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Mendel’s Laws of Genetics

Gregor Mendel (1822–1884), the "father of genetics", was an Augustine monk and mathematician who performed cross-breeding experiments with peas and beans from a monastery garden. Based on the experiments, Mendel deduced hereditary factors may be passed from the parental generation to the filial generation. From the deductions, the father of genetics Genetics Genetics is the study of genes and their functions and behaviors. Basic Terms of Genetics formed Mendel's laws of heredity: the law of segregation, the law of independent assortment, and the law of dominance. Mendel's laws described the inheritance of uncoupled autosomal genes Genes A category of nucleic acid sequences that function as units of heredity and which code for the basic instructions for the development, reproduction, and maintenance of organisms. DNA Types and Structure based on statistical predictions. The gene Gene A category of nucleic acid sequences that function as units of heredity and which code for the basic instructions for the development, reproduction, and maintenance of organisms. Basic Terms of Genetics traits follow the laws of mendelian inheritance.

Last updated: 28 Oct, 2021

Editorial responsibility: Stanley Oiseth, Lindsay Jones, Evelin Maza

Mendel’s Experiments and Punnett Squares

Mendel’s experiments

Mendel chose pea PEA Electrocardiogram (ECG) shows a cardiac rhythm without a palpable pulse. May be organized (with normal-appearing ECG complexes) or unorganized (no discernible complexes on ecg). From electromechanical dissociation, or no cardiac filling (“empty heart”). Cardiac Arrest plants Plants Cell Types: Eukaryotic versus Prokaryotic as the experimental model. Mendel reasoned pea PEA Electrocardiogram (ECG) shows a cardiac rhythm without a palpable pulse. May be organized (with normal-appearing ECG complexes) or unorganized (no discernible complexes on ecg). From electromechanical dissociation, or no cardiac filling (“empty heart”). Cardiac Arrest plants Plants Cell Types: Eukaryotic versus Prokaryotic were a good model of inheritance because of the following properties:

  • Many varieties
  • Easy to grow
  • Short generation time
  • Self- fertilization Fertilization To undergo fertilization, the sperm enters the uterus, travels towards the ampulla of the fallopian tube, and encounters the oocyte. The zona pellucida (the outer layer of the oocyte) deteriorates along with the zygote, which travels towards the uterus and eventually forms a blastocyst, allowing for implantation to occur. Fertilization and First Week
  • Cross- fertilization Fertilization To undergo fertilization, the sperm enters the uterus, travels towards the ampulla of the fallopian tube, and encounters the oocyte. The zona pellucida (the outer layer of the oocyte) deteriorates along with the zygote, which travels towards the uterus and eventually forms a blastocyst, allowing for implantation to occur. Fertilization and First Week

Mendel cross-fertilized plants Plants Cell Types: Eukaryotic versus Prokaryotic truebred (purebred) for certain traits. Mendel selected 7 true-breeding traits:

  • Seed shape (round or wrinkled)
  • Seed color (green or yellow)
  • Flower color (purple or white)
  • Pod shape (constricted or inflated)
  • Pod color (green or yellow)
  • Plant height (tall or dwarf)
  • Flower location ( axial Axial Computed Tomography (CT) or terminal)

True-breeding traits are referred to as homozygous alleles ( genes Genes A category of nucleic acid sequences that function as units of heredity and which code for the basic instructions for the development, reproduction, and maintenance of organisms. DNA Types and Structure). Example of determining true-breeding traits of flower color:

  • Parental (P1) lineage: purple or white flowers
  • Offspring (F1): purple flowers
  • Self-fertilize F1 to produce F2: purple and white flowers
  • Self-fertilize F2 to produce F3: all purple or all white flowers
  • The lineages are not truebred if the self- fertilization Fertilization To undergo fertilization, the sperm enters the uterus, travels towards the ampulla of the fallopian tube, and encounters the oocyte. The zona pellucida (the outer layer of the oocyte) deteriorates along with the zygote, which travels towards the uterus and eventually forms a blastocyst, allowing for implantation to occur. Fertilization and First Week of F2 does not produce a uniform color.

From the experiments, Mendel inferred:

  • Individuals are diploid Diploid The chromosomal constitution of cells, in which each type of chromosome is represented twice. Symbol: 2n or 2x. Basic Terms of Genetics.
  • Gametes are haploid Haploid The chromosomal constitution of cells, in which each type of chromosome is represented once. Symbol: n. Basic Terms of Genetics.
  • Traits have 2 forms (alleles).
  • Individuals could be homozygous or heterozygous.
  • Dominant vs recessive alleles
  • Alleles are discrete. 

Punnett squares

A Punnett square is a statistical analysis tool based on Mendel’s research Research Critical and exhaustive investigation or experimentation, having for its aim the discovery of new facts and their correct interpretation, the revision of accepted conclusions, theories, or laws in the light of newly discovered facts, or the practical application of such new or revised conclusions, theories, or laws. Conflict of Interest:

  • Predicts the probability Probability Probability is a mathematical tool used to study randomness and provide predictions about the likelihood of something happening. There are several basic rules of probability that can be used to help determine the probability of multiple events happening together, separately, or sequentially. Basics of Probability of a phenotype Phenotype The complete genetic complement contained in the DNA of a set of chromosomes in a human. The length of the human genome is about 3 billion base pairs. Basic Terms of Genetics occurrence:
    • Assumes 1 gamete Gamete Gametogenesis from each parent + 2 alleles per trait
    • Each allele Allele Variant forms of the same gene, occupying the same locus on homologous chromosomes, and governing the variants in production of the same gene product. Basic Terms of Genetics has a dominant and a recessive form.
    • The frequency of allele Allele Variant forms of the same gene, occupying the same locus on homologous chromosomes, and governing the variants in production of the same gene product. Basic Terms of Genetics combinations is calculated by determining all possible combinations of male and female gametes.
  • Heterozygous parents produce a 1:2:1 genotypic ratio and a 3:1 phenotypic ratio.
  • Homozygous parents produce a single phenotype Phenotype The complete genetic complement contained in the DNA of a set of chromosomes in a human. The length of the human genome is about 3 billion base pairs. Basic Terms of Genetics and single genotype Genotype The genetic constitution of the individual, comprising the alleles present at each genetic locus. Basic Terms of Genetics.

Mendel’s 1st Law: The Law of Dominance

  • The law of dominance: Genes Genes A category of nucleic acid sequences that function as units of heredity and which code for the basic instructions for the development, reproduction, and maintenance of organisms. DNA Types and Structure with a dominant allele Allele Variant forms of the same gene, occupying the same locus on homologous chromosomes, and governing the variants in production of the same gene product. Basic Terms of Genetics will display the phenotype Phenotype The complete genetic complement contained in the DNA of a set of chromosomes in a human. The length of the human genome is about 3 billion base pairs. Basic Terms of Genetics of the allele Allele Variant forms of the same gene, occupying the same locus on homologous chromosomes, and governing the variants in production of the same gene product. Basic Terms of Genetics.
  • Not all alleles have an equal phenotypic expression:
    • When 2 different alleles are inherited, 1 dominant allele Allele Variant forms of the same gene, occupying the same locus on homologous chromosomes, and governing the variants in production of the same gene product. Basic Terms of Genetics determines the phenotypic characteristics of the organism.
    • The silent characteristic is known as the recessive allele Allele Variant forms of the same gene, occupying the same locus on homologous chromosomes, and governing the variants in production of the same gene product. Basic Terms of Genetics.
    • The expressed allele Allele Variant forms of the same gene, occupying the same locus on homologous chromosomes, and governing the variants in production of the same gene product. Basic Terms of Genetics is known as the dominant allele Allele Variant forms of the same gene, occupying the same locus on homologous chromosomes, and governing the variants in production of the same gene product. Basic Terms of Genetics.
  • Dominance is not inherent (e.g., 1 gene Gene A category of nucleic acid sequences that function as units of heredity and which code for the basic instructions for the development, reproduction, and maintenance of organisms. Basic Terms of Genetics may be dominant to another, but recessive to a 3rd). 
  • Recessive genes Genes A category of nucleic acid sequences that function as units of heredity and which code for the basic instructions for the development, reproduction, and maintenance of organisms. DNA Types and Structure must be inherited in both copies of the gene Gene A category of nucleic acid sequences that function as units of heredity and which code for the basic instructions for the development, reproduction, and maintenance of organisms. Basic Terms of Genetics to be expressed.
  • Monohybrid crosses are used to study dominant relationships:
    • Punnett squares can predict the results of a monohybrid cross.
    • For example, if both homozygous parents carry different alleles of a trait (monohybrid cross), the F1 generation will be uniformly heterozygous and all will express the dominant trait.
  • Nonmendelian variants of dominance:
    • Codominant: Both traits are expressed in the phenotype Phenotype The complete genetic complement contained in the DNA of a set of chromosomes in a human. The length of the human genome is about 3 billion base pairs. Basic Terms of Genetics independently of one another (e.g., blood types AA AA Amyloidosis and BB all have F1 AB).
    • Intermediate: Both alleles influence one another (e.g., the result of red flower color and white flower color is pink flower color in the F1 generation).

Mendel’s 2nd Law: The Principle of Segregation

  • The law of segregation: Alleles separate from one another during gamete Gamete Gametogenesis formation, resulting in offspring inheritence of 1 allele Allele Variant forms of the same gene, occupying the same locus on homologous chromosomes, and governing the variants in production of the same gene product. Basic Terms of Genetics per gene Gene A category of nucleic acid sequences that function as units of heredity and which code for the basic instructions for the development, reproduction, and maintenance of organisms. Basic Terms of Genetics from each parent.
  • Mendel’s hypothesis Hypothesis A hypothesis is a preliminary answer to a research question (i.e., a “guess” about what the results will be). There are 2 types of hypotheses: the null hypothesis and the alternative hypothesis. Statistical Tests and Data Representation: Alleles segregate randomly during meiosis Meiosis The creation of eukaryotic gametes involves a DNA replication phase followed by 2 cellular division stages: meiosis I and meiosis II. Meiosis I separates homologous chromosomes into separate cells (1n, 2c), while meiosis II separates sister chromatids into gametes (1n, 1c). Meiosis.
  • The law is applied and best demonstrated when 2 heterozygotes are crossed.
  • When 2 heterozygotes are crossed (e.g., Aa AA Amyloidosis x Aa AA Amyloidosis):
  • The phenotype Phenotype The complete genetic complement contained in the DNA of a set of chromosomes in a human. The length of the human genome is about 3 billion base pairs. Basic Terms of Genetics of the F2 generation follows a 3:1 ratio:
    • AA AA Amyloidosis expresses the dominant phenotype Phenotype The complete genetic complement contained in the DNA of a set of chromosomes in a human. The length of the human genome is about 3 billion base pairs. Basic Terms of Genetics: 25%
    • Aa AA Amyloidosis expresses the dominant phenotype Phenotype The complete genetic complement contained in the DNA of a set of chromosomes in a human. The length of the human genome is about 3 billion base pairs. Basic Terms of Genetics: 50%
    • aa AA Amyloidosis expresses the recessive phenotype Phenotype The complete genetic complement contained in the DNA of a set of chromosomes in a human. The length of the human genome is about 3 billion base pairs. Basic Terms of Genetics: 25%

Mendel’s 3rd Law: The Law of Independent Assortment

  • The law of independent assortment: Alleles of 2 or more genes Genes A category of nucleic acid sequences that function as units of heredity and which code for the basic instructions for the development, reproduction, and maintenance of organisms. DNA Types and Structure separate into gametes independently of one another.
  • The alleles are passed along separate of one another.
  • Allows for extreme variability of inherited genes Genes A category of nucleic acid sequences that function as units of heredity and which code for the basic instructions for the development, reproduction, and maintenance of organisms. DNA Types and Structure in different offspring
  • Occurs during meiotic metaphase I Metaphase I Meiosis
  • Mendel used dihybrid crosses to analyze inheritance patterns of 2 traits to prove the law.
  • Dihybrid crosses are explained in the following way:
    • The crossing of 2 organisms differing in 2 observed traits 
    • The inheritance of 2 traits results in 16 unique allele Allele Variant forms of the same gene, occupying the same locus on homologous chromosomes, and governing the variants in production of the same gene product. Basic Terms of Genetics combinations.
  • The 1st generation produces phenotypically identical offspring.
  • The 2nd generation produces varying phenotypic appearances with a 9:3:3:1 ratio:
    • 9 offspring show both dominant traits.
    • 3 offspring show the 1st dominant and 2nd recessive trait.
    • 3 offspring show the 1st recessive and 2nd dominant trait.
    • 1 offspring shows both recessive traits.
  • The exception is genetic linkage (neighboring genes Genes A category of nucleic acid sequences that function as units of heredity and which code for the basic instructions for the development, reproduction, and maintenance of organisms. DNA Types and Structure are inherited together).
Dihybrid crosses

A Punnett square showing a dihybrid cross:
All possible genotypic and phenotypic combinations of the 2 alleles from 2 different genes Genes A category of nucleic acid sequences that function as units of heredity and which code for the basic instructions for the development, reproduction, and maintenance of organisms. DNA Types and Structure are shown.

Image by Lecturio.

Clinical Relevance

  • Codominant Inheritance: For heterozygous genotypes, both characteristics are physically manifested in parallel (e.g., the ABO system of blood types). In a genetic heterozygous AB, both A and B characteristics are phenotypically expressed on the surface of erythrocytes Erythrocytes Erythrocytes, or red blood cells (RBCs), are the most abundant cells in the blood. While erythrocytes in the fetus are initially produced in the yolk sac then the liver, the bone marrow eventually becomes the main site of production. Erythrocytes: Histology.
  • Autosomal Dominant Inheritance Autosomal dominant inheritance Autosomal Recessive and Autosomal Dominant Inheritance: One allele Allele Variant forms of the same gene, occupying the same locus on homologous chromosomes, and governing the variants in production of the same gene product. Basic Terms of Genetics is dominant to another allele Allele Variant forms of the same gene, occupying the same locus on homologous chromosomes, and governing the variants in production of the same gene product. Basic Terms of Genetics. If a dominant and nondominant allele Allele Variant forms of the same gene, occupying the same locus on homologous chromosomes, and governing the variants in production of the same gene product. Basic Terms of Genetics are in a heterozygous genotype Genotype The genetic constitution of the individual, comprising the alleles present at each genetic locus. Basic Terms of Genetics, only the dominant allele Allele Variant forms of the same gene, occupying the same locus on homologous chromosomes, and governing the variants in production of the same gene product. Basic Terms of Genetics is manifested (seen with structural diseases (e.g., osteogenesis imperfecta Osteogenesis imperfecta Osteogenesis imperfecta (OI), or “brittle bone disease,” is a rare genetic connective tissue disorder characterized by severe bone fragility. Although OI is considered a single disease, OI includes over 16 genotypes and clinical phenotypes with differing symptom severity. Osteogenesis Imperfecta)).
  • Autosomal Recessive Inheritance Autosomal recessive inheritance Autosomal Recessive and Autosomal Dominant Inheritance: Recessive alleles are only manifested when homozygous. Two copies of an allele Allele Variant forms of the same gene, occupying the same locus on homologous chromosomes, and governing the variants in production of the same gene product. Basic Terms of Genetics are required to phenotypically express the disease. A heterozygote Heterozygote An individual having different alleles at one or more loci regarding a specific character. Basic Terms of Genetics will not express the recessive phenotype Phenotype The complete genetic complement contained in the DNA of a set of chromosomes in a human. The length of the human genome is about 3 billion base pairs. Basic Terms of Genetics (seen with diseases caused by defective enzymes Enzymes Enzymes are complex protein biocatalysts that accelerate chemical reactions without being consumed by them. Due to the body’s constant metabolic needs, the absence of enzymes would make life unsustainable, as reactions would occur too slowly without these molecules. Basics of Enzymes (e.g., cystic Cystic Fibrocystic Change fibrosis Fibrosis Any pathological condition where fibrous connective tissue invades any organ, usually as a consequence of inflammation or other injury. Bronchiolitis Obliterans)).
  • X-linked Recessive Inheritance X-linked recessive inheritance Sex-Linked Inheritance: The allele Allele Variant forms of the same gene, occupying the same locus on homologous chromosomes, and governing the variants in production of the same gene product. Basic Terms of Genetics in question, or the disease-causing mutation Mutation Genetic mutations are errors in DNA that can cause protein misfolding and dysfunction. There are various types of mutations, including chromosomal, point, frameshift, and expansion mutations. Types of Mutations, is recessive and on the X chromosome X chromosome The female sex chromosome, being the differential sex chromosome carried by half the male gametes and all female gametes in human and other male-heterogametic species. Basic Terms of Genetics. Because men have 1 X chromosome X chromosome The female sex chromosome, being the differential sex chromosome carried by half the male gametes and all female gametes in human and other male-heterogametic species. Basic Terms of Genetics, the allele Allele Variant forms of the same gene, occupying the same locus on homologous chromosomes, and governing the variants in production of the same gene product. Basic Terms of Genetics will always be manifested. Because women possess a 2nd X chromosome X chromosome The female sex chromosome, being the differential sex chromosome carried by half the male gametes and all female gametes in human and other male-heterogametic species. Basic Terms of Genetics and expression requires both alleles to have the disease-causing mutation Mutation Genetic mutations are errors in DNA that can cause protein misfolding and dysfunction. There are various types of mutations, including chromosomal, point, frameshift, and expansion mutations. Types of Mutations, the likelihood of manifestation is very low.
  • X-linked Dominant Inheritance X-linked dominant inheritance Sex-Linked Inheritance: Only 1 copy of the diseased allele Allele Variant forms of the same gene, occupying the same locus on homologous chromosomes, and governing the variants in production of the same gene product. Basic Terms of Genetics is required for phenotypic expression (also seen in autosomal dominant inheritance Autosomal dominant inheritance Autosomal Recessive and Autosomal Dominant Inheritance). However, for X-linked dominant inheritance X-linked dominant inheritance Sex-Linked Inheritance, the diseased allele Allele Variant forms of the same gene, occupying the same locus on homologous chromosomes, and governing the variants in production of the same gene product. Basic Terms of Genetics is located on the X chromosome X chromosome The female sex chromosome, being the differential sex chromosome carried by half the male gametes and all female gametes in human and other male-heterogametic species. Basic Terms of Genetics. The diseases are often lethal in men.
  • Mitochondrial Inheritance Mitochondrial inheritance Mitochondria are located in a cell’s cytoplasm and contain circular DNA, called mitochondrial DNA (mtDNA). This DNA exists separately from a cell’s nuclear genome and is inherited solely through the maternal lineage-nonmendelian inheritance. Genetic mutations in mtDNA give rise to various rare diseases. Mitochondrial Inheritance: Genetic defects of the mitochondria Mitochondria Semiautonomous, self-reproducing organelles that occur in the cytoplasm of all cells of most, but not all, eukaryotes. Each mitochondrion is surrounded by a double limiting membrane. The inner membrane is highly invaginated, and its projections are called cristae. Mitochondria are the sites of the reactions of oxidative phosphorylation, which result in the formation of ATP. They contain distinctive ribosomes, transfer RNAs; amino Acyl tRNA synthetases; and elongation and termination factors. Mitochondria depend upon genes within the nucleus of the cells in which they reside for many essential messenger RNAs. Mitochondria are believed to have arisen from aerobic bacteria that established a symbiotic relationship with primitive protoeukaryotes. The Cell: Organelles can cause diseases. Mitochondrial diseases Mitochondrial diseases Diseases caused by abnormal function of the mitochondria. They may be caused by mutations, acquired or inherited, in mitochondrial DNA or in nuclear genes that code for mitochondrial components. They may also be the result of acquired mitochondria dysfunction due to adverse effects of drugs, infections, or other environmental causes. Mitochondrial Myopathies are always inherited from the mother. While all of the mother’s children may be affected, a father with the genetic mutation Mutation Genetic mutations are errors in DNA that can cause protein misfolding and dysfunction. There are various types of mutations, including chromosomal, point, frameshift, and expansion mutations. Types of Mutations cannot transmit the disease. One commonly cited class of mitochondrial diseases Mitochondrial diseases Diseases caused by abnormal function of the mitochondria. They may be caused by mutations, acquired or inherited, in mitochondrial DNA or in nuclear genes that code for mitochondrial components. They may also be the result of acquired mitochondria dysfunction due to adverse effects of drugs, infections, or other environmental causes. Mitochondrial Myopathies is mitochondrial myopathies Mitochondrial myopathies Mitochondrial myopathies are conditions arising from dysfunction of the mitochondria (the energy-producing structures) and are characterized by prominent muscular symptoms and accompanied by various symptoms from organs with high energy requirements. The organs disproportionately affected include the skeletal muscles, brain, and heart. Mitochondrial Myopathies.

References

  1. Lewis, R. G., & Simpson, B. (2021). Genetics, Autosomal Dominant. In StatPearls. Treasure Island (FL): StatPearls Publishing. Available from https://www.ncbi.nlm.nih.gov/books/NBK557512/
  2. Gulani, A., &Weiler T. (2021). Genetics, Autosomal Recessive. In StatPearls. Treasure Island (FL): StatPearls Publishing. Available from https://www.ncbi.nlm.nih.gov/books/NBK546620/
  3. Griffiths, A. J. F., Miller, J. H., Suzuki, D. T., et al. (2000). An Introduction to Genetic Analysis. 7th ed. New York: W. H. Freeman. Mendel’s experiments. Available from https://www.ncbi.nlm.nih.gov/books/NBK22098/
  4. Ellis, T. H. N., Hofer, J. M. I., Swain, M. T., van Dijk, P. J. (2019). Mendel’s pea crosses: varieties, traits and statistics. Hereditas, 156:33. https://doi.org/10.1186/s41065-019-0111-y
  5. Castle, W. E. (1903). Mendel’s Law of Heredity. Science, 18(456), pp. 396–406. https://doi.org/10.1126/science.18.456.396

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