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 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 based on statistical predictions. The gene traits follow the laws of mendelian inheritance.

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Mendel’s Experiments and Punnett Squares

Mendel’s experiments

Mendel chose pea plants as the experimental model. Mendel reasoned pea plants 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 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 or terminal)

True-breeding traits are referred to as homozygous alleles (genes). 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.
  • Gametes are haploid.
  • 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:

  • 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 occurrence:
    • Assumes 1 gamete from each parent + 2 alleles per trait
    • Each allele has a dominant and a recessive form.
    • The frequency of allele 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 and single genotype.

Mendel’s 1st Law: The Law of Dominance

  • The law of dominance: Genes with a dominant allele will display the phenotype of the allele.
  • Not all alleles have an equal phenotypic expression:
    • When 2 different alleles are inherited, 1 dominant allele determines the phenotypic characteristics of the organism.
    • The silent characteristic is known as the recessive allele.
    • The expressed allele is known as the dominant allele.
  • Dominance is not inherent (e.g., 1 gene may be dominant to another, but recessive to a 3rd). 
  • Recessive genes must be inherited in both copies of the gene 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 independently of one another (e.g., blood types AA 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 formation, resulting in offspring inheritence of 1 allele per gene from each parent.
  • Mendel’s hypothesis: 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 x Aa):
    •  The F2 generation is split genotypically according to a 1:2:1 ratio:
      • AA: 25%
      • Aa: 50%
      • aa: 25%
  • The phenotype of the F2 generation follows a 3:1 ratio:
    • AA expresses the dominant phenotype: 25%
    • Aa expresses the dominant phenotype: 50%
    • aa expresses the recessive phenotype: 25%

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

  • The law of independent assortment: Alleles of 2 or more genes separate into gametes independently of one another.
  • The alleles are passed along separate of one another.
  • Allows for extreme variability of inherited genes in different offspring
  • Occurs during meiotic metaphase I
  • 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 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 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 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.
  • Autosomal Dominant Autosomal dominant Autosomal inheritance, both dominant and recessive, refers to the transmission of genes from the 22 autosomal chromosomes. Autosomal dominant diseases are expressed when only 1 copy of the dominant allele is inherited. Autosomal Recessive and Autosomal Dominant Inheritance Inheritance: One allele is dominant to another allele. If a dominant and nondominant allele are in a heterozygous genotype, only the dominant allele 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 Autosomal recessive Autosomal inheritance, both dominant and recessive, refers to the transmission of genes from the 22 autosomal chromosomes. Autosomal recessive diseases are only expressed when 2 copies of the recessive allele are inherited. Autosomal Recessive and Autosomal Dominant InheritanceInheritance: Recessive alleles are only manifested when homozygous. Two copies of an allele are required to phenotypically express the disease. A heterozygote will not express the recessive phenotype (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 fibrosis Cystic fibrosis Cystic fibrosis is an autosomal recessive disorder caused by mutations in the gene CFTR. The mutations lead to dysfunction of chloride channels, which results in hyperviscous mucus and the accumulation of secretions. Common presentations include chronic respiratory infections, failure to thrive, and pancreatic insufficiency. Cystic Fibrosis)).
  • X-linked Recessive Inheritance: The allele 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. Because men have 1 X chromosome, the allele will always be manifested. Because women possess a 2nd X chromosome 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: Only 1 copy of the diseased allele is required for phenotypic expression (also seen in autosomal dominant inheritance). However, for X-linked dominant inheritance, the diseased allele is located on the X chromosome. 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 can cause diseases. Mitochondrial diseases 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 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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