IB DP Biology D3.2 Inheritance Study Notes - New Syllabus -2025
IB DP Biology D3.2 Inheritance Study Notes – New Syllabus
IB DP Biology D3.2 Inheritance Study Notes at IITian Academy focus on specific topic and type of questions asked in actual exam. Study Notes focus on IB Biology syllabus with guiding questions of
- • What patterns of inheritance exist in plants and animals?
• What is the molecular basis of inheritance patterns?
Standard level and higher level: 5 hours
Additional higher level: 3 hours
D3.2.1 – Production of Haploid Gametes and Formation of Diploid Zygote
🌿 Key Concepts of Sexual Reproduction
- Haploid gametes (sperm and egg) are produced by parents through meiosis.
- Each haploid gamete contains one copy of each chromosome, so one set of genes.
- When gametes fuse (fertilization), they form a diploid zygote with two copies of each chromosome-one from each parent.
- This process restores the diploid number and is the basis of genetic inheritance.
🌿 Diploid Cells and Genes
- A diploid cell has two copies of each autosomal gene-these are called alleles.
- This means organisms inherit two versions of each gene, one from the mother and one from the father.
- This pattern is common to all eukaryotes with a sexual life cycle (plants, animals, fungi).
🔍 Why This Matters
- Having two alleles allows for genetic variation (dominant/recessive traits).
- It also provides genetic backup if one allele is faulty.
- Sexual reproduction ensures mixing of genetic material, increasing diversity.
| Term | Definition |
|---|---|
| Haploid (n) | Cell with one set of chromosomes (gametes) |
| Diploid (2n) | Cell with two sets of chromosomes (zygote/body cells) |
| Fertilization | Fusion of haploid gametes to form diploid zygote |
D3.2.2 – Methods for Conducting Genetic Crosses in Flowering Plants
🌿 Basic Terms
- P Generation (Parental generation): The original plants used in a genetic cross.
- F1 Generation (First filial generation): Offspring of the P generation.
- F2 Generation (Second filial generation): Offspring produced by crossing or self-fertilizing F1 plants.
- Punnett Grid: A diagram used to predict the genotypes and phenotypes of offspring from a genetic cross.
🌿 How Genetic Crosses Work in Plants
- Pollination transfers pollen (male gametes) to the ovary (female gametes).
- Many plants (e.g., peas) produce both male and female gametes on the same plant.
- This enables self-pollination and self-fertilization, producing genetically similar offspring.
- Cross-pollination between different plants is needed to study inheritance or breed new varieties.
🌿 Steps in Conducting Genetic Crosses
- Select P generation plants with contrasting traits.
- Cross-pollinate by transferring pollen from one plant to the stigma of another.
- Collect and grow the F1 generation seeds.
- Observe and record the traits of the F1 plants.
- Self-fertilize or cross F1 plants to produce the F2 generation.
- Use a Punnett grid to predict and analyze inheritance patterns.
🌿 Applications
- Breed new crop varieties with desirable traits (e.g., disease resistance, higher yield).
- Breed ornamental plants with specific flower colors or shapes.
| Generation | Description |
|---|---|
| P | Parent plants with contrasting traits |
| F1 | All offspring show one dominant trait |
| F2 | Traits segregate in a predictable ratio (e.g., 3:1) |
Genetic crosses in flowering plants rely on pollination and fertilization of male and female gametes.
P, F1, and F2 generations help track inheritance.
Punnett grids predict offspring genotypes and phenotypes.
These methods are key for plant breeding and understanding genetics.
D3.2.3 – Genotype as the Combination of Alleles Inherited by an Organism
🌿 Key Definitions
- Gene: A segment of DNA that codes for a particular characteristic (e.g., flower color).
- Alleles: Different forms or versions of a gene (e.g., allele for purple flowers or white flowers).
- Genotype: The genetic makeup of an organism—the specific combination of alleles it inherits for a gene.
🌿 Homozygous vs Heterozygous
| Term | Meaning | Example (Flower Color) |
|---|---|---|
| Homozygous | Both alleles for a gene are the same | PP (two purple alleles) or pp (two white alleles) |
| Heterozygous | Two different alleles for a gene | Pp (one purple allele, one white allele) |
🌿 Important Points
- The genotype determines what alleles an organism has.
- Different genotypes can produce different phenotypes (observable traits).
- Homozygous individuals have identical alleles; their offspring genotype depends on the other parent.
- Heterozygous individuals carry two different alleles, often showing the dominant trait.
Genotype = the combination of alleles inherited.
Genes are units of heredity; alleles are variations of genes.
Organisms can be homozygous (same alleles) or heterozygous (different alleles).
Understanding genotype is key to predicting inheritance and traits.
D3.2.4 – Phenotype as the Observable Traits Resulting from Genotype and Environment
What is Phenotype?
Phenotype: The physical or biochemical traits you can observe in an organism.
It results from the interaction between:
- The genotype (genetic makeup)
- The environment (external factors influencing development)
Examples of Traits
| Type of Trait | Explanation | Example |
|---|---|---|
| Genotype only | Traits controlled almost entirely by genes | Blood group (A, B, AB, O) |
| Environment only | Traits influenced solely by environment | Sun-tanned skin due to exposure to sunlight |
| Genotype × Environment interaction | Traits affected by both genes and environment | Height (genes set potential, nutrition affects final height) |
🌿Details:
- Genetic traits: Eye color, blood type – mainly determined by DNA, not much influenced by environment.
- Environmental traits: Scars, muscle build from exercise — caused by surroundings or lifestyle.
- Combined traits: Skin color can be influenced by genes but also by sun exposure; intelligence has genetic components but also depends on education and environment.
Phenotype = observable traits shaped by genes + environment.
Some traits depend only on genotype, others only on environment.
Most traits, especially complex ones, come from an interaction between the two.
D3.2.5 – Effects of Dominant and Recessive Alleles on Phenotype
🌿 Key Terms
- Alleles: Different versions of the same gene.
- Dominant allele: An allele that shows its effect on the phenotype even if only one copy is present.
- Recessive allele: An allele whose effect on phenotype is masked if a dominant allele is present.
- Homozygous: Having two identical alleles for a gene (e.g., AA or aa).
- Heterozygous: Having two different alleles for a gene (e.g., Aa).
🧬How Alleles Affect Phenotype
- A homozygous dominant (AA) individual has two copies of the dominant allele.
- A heterozygous (Aa) individual has one dominant and one recessive allele.
- Both produce the same phenotype because:
- The dominant allele masks the effect of the recessive allele in heterozygotes.
- One dominant allele is enough to show the dominant trait.
🌿 Examples
| Genotype | Phenotype | Reason |
|---|---|---|
| AA (homozygous dominant) | Displays dominant trait | Two dominant alleles present |
| Aa (heterozygous) | Displays dominant trait | Dominant allele masks recessive one |
| aa (homozygous recessive) | Displays recessive trait | No dominant allele present to mask recessive |
Example: Brown eye color (B) is dominant over blue (b). Genotypes BB and Bb both result in brown eyes.
Dominant alleles show their effect even if only one copy is present.
Recessive alleles only affect phenotype if both alleles are recessive.
Homozygous dominant and heterozygous individuals look the same for that trait.
D3.2.6 – Phenotypic Plasticity: Adaptation by Gene Expression Changes
🌿 What is Phenotypic Plasticity?
The ability of an organism to change its traits (phenotype) during its lifetime in response to the environment.
Changes happen without altering the genotype (no changes in DNA sequence).
It involves varying patterns of gene expression – some genes turn on/off or change activity depending on conditions.
🌿 Key Features
- Environmental influence: Different environments can cause different phenotypes from the same genotype.
- Reversibility: Changes can often be reversed if the environment changes again.
- Not genetic change: Phenotypic plasticity affects how genes are expressed, not the genes themselves.
🌿 Examples
| Organism | Trait Showing Plasticity | Environmental Trigger |
|---|---|---|
| Arctic fox | Fur color changes between brown (summer) and white (winter) | Seasonal temperature and daylight |
| Plants | Leaf size/thickness adjusts to sunlight levels | Light intensity |
| Humans | Muscle growth with exercise | Physical activity |
Phenotypic plasticity helps organisms adapt quickly to their environment.
It works through gene expression changes, not DNA mutation.
Allows reversible changes to traits within a single lifetime.
D3.2.7 – Phenylketonuria (PKU): A Recessive Genetic Disorder
⚙️ Cause of PKU
Mutation in the gene coding for phenylalanine hydroxylase enzyme.
This enzyme converts phenylalanine → tyrosine.
Mutation causes enzyme deficiency → phenylalanine builds up.
🧠 Effects on the Body
Excess phenylalanine is toxic to brain development.
Leads to intellectual disabilities and neurological problems if untreated.
👶 Inheritance Pattern
Recessive allele: symptoms appear only if two mutated alleles are inherited.
Carriers (heterozygotes) usually healthy but can pass on the gene.
🍽️ Management
Detected through newborn screening.
Requires a low-phenylalanine diet to prevent toxic buildup.
Early treatment can allow normal development.
PKU is caused by a recessive mutation affecting metabolism.
Untreated PKU causes brain damage, but is manageable.
Example of how genetics influence phenotype and health.
D3.2.8 – Single-Nucleotide Polymorphisms & Multiple Alleles in Gene Pools
🔍 What Are Single-Nucleotide Polymorphisms (SNPs)?
- Smallest type of genetic variation in a population.
- A single base change in the DNA sequence (e.g., A → G).
- SNPs can affect traits or be neutral.
- Common in genomes and useful for studying genetic diversity.
🌈 Multiple Alleles in a Gene Pool
- A gene pool is the total collection of all alleles in a population.
- There can be more than two alleles for a gene in the gene pool.
- Example: The ABO blood group system has three alleles — IA, IB, and i.
👥 Inheritance in Individuals
Although many alleles exist in the population, an individual inherits only two alleles — one from each parent.
This means each person’s genotype for a gene is made of two alleles (could be the same or different).
📊 Why This Matters
- SNPs and multiple alleles increase genetic diversity.
- This diversity allows populations to adapt and evolve.
- Understanding allele variation is important for genetics, medicine, and breeding.
| Term | Definition | Example |
|---|---|---|
| Single-Nucleotide Polymorphism (SNP) | Change of a single DNA base in a gene sequence | A → G base change |
| Multiple Alleles | More than two alleles of a gene present in a population | ABO blood group alleles IA, IB, i |
| Individual Genotype | Two alleles inherited for each gene | IA and IB in blood group AB |
SNPs are tiny DNA base changes increasing genetic diversity.
Multiple alleles in a population broaden gene variation.
Individuals inherit only two alleles per gene, shaping their genotype.
3.2.9 – ABO Blood Groups as an Example of Multiple Alleles
🩺 What Is the ABO Blood Group System?
- A classic example of multiple alleles in humans.
- Determines the type of antigens on red blood cells (RBCs).
- Controlled by one gene with three alleles: IA, IB, and i.
🧬 The Alleles and Their Effects
- IA allele: Produces A antigen on RBC surface.
- IB allele: Produces B antigen on RBC surface.
- i allele: Produces no antigen (O blood group).
🧩 Genotypes and Phenotypes
| Genotype | Phenotype (Blood Group) | Antigens on RBCs | Antibodies in Plasma |
|---|---|---|---|
| IAIA or IAi | Blood group A | A antigen | Anti-B antibodies |
| IBIB or IBi | Blood group B | B antigen | Anti-A antibodies |
| IAIB | Blood group AB | Both A and B antigens | No antibodies |
| ii | Blood group O | No antigens | Both Anti-A and Anti-B antibodies |
🤝 Codominance and Recessiveness
IA and IB are codominant: Both expressed equally in IAIB genotype → blood group AB.
i is recessive: Only expressed if both alleles are i (ii genotype).
🌍 Importance of ABO Blood Groups
Crucial for safe blood transfusions.
Determines compatibility between donor and recipient.
Helps in forensic science and paternity testing.
ABO blood group gene has 3 alleles (IA, IB, i).
IA and IB are codominant, i is recessive.
Blood groups identified by antigens on RBCs and antibodies in plasma.
D3.2.10 – Incomplete Dominance and Codominance
⚖️ Inheritance Patterns Beyond Simple Dominance
🧩 Codominance 
- Both alleles in a heterozygote are fully expressed together.
- The phenotype shows both traits equally (dual phenotype).
- Example: AB blood group (IAIB) – both A and B antigens appear on red blood cells.
- No blending; both alleles’ traits are visible side by side.
🌈 Incomplete Dominance
- The heterozygote shows a blend or intermediate phenotype between the two homozygous phenotypes.
- Neither allele is completely dominant over the other.
- Example: Four o’clock flower (Mirabilis jalapa)
- Red-flowered plant (RR) × White-flowered plant (WW) → Pink flowers (RW).
- The heterozygous flowers have a mix of red and white pigments, producing pink.
🔍 Comparing Codominance and Incomplete Dominance
| Feature | Codominance | Incomplete Dominance |
|---|---|---|
| Phenotype in heterozygote | Both traits visible simultaneously (dual) | Intermediate, blended trait |
| Example | AB blood group (IAIB) | Four o’clock flower (pink RW) |
| Trait expression | Both alleles expressed fully | Alleles partially expressed |
Codominance = both alleles show up fully (no blending).
Incomplete dominance = blended phenotype in heterozygotes.
Both differ from simple dominant-recessive inheritance.
D3.2.11 – Sex Determination in Humans & Sex Chromosome Gene Inheritance
🧬 Sex Chromosomes and Determining Biological Sex
- Humans have two sex chromosomes:
- X chromosome (large, many genes)
- Y chromosome (smaller, fewer genes)
- Females have two X chromosomes (XX).
- Males have one X and one Y chromosome (XY).
🧑🤝🧑 Role of the Sperm’s Sex Chromosome
- The sex of the zygote depends on the sperm:
- Sperm carrying X → offspring typically develops female characteristics (XX).
- Sperm carrying Y → offspring typically develops male characteristics (XY).
- The egg always contributes an X chromosome.
🧩 Genes on Sex Chromosomes
- The X chromosome carries far more genes than the Y chromosome.
- Many genes important for non-sexual traits are found on the X chromosome.
- The Y chromosome mostly carries genes related to male sex determination and sperm production.
⚠️ Implications for Inheritance
- X-linked genes can show different inheritance patterns, especially in males who have only one X chromosome (hemizygous).
- Conditions like color blindness and hemophilia are often caused by mutations on the X chromosome.
Sex is determined by whether sperm carries an X or Y chromosome.
X chromosome is gene-rich; Y chromosome is gene-poor but crucial for male traits.
Genes on sex chromosomes affect inheritance patterns differently than autosomal genes.
D3.2.12 – Haemophilia: A Sex-Linked Genetic Disorder
🔬 What is Haemophilia?
- Haemophilia is a sex-linked recessive disorder affecting blood clotting.
- Caused by a faulty allele on the X chromosome that codes for a clotting factor protein.
- People with haemophilia bleed longer because their blood does not clot properly.
Inheritance of Haemophilia
The gene for haemophilia is located on the X chromosome.
Use notation:
Xᴴ = normal allele (dominant)
Xʰ = haemophilia allele (recessive)
👩👧 Genotypes and Phenotypes
| Genotype | Description | Phenotype |
|---|---|---|
| Females: | ||
| XᴴXᴴ | Homozygous normal | Healthy (no haemophilia) |
| XᴴXʰ | Heterozygous carrier | Healthy but carrier |
| XʰXʰ | Homozygous affected | Haemophilia |
| Males: | ||
| XᴴY | Normal | Healthy |
| XʰY | Affected | Haemophilia |
⚠️ Key Points
- Males are more likely to have haemophilia because they have only one X chromosome.
- A male with the haemophilia allele (XʰY) will have the disorder since there is no second X to mask the faulty gene.
- Females need two copies of the haemophilia allele (XʰXʰ) to express the disorder; otherwise, they are carriers.
- Carrier mothers can pass the haemophilia allele to their sons, who will be affected.
Haemophilia is a classic example of a sex-linked recessive disorder.
It illustrates how gene location on sex chromosomes affects inheritance patterns and disease expression.
D3.2.13 – Pedigree Charts & Genetic Inheritance Patterns
🔍 What Are Pedigree Charts?
- Diagrams showing the inheritance of traits or disorders across generations in a family.
- Help identify whether a trait is dominant, recessive, sex-linked, or autosomal.
- Used to deduce genotypes of family members based on observed phenotypes.
🧬 Why Use Pedigree Charts?
- To understand how genetic disorders are passed on.
- To identify carriers of recessive or sex-linked disorders.
- To help predict the risk of inheritance in future generations.
🚫 Genetic Basis for Prohibition of Marriage Between Close Relatives
- Close relatives have a higher chance of sharing similar alleles, including harmful recessive alleles.
- Marriage between close relatives increases the risk of offspring inheriting recessive genetic disorders.
- This is a biological reason behind social prohibitions in many cultures.
Inductive vs Deductive Reasoning in Genetics
| Reasoning Type | Explanation | Example in Pedigree Analysis |
|---|---|---|
| Inductive | Drawing general conclusions from specific observations. | Observing some family members with a disorder and proposing a mode of inheritance. |
| Deductive | Applying general principles to predict specific cases. | Using inheritance patterns to deduce genotypes of untested individuals in the pedigree. |
Pedigree charts are powerful tools to trace genetic disorders and inheritance patterns.
Understanding these patterns can inform medical advice and genetic counseling.
Differentiating inductive and deductive reasoning is key to interpreting pedigrees correctly.
D3.2.14 – Continuous Variation & Polygenic Inheritance
🌿 What is Continuous Variation?
- Traits that show a range of phenotypes, not just distinct categories.
- Examples: Skin colour, height, weight in humans.
- These traits usually result from polygenic inheritance (many genes involved) and/or environmental influences.
🧬 Polygenic Inheritance
- Multiple genes (polygenes) contribute to the phenotype.
- Each gene adds a small effect.
- Produces a gradual distribution of traits instead of discrete groups.
🌞 Environmental Factors
- Environmental influences (like sunlight exposure for skin colour) modify the expression of polygenic traits.
- Result: Phenotypes can shift within a population due to environment, e.g., tanning.
📊 Continuous vs Discrete Variables
| Feature | Continuous Variation | Discrete Variation |
|---|---|---|
| Example | Skin colour, height | ABO blood group, tongue rolling ability |
| Range of Phenotypes | Many intermediate phenotypes | Distinct categories only |
| Genetic Basis | Polygenic inheritance & environment | Typically single gene with clear alleles |
📏 Application of Skills: Measures of Central Tendency
- Mean: Average value (sum of all values ÷ number of values).
- Median: Middle value when data is ordered.
- Mode: Most frequently occurring value.
- This help summarize data on continuous traits like skin colour measurements.
Continuous variation results from multiple genes and environmental effects.
Traits like skin colour vary in a gradient, not fixed groups.
Understanding continuous vs discrete traits helps in data analysis and interpretation.
D3.2.15 – Box-and-Whisker Plots for Continuous Data
📚 What is a Box-and-Whisker Plot?
- A graphical way to display data for continuous variables (e.g., student height).
- Shows the distribution and spread of data clearly.
📌 Six Key Aspects Displayed:
| Aspect | Description |
|---|---|
| Minimum | Smallest data value (excluding outliers) |
| First Quartile (Q1) | Value below which 25% of data lie |
| Median (Q2) | Middle value dividing data into two equal halves |
| Third Quartile (Q3) | Value below which 75% of data lie |
| Maximum | Largest data value (excluding outliers) |
| Outliers | Data points unusually distant from the rest |
⚠️ Outliers Defined
- Outliers are data points that lie:
- More than 1.5 × IQR above the third quartile (Q3 + 1.5 × IQR), or
- More than 1.5 × IQR below the first quartile (Q1 − 1.5 × IQR).
- IQR (Interquartile Range) = Q3 − Q1
🎯 Why Use Box-and-Whisker Plots?
- To visualize the central tendency and spread of data.
- To identify skewness or presence of outliers.
- Useful in comparing datasets side by side.
📊 Example Application
- Plotting heights of students to analyze variations and spot unusual values.
Additional Higher Level
D3.2.16 – Segregation and Independent Assortment of Unlinked Genes in Meiosis
🧩 Key Concepts
1. Segregation of Alleles
- During meiosis I, homologous chromosomes separate (segregate) into different gametes.
- Each gamete receives one allele of each gene.
- This explains Mendel’s Law of Segregation.
2. Independent Assortment of Unlinked Genes
- Genes located on different chromosomes (unlinked) assort independently.
- The orientation of each pair of homologous chromosomes during meiosis I is random and independent of others.
- Results in many possible combinations of alleles in gametes.
🔄 Link to Dihybrid Crosses
Dihybrid crosses involve two different genes.
For unlinked genes, the alleles assort independently.
This produces a typical 9:3:3:1 phenotypic ratio in the F2 generation when crossing heterozygotes.
🎲 Example
Gene 1: A or a
Gene 2: B or b
During meiosis:
Possible gametes from AaBb individual: AB, Ab, aB, ab
Gametes combine randomly during fertilization → genetic variation.
🔍 Why It Matters
Explains genetic diversity in sexually reproducing organisms.
Helps predict offspring genotypes and phenotypes in genetics.
D3.2.17 – Punnett Grids for Dihybrid Crosses with Unlinked Autosomal Genes
🧩 Key Concepts
- Purpose of Punnett Grids
Used to predict genotypic and phenotypic ratios in offspring.
Especially helpful in dihybrid crosses involving two genes. - Unlinked Genes
Genes located on different chromosomes or far apart on the same chromosome.
Alleles of these genes assort independently during meiosis.
🔢 Common Ratios from Dihybrid Crosses
- 9:3:3:1 Phenotypic Ratio
Occurs when crossing two heterozygous parents (AaBb × AaBb).
Phenotypes:- 9 show both dominant traits (A-B-)
- 3 show dominant for gene 1, recessive for gene 2 (A-bb)
- 3 show recessive for gene 1, dominant for gene 2 (aaB-)
- 1 shows both recessive traits (aabb)
- 1:1:1:1 Phenotypic or Genotypic Ratio
Occurs in a cross between a heterozygote and a homozygous recessive (AaBb × aabb).
Equal proportions of the four possible gametes and phenotypes.
🧮 How Ratios Are Derived
- Construct a 4×4 Punnett grid with gametes from each parent:
- For AaBb, gametes: AB, Ab, aB, ab.
- Fill in the grid with combinations.
- Count the genotypes and group into phenotypes.
- Calculate ratios by dividing counts by total offspring number.
⚠️ Important Note (NOS)
These ratios illustrate Mendel’s second law (Law of Independent Assortment).
This law holds true only if:
- Genes are on different chromosomes, or
- Genes are far apart on the same chromosome (recombination frequency ~50%).
There are exceptions, such as linked genes (close together on the same chromosome).
📊 Summary Table Example:
| Genotype | Phenotype | Count | Ratio |
|---|---|---|---|
| A-B- | Both dominant traits | 9 | 9/16 |
| A-bb | Dom. gene 1 only | 3 | 3/16 |
| aaB- | Dom. gene 2 only | 3 | 3/16 |
| aabb | Both recessive | 1 | 1/16 |
D3.2.18 – Loci of Human Genes and Their Polypeptide Products
What is a Locus?
- A locus (plural: loci) is the specific physical location of a gene on a chromosome.
- Each gene at a locus codes for a polypeptide (a chain of amino acids that makes up proteins).
Exploring Genes and Their Products
- Scientists use gene databases (like NCBI, Ensembl, or UniProt) to:
- Find the exact locus of a gene on a chromosome.
- Learn about the polypeptide that gene produces.
Key Concepts for Students
- Pairs of genes on different chromosomes:
- These genes are unlinked and assort independently.
- Example: A gene on chromosome 1 and another on chromosome 12.
- Pairs of genes close together on the same chromosome:
- These genes are linked and often inherited together.
- The closer they are, the less likely they are to be separated by recombination during meiosis.
D3.2.19 – Autosomal Gene Linkage
What is Gene Linkage?
- Gene linkage happens when two or more genes are located close together on the same autosome (non-sex chromosome).
- These genes tend to be inherited together because their alleles are physically linked on the same chromosome.
Why Does Linkage Affect Inheritance?
- During meiosis, homologous chromosomes pair up, and linked genes are less likely to be separated by crossing over (recombination).
- Because they travel together, the alleles of linked genes do not assort independently, which is an exception to Mendel’s second law.
How to Represent Linked Genes in Genetic Crosses
| Chromosome 1 | Chromosome 2 |
| A — B | a — b |
Key Points
- Linked alleles tend to be inherited as a package deal unless crossing over separates them.
- The closer two genes are on a chromosome, the stronger the linkage and the less frequent the recombination.
Gene linkage = genes close on the same autosome.
Linked alleles do not assort independently.
Vertical lines show linkage on homologous chromosomes.
D3.2.20 – Recombinants in Crosses Involving Two Linked or Unlinked Genes
Context:
Cross between:
– Individual heterozygous for both genes (e.g., AaBb)
– Individual homozygous recessive for both genes (aabb)
What Are Recombinants?
Recombinant gametes or offspring have new combinations of alleles different from the parental combinations.
Result from independent assortment (unlinked genes) or crossing over (linked genes).
Cross Outcomes:
| Type of Genes | Gametes from AaBb parent | Parental vs Recombinant Gametes | Resulting offspring genotypes | Explanation |
|---|---|---|---|---|
| Unlinked genes | AB, Ab, aB, ab (all equally likely) | Parental: AB, ab; Recombinant: Ab, aB | All four genotypes in 1:1:1:1 ratio | Independent assortment → 50% recombinants |
| Linked genes | Mostly AB, ab (parental) and fewer Ab, aB (recombinant) | Parental: AB, ab; Recombinant: Ab, aB | More parental genotype offspring than recombinant | Linkage reduces recombination frequency |
Identifying Recombinants:
- In gametes: Recombinant gametes carry allele combinations not present in the parents’ chromosomes due to crossing over.
- In offspring genotypes: Offspring with allele combinations different from parental types are recombinant genotypes.
- In offspring phenotypes: Recombinant phenotypes show traits in new combinations, different from parents.
Recombinants result from new allele combinations.
Unlinked genes → 50% recombinants (due to independent assortment).
Linked genes → fewer recombinants (due to crossing over).
Cross AaBb × aabb helps observe parental vs recombinant offspring.
D3.2.21 – Using a Chi-Squared Test on Dihybrid Cross Data
🧬What is a Chi-Squared Test?
A statistical test used to check if observed experimental results fit the expected results.
Helps determine if differences between observed and expected data are due to chance or something else.
Key Terms:
- Null Hypothesis (H₀): Assumes there is no real difference between observed and expected results (any difference is by chance).
- Alternative Hypothesis (H₁): Assumes there is a real difference between observed and expected results.
How It Works:
Compare the observed data (actual offspring counts from your dihybrid cross) to the expected data (predicted ratios, e.g., 9:3:3:1).
Calculate a chi-squared (χ²) value using:
χ² = ∑ ((O − E)² / E)
where O = observed frequency, E = expected frequency for each category.
Interpreting the Result:
Use a chi-squared table with degrees of freedom (usually number of categories − 1) to find the critical value at the p = 0.05 significance level.
– If χ² is less than the critical value → fail to reject null hypothesis → observed differences are likely due to chance.
– If χ² is greater than the critical value → reject null hypothesis → observed differences are statistically significant (unlikely due to chance).
Statistical significance (p = 0.05): There is a 5% chance that the differences occurred by random chance — this is the cutoff for significance.
The F2 generation data is a sample representing a larger population of offspring.
Results are more reliable when experiments are replicated or repeated