What is the difference between genotype and phenotype?

Genotype refers to the genetic makeup of an organism (e.g., BB, Bb, bb), while phenotype is the observable characteristic (e.g., brown eyes). The phenotype results from the interaction of the genotype with the environment. For example, identical twins (same genotype) may have different weights due to diet.

How do you solve a dihybrid cross problem?

First, determine the genotypes of the parents for both traits. Then, list all possible gametes each parent can produce (using the FOIL method for two genes). Set up a 4x4 Punnett square and fill in the offspring genotypes. Finally, count the genotypic and phenotypic ratios. Remember that independent assortment only applies if the genes are on different chromosomes.

Why do some genetic disorders skip generations?

This often occurs with autosomal recessive disorders. Affected individuals are homozygous recessive, but carriers (heterozygotes) are unaffected. If two carriers have children, there is a 25% chance of an affected child. The disorder may appear to skip generations if carriers do not have affected children. For X-linked recessive disorders, it can skip generations if the allele is passed through carrier females.

What is epistasis and how does it affect phenotypic ratios?

Epistasis occurs when one gene masks or modifies the expression of another gene. For example, in Labrador retrievers, the B gene controls black (B) vs brown (b) coat, but the E gene determines whether pigment is deposited (E) or not (ee). A dog with ee genotype is yellow regardless of B/b. This modifies the typical 9:3:3:1 dihybrid ratio to 9:3:4 (black:brown:yellow).

How do you calculate the probability of a child inheriting a sex-linked disorder?

For X-linked recessive disorders (e.g., haemophilia), a carrier mother (XHXh) and unaffected father (XHY) have: 50% of sons affected (XhY), 50% of sons unaffected (XHY), 0% of daughters affected (they need two recessive alleles), but 50% of daughters are carriers (XHXh). For X-linked dominant disorders, affected fathers pass the allele to all daughters but no sons.

What is the chi-squared test used for in genetics?

The chi-squared test is used to determine if observed results from a genetic cross differ significantly from expected Mendelian ratios. For example, if you cross two heterozygotes and observe 75 dominant and 25 recessive phenotypes, the chi-squared test can tell you whether this fits a 3:1 ratio. If the calculated value is less than the critical value, you accept the null hypothesis (no significant difference).

Patterns of inheritance

OCR

A-Level

This topic explores the mechanisms of inheritance, focusing on how genetic information is passed from parents to offspring. It covers the roles of environmental and genetic factors in phenotypic variation, the use of genetic diagrams to predict inheritance patterns, and the application of statistical tests to analyze genetic data.

Objectives

Exam Tips

Pitfalls

Key Terms

Mark Points

Topic Overview

Patterns of inheritance, also known as genetics, explores how traits are passed from parents to offspring. This topic is central to understanding biological variation, evolution, and many human diseases. In OCR A-Level Biology, you will study Mendelian genetics, including monohybrid and dihybrid crosses, sex linkage, codominance, multiple alleles, and epistasis. You will also learn how to interpret pedigree diagrams and use chi-squared tests to analyse genetic data. Mastering these concepts is essential for explaining real-world phenomena like cystic fibrosis, Huntington's disease, and blood group inheritance.

This topic builds on GCSE knowledge of DNA, genes, and chromosomes, and connects to later topics such as gene technology, evolution, and speciation. Understanding inheritance patterns allows biologists to predict the likelihood of traits appearing in offspring, which has applications in medicine, agriculture, and conservation. For example, genetic counsellors use pedigree analysis to advise families on the risks of inherited disorders. By the end of this topic, you should be able to construct genetic crosses, calculate probabilities, and evaluate the role of genes and environment in shaping phenotypes.

In the OCR specification, this topic appears in Module 6 (Genetics, Evolution and Ecosystems). You will need to recall specific terminology (e.g., homozygous, heterozygous, allele, locus) and apply mathematical skills to genetic ratios. Exam questions often require you to explain deviations from expected Mendelian ratios, such as those caused by lethal alleles or linkage. A strong grasp of patterns of inheritance is also vital for understanding the principles of natural selection and how genetic variation arises.

Key Concepts

Core ideas you must understand for this topic

→Mendel's laws: Law of Segregation (alleles separate during gamete formation) and Law of Independent Assortment (genes on different chromosomes assort independently).
→Monohybrid and dihybrid crosses: Use of Punnett squares to predict genotypic and phenotypic ratios (e.g., 3:1, 9:3:3:1).
→Sex linkage: Genes on the X chromosome (e.g., haemophilia, colour blindness) show different inheritance patterns in males and females.
→Codominance and multiple alleles: Both alleles are expressed in heterozygotes (e.g., AB blood group) and more than two alleles exist in a population (e.g., ABO system).
→Epistasis: One gene masks or modifies the expression of another gene (e.g., coat colour in Labrador retrievers).

What You Need to Demonstrate

Key skills and knowledge for this topic

Distinction between environmental and genetic contributions to phenotypic variation
Use of genetic diagrams for monogenic, dihybrid, multiple alleles, sex linkage, and codominance
Identification of linkage (autosomal and sex linkage) and epistasis using phenotypic ratios
Application of the chi-squared test to determine the significance of differences between observed and expected results
Understanding the genetic basis of continuous and discontinuous variation
Factors affecting evolution including selection types, genetic drift, bottleneck, and founder effect
Use of Hardy-Weinberg principle to calculate allele frequencies
Role of isolating mechanisms in speciation (allopatric and sympatric)

Marking Points

Key points examiners look for in your answers

Distinction between environmental and genetic contributions to phenotypic variation
Use of genetic diagrams for monogenic, dihybrid, multiple alleles, sex linkage, and codominance
Identification of linkage (autosomal and sex linkage) and epistasis using phenotypic ratios
Application of the chi-squared test to determine the significance of differences between observed and expected results
Understanding the genetic basis of continuous and discontinuous variation
Factors affecting evolution including selection types, genetic drift, bottleneck, and founder effect
Use of Hardy-Weinberg principle to calculate allele frequencies
Role of isolating mechanisms in speciation (allopatric and sympatric)
Principles and ethical considerations of artificial selection

Examiner Tips

Expert advice for maximising your marks

💡Ensure you can clearly distinguish between continuous and discontinuous variation with examples
💡Practice setting up Punnett squares for complex inheritance patterns like codominance and sex linkage
💡Be prepared to explain how epistasis alters expected Mendelian ratios
💡Memorize the steps for the chi-squared test and how to use the provided formula and critical values table
💡When using Hardy-Weinberg, always define what p and q represent before starting calculations
💡Use specific biological terminology when describing evolutionary mechanisms
💡Always define your alleles clearly (e.g., let B = brown eyes, b = blue eyes) and state whether the trait is autosomal or sex-linked. This avoids ambiguity and shows the examiner you understand the genetic basis.
💡When answering questions about dihybrid crosses, remember that independent assortment only applies to genes on different chromosomes or far apart on the same chromosome. If genes are linked, you must consider crossing over.
💡For chi-squared tests, state the null hypothesis (e.g., 'there is no significant difference between observed and expected ratios'), calculate the chi-squared value, compare to the critical value at the appropriate degrees of freedom, and conclude whether to accept or reject the null hypothesis.

Common Mistakes

Pitfalls to avoid in your exam answers

Confusing the roles of environmental and genetic factors in determining phenotype
Incorrectly setting up genetic diagrams for dihybrid or sex-linked crosses
Misinterpreting phenotypic ratios when linkage or epistasis is present
Errors in calculating chi-squared values or misinterpreting degrees of freedom
Misapplying the Hardy-Weinberg equations or failing to identify the correct variables (p and q)
Confusing allopatric and sympatric speciation mechanisms
Misconception: Dominant alleles are always more common in populations. Correction: Dominance refers to expression in heterozygotes, not frequency. For example, the recessive allele for cystic fibrosis is rare, but dominant alleles for Huntington's disease are also rare.
Misconception: If both parents have a recessive condition, all children will be affected. Correction: This is true only if the condition is autosomal recessive and both parents are homozygous recessive. For X-linked recessive conditions, a father cannot pass the allele to sons.
Misconception: A 3:1 ratio always indicates simple dominance. Correction: Other factors like lethal alleles or incomplete penetrance can alter ratios. Always consider the possibility of epistasis or linkage.

Frequently Asked Questions

Common questions students ask about this topic

Before You Start

Prior knowledge that will help with this topic

•Basic understanding of DNA, genes, chromosomes, and the cell cycle (mitosis and meiosis).
•Knowledge of probability and ratios from GCSE mathematics.
•Familiarity with the terms allele, genotype, phenotype, dominant, and recessive from GCSE Biology.

Likely Command Words

How questions on this topic are typically asked

Explain

Calculate

Describe

Compare

Evaluate

Predict

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Practice questions tailored to this topic

Patterns of inheritance

Topic Overview

Key Concepts

What You Need to Demonstrate

Marking Points

Examiner Tips

Common Mistakes

Frequently Asked Questions

Before You Start

Likely Command Words

Ready to test yourself?

Related Topics in OCR A-Level Biology

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How to Revise Patterns of inheritance — OCR A-Level Biology

Examiner Tips for Patterns of inheritance

Common Mistakes in Patterns of inheritance

Key Marking Points

Patterns of inheritance

Topic Overview

Key Concepts

What You Need to Demonstrate

Marking Points

Examiner Tips

Common Mistakes

Frequently Asked Questions

What is the difference between genotype and phenotype?

How do you solve a dihybrid cross problem?

Why do some genetic disorders skip generations?

What is epistasis and how does it affect phenotypic ratios?

How do you calculate the probability of a child inheriting a sex-linked disorder?

What is the chi-squared test used for in genetics?

Before You Start

Likely Command Words

Ready to test yourself?

Related Topics in OCR A-Level Biology

Analysis

Biodiversity

Biological membranes

Biological molecules