Gregor Mendel's Experiments with Biological Inheritance BIO Lesson 12 by Owen Borville August 29, 2024
Gregor Mendel (1822-1884) was an Austrian friar, biologist, naturalist, and scientist. In 1865, Mendel presented the results of his experiments with nearly 30, 000 pea plants to the local Natural History Society. In 1866, he published his work, Experiments in Plant Hybridization, to the Natural History Society of Brunn.
Mendel's Model System
A model system is a system with convenient characteristics used to study a specific biological phenomenon to be applied to other systems.
Blending theory of inheritance was the accepted theory of biological inheritance at the time, which said that the original parental traits were lost or absorbed by the blending in the offspring, but today, biologists do not accept this theory.
This theory appeared to be correct because of continuous variation, which results from the action of many genes to determine a characteristic such as human height. Biological offspring appear to be a blend of their parent's traits. Continuous variation is a pattern in which a character shows a range of trait values with small gradations rather than large gaps between them.
Discontinuous variation, which Mendel worked with, results from traits that are inherited in distinct classes that are transmitted independently from each other. Mendel was able to see through his experiments that traits were not blended in the offspring, or absorbed, but rather the traits kept their distinctness and could be passed along.
Mendel used the pea plant to study inheritance.
Mendelian Crosses: Mendel performed hybridizations, which are crosses or mating two-true breeding individual organisms that have different traits.
In his experiments, first-generation crosses were labeled P0 for parental generation.
The offspring were called F1, for filial generation, or first filial generation in a cross. The F1 offspring were called F2, for second generation or second filial generation. He continued this process to F3, F4, ...etc. The ratio of the characteristics in P0-F1-F2 was the basis for Mendel's theories.
The Basics of Heredity
A trait is a variation in the physical appearance of a heritable characteristic. The traits that Mendel worked with include plant height, seed texture, seed color, flower color, pea pod size, pea pod color, and flower position.
A reciprocal cross is a paired cross in which the respective traits of the male and female in one cross become the respective traits of the female and the male in the other cross.
Dominant traits are inherited traits that are unchanged in a hybridization and has the same physical appearance whether an individual has two copies of the trait or one copy of the dominant trait and one copy of the recessive trait. Recessive traits are traits that appear "latent" or non-expressed when the individual also carries a dominant trait for that same characteristic. When present as two identical copies, the recessive trait is expressed.
Probability Basics
Mendel repeatedly obtained 3 to 1 ratios in his crosses.
The product rule is the probability of two independent events occurring simultaneously and can be calculated by multiplying the individual probabilities of each event occurring alone. The sum rule is the probability of the occurrence of at least one of two mutually exclusive events is the sum of their individual probabilities.
Alleles are gene variations that arise by mutation and exist at the same relative locations on homologous chromosomes. Phenotypes are the observable traits expressed by an organism. Genotypes are the underlying genetic makeup, consisting of both physically visible and non-expressed alleles, of an organism.
Homozygous refers to having two identical alleles for a given gene on the homologous chromosome. Heterozygous refers to having two different alleles for a given gene on the homologous chromosome.
Monohybrid is the result of a cross between two true-breeding parents that express different traits for only one characteristic.
A Punnett Square (by British geneticist Reginald Punnett) is a visual representation of a cross between two individuals in which the gametes of each individual are denoted along the top and the side of a grid, respectively, and the possible zygotic genotypes are recombined at each box in the grid.
A test cross is cross between a dominant expressing individual with an unknown genotype and a homozygous recessive individual; the offspring phenotypes indicate whether the unknown parent is heterozygous or homozygous for the dominant trait. This test is still used by plant and animal breeders.
Because doing a test cross in humans is unethical and impractical, a pedigree analysis is commonly performed.
A pedigree analysis is a process that uses a pedigree chart to show how a trait or health condition is passed down through generations of a family. Pedigrees are most commonly used for humans and can help determine disease inheritance patterns.
Incomplete dominance occurs in a heterozygote when the expression of two contrasting alleles is such that the individual displays an intermediate phenotype, so that two different alleles are partially expressed, and result in a phenotype that is distinct or intermediate.
Codominance in a heterozygote is the complete and simultaneous expression of both alleles for the same characteristic.
When multiple alleles exist, biologists classify the most common phenotype or genotype as the wild type, which is commonly abbreviated with a plus (+) sign. All other phenotypes and genotypes are considered variants of the standard, or deviations from the wild type, and may be recessive or dominant to the wild type.
Autosomes are non-sex chromosomes that have inheritance patterns.
Human females have a pair of homologous X chromosomes, while human males have an XY chromosome pair.
X-linked genes are genes present on the X chromosome but not on the Y chromosome.
Hemizygous refers to the presence of only one allele for a characteristic, or one copy of a gene, as in X-linkage. Hemizygous presence makes descriptions of dominance and recessive irrelevant.
Recessive lethal is an inheritance pattern in which an allele is only lethal in the homozygous form. The heterozygote may be normal or have some altered, non-lethal phenotype.
Dominant lethal is an inheritance pattern in which an allele is lethal both in the homozygote and heterozygote. This allele can only be transmitted if the lethality phenotype occurs after reproductive age.
Mendel's law of dominance states that in a heterozygote, one trait will conceal the presence of another trait for the same characteristic.
Mendel's law of segregation states that paired unit factors (such as genes) segregate equally into gametes such that offspring have an equal likelihood of inheriting any combination of factors.
Mendel's law of independent assortment states that genes do not influence each other with regard to sorting of alleles into gametes. Every possible combination of alleles is equally likely to occur. The independent assortment of genes can be illustrated by the dihybrid cross, which is a cross between two true-breeding parents that express different traits for two characteristics.
The Forked-Line method is used when more than two genes are considered, and the Punnett Square becomes difficult. In addition, the Probability method gives the proportions of offspring expected to exhibit each phenotype or genotype without added visual assistance. Both methods make use of the product rule and consider the alleles for each gene separately.
Predicting genotypes and phenotypes of offspring from given crosses is the best way to test knowledge of Mendelian genetics.
Linkage is a phenomenon in biology in which alleles that are located in close proximity to each other on the same chromosome are more likely to be inherited together.
Epistasis is the antagonistic interaction between genes such that one gene masks or interferes with the expression of another. The alleles that are being masked are hypostatic.
Gregor Mendel (1822-1884) was an Austrian friar, biologist, naturalist, and scientist. In 1865, Mendel presented the results of his experiments with nearly 30, 000 pea plants to the local Natural History Society. In 1866, he published his work, Experiments in Plant Hybridization, to the Natural History Society of Brunn.
Mendel's Model System
A model system is a system with convenient characteristics used to study a specific biological phenomenon to be applied to other systems.
Blending theory of inheritance was the accepted theory of biological inheritance at the time, which said that the original parental traits were lost or absorbed by the blending in the offspring, but today, biologists do not accept this theory.
This theory appeared to be correct because of continuous variation, which results from the action of many genes to determine a characteristic such as human height. Biological offspring appear to be a blend of their parent's traits. Continuous variation is a pattern in which a character shows a range of trait values with small gradations rather than large gaps between them.
Discontinuous variation, which Mendel worked with, results from traits that are inherited in distinct classes that are transmitted independently from each other. Mendel was able to see through his experiments that traits were not blended in the offspring, or absorbed, but rather the traits kept their distinctness and could be passed along.
Mendel used the pea plant to study inheritance.
Mendelian Crosses: Mendel performed hybridizations, which are crosses or mating two-true breeding individual organisms that have different traits.
In his experiments, first-generation crosses were labeled P0 for parental generation.
The offspring were called F1, for filial generation, or first filial generation in a cross. The F1 offspring were called F2, for second generation or second filial generation. He continued this process to F3, F4, ...etc. The ratio of the characteristics in P0-F1-F2 was the basis for Mendel's theories.
The Basics of Heredity
A trait is a variation in the physical appearance of a heritable characteristic. The traits that Mendel worked with include plant height, seed texture, seed color, flower color, pea pod size, pea pod color, and flower position.
A reciprocal cross is a paired cross in which the respective traits of the male and female in one cross become the respective traits of the female and the male in the other cross.
Dominant traits are inherited traits that are unchanged in a hybridization and has the same physical appearance whether an individual has two copies of the trait or one copy of the dominant trait and one copy of the recessive trait. Recessive traits are traits that appear "latent" or non-expressed when the individual also carries a dominant trait for that same characteristic. When present as two identical copies, the recessive trait is expressed.
Probability Basics
Mendel repeatedly obtained 3 to 1 ratios in his crosses.
The product rule is the probability of two independent events occurring simultaneously and can be calculated by multiplying the individual probabilities of each event occurring alone. The sum rule is the probability of the occurrence of at least one of two mutually exclusive events is the sum of their individual probabilities.
Alleles are gene variations that arise by mutation and exist at the same relative locations on homologous chromosomes. Phenotypes are the observable traits expressed by an organism. Genotypes are the underlying genetic makeup, consisting of both physically visible and non-expressed alleles, of an organism.
Homozygous refers to having two identical alleles for a given gene on the homologous chromosome. Heterozygous refers to having two different alleles for a given gene on the homologous chromosome.
Monohybrid is the result of a cross between two true-breeding parents that express different traits for only one characteristic.
A Punnett Square (by British geneticist Reginald Punnett) is a visual representation of a cross between two individuals in which the gametes of each individual are denoted along the top and the side of a grid, respectively, and the possible zygotic genotypes are recombined at each box in the grid.
A test cross is cross between a dominant expressing individual with an unknown genotype and a homozygous recessive individual; the offspring phenotypes indicate whether the unknown parent is heterozygous or homozygous for the dominant trait. This test is still used by plant and animal breeders.
Because doing a test cross in humans is unethical and impractical, a pedigree analysis is commonly performed.
A pedigree analysis is a process that uses a pedigree chart to show how a trait or health condition is passed down through generations of a family. Pedigrees are most commonly used for humans and can help determine disease inheritance patterns.
Incomplete dominance occurs in a heterozygote when the expression of two contrasting alleles is such that the individual displays an intermediate phenotype, so that two different alleles are partially expressed, and result in a phenotype that is distinct or intermediate.
Codominance in a heterozygote is the complete and simultaneous expression of both alleles for the same characteristic.
When multiple alleles exist, biologists classify the most common phenotype or genotype as the wild type, which is commonly abbreviated with a plus (+) sign. All other phenotypes and genotypes are considered variants of the standard, or deviations from the wild type, and may be recessive or dominant to the wild type.
Autosomes are non-sex chromosomes that have inheritance patterns.
Human females have a pair of homologous X chromosomes, while human males have an XY chromosome pair.
X-linked genes are genes present on the X chromosome but not on the Y chromosome.
Hemizygous refers to the presence of only one allele for a characteristic, or one copy of a gene, as in X-linkage. Hemizygous presence makes descriptions of dominance and recessive irrelevant.
Recessive lethal is an inheritance pattern in which an allele is only lethal in the homozygous form. The heterozygote may be normal or have some altered, non-lethal phenotype.
Dominant lethal is an inheritance pattern in which an allele is lethal both in the homozygote and heterozygote. This allele can only be transmitted if the lethality phenotype occurs after reproductive age.
Mendel's law of dominance states that in a heterozygote, one trait will conceal the presence of another trait for the same characteristic.
Mendel's law of segregation states that paired unit factors (such as genes) segregate equally into gametes such that offspring have an equal likelihood of inheriting any combination of factors.
Mendel's law of independent assortment states that genes do not influence each other with regard to sorting of alleles into gametes. Every possible combination of alleles is equally likely to occur. The independent assortment of genes can be illustrated by the dihybrid cross, which is a cross between two true-breeding parents that express different traits for two characteristics.
The Forked-Line method is used when more than two genes are considered, and the Punnett Square becomes difficult. In addition, the Probability method gives the proportions of offspring expected to exhibit each phenotype or genotype without added visual assistance. Both methods make use of the product rule and consider the alleles for each gene separately.
Predicting genotypes and phenotypes of offspring from given crosses is the best way to test knowledge of Mendelian genetics.
Linkage is a phenomenon in biology in which alleles that are located in close proximity to each other on the same chromosome are more likely to be inherited together.
Epistasis is the antagonistic interaction between genes such that one gene masks or interferes with the expression of another. The alleles that are being masked are hypostatic.