucleotide (silent, missense, nonsense mutations)
Frameshift mutations result from the insertion or deletion of a number of nucleotides not divisible by three, altering the reading frame
Chromosomal abnormalities involve large-scale changes in chromosome structure or number
Aneuploidy is an abnormal number of chromosomes (Down syndrome, Turner syndrome)
Translocations occur when a segment of one chromosome is transferred to another chromosome
Mutations can be spontaneous (occurring naturally) or induced by mutagens (radiation, chemicals)
Genetic variation arises from mutations, recombination during meiosis, and the random assortment of chromosomes
Variation is essential for evolution by natural selection and contributes to the diversity of life
Beneficial mutations can increase an organism's fitness and be favored by natural selection
Neutral mutations have no effect on fitness and can accumulate in a population through genetic drift
Deleterious mutations reduce an organism's fitness and are typically selected against
Inheritance Patterns in Humans
Autosomal dominant disorders (Huntington's disease) manifest when an individual has at least one dominant allele
Affected individuals typically have an affected parent, and there is a 50% chance of passing the allele to offspring
Autosomal recessive disorders (cystic fibrosis) require two copies of the recessive allele to manifest
Carriers (heterozygotes) are typically unaffected but can pass the allele to their offspring
Affected individuals usually have unaffected parents who are both carriers
X-linked disorders are caused by mutations on the X chromosome
X-linked recessive disorders (hemophilia) are more common in males because they only have one X chromosome
X-linked dominant disorders (Rett syndrome) affect both males and females, but males may have more severe symptoms
Mitochondrial disorders are caused by mutations in mtDNA and are inherited exclusively from the mother
Genetic testing can identify carriers of recessive disorders and predict the likelihood of passing the disorder to offspring
Pedigree analysis is used to study the inheritance of human traits and disorders within families
Applications and Modern Techniques
Genetic engineering involves the direct manipulation of an organism's DNA to alter its characteristics
Recombinant DNA technology allows for the insertion of genes from one organism into another (insulin production in bacteria)
CRISPR-Cas9 is a powerful gene-editing tool that enables precise modifications to DNA sequences
Genetically modified organisms (GMOs) have had their DNA altered through genetic engineering (pest-resistant crops)
Gene therapy is a technique that introduces functional genes into cells to replace or correct defective genes (treatment of genetic disorders)
DNA fingerprinting uses variations in DNA sequences to identify individuals and establish biological relationships
Short tandem repeats (STRs) are highly variable DNA sequences used in forensic analysis and paternity testing
Personalized medicine utilizes an individual's genetic information to tailor medical treatments and preventive strategies
Genome-wide association studies (GWAS) identify genetic variations associated with complex traits and diseases by comparing the genomes of affected and unaffected individuals
Next-generation sequencing technologies have revolutionized genetics by enabling rapid, high-throughput DNA sequencing at reduced costs