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Semester 3: Genetics
Mendelian Genetics and Inheritance
Introduction to Mendelian Genetics
Mendelian genetics is the study of how traits are passed from parents to offspring based on the laws formulated by Gregor Mendel. Mendel performed experiments on pea plants and discovered patterns of inheritance.
Mendel's Laws of Inheritance
Mendel proposed two key laws: the Law of Segregation, which states that alleles for a trait separate during gamete formation, and the Law of Independent Assortment, which states that alleles of different traits are distributed independently of one another.
Genotypes and Phenotypes
Genotype refers to the genetic makeup of an organism, while phenotype refers to the observable characteristics. The interaction between genotype and environment influences phenotype.
Punnett Squares
Punnett squares are a tool used to predict the genetic outcomes of a cross between two organisms. They illustrate the possible genetic combinations of offspring based on parental genotypes.
Dominant and Recessive Traits
In Mendelian genetics, dominant traits mask the expression of recessive traits. An individual needs two copies of a recessive allele for the trait to be expressed.
Applications of Mendelian Genetics
Mendelian genetics has applications in various fields including agriculture, medicine, and conservation biology. Understanding inheritance patterns can improve crop yield and predict genetic diseases.
Mendelian experiments, laws of Mendel, Monohybrid, Dihybrid, back and test cross
Mendelian Experiments and Laws of Mendel
Introduction to Mendel's Experiments
Gregor Mendel conducted experiments on pea plants to understand inheritance patterns. He used controlled breeding to analyze how traits are passed from parents to offspring.
Laws of Mendel
Mendel's principles include the Law of Segregation, which states that allele pairs separate during gamete formation, and the Law of Independent Assortment, which states that genes for different traits can segregate independently during the formation of gametes.
Monohybrid Cross
A monohybrid cross examines the inheritance of a single trait. Mendel crossed true-breeding plants that differed in one trait, observing the F1 generation and the F2 generation to deduce the inheritance pattern.
Dihybrid Cross
A dihybrid cross studies the inheritance of two different traits simultaneously. Mendel's experiments with pea plants led to the conclusion that the inheritance of one trait does not affect the inheritance of another, demonstrating the Law of Independent Assortment.
Back Cross
A back cross, or test cross, involves crossing an F1 individual with one of the parental genotypes, usually the recessive type, to determine the genotype of the F1 generation and to identify the genotype of unknown individuals.
Test Cross
A test cross helps to reveal the genotype of an individual expressing a dominant trait. The individual is crossed with a homozygous recessive individual, allowing observation of offspring phenotypes to infer the unknown genotype.
Interaction of genes Incomplete dominance, co dominance, complementary genes, supplementary genes, inhibiting genes, lethal genes and atavism
Interaction of genes: Incomplete dominance, co-dominance, complementary genes, supplementary genes, inhibiting genes, lethal genes, and atavism
Incomplete dominance
Incomplete dominance is a genetic situation in which one allele does not completely dominate another allele, resulting in a new phenotype. For example, in a cross between red and white flowers, the offspring may show a pink color.
Co-dominance
Co-dominance occurs when both alleles in a heterozygous organism are fully expressed, resulting in a phenotype that showcases both traits. An example is the AB blood type in humans, where both A and B alleles are expressed.
Complementary genes
Complementary genes are pairs of genes where each gene contributes to a single phenotype. The presence of both genes is necessary for expression of a particular trait, such as flower color in certain plants.
Supplementary genes
Supplementary genes are those whose effect is to modify the expression of a trait controlled by another pair of genes. If a supplementary gene is present, it can enhance or reduce the expression of the other gene's traits.
Inhibiting genes
Inhibiting genes are genes that suppress the expression of other genes. These genes can control processes such as pigment synthesis by inhibiting the effects of genes responsible for color.
Lethal genes
Lethal genes are those whose expression leads to the death of the organism. They can be recessive or dominant and often influence population dynamics and evolution.
Atavism
Atavism refers to the reappearance of traits from ancestral forms that were previously lost in a lineage. This phenomenon can result from genetic mutations or the activation of dormant genes.
Inheritance Polygenic inheritance - skin colour ABO blood groups - sex linked inheritance - eye colour in Drosophila, colour blindness and hemophilia in man
Genetics
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Polygenic inheritance
Polygenic inheritance involves multiple genes contributing to a single trait. Traits such as skin color and height are regulated by several genes, resulting in a continuous range of phenotypes. Skin color is influenced by multiple pairs of alleles, leading to various shades depending on the combination of inherited genes.
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ABO blood groups
The ABO blood group system is a classic example of multiple alleles and codominance. There are four main blood types: A, B, AB, and O, determined by the presence or absence of antigens on the surface of red blood cells. The A and B alleles are codominant, while O is recessive.
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Sex-linked inheritance
Sex-linked traits are associated with genes located on sex chromosomes. Male and female offspring inherit these traits differently due to the distinct combinations of X and Y chromosomes. Common examples include color blindness and hemophilia, which are primarily found in males due to the presence of a single X chromosome.
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Eye color in Drosophila
In Drosophila melanogaster (fruit flies), eye color is a well-studied genetic trait. The red eye color is dominant over white. Mutations in the genes regulating pigment production result in variations in eye color, showcasing Mendelian inheritance patterns.
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Color blindness in man
Color blindness is a sex-linked trait caused by mutations in genes related to color vision located on the X chromosome. The most common forms include red-green color blindness, emphasizing the higher incidence in males due to their XY genotype, resulting in a lack of a second X to mask the trait.
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Hemophilia in man
Hemophilia is a genetic disorder characterized by a deficiency in clotting factors, leading to excessive bleeding. It is predominantly inherited in an X-linked recessive pattern, primarily affecting males. Female carriers can pass the trait to their offspring, potentially resulting in hemophilia in males.
Linkage and Crossing Over
Linkage and Crossing Over
Introduction to Linkage
Linkage refers to the tendency of genes located close to each other on the same chromosome to be inherited together. This phenomenon occurs because linked genes do not assort independently during meiosis.
Types of Linkage
There are two main types of linkage: complete and incomplete. Complete linkage occurs when genes are so close that recombination does not occur. Incomplete linkage allows for some degree of recombination.
Crossing Over Mechanism
Crossing over is the exchange of genetic material between homologous chromosomes during prophase I of meiosis. It results in the recombination of alleles, leading to genetic diversity in offspring.
Significance of Crossing Over
Crossing over increases genetic variation within a population, which is crucial for evolution and adaptation. It can also have important implications in breeding and genetic research.
Relationship between Linkage and Crossing Over
Linkage and crossing over are interconnected. While linked genes are inherited together, crossing over can separate them, resulting in recombinant phenotypes. The frequency of crossing over can be used to map gene locations on chromosomes.
Practical Applications
Understanding linkage and crossing over is essential in genetics for traits mapping, understanding inheritance patterns, and in fields such as agriculture and medicine for selective breeding.
Linkage Linked genes, complete and incomplete linkage
Linkage and Linked Genes
Introduction to Linkage
Linkage refers to the tendency of genes located close to each other on the same chromosome to be inherited together during meiosis. This phenomenon contrasts with the independent assortment of genes on different chromosomes.
Linked Genes
Linked genes are genes that are located on the same chromosome and are inherited together. They do not assort independently and can lead to variations in genetic inheritance.
Complete Linkage
Complete linkage occurs when two genes are located very close to each other on the same chromosome, resulting in their alleles being inherited together without any recombination. This leads to offspring that exhibit parental combinations of traits.
Incomplete Linkage
Incomplete linkage occurs when two genes are located on the same chromosome but are far enough apart to allow for some crossing over during meiosis. This results in a mix of parental and recombinant allele combinations in the offspring.
Significance of Linkage
Understanding linkage is crucial in genetics for mapping genes and studying inheritance patterns. It helps in predicting the likelihood of certain traits being passed on together and has implications in fields like medicine and agriculture.
Crossing over molecular mechanisms of crossing over, kinds of crossing over, models of recombination
Crossing Over and Its Mechanisms
Molecular Mechanisms of Crossing Over
Crossing over is a biological process that occurs during meiosis, specifically in prophase I, where homologous chromosomes exchange genetic material. The key molecular players involved include: 1. **Double Strand Breaks (DSBs)**: The process begins with the introduction of double strand breaks in one of the homologous chromosomes, facilitated by enzymes like Spo11. 2. **End Resection**: After DSBs, the ends of the breaks are processed to create single-stranded DNA ends, allowing for strand invasion. 3. **Strand Invasion**: Single-stranded DNA from one chromosome invades a homologous region of the sister chromosome, leading to the formation of a displacement loop (D-loop). 4. **DNA Synthesis and Repair**: DNA polymerases synthesize new DNA, using the invaded strand as a template, repairing the break and forming Holliday junctions. 5. **Resolution of Holliday Junctions**: The interconnected junctions can be resolved in different ways, leading to either crossover or non-crossover outcomes, influencing genetic diversity.
Types of Crossing Over
There are several types of crossing over, distinguished by the mechanisms and outcomes involved: 1. **Homologous Recombination**: The most common type, where genetic material is exchanged between homologous chromosomes during meiosis. 2. **Non-Homologous End Joining (NHEJ)**: A repair mechanism that can join ends of two broken chromosomes without a homologous template, potentially leading to mutations. 3. **Sister Chromatid Exchange**: Involves the exchange between sister chromatids, which does not produce genetic variation, but is a form of repair. 4. **Gene Conversion**: A non-reciprocal exchange that results in the alteration of one of the alleles, which can affect the phenotypic expression of traits.
Models of Recombination
Various models attempt to explain the mechanics behind recombination and crossing over: 1. **SDSA Model (Synthesis-Dependent Strand Annealing)**: This model proposes that after strand invasion, DNA synthesis occurs and the newly synthesized strand can anneal back to the original DNA, typically resulting in non-crossover products. 2. **DSBR Model (Double Strand Break Repair)**: In this model, DSBs form and are repaired by incorporating the homologous chromosome, leading to either crossover or non-crossover products, depending on how the Holliday junctions are resolved. 3. **Classical Homologous Recombination Model**: Integrating components of both the SDSA and DSBR models, it emphasizes the role of multiple proteins and enzymatic activities in ensuring proper recombination takes place during meiosis.
Chromosome mapping inference and coincidence, haploid mapping, somatic cell hybridization
Chromosome Mapping Inference and Coincidence, Haploid Mapping, Somatic Cell Hybridization
Introduction to Chromosome Mapping
Chromosome mapping is the method used to determine the location and arrangement of genes on chromosomes. It is essential for understanding genetic linkage and for locating genes associated with diseases.
Mapping Inference and Coincidence
Mapping inference involves using statistical methods to deduce the location of genes based on observed recombination frequencies. Coincidence refers to the occurrence of simultaneous crossover events and is measured by comparing observed double crossovers to expected frequencies.
Haploid Mapping
Haploid mapping is performed using organisms with haploid genomes, such as certain fungi and plants. It simplifies the analysis of genetic traits, making it easier to link traits to specific genes.
Somatic Cell Hybridization
Somatic cell hybridization is a technique in which two different somatic cells are fused to create hybrid cells. This method is used in mapping genes and analyzing gene expression, especially in the study of human diseases.
Applications of Chromosome Mapping
The methods of chromosome mapping have applications in genetics, breeding programs, and medical research. Accurate mapping of chromosomes helps in identifying genes responsible for genetic disorders and in developing gene therapies.
Cytogenetics Variation in chromosome number and structure position effect, chromosomal mutation and evolution
Cytogenetics Variation in Chromosome Number and Structure
Chromosome Number Variation
Chromosome number variation refers to changes in the number of chromosomes in an organism's cells. This can occur through processes such as aneuploidy, where there is an abnormal number of chromosomes due to nondisjunction during meiosis, resulting in conditions such as Down syndrome. Polyploidy, where an organism has more than two sets of chromosomes, is common in plants and can lead to greater genetic diversity.
Structure Position Effect
Position effect refers to the phenomenon where the expression of a gene is altered by its position within the chromosome. This can occur due to changes in chromatin structure or gene rearrangements, such as translocations. The position effect can lead to gene activation or silencing, affecting phenotypic traits and potentially leading to evolutionary advantages or disadvantages.
Chromosomal Mutation
Chromosomal mutations involve alterations to the structure or number of chromosomes. Types of mutations include deletions, duplications, inversions, and translocations. These mutations can lead to significant changes in gene expression and phenotype. In some cases, they are beneficial and can provide new traits that contribute to an organism's fitness.
Evolution and Chromosomal Changes
Chromosomal variations and mutations play a crucial role in evolution. They can lead to speciation events, where populations diverge genetically due to differences in chromosome structure or number. Natural selection may favor certain chromosomal configurations that enhance survival or reproductive success in specific environments, influencing the evolutionary trajectory of species.
Gene mutation types, molecular basis of mutation, mutational hot spots, reversion radiation and chemical agents as mutagens
Gene Mutation Types, Molecular Basis of Mutation, Mutational Hot Spots, Reversion, Radiation and Chemical Agents as Mutagens
Gene mutations are alterations in the nucleotide sequence of DNA. They can be classified into several types: point mutations, insertions, deletions, and duplications.
These involve the substitution of a single nucleotide base pair, leading to transitions or transversions.
This type involves the addition of one or more nucleotide pairs, potentially causing a frameshift.
These are the removal of nucleotide pairs, which can also lead to frameshift mutations.
This refers to the repetition of a gene or a segment of DNA, leading to increased gene dosage.
Mutations can occur due to various processes at the molecular level, including errors during DNA replication, exposure to radiation, or chemical damage.
Mistakes made by DNA polymerases can result in incorrect nucleotides being incorporated.
These may arise due to inherent chemical instability of nucleotides or during biological processes.
Mutagens can cause changes in DNA structure, leading to mutations.
Certain regions of DNA are more prone to mutations. These areas are known as mutational hot spots.
Hot spots often contain repetitive sequences or are regions that are difficult for DNA repair mechanisms to navigate.
Common examples of mutations in these regions include microsatellite instability.
Reversion is the process by which a mutated gene returns to its original sequence or function.
This can happen through second-site mutations that compensate for the original mutation or through precise repair processes.
Radiation is a significant source of mutagens, with different types affecting DNA in varied ways.
Includes X-rays and gamma rays which can cause breaks in the DNA strand.
UV light can cause the formation of thymine dimers, which distort the DNA helix and can lead to replication errors.
Chemical agents can also induce mutations through various mechanisms.
These add alkyl groups to nucleotide bases, leading to mispairing during DNA replication.
These slip between DNA bases, resulting in insertions or deletions during replication.
These mimic nucleotide bases and can lead to false incorporation during DNA synthesis.
Human and Microbial Genetics
Human and Microbial Genetics
Introduction to Genetics
Genetics is the study of heredity and variation in organisms. It explores how traits are passed from one generation to the next through genes.
Human Genetics
Human genetics focuses on the inheritance of traits in humans. It involves the study of chromosomes, DNA sequences, and the role of genes in health and disease.
Microbial Genetics
Microbial genetics examines the genetic makeup of microorganisms. It includes the study of bacteria, viruses, fungi, and their genetic mechanisms.
Genetic Variation
Genetic variation refers to differences in DNA among individuals. It is essential for evolution and population adaptability.
Applications of Genetics
Genetics has numerous applications, including medicine, agriculture, and biotechnology. Understanding genetic principles aids in developing therapies and improved crop varieties.
Gene Expression and Regulation
Gene expression involves the transcription and translation of genes into proteins. Regulation of gene expression is crucial for cellular function and response to environmental changes.
Ethical Considerations in Genetics
Ethical issues arise in genetics concerning privacy, consent, and potential discrimination based on genetic information.
Human genetics Karyotype and ideogram sex determination - Barr body technique, drumstick method chromosomal abnormalities in humans, Pedigree analysis diagnosis of genetic abnormalities
Human genetics Karyotype and ideogram sex determination
Karyotype
A karyotype is the complete set of chromosomes in an individual, organized in pairs and sorted by size. It typically helps in identifying chromosomal abnormalities.
Ideogram
An ideogram is a graphical representation of the karyotype which shows the chromosomes arranged in a standard format. It helps visualize the structure and number of chromosomes.
Sex Determination
Human sex determination is primarily based on the presence of sex chromosomes. Females have two X chromosomes (XX) while males have one X and one Y chromosome (XY).
Barr Body Technique
The Barr body technique is used to identify the presence of an inactive X chromosome in female cells. It is essential for understanding conditions related to dosage compensation.
Drumstick Method
The drumstick method is a cytogenetic technique used to identify sex chromosomes, particularly in females. The presence of a drumstick-shaped structure indicates an X chromosome.
Chromosomal Abnormalities
Chromosomal abnormalities can occur due to nondisjunction, translocations, or deletions, leading to conditions such as Down syndrome, Turner syndrome, and Klinefelter syndrome.
Pedigree Analysis
Pedigree analysis is a diagrammatic method to trace inheritance patterns of genetic traits within families. It helps in understanding the inheritance of genetic disorders.
Diagnosis of Genetic Abnormalities
Genetic abnormalities can be diagnosed through various techniques including karyotyping, molecular analysis, and pedigree studies to assess risks for inherited conditions.
Population genetics and evolution gene pool, gene frequency and genotype frequency
Population genetics and evolution
Gene pool refers to the total collection of genes and their variations present in a population. It encompasses all alleles for every gene in the population and is essential for understanding genetic diversity and evolutionary potential.
Gene frequency, or allele frequency, is the proportion of a specific allele among all alleles for a particular gene in a given population. It is a critical component in studying population genetics, as changes in gene frequency can indicate evolutionary processes in action.
Genotype frequency is the proportion of a specific genotype in a population. Understanding genotype frequency helps to evaluate how individuals in a population are distributed with respect to their genetic makeup, which can inform about evolutionary pressures and genetic diversity.
Evolution is the process through which populations of organisms change over generations. It is driven by mechanisms such as natural selection, genetic drift, mutation, and gene flow. The study of population genetics provides insights into the genetic basis of evolutionary change.
