Detailed Notes -2 -Chapter 4 Principles of Inheritance and Variation

4.3 INHERITANCE OF TWO GENES 

1. Experimental Setup:

   • Mendel crossed pea plants differing in two distinct characteristics: seed color and seed shape.
   • One parent had yellow-colored and round-shaped seeds, while the other had green-colored and wrinkled-shaped seeds.

2. Observations:

   • The seeds resulting from the cross exhibited yellow color and round shape.
   • This indicates that yellow color and round shape were dominant traits in this cross.

3. Determination of Dominance:

   • Mendel concluded that yellow color was dominant over green, and round shape was dominant over wrinkled.
   • These findings were consistent with separate monohybrid crosses Mendel conducted for each trait.

4. Genotypic Symbols:

   • Using genotypic symbols, Mendel represented the dominant and recessive alleles as follows:
     • Yellow color: Y (dominant), green color: y (recessive)
     • Round shape: R (dominant), wrinkled shape: r (recessive)
   • The genotype of the parents in this cross could be written as RRYY and rryy, representing homozygous dominant and recessive genotypes, respectively.

5. Cross and F1 Generation:

   • The cross between the two parent plants resulted in an F1 hybrid with the genotype RrYy.
   • This hybrid inherited one dominant allele for each trait from each parent.

6. F2 Generation:

   • Mendel self-hybridized the F1 plants and observed the F2 generation.
   • In the F2 generation, the traits segregated independently in a 3:1 ratio, similar to a monohybrid cross for each trait.
   • 3/4 of the F2 plants had yellow seeds, while 1/4 had green seeds.
   • Similarly, 3/4 of the F2 plants had round seeds, while 1/4 had wrinkled seeds.

4.3.1 Law of Independent Assortment

1. Explanation of the Law of Independent Assortment:

   • Mendel’s Law of Independent Assortment states that when two pairs of traits are combined in a hybrid, the segregation of one pair of characters is independent of the other pair of characters.
   • In other words, the inheritance of one trait does not influence the inheritance of another trait when considering two different gene pairs.

2. Derivation of the 9:3:3:1 Ratio:

   • The observed ratio of 9:3:3:1 in dihybrid crosses can be derived as a combination series of 3 yellow : 1 green, with 3 round : 1 wrinkled.
   • This combination can be expressed as (3 Round : 1 Wrinkled) (3 Yellow : 1 Green) = 9 Round, Yellow : 3 Wrinkled, Yellow : 3 Round, Green : 1 Wrinkled, Green.

3. Application of the Law using Punnett Square:

   • In a dihybrid cross involving traits for seed color (Y/y) and seed shape (R/r), the segregation of alleles for one pair of genes (R/r) is independent of the segregation of alleles for the other pair (Y/y).
   • When constructing a Punnett square for the F1 generation (RrYy), segregation of alleles for each gene pair occurs independently.
   • Each gamete produced by the F1 plant will have a 50% chance of containing either the dominant or recessive allele for each gene pair.
   • Thus, there are four possible combinations of alleles in gametes: RY, Ry, rY, and ry, each with a frequency of 25% or 1/4th of the total gametes produced.

4. Genotypic and Phenotypic Ratios in the F2 Generation:

   • By crossing the gametes in a Punnett square, we can determine the genotypic and phenotypic ratios in the F2 generation.
   • The genotypic and phenotypic ratios are not necessarily 9:3:3:1, as they depend on the specific traits being studied and the dominance relationships between alleles.
   • However, the independent assortment of alleles for each gene pair leads to the generation of multiple genotypes and phenotypes in the F2 generation.

5. Calculation of Genotypic Ratio in the F2 Stage:

   • Using the Punnett square data, the genotypic ratio in the F2 stage can be calculated by counting the number of each genotype and expressing it as a ratio.

6. Conclusion:

   • The Law of Independent Assortment explains how different gene pairs segregate independently during gamete formation, leading to the inheritance of multiple traits in offspring.
   • While the genotypic ratio may not always be 9:3:3:1, the principle of independent assortment holds true in various genetic crosses, contributing to the diversity of traits observed in populations.

4.3.2 Chromosomal Theory of Inheritance

1. Rediscovery of Mendel’s Work:

   • Around 35 years after Mendel’s initial publication on inheritance, three scientists independently rediscovered his findings in 1900.
   • This rediscovery brought attention back to Mendel’s principles, which had largely been overlooked since their initial publication in 1865.

2. Advancements in Microscopy:

   • Advances in microscopy techniques allowed scientists to observe cellular processes with greater detail.
   • Through microscopic observation, structures within the nucleus called chromosomes were discovered. These structures seemed to play a role in cell division.

3. Chromosomes and Genes:

   • Walter Sutton and Theodore Boveri noted that chromosomes occurred in pairs and carried genes.
   • They observed that alleles of a gene were located on homologous chromosomes, suggesting a connection between chromosomes and Mendel’s factors (genes).

4. Chromosomal Segregation:

   • During the process of meiosis, chromosomes segregate independently of each other.
   • This means that different chromosome pairs can align and separate randomly during cell division, leading to a random assortment of chromosomes in gametes.

5. Synthesis of Ideas:

   • Sutton and Boveri synthesized the knowledge of chromosomal segregation with Mendelian principles, proposing the Chromosomal Theory of Inheritance.
   • They suggested that the pairing and separation of chromosomes during meiosis lead to the segregation of the genes they carry.

6. Experimental Verification:

   • Thomas Hunt Morgan and his colleagues provided experimental evidence to support the Chromosomal Theory of Inheritance.
   • They conducted experiments with fruit flies (Drosophila melanogaster) and observed how traits were inherited, linking these observations to the behavior of chromosomes during cell division.

7. Significance:

   • The Chromosomal Theory of Inheritance provided a physical basis for understanding how traits are transmitted from parents to offspring.
   • It laid the groundwork for modern genetics, leading to further discoveries about the structure and function of genes on chromosomes.

4.3.3 Linkage and Recombination

1. Experimental Setup:

   • Morgan conducted dihybrid crosses in Drosophila, similar to Mendel’s pea experiments.
   • He crossed yellow-bodied, white-eyed females with brown-bodied, red-eyed males and intercrossed their F1 progeny.

2. Observations:

   • Morgan observed that the two genes did not segregate independently, leading to a deviation from the expected 9:3:3:1 ratio in the F2 generation.
   • He attributed this to the physical association or linkage of the two genes on the same chromosome.

3. Linkage and Recombination:

   • Morgan coined the term “linkage” to describe the physical association of genes on a chromosome.
   • He also introduced the term “recombination” to describe the generation of non-parental gene combinations during crossing over.

4. Types of Linkage:

   • Morgan found that genes on the same chromosome could exhibit different levels of linkage.
   • Some genes were tightly linked, showing low recombination rates, while others were loosely linked, showing higher recombination rates.

5. Example of Linkage Strength:

   • For instance, Morgan observed that the genes for white eye color and yellow body color were tightly linked, with only 1.3% recombination.
   • On the other hand, the genes for white eye color and miniature wing size showed 37.2% recombination, indicating looser linkage.

6. Mapping of Genes:

   • Morgan’s student, Alfred Sturtevant, used the frequency of recombination between gene pairs to estimate the distance between them on the chromosome.
   • This approach allowed them to create genetic maps, which are still used today to understand the arrangement of genes on chromosomes.

7. Applications of Genetic Mapping:

   • Genetic maps serve as a foundation for projects like the Human Genome Sequencing Project, aiding in the sequencing of entire genomes by providing a framework for locating genes.

4.4 POLYGENIC INHERITANCE

1. Definition of Polygenic Traits:

   • Polygenic traits are those traits that are controlled by three or more genes.
   • Unlike Mendel’s traits, which have distinct alternate forms (e.g., purple or white flower color), polygenic traits exhibit a continuous range of variation.

2. Examples of Polygenic Traits:

   • Human height is a classic example of a polygenic trait. Instead of just tall or short individuals, there is a wide spectrum of possible heights.
   • Another example is human skin color, which is controlled by multiple genes and influenced by environmental factors.

3. Genetic Basis:

   • Polygenic traits involve the additive effects of multiple alleles.
   • Each allele contributes to the phenotype, and the phenotype reflects the cumulative contribution of all alleles involved.

4. Illustrative Example:

   • Suppose three genes, A, B, and C, control skin color in humans.
   • The dominant forms (A, B, and C) result in darker skin color, while the recessive forms (a, b, and c) lead to lighter skin color.
   • Individuals with genotypes containing all dominant alleles (AABBCC) will have the darkest skin color, while those with all recessive alleles (aabbcc) will have the lightest skin color.
   • Intermediate skin colors result from genotypes with a mix of dominant and recessive alleles.
   • The darkness or lightness of skin color is determined by the combination and quantity of each type of allele in an individual’s genotype.

4.5 PLEIOTROPY

1. Definition of Pleiotropy:

   • Pleiotropy refers to the phenomenon where a single gene influences multiple phenotypic traits or characteristics.

2. Mechanism of Pleiotropy:

   • The underlying mechanism of pleiotropy often involves the gene’s effect on various metabolic pathways.
   • A single gene may impact different aspects of an organism’s biology, leading to diverse phenotypic expressions.

3. Example of Pleiotropy: Phenylketonuria (PKU):

   • PKU is a disorder in humans caused by a mutation in a single gene that codes for the enzyme phenylalanine hydroxylase.
   • This enzyme is involved in the metabolic pathway that breaks down the amino acid phenylalanine.
   • Due to the mutation, phenylalanine cannot be properly metabolized, leading to its accumulation in the body.
   • The pleiotropic effects of this mutation result in various phenotypic expressions, including:
     • Mental retardation: Accumulation of phenylalanine can lead to neurological damage, causing cognitive impairment.
     • Reduction in hair and skin pigmentation: Phenylalanine is a precursor for melanin, the pigment responsible for hair and skin color. Its altered metabolism can result in reduced pigmentation.

4. Significance of Pleiotropy:

   • Pleiotropy highlights the complexity of gene function and the interconnectedness of biological processes.
   • Understanding pleiotropy is essential for unraveling the genetic basis of complex traits and diseases, as a single gene can have widespread effects throughout the organism.