Lsn 15 Study Notes on Mendel's Principles of Inheritance
Mendel's Principles of Inheritance
Learning Objectives
LO15.1: Describe the experimental design, model system, and scientific approach that led to Mendel's success in uncovering patterns of inheritance.
LO15.2: Define and distinguish key genetics terms including allele, trait, genotype, phenotype, homozygous, heterozygous, and zygote.
LO15.3: Predict the outcomes of monohybrid and dihybrid genetic crosses using Punnett squares, product rule, and sum rule.
LO15.4: Explain Mendel's Laws of Segregation and Independent Assortment and how they contribute to genetic variation.
LO15.5: Describe the molecular basis of dominant and recessive alleles and define pleiotropy with reference to single-gene disorders such as cystic fibrosis.
LO15.6: Use the forked-line method and probability rules to calculate phenotypic ratios from multihybrid genetic crosses.
LO15.7: Distinguish between Mendelian and non-Mendelian inheritance patterns, including incomplete dominance, codominance, lethal alleles, and multiple alleles.
LO15.8: Predict the outcomes of genetic crosses involving sex-linked traits and explain why X-linked recessive traits are more common in males.
Key Genetics Terms
Dominant Trait: Traits that are displayed in hybrid individuals.
Recessive Trait: Traits that are masked by dominant traits.
Genotype: The genetic composition of an organism.
Phenotype: An organism's observable characteristics.
Homozygous: Having two identical alleles for a particular gene (e.g., AA or aa).
Heterozygous: Having two different alleles for a particular gene (e.g., Aa).
Zygote: The first diploid cell formed by fertilization.
Mendel’s Experiments with Pea Plants
Mendel selected pea plants (Pisum sativum) for his experiments due to several advantages:
They are easy to grow and have a short life cycle.
They possess many simple, distinguishable traits such as flower color and seed shape.
Pea plants are monoecious, possessing both male and female reproductive organs, facilitating controlled breeding.
Mendel performed controlled breeding by manually transferring pollen between plants.
Statistical methods were employed to analyze data and discern patterns.
Mendel's Laws of Inheritance
Law of Segregation: An organism inherits two copies of each gene, one from each parent, but only one copy is passed to each offspring during gamete formation. This is explained by the segregation of homologous chromosomes during meiosis.
Law of Independent Assortment: Genes for different traits are inherited independently of each other, as they are located on different chromosomes.
Molecular Basis of Genetic Dominance
Dominant Alleles: Typically encode functional proteins; the presence of one dominant allele is usually sufficient for normal phenotype expression.
Recessive Alleles: Often result from loss-of-function mutations in proteins.
Gene expression regulation can affect protein production to maintain function in homozygous or heterozygous individuals.
Genotype and Phenotype Ratios
When examining genetic crosses:
Genotype ratio for a monohybrid cross (e.g., TT, Tt, tt) is 1:2:1.
Phenotype ratio for a monohybrid cross typically is 3:1 (dominant:recessive).
Probability in Genetic Crosses
Punnett Square: A tool used to predict the possible genotypes and phenotypes of offspring from genetic crosses.
Product Rule: The probability that two or more independent events will occur is the product of their individual probabilities.
Example: For AaBBCcDdee from AaBbCcDdEe crosses: = $(0.5)(0.25)(0.5)(0.5)(0.25) = 7.8 imes 10^{-3}$ or 0.78% (1 in 128).
Sum Rule: The probability of one of two or more mutually exclusive outcomes occurring is the sum of their individual probabilities.
Example: Probability of producing a phenotype of all dominant or all recessive traits when crossing AaBbCcDdEe with AaBbCcDdEe = $(0.75)^5 + (0.25)^5 = 0.238$ or 23.8% (244 in 1024).
Test Cross Method
Test Cross: A breeding experiment used to determine the genotype of an individual displaying a dominant trait by crossing it with a homozygous recessive individual.
This can reveal if the dominant display results from a homozygous dominant or heterozygous genotype.
Non-Mendelian Inheritance Patterns
Incomplete Dominance: Neither allele is completely dominant, resulting in an intermediate phenotype. Example: Red and white snapdragons producing pink offspring.
Codominance: Both alleles are fully expressed in heterozygotes. Example: Human ABO blood types, where both A and B antigens are expressed in AB blood type.
Lethal Alleles: Alleles that cause death when inherited in a certain combination. Example: Manx cats have a lethal allele causing mortality if homozygous for certain tail mutations.
Multiple Alleles: A gene exists in more than two forms within a population, although an individual can only have two of these alleles.
Epistasis
Epistasis: The effect of one gene masks or modifies the expression of another gene, leading to deviations from expected Mendelian inheritance patterns.
Sex-Linked Traits and X-Linked Inheritance
X-Linked Inheritance: Traits located on the X chromosome show different inheritance patterns in males and females due to males having a single X chromosome (XY) and females possessing two (XX).
Males are more prone to X-linked recessive disorders as they only carry one X chromosome, lacking a second to mask the effect.
Example: Hemophilia and color blindness in humans.
Pedigree Analysis
Method to study the inheritance patterns of traits or diseases across generations within related individuals.
Conclusions from Mendel's Work
Dominance does not blend traits, confirming particulate inheritance. Dominant traits can obscure recessive ones but can still be passed on. Mendel's ratios consistently reflect this through the outcomes of his crosses, including the 3:1 ratio in F₂ generations following the monohybrid cross.