MENDELIAN INHERITENCE
Mendelian inheritance is a type of biological inheritance that follows the laws originally proposed by Gregor Mendel in 1865 and 1866 and re-discovered in 1900. These laws were initially very controversial. When Mendel’s theories were integrated with the Boveri–Sutton chromosome theory of inheritance by Thomas Hunt Morgan in 1915, they became the core of classical genetics. Ronald Fisher later combined these ideas with the theory of natural selection in his 1930 book The Genetical Theory of Natural Selection, putting evolution onto a mathematical footing and forming the basis for Population genetics and the modern evolutionary synthesis.
Mendel’s laws
Mendel discovered that, when he crossed purebred white flower and purple flower pea Plants (the parental or P generation), the result was not a blend. Rather than being a mix of the two, the offspring (known as the F1 generation) was purple-flowered. When Mendel self-fertilized the F1 generation pea plants, he obtained a purple flower to white flower ratio in the F2 generation of 3 to 1. The results of this cross are tabulated in the Punnett square to the right.
Law of Segregation of genes: The Law of Segregation states that every individual organism contains two alleles for each trait, and that these alleles segregate (separate) during meiosis such that each gamete contains only one of the alleles. An offspring thus receives a pair of alleles for a trait by inheriting homologous Chromosomes from the parent organisms: one allele for each trait from each parent.[6] Molecular proof of this principle was subsequently found through observation of meiosis by two scientists independently, the German botanist Oscar Hertwig in 1876, and the Belgian zoologist Edouard Van Beneden in 1883. Paternal and maternal chromosomes get separated in meiosis and the alleles with the traits of a character are segregated into two different gametes. Each parent contributes a single gamete, and thus a single, randomly successful allele copy to their offspring and fertilization.
Law of Independent Assortment
Mendel’s law of independent assortment, states that allele pairs separate independently during the formation of gametes. This means that traits are transmitted to offspring independently of one another. Mendel formulated this principle after performing dihybrid crosses between plants that differed in two traits, such as seed color and pod color. After these plants were allowed to self pollinate, he noticed that the same ratio of 9:3:3:1 appeared among the offspring. Mendel concluded that traits are transmitted to offspring independently.
Law of Dominance
Mendel’s Law of Dominance states that recessive alleles will always be masked by dominant alleles. Therefore, a cross between a homozygous dominant and a homozygous recessive will always express the dominant phenotype, while still having a heterozygous genotype. Law of Dominance can be explained easily with the help of a mono hybrid cross experiment:- In a cross between two organisms pure for any pair (or pairs) of contrasting traits (characters), the character that appears in the F1 generation is called “dominant” and the one which is suppressed (not expressed) is called “recessive.” Each character is controlled by a pair of dissimilar factors. Only one of the characters expresses. The one which expresses in the F1 generation is called Dominant. It is important to note however, that the law of dominance is significant and true but is not universally applicable. According to the latest revisions, only two of these rules are considered to be laws. The third one is considered as a basic principle but not a genetic law of Mendel.
Monohybrid cross
A monohybrid cross is a breeding experiment between P generation (parental generation) organisms that differ in a single given trait. The P generation organisms are homozygous for the given trait, however each parent possesses different alleles for that particular trait. A Punnett square may be used to predict the possible genetic outcomes of a monohybrid cross based on Probability. This type of genetic analysis can also be performed in a dihybrid cross, a genetic cross between parental generations that differ in two traits. Traits are characteristics that are determined by discrete segments of DNA called genes. Individuals typically inherit two alleles for each gene. An allele is an alternate version of a gene that is inherited (one from each parent) during sexual Reproduction. Male and female gametes, produced by meiosis, have a single allele for each trait. These alleles are randomly united at fertilization. Example: In the image above, the single trait being observed is pod color. The organisms in this monohybrid cross are true-breeding for pod color. True-breeding organisms have homozygous alleles for specific traits. In this cross, the allele for green pod color (G) is completely dominant over the recessive allele for yellow pod color (g). The genotype for the green pod plant is (GG) and the genotype for the yellow pod plant is (gg). Cross-pollination between the true-breeding homozygous dominant green pod plant and the true-breeding homozygous recessive yellow pod plant results in offspring with phenotypes of green pod color. All genotypes are (Gg). The offspring or F1 generation are all green because the dominant green pod color obscures the recessive yellow pod color in the heterozygous genotype.
Monohybrid cross: F2 generation
The F2 generation would have genotypes of (GG, Gg, and gg) and a genotypic ratio of 1:2:1. One-fourth of the F2 generation would be homozygous dominant (GG), one-half would be heterozygous (Gg), and one-fourth would be homozygous recessive (gg). The phenotypic ratio would be 3:1, with three-fourths having green pod color (GG and Gg) and one-fourth having yellow pod color (gg).
Dihybrid cross
A dihybrid cross is a breeding experiment between P generation (parental generation) organisms that differ in two traits. The individuals in this type of cross are homozygous for a specific trait. Traits are characteristics that are determined by segments of DNA called genes. Diploid organisms inherit two alleles for each gene. An allele is an alternate version of a gene that is inherited (one from each parent) during sexual reproduction. In a dihybrid cross, the parent organisms have different pairs of alleles for each trait being studied. One parent possesses homozygous dominant alleles and the other possesses homozygous recessive alleles. The offspring, or F1 generation, produced from the genetic cross of such individuals are all heterozygous for the specific traits. This means that all of the F1 individuals possess a hybrid genotype and express the dominant phenotypes for each trait.
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Monohybrid Cross
A monohybrid cross is a genetic cross between two individuals that are heterozygous for a single trait. The trait can be any characteristic that is controlled by a single gene, such as eye color, hair color, or height.
The offspring of a monohybrid cross will always be heterozygous for the trait, meaning that they will have two different alleles for the gene. The alleles can be dominant or recessive. The dominant allele will be expressed in the phenotype, while the recessive allele will be hidden.
For example, if a pea plant with purple flowers is crossed with a pea plant with white flowers, the offspring will all have purple flowers. This is because the purple flower color is dominant over the white flower color.
The law of segregation states that the two alleles for a gene will segregate (separate) during meiosis and will be passed down to the offspring in separate gametes. This means that each offspring will have a 50% chance of inheriting the dominant allele and a 50% chance of inheriting the recessive allele.
A test cross is a cross between an individual with an unknown genotype and an individual with a known recessive genotype. The purpose of a test cross is to determine the genotype of the individual with the unknown genotype.
For example, if a pea plant with purple flowers is crossed with a pea plant with white flowers, the offspring will all have purple flowers. This does not necessarily mean that the purple flowered plant is homozygous dominant for the purple flower color gene. It is possible that the plant is heterozygous for the gene, meaning that it has one dominant allele and one recessive allele.
To determine the genotype of the purple flowered plant, a test cross can be performed. The purple flowered plant is crossed with a white flowered plant. If all of the offspring have purple flowers, then the purple flowered plant is homozygous dominant for the purple flower color gene. If any of the offspring have white flowers, then the purple flowered plant is heterozygous for the gene.
Dihybrid Cross
A dihybrid cross is a genetic cross between two individuals that are heterozygous for two different traits. The traits can be any characteristic that is controlled by a single gene, such as eye color, hair color, or height.
The offspring of a dihybrid cross will always be heterozygous for the two traits, meaning that they will have two different alleles for each gene. The alleles can be dominant or recessive. The dominant allele will be expressed in the phenotype, while the recessive allele will be hidden.
For example, if a pea plant with purple flowers and round seeds is crossed with a pea plant with white flowers and wrinkled seeds, the offspring will all have purple flowers and round seeds. This is because the purple flower color and round seed shape are both dominant over the white flower color and wrinkled seed shape.
The law of independent assortment states that the two alleles for one gene will assort independently of the two alleles for another gene. This means that the offspring of a dihybrid cross will have a 25% chance of inheriting the dominant allele for both genes, a 25% chance of inheriting the recessive allele for both genes, a 25% chance of inheriting the dominant allele for one gene and the recessive allele for the other gene, and a 25% chance of inheriting the recessive allele for one gene and the dominant allele for the other gene.
The chi-square test is a statistical test that can be used to determine if the results of a genetic cross are significantly different from the expected results. The chi-square test is calculated by comparing the observed number of offspring with the expected number of offspring. If the chi-square value is greater than the critical value, then the results of the genetic cross are significantly different from the expected results.
Conclusion
Monohybrid and dihybrid crosses are two types of genetic crosses that can be used to study the inheritance of traits. The law of segregation and the law of independent assortment are two important concepts in genetics that can be used to explain the results of genetic crosses. The chi-square test is a statistical test that can be used to determine if the results of a genetic cross are significantly different from the expected results.
What is inheritance?
Inheritance is the passing of genetic information from parents to offspring. This information is encoded in genes, which are located on chromosomes.
What is a monohybrid cross?
A monohybrid cross is a cross between two individuals that are heterozygous for a single gene. The offspring of a monohybrid cross will be in a 3:1 ratio of dominant to recessive phenotypes.
What is a dihybrid cross?
A dihybrid cross is a cross between two individuals that are heterozygous for two genes. The offspring of a dihybrid cross will be in a 9:3:3:1 ratio of phenotypes.
What is a Punnett square?
A Punnett square is a tool used to predict the possible genotypes and phenotypes of the offspring of a genetic cross.
What is a dominant allele?
A dominant allele is an allele that will be expressed in the phenotype of an individual if it is present in the genotype.
What is a recessive allele?
A recessive allele is an allele that will only be expressed in the phenotype of an individual if it is present in the genotype in two copies.
What is a phenotype?
A phenotype is the physical appearance of an individual.
What is a genotype?
A genotype is the genetic makeup of an individual.
What is a gene?
A gene is a unit of heredity that is located on a chromosome.
What is a chromosome?
A chromosome is a structure in the nucleus of a cell that contains genes.
What is DNA?
DNA is a molecule that contains the genetic information of an organism.
What is RNA?
RNA is a molecule that is involved in the process of Protein Synthesis.
What is a mutation?
A mutation is a change in the DNA of an organism.
What is a gene mutation?
A gene mutation is a change in the DNA of a gene.
What is a chromosomal mutation?
A chromosomal mutation is a change in the structure of a chromosome.
What is a point mutation?
A point mutation is a change in a single base pair of DNA.
What is a frameshift mutation?
A frameshift mutation is a change in the number of base pairs in a gene.
What is a deletion mutation?
A deletion mutation is the loss of one or more base pairs from a gene.
What is an insertion mutation?
An insertion mutation is the addition of one or more base pairs to a gene.
What is a substitution mutation?
A substitution mutation is the replacement of one base pair with another.
What is a silent mutation?
A silent mutation is a mutation that does not change the amino acid sequence of a protein.
What is a missense mutation?
A missense mutation is a mutation that changes the amino acid sequence of a protein.
What is a nonsense mutation?
A nonsense mutation is a mutation that changes a codon for an amino acid into a stop codon.
What is a frameshift mutation?
A frameshift mutation is a mutation that changes the reading frame of a gene.
What is a deletion mutation?
A deletion mutation is a mutation that results in the loss of one or more nucleotides from a gene.
What is an insertion mutation?
An insertion mutation is a mutation that results in the addition of one or more nucleotides to a gene.
What is a substitution mutation?
A substitution mutation is a mutation that results in the replacement of one nucleotide with another.
What is a silent mutation?
A silent mutation is a mutation that does not change the amino acid sequence of a protein.
What is a missense mutation?
A missense mutation is a mutation that changes the amino acid sequence of a protein.
What is a nonsense mutation?
A nonsense mutation is a mutation that changes a codon for an amino acid into a stop codon.
What is a frameshift mutation?
A frameshift mutation is a mutation that changes the reading frame of a gene. This can result in the production of a protein that is non-functional.
What is a deletion mutation?
A deletion mutation is a mutation that results in the loss of one or more nucleotides from a gene. This can result in the production of a protein that is shorter than the normal protein.
What is an insertion mutation?
An insertion mutation is a mutation that results in the addition of one
Question 1
In a monohybrid cross, the F1 generation is heterozygous for the trait in question. This means that they have one dominant allele and one recessive allele. The F2 generation is produced by crossing two F1 individuals. The F2 generation will have a 3:1 ratio of dominant to recessive phenotypes.
Which of the following is an example of a monohybrid cross?
(A) A cross between a tall pea plant and a short pea plant
(B) A cross between a red flowered pea plant and a white flowered pea plant
(C) A cross between a homozygous dominant pea plant and a homozygous recessive pea plant
(D) A cross between a heterozygous pea plant and a homozygous recessive pea plant
Answer
The correct answer is (A). A cross between a tall pea plant and a short pea plant is a monohybrid cross because the trait in question is height. The F1 generation will be heterozygous for height, and the F2 generation will have a 3:1 ratio of tall to short phenotypes.
Question 2
In a dihybrid cross, the F1 generation is heterozygous for two traits. This means that they have one dominant allele for each trait and one recessive allele for each trait. The F2 generation is produced by crossing two F1 individuals. The F2 generation will have a 9:3:3:1 ratio of phenotypes.
Which of the following is an example of a dihybrid cross?
(A) A cross between a tall pea plant with purple flowers and a short pea plant with white flowers
(B) A cross between a tall pea plant with purple flowers and a tall pea plant with white flowers
(C) A cross between a short pea plant with purple flowers and a short pea plant with white flowers
(D) A cross between a tall pea plant with purple flowers and a short pea plant with purple flowers
Answer
The correct answer is (A). A cross between a tall pea plant with purple flowers and a short pea plant with white flowers is a dihybrid cross because the traits in question are height and flower color. The F1 generation will be heterozygous for both traits, and the F2 generation will have a 9:3:3:1 ratio of phenotypes.
Question 3
The law of segregation states that during meiosis, the two alleles for a given trait segregate into separate gametes. This means that each gamete will only contain one allele for each trait.
Which of the following is an example of the law of segregation?
(A) A cross between a tall pea plant and a short pea plant
(B) A cross between a red flowered pea plant and a white flowered pea plant
(C) A cross between a homozygous dominant pea plant and a homozygous recessive pea plant
(D) A cross between a heterozygous pea plant and a homozygous recessive pea plant
Answer
The correct answer is (D). A cross between a heterozygous pea plant and a homozygous recessive pea plant is an example of the law of segregation because the heterozygous pea plant will produce gametes with both dominant and recessive alleles. The homozygous recessive pea plant will only produce gametes with recessive alleles.
Question 4
The law of independent assortment states that during meiosis, the segregation of alleles for one trait is independent of the segregation of alleles for another trait. This means that the gametes produced by an individual will contain a random combination of alleles for all of the traits that the individual is heterozygous for.
Which of the following is an example of the law of independent assortment?
(A) A cross between a tall pea plant with purple flowers and a short pea plant with white flowers
(B) A cross between a tall pea plant with purple flowers and a tall pea plant with white flowers
(C) A cross between a short pea plant with purple flowers and a short pea plant with white flowers
(D) A cross between a tall pea plant with purple flowers and a short pea plant with purple flowers
Answer
The correct answer is (A). A cross between a tall pea plant with purple flowers and a short pea plant with white flowers is an example of the law of independent assortment because the F2 generation will have a 9:3:3:1 ratio of phenotypes. This ratio is the result of the independent assortment of the alleles for height and flower color.
Question 5
The Punnett square is a tool that can be used to predict the possible genotypes and phenotypes of the offspring of a genetic cross.
Which of the following is an example of a Punnett square?
(A) A cross between a tall pea plant and a short pea plant
(B) A cross between a red flowered pea plant and a white flowered pea plant
(C