13.1 Chromosomal Theory and Genetic Linkage
3 min read•june 14, 2024
Chromosomes are the carriers of genetic material, housing genes along their length. They come in pairs, one from each parent, and play a crucial role in inheritance patterns. Understanding how chromosomes behave during cell division is key to grasping genetic inheritance.
occurs when genes are close together on a chromosome, making them more likely to be inherited together. This concept, along with during , helps explain why some traits don't follow Mendel's laws of inheritance perfectly.
Chromosomal Theory and Genetic Linkage
Principles of chromosomal inheritance
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- Chromosomes carry genetic material and contain many genes located along their length
- Chromosomes exist in homologous pairs, one from each parent, carry same genes but may have different (e.g. one chromosome may have allele for brown eyes, other blue eyes)
- Chromosomes segregate during meiosis, each (egg or sperm) receives one chromosome from each homologous pair, explains Mendel's law of
- Chromosomes assort independently during meiosis, explains Mendel's law of (e.g. chromosome with gene for eye color assorts independently from chromosome with gene for hair color)
Genetic linkage and inheritance patterns
- Genetic linkage is the tendency of genes located close together on a chromosome to be inherited together, do not follow Mendel's law of independent assortment
- Linked genes are more likely to be inherited together than unlinked genes (e.g. genes for petal color and pollen shape in pea plants)
- Linkage reduces frequency of between genes; closer the genes, stronger the linkage
- Linkage can be broken by crossing over during meiosis, allows for exchange of genetic material between (e.g. linked genes on chromosome 1 can be separated by crossing over and end up on different chromosomes in gametes)
- refers to the non-random association of alleles at different loci
Crossing over in recombination
- Crossing over occurs during of meiosis
- Homologous chromosomes pair up, form
- Non- of homologous chromosomes exchange genetic material at specific points called
- Crossing over results in genetic recombination, new combinations of alleles formed on chromosomes, increases genetic diversity in offspring
- Frequency of crossing over depends on distance between genes; farther apart, higher probability of crossing over (e.g. genes at opposite ends of chromosome more likely to recombine than adjacent genes)
- Crossing over frequency can be used to map relative positions of genes on chromosome
Stages of chromosome formation
- : Chromosomes decondensed, not visible, DNA replication occurs during S phase
- : Chromosomes condense, become visible, each consists of two sister chromatids held together by
- : Chromosomes align at equatorial plate, attach to centromeres
- : Sister chromatids separate, move towards opposite poles, each chromatid becomes individual chromosome
- : Chromosomes decondense, nuclear envelope reforms, cytokinesis occurs dividing cytoplasm, forming two daughter cells (e.g. mitosis produces two genetically identical daughter cells, meiosis produces four genetically unique gametes)
Gene mapping through test crosses
- involves three genes on same chromosome, allows for determination of gene order, relative distances
- Procedure:
- Cross triple heterozygote (ABC/abc) with triple recessive (abc/abc)
- Analyze phenotypes, proportions of offspring
- Calculate recombination frequencies between each pair of genes
- Gene order determined by comparing recombination frequencies, order is one that minimizes total number of recombinants (e.g. if between A and B is 10%, B and C is 25%, A and C is 35%, then order is A-B-C)
- distances calculated using recombination frequencies
- One map unit (, cM) equals 1%
- between two genes:
- E.g. if 20 recombinants out of 1000 offspring, then map distance is
Genetic concepts and terminology
- : The specific location of a gene on a chromosome
- : Having a single set of chromosomes, typical of gametes
- : Having two sets of chromosomes, one from each parent, typical of somatic cells
- : The genetic makeup of an organism, determined by the alleles present at a given locus
- : The observable characteristics of an organism, resulting from the interaction between its genotype and the environment
- : A pioneering geneticist who made significant contributions to the through his work with Drosophila melanogaster
Key Terms to Review (45)
Alleles: Alleles are different versions of a gene that exist at a specific locus on a chromosome. They can determine distinct traits or characteristics in an organism, influencing everything from flower color to disease resistance. The interaction between alleles—such as dominant and recessive relationships—plays a crucial role in inheritance patterns and genetic diversity.
Anaphase: Anaphase is a stage in cell division where the sister chromatids of each chromosome are pulled apart and moved towards opposite poles of the cell. This critical phase ensures that each daughter cell will receive an identical set of chromosomes, contributing to genetic stability during cell division. Anaphase occurs following metaphase and is crucial in both mitosis and meiosis, facilitating accurate chromosome segregation.
CentiMorgan: A centiMorgan (cM) is a unit of measurement used in genetics to describe the genetic distance between loci on a chromosome. It is defined as the distance at which one percent of crossover events occur between two genes during meiosis. The concept is crucial for understanding genetic linkage and how closely genes are inherited together, which ties back to the chromosomal theory of inheritance.
Centimorgans (cM): Centimorgans (cM) are units of measure for genetic linkage that describe the distance between chromosome positions. One centimorgan represents a 1% chance that a marker at one genetic locus will be separated from a marker at another locus due to crossing over in a single generation.
Centromere: The centromere is a region of a chromosome that links sister chromatids. It plays a crucial role in chromosome segregation during cell division.
Centromere: A centromere is a specialized region of a chromosome that serves as the attachment point for spindle fibers during cell division, ensuring proper segregation of chromosomes into daughter cells. It plays a critical role in both mitosis and meiosis, contributing to the accurate distribution of genetic material. The centromere is essential for maintaining chromosome stability and integrity throughout the cell cycle.
Chiasmata: Chiasmata are the points where homologous chromosomes exchange genetic material during meiosis, specifically during prophase I. This process, known as crossing over, results in genetic recombination, which increases genetic diversity in offspring. Chiasmata play a crucial role in the proper segregation of chromosomes, ensuring that each gamete receives a mix of maternal and paternal genes.
Chromosomal theory: The chromosomal theory of inheritance states that genes are located on chromosomes, and that the behavior of chromosomes during meiosis and fertilization accounts for the inheritance patterns observed in organisms. This theory connects the concepts of genetics and cell biology by explaining how genetic information is transmitted from one generation to the next through the segregation and independent assortment of chromosomes during gamete formation.
Chromosomal Theory of Inheritance: Chromosomal Theory of Inheritance states that genes are located on chromosomes, which are the basis for all genetic inheritance. It integrates Mendelian genetics with chromosome behavior during meiosis.
Crossing over: Crossing over is the process during meiosis where homologous chromosomes exchange segments of genetic material, leading to genetic variation in the resulting gametes. This genetic shuffling is crucial for sexual reproduction, enhancing diversity and influencing inheritance patterns, as it allows for new combinations of alleles to be passed on to offspring.
Diploid: A diploid cell contains two complete sets of chromosomes, one from each parent. In humans, diploid cells have 46 chromosomes.
Diploid: Diploid refers to a cell or organism that contains two complete sets of chromosomes, one inherited from each parent. This condition is vital for sexual reproduction and genetic diversity, as it ensures that offspring receive a mix of genetic material from both parents.
G2 of Interphase: G2 of Interphase is the final sub-phase of Interphase where the cell undergoes rapid growth and prepares for mitosis. It involves the synthesis of proteins and organelles needed for cell division.
Gamete: A gamete is a specialized reproductive cell that carries half the genetic information of an organism, combining with another gamete during fertilization to form a new organism. Gametes are crucial for sexual reproduction and play a key role in genetic diversity and inheritance.
Genetic linkage: Genetic linkage refers to the tendency of genes located close to each other on a chromosome to be inherited together during meiosis. This phenomenon is a crucial aspect of inheritance patterns and helps explain why certain traits tend to be passed on as a group, rather than independently. The closer two genes are on a chromosome, the higher the likelihood that they will be inherited together, which can influence genetic diversity and the study of inheritance.
Genetic map: A genetic map is a representation of the arrangement of genes and genetic markers on a chromosome, showing the relative distances between them based on the frequency of recombination during meiosis. This type of map is crucial for understanding how traits are inherited and can help in locating genes associated with specific diseases, as well as in breeding programs for plants and animals.
Genotype: A genotype refers to the specific genetic makeup of an organism, represented by the alleles inherited from its parents. It determines various traits and characteristics that an organism may express, linking it to patterns of inheritance and genetic diversity within populations.
Haploid: Haploid describes a cell that contains a single set of chromosomes. In humans, haploid cells are typically gametes, such as sperm and eggs, which contain 23 chromosomes each.
Haploid: Haploid refers to a cell or organism that has only one complete set of chromosomes, which is half the diploid number typical for a species. This condition is crucial in the process of sexual reproduction, where haploid gametes unite during fertilization to form a diploid zygote, ultimately leading to the development of a new organism.
Homologous chromosomes: Homologous chromosomes are pairs of chromosomes in a diploid organism that have the same genes at the same loci, but may have different alleles. These pairs consist of one chromosome inherited from each parent, and they play a crucial role during meiosis as they ensure genetic diversity through processes like crossing over.
Homologous recombination: Homologous recombination is a process where homologous chromosomes exchange genetic material during meiosis. This results in genetic variation by producing new combinations of alleles.
Independent assortment: Independent assortment is the principle that states that alleles for different traits segregate independently of one another during gamete formation. This means that the inheritance of one trait will not affect the inheritance of another trait, allowing for a variety of genetic combinations in offspring and playing a critical role in increasing genetic diversity.
Interphase: Interphase is the stage in the cell cycle where the cell prepares for division, encompassing the growth and DNA replication necessary for successful cell division. It is crucial for cellular function as it allows the cell to grow, replicate its DNA, and produce proteins needed for mitosis or meiosis.
Linkage disequilibrium: Linkage disequilibrium refers to the non-random association of alleles at different loci on a chromosome, meaning certain combinations of alleles occur together more or less often than expected by chance. This phenomenon is crucial for understanding how genes are inherited, especially when considering the role of genetic linkage and the chromosomal theory of inheritance. It highlights the impact of physical proximity on allele combinations, which can affect traits and disease susceptibility across populations.
Locus: A locus is the specific, fixed position on a chromosome where a particular gene or genetic marker is located. Understanding the concept of a locus is crucial for grasping how genes are inherited and expressed, as it helps establish the relationship between genetic traits and their physical locations on chromosomes, which is key to the principles of genetic linkage and chromosomal theory.
Map distance: Map distance is a measurement used in genetics to indicate the relative distance between genes on a chromosome, typically expressed in centimorgans (cM). It reflects the likelihood of recombination occurring between two genes during meiosis; the greater the distance, the higher the chance that crossover events will separate the alleles of those genes. This concept connects to chromosomal theory and genetic linkage, illustrating how genes located close to each other tend to be inherited together.
Meiosis: Meiosis is a specialized form of cell division that reduces the chromosome number by half, resulting in the production of four genetically diverse gametes, or sex cells. This process is crucial for sexual reproduction, as it ensures genetic diversity and maintains the species' chromosome number across generations.
Metaphase: Metaphase is a stage in cell division where chromosomes align at the cell's equatorial plane, preparing for separation into daughter cells. During this critical phase, spindle fibers attach to the centromeres of the chromosomes, ensuring that each new cell will receive an accurate copy of the genetic material. The proper alignment of chromosomes is vital for maintaining genetic stability and preventing disorders related to improper chromosome distribution.
Nonparental types: Nonparental types are offspring whose genetic combinations are different from those of their parents. This occurs due to the process of recombination during meiosis.
Parental types: Parental types are offspring that inherit a combination of traits that exactly match one of the two parents. This occurs due to the inheritance of linked genes that do not undergo recombination during meiosis.
Phenotype: A phenotype is the observable physical or biochemical characteristics of an organism, determined by both genetic makeup and environmental influences. It encompasses traits such as appearance, behavior, and physiological properties, highlighting how genes interact with the environment to shape an organism's characteristics.
Prophase: Prophase is the first stage of mitosis and meiosis, where chromatin condenses into visible chromosomes and the mitotic spindle begins to form. During this phase, the nuclear envelope starts to break down, allowing the chromosomes to become more accessible for segregation. Prophase is crucial for proper cell division as it sets the stage for chromosome alignment and separation in later phases.
Prophase I: Prophase I is the first stage of meiosis I, where homologous chromosomes pair up and undergo genetic recombination through a process called crossing over. This stage is crucial for genetic diversity, as it sets the stage for the subsequent separation of chromosomes, and is a key part of sexual reproduction, influencing how traits are inherited across generations.
Recombination: Recombination is the process by which genetic material is exchanged between homologous chromosomes during meiosis, resulting in new combinations of alleles. This process plays a vital role in increasing genetic diversity within populations, as it allows for the shuffling of genes, creating unique combinations that can be passed down to offspring and influencing traits across generations.
Recombination frequency: Recombination frequency is the proportion of recombinant offspring produced in a genetic cross. It measures the likelihood of crossing over between two genes during meiosis.
Recombination frequency: Recombination frequency is the measure of the likelihood that two alleles will be separated during meiosis due to the exchange of genetic material between homologous chromosomes. This concept is crucial in understanding genetic linkage, as it helps determine the distance between genes on a chromosome. A higher recombination frequency indicates that genes are farther apart, while a lower frequency suggests they are closer together.
Segregation: Segregation is the process during meiosis where alleles for a gene separate from each other so that each gamete receives only one allele for each gene. This fundamental principle underlies the inheritance patterns described by the chromosomal theory, where chromosomes segregate during cell division, ensuring that offspring inherit a mix of traits from both parents. Segregation also plays a key role in genetic linkage, as it affects how alleles are passed on together when genes are located on the same chromosome.
Sister chromatids: Sister chromatids are identical copies of a single chromosome that are connected by a centromere and are crucial during cell division. They form during the S phase of the cell cycle, when DNA is replicated, ensuring that each new cell receives an identical set of chromosomes. Understanding sister chromatids is vital for grasping how genetic information is accurately distributed during both mitosis and meiosis.
Spindle fibers: Spindle fibers are dynamic protein structures that form during cell division, specifically during mitosis and meiosis. They are crucial for separating chromosomes and ensuring that each daughter cell receives the correct number of chromosomes. Spindle fibers attach to the centromeres of chromosomes and help guide their movement to opposite poles of the cell.
Synapsis: Synapsis is the process during meiosis where homologous chromosomes pair up and align closely together, facilitating genetic exchange through crossing over. This critical step ensures that each gamete receives a mix of genetic material from both parents, promoting genetic diversity. Synapsis occurs during prophase I of meiosis, allowing for accurate chromosome segregation and the formation of recombinant chromosomes.
Telophase: Telophase is the final stage of mitosis, where two daughter nuclei form in a cell. Chromosomes decondense and the nuclear envelope reappears around each set of chromosomes.
Telophase: Telophase is the final stage of cell division, specifically during mitosis and meiosis, where the separated chromosomes reach opposite poles of the cell and begin to de-condense, ultimately leading to the formation of two distinct nuclei. This phase marks the end of nuclear division and prepares the cell for cytokinesis, where the cytoplasm divides, completing the overall cell division process.
Test cross: A test cross is a breeding experiment used to determine the genotype of an individual with a dominant phenotype by crossing it with a homozygous recessive individual. This method helps reveal whether the dominant individual is homozygous or heterozygous for a specific trait, thus providing insight into the inheritance patterns observed in offspring. The results can clarify the underlying genetic mechanisms involved in traits, as well as their expression in different generations.
Thomas Hunt Morgan: Thomas Hunt Morgan was an American geneticist known for his pioneering work in the field of genetics, particularly in understanding the role of chromosomes in heredity. He conducted crucial experiments with the fruit fly Drosophila melanogaster, demonstrating that genes are located on chromosomes and laying the groundwork for the chromosomal theory of inheritance, which connects closely to the mechanisms of genetic variation observed during meiosis.
Three-point test cross: A three-point test cross is a genetic technique used to determine the arrangement of genes on a chromosome by crossing an individual heterozygous for three traits with a homozygous recessive individual for the same traits. This method allows researchers to analyze offspring phenotypes to deduce gene linkage and map the order of genes. It is essential for understanding how genes are inherited together, shedding light on genetic linkage and chromosomal organization.