All Topics
biology-hl | ib
1.
Interaction and Interdependence
2.
Continuity and Change
3.
Unity and Diversity
4.
Form and Function
Mendelian inheritance
Topic 2/3
Mendelian Inheritance
Introduction
Mendelian inheritance forms the cornerstone of classical genetics, elucidating how traits are transmitted from one generation to the next. Rooted in the pioneering work of Gregor Mendel, this foundational concept is integral to the International Baccalaureate (IB) Biology Higher Level (HL) curriculum under the unit "Continuity and Change." Understanding Mendelian principles not only enhances comprehension of biological inheritance patterns but also provides a framework for exploring more complex genetic phenomena.
Key Concepts
Gregor Mendel and His Experiments
Gregor Johann Mendel, often dubbed the "father of genetics," conducted groundbreaking experiments in the mid-19th century that laid the foundation for our understanding of genetic inheritance. Mendel meticulously cross-pollinated pea plants (`Pisum sativum`) and observed the transmission of specific traits across generations. By focusing on traits such as flower color, seed shape, and pod color, Mendel was able to discern consistent patterns in the inheritance of these characteristics.
Laws of Inheritance
Mendel's experiments led to the formulation of three fundamental laws of inheritance:
- Law of Segregation: Each individual possesses two alleles for a particular trait, which segregate during the formation of gametes, ensuring that each gamete carries only one allele.
- Law of Independent Assortment: Alleles of different genes assort independently of one another during gamete formation, allowing for genetic variation.
- Law of Dominance: In a heterozygous pair, one allele may mask the expression of another, leading to the dominance of one trait over another.
Alleles and Genotypes
An allele is one of two or more versions of a gene that occupy a specific position on a chromosome. Alleles can be dominant or recessive, determining the expression of a trait.
- Homozygous: An organism with two identical alleles for a trait (e.g., AA or aa).
- Heterozygous: An organism with two different alleles for a trait (e.g., Aa).
Genotypic Ratio
The genotypic ratio refers to the proportion of different genotypes in a population. For instance, in a monohybrid cross between two heterozygous individuals (Aa × Aa), the expected genotypic ratio is:
$$
1\ AA : 2\ Aa : 1\ aa
$$
This ratio indicates that 25% of the offspring are homozygous dominant, 50% are heterozygous, and 25% are homozygous recessive.
Punnett Squares
Punnett squares are a graphical representation used to predict the genotypic and phenotypic outcomes of genetic crosses. By outlining all possible allele combinations from parental gametes, Punnett squares facilitate the calculation of probabilities for various genetic outcomes.
Example: Monohybrid Cross
Consider a cross between two heterozygous (`Aa`) pea plants. The Punnett square is constructed as follows:
The resulting genotypic ratio is 1 AA : 2 Aa : 1 aa, yielding a phenotypic ratio of 3 dominant : 1 recessive trait expression.
| A | a | |
|---|---|---|
| A | AA | Aa |
| a | Aa | aa |
Monohybrid and Dihybrid Crosses
Mendelian inheritance examines both monohybrid and dihybrid crosses to understand single and multiple trait inheritance.
- Monohybrid Cross: Involves a single trait. For example, crossing `Tt` (tall) with `tt` (short) plants results in a phenotypic ratio of 1 tall: 1 short.
- Dihybrid Cross: Involves two traits. For instance, crossing `AaBb` with `AaBb` (where A and B represent different traits) follows the law of independent assortment, producing a phenotypic ratio of 9:3:3:1.
Dihybrid Punnett Square
A dihybrid cross can be represented using a 4x4 Punnett square to account for all possible allele combinations. This method illustrates the independent assortment of genes, leading to genetic diversity among offspring.
Deviation from Mendelian Ratios
While Mendel's laws provide a foundational understanding, real-world genetics often exhibits deviations due to factors such as:
- Linkage: Genes located close together on the same chromosome may be inherited together, violating independent assortment.
- Incomplete Penetrance: The presence of a genotype does not always result in the expected phenotype.
- Lethal Alleles: Certain allele combinations may be non-viable, reducing their observed frequency in populations.
Incomplete Dominance and Codominance
Mendelian inheritance primarily addresses complete dominance, where one allele fully masks another. However, other patterns such as incomplete dominance and codominance also play pivotal roles.
- Incomplete Dominance: Neither allele is completely dominant, resulting in an intermediate phenotype. For example, crossing red (`RR`) and white (`WW`) snapdragon flowers yields pink (`RW`) offspring.
- Codominance: Both alleles are fully expressed in the phenotype. A classic example is the AB blood type in humans, where both A and B alleles are expressed simultaneously.
Sex-linked Inheritance
Sex-linked inheritance involves genes located on sex chromosomes, typically the X chromosome. This leads to distinct patterns of trait transmission between males and females.
- Hemizygous Expression: Males, possessing one X and one Y chromosome, express alleles on the X chromosome without a corresponding pair.
- Recessive Disorders: Traits such as hemophilia and color blindness are often sex-linked, manifesting predominantly in males.
Advanced Concepts
Pedigree Analysis
Pedigree analysis is a tool used to trace the inheritance of traits through generations within a family. By mapping familial relationships and trait expressions, genotypic and phenotypic patterns can be deduced.
- Symbols: Squares represent males, circles represent females, shaded symbols indicate affected individuals.
- Generation Labels: Roman numerals denote generations (e.g., I, II, III).
- Carrier Status: Unshaded individuals can be carriers if they possess recessive alleles without expressing the trait.
Gene Linkage and Recombination
Gene linkage refers to the tendency of genes located close to each other on the same chromosome to be inherited together. This proximity reduces the likelihood of recombination during meiosis, affecting independent assortment.
- Recombination Frequency: The probability of crossover between genes, expressed as a percentage. A recombination frequency of less than 50% indicates linkage.
- Genetic Mapping: By analyzing recombination frequencies, the positions of genes on chromosomes can be determined.
Multiple Alleles
While Mendel's principles often consider two alleles per gene, some genes exhibit multiple alleles, providing a broader range of phenotypic expressions.
- Blood Types: The ABO blood group system in humans involves three alleles (IA, IB, i), resulting in four blood types (A, B, AB, O).
- Coat Colors in Rabbits: Multiple alleles can influence the diverse coat color expressions observed in rabbit populations.
Pleiotropy and Epistasis
These concepts delve into interactions between different genes and their effects on phenotypes.
- Pleiotropy: A single gene influences multiple phenotypic traits. For example, the gene responsible for Marfan syndrome affects connective tissue, impacting skeletal structure, vision, and cardiovascular health.
- Epistasis: The interaction between different genes, where one gene masks or modifies the effect of another. An example is coat color in Labrador retrievers, where one gene determines pigment color and another determines whether it's deposited in the coat.
Quantitative Traits and Polygenic Inheritance
Quantitative traits are influenced by multiple genes, often resulting in a continuous range of phenotypes.
- Polygenic Inheritance: Traits such as height, skin color, and intelligence are controlled by several genes, each contributing additively to the phenotype.
- Environmental Influence: Non-genetic factors often interact with polygenic traits, further contributing to phenotypic variation.
Molecular Basis of Mendelian Inheritance
At the molecular level, Mendelian inheritance is governed by the transmission of `DNA` sequences from parents to offspring. Genes, composed of nucleotides, encode proteins that determine an organism's traits.
- DNA Replication: Ensures accurate transmission of genetic information during cell division.
- Mutation: Alterations in DNA sequences can introduce new alleles, contributing to genetic diversity.
- Gene Expression: Regulatory mechanisms control when and how genes are expressed, influencing phenotypic outcomes.
Modern Applications of Mendelian Genetics
Mendelian genetics extends beyond theoretical frameworks, finding applications in various fields:
- Genetic Counseling: Assists individuals and families in understanding their risks for inherited disorders.
- Agriculture: Facilitates selective breeding programs to enhance desirable traits in crops and livestock.
- Medicine: Informs the development of therapies for genetic diseases by targeting specific genetic mutations.
Comparison Table
| Aspect | Mendelian Inheritance | Non-Mendelian Inheritance |
|---|---|---|
| Fundamental Principle | Inheritance follows predictable patterns based on dominant and recessive alleles. | Inheritance involves complex patterns such as incomplete dominance, codominance, and multiple alleles. |
| Number of Alleles | Typically two alleles per gene. | Can involve multiple alleles. |
| Gene Interaction | Single gene influences one trait. | Multiple genes can interact to influence one trait (polygenic), or affect multiple traits (pleiotropy). |
| Environmental Influence | Minimal, focuses on genetic factors. | Significant, with gene-environment interactions influencing traits. |
| Applications | Basic genetic predictions, understanding inheritance. | Complex trait analysis, understanding non-Mendelian diseases. |
| Example | Pea plant flower color. | Human blood type (ABO system). |
Summary and Key Takeaways
- Mendelian inheritance provides a foundational framework for understanding how traits are transmitted across generations.
- Key principles include the laws of segregation, independent assortment, and dominance.
- Advanced concepts such as gene linkage, multiple alleles, and polygenic inheritance highlight the complexity beyond basic Mendelian patterns.
- Pedigree analysis and Punnett squares are essential tools for predicting and analyzing genetic outcomes.
- Mendelian genetics has profound applications in fields like medicine, agriculture, and genetic counseling.
Coming Soon!
Examiner Tip
Tips
Use the mnemonic “G. Mendel’s PAGs” to remember the key concepts: Punnett Squares, Allowances of segregation, and Genes assortment. Practicing with multiple Punnett Square examples and regularly reviewing the laws of inheritance can solidify your understanding and boost your confidence for exams.
Did You Know
Did You Know
Mendelian inheritance principles extend beyond pea plants. For example, human traits like widow's peak hairline and earlobe attachment follow Mendelian patterns. Additionally, certain inherited diseases, such as Huntington's disease, are directly explained by Mendelian genetics, showcasing the real-world impact of these foundational principles.
Common Mistakes
Common Mistakes
Incorrect Punnett Square Setup: Students often mix up parental alleles, leading to inaccurate predictions.
Incorrect: Placing both alleles of one parent on the same side.
Correct: Separating each allele of a parent into different columns or rows.
Confusing Genotype and Phenotype: Mistaking the genetic makeup (genotype) for the physical trait (phenotype).
Incorrect: Saying "AA" is a dominant trait.
Correct: "AA" is the genotype that results in the dominant phenotype.
FAQ
What is Mendelian inheritance?
Mendelian inheritance refers to the set of principles established by Gregor Mendel that describe how traits are passed from parents to offspring through dominant and recessive alleles.
What are the main laws of Mendelian inheritance?
Mendel's main laws include the Law of Segregation, the Law of Independent Assortment, and the Law of Dominance, which explain how alleles separate and combine during reproduction.
How do Punnett Squares work?
Punnett Squares are diagrams used to predict the possible genotypes and phenotypes of offspring resulting from a cross between parents, based on their genotype combinations.
What is the difference between genotype and phenotype?
Genotype refers to the genetic makeup of an organism (the alleles present), while phenotype is the observable physical or biochemical characteristics resulting from the genotype.
Can Mendelian inheritance explain all genetic traits?
No, while Mendelian inheritance provides a fundamental framework, many traits are influenced by multiple genes (polygenic inheritance) and environmental factors, leading to non-Mendelian patterns.
1.
Interaction and Interdependence
2.
Continuity and Change
3.
Unity and Diversity
4.
Form and Function
Get PDF
