Heredity and Variation

What you'll learn

This revision guide covers heredity and variation as examined in the CXC CSEC Human and Social Biology syllabus. You will understand how characteristics pass from parents to offspring through genetic material, and why variation exists within populations. This topic is essential for Section B of Paper 02 and frequently appears in Paper 01 multiple-choice questions.

Key terms and definitions

Heredity — the transmission of genetic characteristics from parents to offspring through genes.

Variation — differences in characteristics between individuals of the same species, caused by genetic factors, environmental factors, or both.

Gene — a section of DNA that codes for a specific characteristic or protein.

Allele — alternative forms of the same gene; for example, the gene for eye colour may have brown or blue alleles.

Chromosome — a thread-like structure in the nucleus made of DNA and protein, carrying genetic information.

Dominant allele — an allele that expresses its characteristic even when only one copy is present; represented by a capital letter (e.g., T).

Recessive allele — an allele that only expresses its characteristic when two copies are present; represented by a lowercase letter (e.g., t).

Genotype — the genetic makeup of an organism; the combination of alleles present (e.g., TT, Tt, tt).

Phenotype — the observable characteristics of an organism resulting from the interaction between genotype and environment.

Core concepts

Structure and function of genetic material

The nucleus of every cell (except mature red blood cells) contains chromosomes. Humans have 46 chromosomes arranged in 23 pairs. One chromosome from each pair comes from the mother, and one from the father.

Each chromosome consists of:

• Long strands of DNA (deoxyribonucleic acid)
• Protein molecules that support the DNA structure
• Genes positioned at specific locations along the DNA

DNA structure:

• Double helix shape (twisted ladder)
• Two strands held together by complementary base pairs
• Four bases: Adenine (A), Thymine (T), Guanine (G), Cytosine (C)
• Base pairing rules: A pairs with T, G pairs with C

Genes control characteristics by:

• Coding for specific proteins
• Proteins determining physical and biochemical characteristics
• Each gene occupying a fixed position on a particular chromosome

Cell division: mitosis and meiosis

Mitosis produces two identical daughter cells for growth and repair:

• Occurs in body (somatic) cells
• Chromosome number stays the same (46 in humans)
• One division produces two diploid cells
• Daughter cells are genetically identical to parent cell

Meiosis produces four sex cells (gametes) for reproduction:

• Occurs only in reproductive organs (testes and ovaries)
• Chromosome number halves (from 46 to 23 in humans)
• Two divisions produce four haploid cells
• Daughter cells are genetically different from each other and parent cell
• Creates variation through random assortment and crossing over

The importance of meiosis for variation:

• Gametes receive random selection of chromosomes from each pair
• Crossing over exchanges genetic material between paired chromosomes
• Fertilisation randomly combines gametes from two parents

Patterns of inheritance

Monohybrid inheritance involves the inheritance of a single characteristic controlled by one gene with two alleles.

Key principles:

• Each parent contributes one allele to offspring
• Dominant alleles mask recessive alleles in heterozygotes
• Homozygous organisms have two identical alleles (TT or tt)
• Heterozygous organisms have two different alleles (Tt)

Mendel's Law of Segregation states that allele pairs separate during gamete formation, and randomly unite at fertilisation.

Codominance occurs when both alleles in a heterozygote express themselves equally. In Caribbean populations, sickle cell trait demonstrates this:

• HbA HbA — normal red blood cells
• HbA HbS — both normal and sickle-shaped cells present (sickle cell trait)
• HbS HbS — all sickle-shaped cells (sickle cell anaemia)

Neither allele is completely dominant; both are expressed in heterozygotes.

Sex determination:

• Sex chromosomes: X and Y
• Females: XX (homogametic)
• Males: XY (heterogametic)
• Males determine offspring sex through sperm carrying X or Y
• 50% probability of male or female offspring

Variation in populations

Variation arises from two sources:

Genetic variation results from:

• Different combinations of alleles inherited from parents
• Mutations (random changes in genes or chromosomes)
• Independent assortment during meiosis
• Crossing over during meiosis
• Random fertilisation

Examples include blood group, eye colour, and ability to roll tongue.

Environmental variation results from:

• Diet and nutrition
• Climate and weather conditions
• Physical activity and lifestyle
• Disease and injury
• Educational opportunities

Examples include body mass, muscle development, and spoken language.

Continuous variation shows a range of phenotypes between extremes:

• Controlled by multiple genes (polygenic inheritance)
• Strongly influenced by environment
• Forms a normal distribution curve when plotted
• Examples: height, skin colour, body mass

Discontinuous variation shows distinct categories with no intermediates:

• Usually controlled by a single gene
• Little environmental influence
• Examples: ABO blood groups, ability to roll tongue, earlobe attachment

Caribbean examples of variation:

• Skin pigmentation in Caribbean populations shows continuous variation influenced by ancestry and sun exposure
• Sickle cell trait frequency varies across Caribbean islands due to historical malaria prevalence
• Body mass in Caribbean youth varies due to genetic factors and dietary patterns (traditional vs. Western diets)

Mutations and their effects

A mutation is a random change in genetic material that can be inherited if it occurs in gametes.

Types of mutations:

Gene mutations:

• Changes in DNA base sequence
• May alter protein structure and function
• Can be beneficial, harmful, or neutral

Chromosome mutations:

• Changes in chromosome number or structure
• Often cause severe developmental problems
• Example: Down syndrome (extra chromosome 21)

Causes of mutations:

• Spontaneous errors during DNA replication
• Ionising radiation (X-rays, gamma rays, UV light)
• Chemical mutagens (tobacco smoke, certain pesticides used in Caribbean agriculture)
• Biological agents (some viruses)

Effects in Caribbean context:

• Increased UV radiation in tropical regions raises mutation risk in skin cells
• Agricultural workers exposed to chemical pesticides face increased mutation rates
• Sickle cell mutation provides malaria resistance (historically important in Caribbean)

Selective breeding and genetic modification

Selective breeding involves choosing parents with desirable characteristics to produce improved offspring.

Process:

1. Select individuals with desired traits
2. Breed them together
3. Select best offspring and breed again
4. Repeat over many generations

Caribbean applications:

• Breeding cattle resistant to tropical diseases and heat (Jamaica Red cattle)
• Developing sugar cane varieties with higher sucrose content and disease resistance
• Breeding breadfruit and mango varieties suited to Caribbean climate
• Improving broiler chickens for faster growth in Caribbean poultry industry

Advantages:

• Produces organisms with desired characteristics
• Uses natural reproductive processes
• Proven over centuries of agricultural practice

Disadvantages:

• Time-consuming (many generations needed)
• Reduces genetic diversity
• Increases vulnerability to diseases
• Cannot introduce entirely new characteristics

Genetic modification involves directly transferring genes between organisms.

While not examined in detail at CSEC level, you should know:

• Involves inserting genes from one species into another
• Creates organisms with new characteristics
• Faster than selective breeding
• Raises ethical and environmental concerns

Worked examples

Example 1: Monohybrid cross

Question: In humans, the ability to taste PTC (a bitter chemical) is dominant (T) over the inability to taste it (t). A heterozygous taster marries a non-taster.

(a) State the genotypes of both parents. (2 marks) (b) Draw a genetic diagram to show the possible offspring. (4 marks) (c) What percentage of offspring will be tasters? (1 mark)

Solution:

(a) Heterozygous taster: Tt ✓ Non-taster: tt ✓

(b)

Parents:        Tt    ×    tt
Gametes:      T   t      t   t
                    ✓         ✓

Offspring:   [Punnett square showing:]
             T from parent 1 × t from parent 2 = Tt
             t from parent 1 × t from parent 2 = tt
             T from parent 1 × t from parent 2 = Tt
             t from parent 1 × t from parent 2 = tt
                    ✓                ✓

Genotypes: Tt : tt in ratio 2:1 (or 50% Tt, 50% tt)
Phenotypes: Taster : Non-taster in ratio 1:1 (or 50% each)

(c) 50% will be tasters ✓

Example 2: Continuous vs discontinuous variation

Question: The table shows data collected from 100 CSEC students at a Caribbean secondary school.

Characteristic	Number of categories observed
Blood group	4
Height range	Continuous from 150-185 cm
Eye colour	2
Body mass	Continuous from 45-90 kg

(a) Identify TWO characteristics showing discontinuous variation. (2 marks) (b) Explain why height shows continuous variation. (3 marks)

Solution:

(a) Blood group ✓ Eye colour ✓

(b) Height is controlled by many genes (polygenic inheritance) ✓ Environmental factors (diet, health, exercise) also influence height ✓ This produces a range of phenotypes with no distinct categories ✓

Example 3: Sex determination

Question: Using a genetic diagram, explain why approximately equal numbers of males and females are born. (5 marks)

Solution:

Parents:        XX (female)  ×  XY (male)  ✓
Gametes:        X    X           X   Y     ✓

Offspring:      XX    XX    XY   XY         ✓
                (female)    (male)

Ratio:          50% XX (female) : 50% XY (male)  ✓

Each fertilisation has equal probability of X or Y sperm fertilising the egg, producing approximately equal numbers of males and females in large populations ✓

Common mistakes and how to avoid them

• Confusing genotype with phenotype. Remember: genotype is the alleles present (letters); phenotype is the observable characteristic (description). Always use the correct term in answers.

• Using incorrect notation in genetic diagrams. Always use capital letters for dominant alleles and lowercase for recessive. Be consistent throughout your diagram. Show parents, gametes, and offspring clearly.

• Forgetting that both parents contribute alleles. Each parent gives one allele for each characteristic. Never write offspring genotypes with three or more alleles for a single gene.

• Stating percentages incorrectly. A 3:1 ratio means 75% and 25%, not 3% and 1%. Always calculate the percentage of the total.

• Confusing mitosis and meiosis. Mitosis produces identical cells for growth (46 chromosomes). Meiosis produces different gametes for reproduction (23 chromosomes). Learn the differences thoroughly.

• Claiming that all variation is genetic. Most characteristics result from both genetic and environmental factors. Distinguish clearly between genetic, environmental, and combined causes.

Exam technique for Heredity and Variation

• Command words matter. "State" requires a simple answer (1 mark). "Explain" requires reasons or mechanisms (usually 2-3 marks per point). "Describe" needs observable features without explanation.

• Genetic diagrams must be complete. Always show: parental genotypes and phenotypes, gametes (in circles or stated clearly), offspring genotypes, and offspring phenotypes. Use a Punnett square or systematic list. Incomplete diagrams lose marks even if your answer is correct.

• Use correct genetic terminology. Replace everyday words: use "genotype" not "genes," "allele" not "version," "phenotype" not "appearance," "heterozygous" not "mixed." Examiners award marks for precise scientific language.

• In extended response questions, structure your answer logically. Define key terms, provide examples, and link concepts. A 6-mark question typically requires 6 distinct points—plan before writing.

Quick revision summary

Heredity transmits characteristics from parents to offspring through genes located on chromosomes. Genes exist as different alleles; dominant alleles express in heterozygotes while recessive alleles need two copies. Meiosis produces genetically different gametes, creating variation. Variation arises from genetic factors (alleles, mutations, meiosis) and environmental factors (diet, climate, lifestyle). Continuous variation shows a range of phenotypes; discontinuous variation shows distinct categories. Monohybrid inheritance follows predictable patterns shown in genetic diagrams. Selective breeding improves organisms over generations by choosing parents with desired traits.