Sciences · Topic 2.1
Genetics & Body Systems
Understand how traits are inherited through DNA, genes, and alleles. Explore how the body generates energy through respiration and how the circulatory and respiratory systems work together to sustain life.
What You'll Learn
- Describe the structure and function of DNA, genes, and chromosomes
- Distinguish between dominant and recessive alleles, genotype and phenotype
- Use Punnett squares to predict offspring genotype and phenotype ratios
- Compare aerobic and anaerobic respiration with word and symbol equations
- Explain how the circulatory and respiratory systems deliver oxygen and remove waste
- Link cellular respiration to energy demands of the body
IB Assessment Focus
Criterion A — Knowing: Define key genetic terms and state respiration equations accurately.
Criterion B — Inquiring: Design experiments to test inheritance patterns; form testable hypotheses.
Criterion C — Processing: Construct and interpret Punnett squares; analyse respiration data.
Criterion D — Reflecting: Discuss ethical implications of genetic technologies; evaluate real-world applications.
Key Vocabulary
| Term | Definition |
|---|---|
| DNA | Deoxyribonucleic acid — a double-helix molecule that carries all genetic instructions in living organisms |
| Gene | A segment of DNA that codes for a specific protein or trait |
| Chromosome | A structure of tightly coiled DNA found in the cell nucleus; humans have 46 (23 pairs) |
| Allele | A version (variant) of a gene; each individual inherits two alleles for each gene, one from each parent |
| Dominant allele | The version of a gene expressed whenever present (written in capital letters, e.g. T) |
| Recessive allele | The version expressed only when two copies are present — homozygous recessive (lowercase, e.g. t) |
| Genotype | The genetic makeup of an organism (e.g. TT, Tt, tt) |
| Phenotype | The observable physical characteristic (e.g. tall, short, brown eyes) |
| Homozygous | Having two identical alleles for a gene (TT or tt) |
| Heterozygous | Having two different alleles for a gene (Tt) — also called a "carrier" for the recessive trait |
Sciences · Topic 2.1
DNA & Chromosomes
DNA is the molecule of inheritance. Its structure determines how genetic information is stored, copied, and passed from parent to offspring.
The Structure of DNA
DNA stands for deoxyribonucleic acid. It is shaped like a twisted ladder, called a double helix. The two long strands form the "sides" of the ladder, while the "rungs" are made of pairs of chemical bases.
The Four Bases
DNA contains four nitrogenous bases that pair in a specific way:
- Adenine (A) always pairs with Thymine (T)
- Cytosine (C) always pairs with Guanine (G)
This is called complementary base pairing. The bases are held together by hydrogen bonds. If you know the sequence on one strand, you can work out the other.
Example: If one strand reads A-T-C-G-G-A, the complementary strand reads:
T-A-G-C-C-T
From DNA to Chromosomes
The hierarchy of genetic organisation is:
- Base pairs → form the rungs of the DNA double helix
- Genes → sections of DNA that code for a specific protein (typically hundreds to thousands of base pairs long)
- Chromosomes → long, tightly coiled structures of DNA; each chromosome contains many genes
- Genome → the complete set of all DNA in an organism
Human Chromosomes
Human body cells contain 46 chromosomes, arranged in 23 pairs. One chromosome in each pair comes from the mother and one from the father.
| Cell Type | Number of Chromosomes | Description |
|---|---|---|
| Body cells (somatic) | 46 (23 pairs) — diploid | Contain the full set; formed by mitosis |
| Sex cells (gametes) | 23 (half) — haploid | Egg and sperm; formed by meiosis |
When a sperm (23) fertilises an egg (23), the resulting zygote has 46 chromosomes — a full set. The 23rd pair determines biological sex: XX = female, XY = male.
Key Distinction: Mitosis produces genetically identical body cells (46 chromosomes). Meiosis produces genetically unique sex cells (23 chromosomes). The halving of chromosome number in meiosis is essential — without it, the chromosome number would double every generation.
Sciences · Topic 2.1
Inheritance & Punnett Squares
Genetics follows predictable patterns. Punnett squares allow us to predict the probability of offspring inheriting specific traits.
Alleles and Inheritance
For every gene, an organism inherits two alleles — one from each parent. These alleles can be:
| Genotype | Type | Phenotype Expressed |
|---|---|---|
| TT | Homozygous dominant | Dominant trait (e.g. tall) |
| Tt | Heterozygous | Dominant trait (e.g. tall) — the dominant allele "masks" the recessive |
| tt | Homozygous recessive | Recessive trait (e.g. short) |
Critical Rule: A dominant allele does NOT mean it is the most common allele in a population. It simply means its trait is expressed when the allele is present. A recessive trait is only visible when the organism is homozygous recessive (tt).
How to Construct a Punnett Square
A Punnett square is a grid used to predict the probability of offspring genotypes and phenotypes from a genetic cross.
Step-by-step method:
- Identify the parents' genotypes (e.g. Tt × Tt)
- Write each parent's alleles along the top and side of a 2×2 grid
- Fill in each box by combining the alleles from the row and column
- Count the genotypes to determine ratios
- Determine phenotypes from the genotypes
Example: Monohybrid Cross (Tt × Tt)
Two heterozygous tall pea plants are crossed. T = tall (dominant), t = short (recessive).
| T | t | |
|---|---|---|
| T | TT | Tt |
| t | Tt | tt |
Results:
- Genotype ratio: 1 TT : 2 Tt : 1 tt
- Phenotype ratio: 3 tall : 1 short (75% tall, 25% short)
- Probability of a tall offspring = 3/4 = 75%
- Probability of a short offspring = 1/4 = 25%
Example: Test Cross (Tt × tt)
A test cross is used to determine whether an organism showing the dominant trait is homozygous (TT) or heterozygous (Tt). Cross the unknown with a homozygous recessive (tt).
| T | t | |
|---|---|---|
| t | Tt | tt |
| t | Tt | tt |
Results:
- Genotype ratio: 1 Tt : 1 tt
- Phenotype ratio: 1 tall : 1 short (50% : 50%)
- If ANY recessive offspring appear, the unknown parent must be heterozygous (Tt)
Co-dominance (Extension)
In some cases, neither allele is fully dominant. When both alleles are expressed equally, this is called co-dominance. For example, in some flower species, a cross between red (RRRR) and white (RWRW) produces pink flowers (RRRW) — both alleles contribute to the phenotype.
Sciences · Topic 2.1
Cellular Respiration
Respiration is the process by which cells release energy from glucose. It happens in every living cell, all the time — it is NOT the same as breathing.
Common Misconception: Respiration is NOT breathing. Breathing (ventilation) is the physical movement of air in and out of the lungs. Respiration is the chemical process inside cells that releases energy from glucose.
Aerobic Respiration
Aerobic respiration uses oxygen to break down glucose completely, releasing a large amount of energy. It occurs in the mitochondria of cells.
Word Equation
Glucose + Oxygen → Carbon dioxide + Water + Energy (ATP)
Symbol Equation
C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy
Key features of aerobic respiration:
- Requires oxygen
- Occurs in the mitochondria
- Produces approximately 30–32 ATP per glucose molecule (high energy yield)
- Products: carbon dioxide and water (waste products expelled by breathing and excretion)
- This is the body's preferred method of energy production
Anaerobic Respiration
Anaerobic respiration occurs when oxygen is insufficient or absent. It takes place in the cytoplasm and releases much less energy.
| Feature | In Animals | In Yeast (Fermentation) |
|---|---|---|
| Equation | Glucose → Lactic acid + Energy | Glucose → Ethanol + Carbon dioxide + Energy |
| Products | Lactic acid (causes muscle fatigue and cramp) | Ethanol (alcohol) + CO2 |
| Energy yield | Low (2 ATP) | Low (2 ATP) |
| Uses | Intense exercise (sprinting) | Bread-making (CO2 makes dough rise), brewing |
Comparison Table
| Feature | Aerobic | Anaerobic |
|---|---|---|
| Oxygen required? | Yes | No |
| Location | Mitochondria | Cytoplasm |
| Energy yield | High (~30–32 ATP) | Low (2 ATP) |
| Products | CO2 + Water | Lactic acid (animals) or Ethanol + CO2 (yeast) |
| Glucose breakdown | Complete | Incomplete (energy still trapped in products) |
| When used | Normal activity; resting | Intense exercise; low oxygen conditions |
Oxygen Debt
After intense exercise, you continue to breathe heavily. This is to repay the oxygen debt — the extra oxygen needed to break down the lactic acid that accumulated during anaerobic respiration. The lactic acid is transported in the blood to the liver, where it is converted back to glucose.
Sciences · Topic 2.1
Circulatory & Respiratory Systems
The circulatory and respiratory systems work together to deliver oxygen to cells and remove carbon dioxide — supporting the continuous process of cellular respiration.
The Respiratory System
The respiratory system is responsible for gas exchange — getting oxygen into the blood and removing carbon dioxide.
Pathway of air:
- Nose/mouth → air is warmed, moistened, and filtered
- Trachea (windpipe) → supported by C-shaped cartilage rings
- Bronchi → two tubes branching into each lung
- Bronchioles → smaller branches within the lungs
- Alveoli → tiny air sacs where gas exchange occurs
Gas Exchange in the Alveoli
The alveoli are adapted for efficient gas exchange:
| Adaptation | How It Helps |
|---|---|
| Large surface area (millions of alveoli) | More space for gas exchange to occur |
| Very thin walls (one cell thick) | Short diffusion distance for gases |
| Rich blood supply (dense capillary network) | Maintains a steep concentration gradient |
| Moist lining | Gases dissolve before diffusing across |
Oxygen diffuses from the alveoli (high concentration) into the blood capillaries (low concentration). Carbon dioxide diffuses the opposite way — from blood into the alveoli, then is exhaled.
The Circulatory System
The circulatory system transports substances around the body. Humans have a double circulatory system — blood passes through the heart twice per complete circuit.
Two circuits:
- Pulmonary circuit: Heart → Lungs → Heart (picks up oxygen, releases CO2)
- Systemic circuit: Heart → Body → Heart (delivers oxygen to cells, collects CO2)
The Heart
The heart has four chambers:
| Chamber | Function |
|---|---|
| Right atrium | Receives deoxygenated blood from the body (via vena cava) |
| Right ventricle | Pumps deoxygenated blood to the lungs (via pulmonary artery) |
| Left atrium | Receives oxygenated blood from the lungs (via pulmonary vein) |
| Left ventricle | Pumps oxygenated blood to the whole body (via aorta) — has the thickest muscular wall |
Common Mistake: The left ventricle wall is thicker than the right because it must pump blood around the entire body (systemic circuit), while the right ventricle only pumps to the nearby lungs. Also note: the pulmonary artery carries deoxygenated blood (the only artery that does), and the pulmonary vein carries oxygenated blood (the only vein that does).
Blood Vessels
| Vessel | Structure | Function |
|---|---|---|
| Arteries | Thick, muscular, elastic walls; small lumen | Carry blood AWAY from the heart at high pressure |
| Veins | Thinner walls; larger lumen; contain valves | Carry blood TOWARD the heart at low pressure; valves prevent backflow |
| Capillaries | One cell thick; tiny | Where exchange of substances occurs between blood and tissues |
Components of Blood
| Component | Function |
|---|---|
| Red blood cells | Contain haemoglobin; transport oxygen; no nucleus (more room for haemoglobin); biconcave shape increases surface area |
| White blood cells | Fight infection (phagocytes engulf pathogens; lymphocytes produce antibodies) |
| Platelets | Cell fragments that help blood clot at wound sites |
| Plasma | Yellow liquid; transports dissolved substances (glucose, CO2, urea, hormones, antibodies) |
Sciences · Topic 2.1
Worked Examples
These examples demonstrate the kind of structured reasoning expected in IB MYP Sciences. Use the word "because" to connect observations to explanations.
EXAMPLE 1Two heterozygous brown-eyed parents (Bb × Bb) have a child. What is the probability of the child having blue eyes?
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Full Solution
Step 1: Identify alleles. B = brown (dominant), b = blue (recessive).
Step 2: Draw the Punnett square:
| B | b
B | BB | Bb
b | Bb | bb
Step 3: Count genotypes: 1 BB : 2 Bb : 1 bb
Step 4: Only bb shows blue eyes (homozygous recessive).
Answer: Probability = 1/4 = 25%
Explanation: Blue eyes only appear when both alleles are recessive (bb), because the dominant allele B masks the recessive allele b in heterozygous (Bb) individuals.
Step 2: Draw the Punnett square:
| B | b
B | BB | Bb
b | Bb | bb
Step 3: Count genotypes: 1 BB : 2 Bb : 1 bb
Step 4: Only bb shows blue eyes (homozygous recessive).
Answer: Probability = 1/4 = 25%
Explanation: Blue eyes only appear when both alleles are recessive (bb), because the dominant allele B masks the recessive allele b in heterozygous (Bb) individuals.
EXAMPLE 2A DNA strand has the base sequence A-T-G-C-C-A. What is the complementary strand?
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Full Solution
Using complementary base pairing rules:
A pairs with T, T pairs with A, G pairs with C, C pairs with G.
Original: A - T - G - C - C - A
Complement: T - A - C - G - G - T
Justification: DNA uses complementary base pairing (A-T and C-G) because of the specific hydrogen bonding between these base pairs. Adenine and thymine form 2 hydrogen bonds; cytosine and guanine form 3 hydrogen bonds.
A pairs with T, T pairs with A, G pairs with C, C pairs with G.
Original: A - T - G - C - C - A
Complement: T - A - C - G - G - T
Justification: DNA uses complementary base pairing (A-T and C-G) because of the specific hydrogen bonding between these base pairs. Adenine and thymine form 2 hydrogen bonds; cytosine and guanine form 3 hydrogen bonds.
EXAMPLE 3Explain why a sprinter breathes heavily after a 100m race.
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Full Solution
During the race: The muscles require energy faster than aerobic respiration can supply it, so the body also uses anaerobic respiration. This produces lactic acid in the muscles.
After the race: The sprinter breathes heavily to take in extra oxygen. This oxygen is used to break down the accumulated lactic acid in the liver — converting it back to glucose. The extra oxygen required is called the oxygen debt.
Key link: Heavy breathing occurs because the body needs to repay the oxygen debt, and the increased breathing rate ensures more oxygen reaches the blood and is delivered to the liver to process lactic acid.
After the race: The sprinter breathes heavily to take in extra oxygen. This oxygen is used to break down the accumulated lactic acid in the liver — converting it back to glucose. The extra oxygen required is called the oxygen debt.
Key link: Heavy breathing occurs because the body needs to repay the oxygen debt, and the increased breathing rate ensures more oxygen reaches the blood and is delivered to the liver to process lactic acid.
EXAMPLE 4Explain why the left ventricle has a thicker muscular wall than the right ventricle.
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Full Solution
The left ventricle pumps oxygenated blood to the entire body (systemic circulation), which requires high pressure to force blood through a longer distance of blood vessels. The right ventricle only pumps blood to the nearby lungs (pulmonary circulation), a much shorter distance.
Therefore, the left ventricle has a thicker muscular wall because it needs to generate greater force to contract more powerfully and maintain higher blood pressure for the systemic circuit.
Therefore, the left ventricle has a thicker muscular wall because it needs to generate greater force to contract more powerfully and maintain higher blood pressure for the systemic circuit.
EXAMPLE 5Explain how the alveoli are adapted for efficient gas exchange.
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Full Solution
The alveoli have several adaptations:
1. Large surface area: There are millions of alveoli, providing a huge total surface area for gas exchange to occur across.
2. Thin walls: Alveoli walls are only one cell thick, minimising the diffusion distance so gases can cross quickly.
3. Rich blood supply: A dense network of capillaries surrounds each alveolus, maintaining a steep concentration gradient — oxygen-rich air in the alveolus and oxygen-poor blood in the capillary.
4. Moist lining: The thin layer of moisture allows gases to dissolve before diffusing across the membrane.
All these features maximise the rate of diffusion of oxygen into the blood and carbon dioxide out of the blood.
1. Large surface area: There are millions of alveoli, providing a huge total surface area for gas exchange to occur across.
2. Thin walls: Alveoli walls are only one cell thick, minimising the diffusion distance so gases can cross quickly.
3. Rich blood supply: A dense network of capillaries surrounds each alveolus, maintaining a steep concentration gradient — oxygen-rich air in the alveolus and oxygen-poor blood in the capillary.
4. Moist lining: The thin layer of moisture allows gases to dissolve before diffusing across the membrane.
All these features maximise the rate of diffusion of oxygen into the blood and carbon dioxide out of the blood.
Sciences · Topic 2.1
Practice Q&A
Attempt each question before revealing the model answer. Focus on using "because" to connect your observations to explanations.
PREDICTA homozygous dominant tall plant (TT) is crossed with a homozygous recessive short plant (tt). Predict the genotype and phenotype ratios of the F1 generation.
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Model Answer
Punnett square (TT × tt):
All offspring are Tt (heterozygous).
Genotype ratio: 100% Tt
Phenotype ratio: 100% tall
All offspring show the dominant phenotype because every individual has at least one dominant allele (T), which masks the recessive allele (t).
All offspring are Tt (heterozygous).
Genotype ratio: 100% Tt
Phenotype ratio: 100% tall
All offspring show the dominant phenotype because every individual has at least one dominant allele (T), which masks the recessive allele (t).
EXPLAINWhy does aerobic respiration produce more energy than anaerobic respiration?
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Model Answer
Aerobic respiration produces more energy because glucose is completely broken down using oxygen in the mitochondria, yielding approximately 30–32 ATP molecules per glucose. In anaerobic respiration, glucose is only partially broken down in the cytoplasm without oxygen, yielding only 2 ATP. The remaining energy is still trapped in the products (lactic acid or ethanol), meaning most of the glucose's energy is not released.
COMPARECompare arteries and veins in terms of structure and function.
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Model Answer
Arteries carry blood away from the heart at high pressure. They have thick, muscular, elastic walls and a small lumen to withstand and maintain the high pressure.
Veins carry blood toward the heart at low pressure. They have thinner walls, a larger lumen, and contain valves to prevent the backflow of blood.
Both are blood vessels that form part of the circulatory system, but their structural differences reflect the different pressures at which they operate.
Veins carry blood toward the heart at low pressure. They have thinner walls, a larger lumen, and contain valves to prevent the backflow of blood.
Both are blood vessels that form part of the circulatory system, but their structural differences reflect the different pressures at which they operate.
EXPLAINA person with genotype Bb has brown eyes but carries the allele for blue eyes. Explain how they could have a child with blue eyes.
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Model Answer
If both parents are heterozygous (Bb × Bb), there is a 1 in 4 (25%) chance the child will be homozygous recessive (bb) and have blue eyes. This is because each parent can pass on either allele. When both parents pass on the recessive allele (b), the child has genotype bb and expresses the recessive blue-eye phenotype. If one parent is Bb and the other is bb, there is a 1 in 2 (50%) chance the child will have blue eyes.
DESCRIBEDescribe the journey of oxygen from the air to a muscle cell during exercise.
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Model Answer
1. Air enters through the nose/mouth and travels down the trachea.
2. It passes through the bronchi into the bronchioles, and reaches the alveoli.
3. Oxygen diffuses across the thin alveolar wall into the blood capillaries (down the concentration gradient).
4. Oxygen binds to haemoglobin in red blood cells, forming oxyhaemoglobin.
5. The oxygenated blood returns to the heart via the pulmonary vein, enters the left atrium, then left ventricle.
6. The heart pumps the blood through arteries to the muscles.
7. At the muscle capillaries, oxygen dissociates from haemoglobin and diffuses into the muscle cells.
8. In the mitochondria of the muscle cell, oxygen is used in aerobic respiration to release energy.
2. It passes through the bronchi into the bronchioles, and reaches the alveoli.
3. Oxygen diffuses across the thin alveolar wall into the blood capillaries (down the concentration gradient).
4. Oxygen binds to haemoglobin in red blood cells, forming oxyhaemoglobin.
5. The oxygenated blood returns to the heart via the pulmonary vein, enters the left atrium, then left ventricle.
6. The heart pumps the blood through arteries to the muscles.
7. At the muscle capillaries, oxygen dissociates from haemoglobin and diffuses into the muscle cells.
8. In the mitochondria of the muscle cell, oxygen is used in aerobic respiration to release energy.
EVALUATEA student says "Anaerobic respiration is bad for the body." Evaluate this statement.
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Model Answer
This statement is partially incorrect. While anaerobic respiration produces lactic acid (which causes muscle fatigue and cramps), it is actually an essential survival mechanism. During intense exercise, the body cannot deliver oxygen fast enough, so anaerobic respiration provides the energy muscles need to keep functioning. Without it, the muscles would simply stop working. The lactic acid produced is temporary — it is broken down after exercise when the oxygen debt is repaid. Therefore, anaerobic respiration is a beneficial short-term adaptation, not simply "bad."
Sciences · Review
Flashcard Review
Tap each card to reveal the answer. Try to answer from memory first.
What is DNA?
Deoxyribonucleic acid — a double-helix molecule that carries genetic instructions. Its "rungs" are made of complementary base pairs: A-T and C-G.
Tap to reveal
What is the difference between a gene and a chromosome?
A gene is a segment of DNA coding for one protein/trait. A chromosome is a long, coiled structure of DNA containing many genes. Humans have 46 chromosomes (23 pairs).
Tap to reveal
What are the complementary base pairing rules?
Adenine (A) pairs with Thymine (T). Cytosine (C) pairs with Guanine (G). A-T have 2 hydrogen bonds; C-G have 3.
Tap to reveal
Genotype vs Phenotype?
Genotype = the genetic makeup (e.g. Tt, TT, tt). Phenotype = the observable physical characteristic (e.g. tall, short). Genotype determines phenotype.
Tap to reveal
What is the genotype ratio from Tt × Tt?
1 TT : 2 Tt : 1 tt. Phenotype ratio = 3 dominant : 1 recessive (75% : 25%).
Tap to reveal
What is the word equation for aerobic respiration?
Glucose + Oxygen → Carbon dioxide + Water + Energy (ATP). Occurs in the mitochondria.
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What is anaerobic respiration in animals?
Glucose → Lactic acid + Energy. Occurs in the cytoplasm without oxygen. Only produces 2 ATP per glucose (vs ~30–32 for aerobic).
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What is oxygen debt?
The extra oxygen needed after intense exercise to break down lactic acid (produced by anaerobic respiration) in the liver. This is why you breathe heavily after sprinting.
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Name the four chambers of the heart.
Right atrium, right ventricle, left atrium, left ventricle. The left ventricle has the thickest wall because it pumps blood to the entire body.
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What are the three types of blood vessel?
Arteries (thick walls, carry blood away from heart at high pressure), veins (thinner walls, valves, carry blood to heart), capillaries (one cell thick, where exchange occurs).
Tap to reveal
How are alveoli adapted for gas exchange?
Large surface area (millions of alveoli), thin walls (one cell), rich blood supply (steep concentration gradient), moist lining (gases dissolve).
Tap to reveal
What is the function of red blood cells?
Transport oxygen by binding it to haemoglobin (forming oxyhaemoglobin). They have no nucleus (more room for haemoglobin) and a biconcave shape (larger surface area).
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What is a double circulatory system?
Blood passes through the heart twice per circuit: (1) pulmonary circuit — heart to lungs and back; (2) systemic circuit — heart to body and back. This maintains high blood pressure for efficient delivery.
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Homozygous vs Heterozygous?
Homozygous = two identical alleles (TT or tt). Heterozygous = two different alleles (Tt). A heterozygous organism is a "carrier" of the recessive trait.
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What is the difference between mitosis and meiosis?
Mitosis produces 2 identical diploid cells (46 chromosomes) for growth/repair. Meiosis produces 4 unique haploid cells (23 chromosomes) for sexual reproduction (gametes).
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Sciences · Practice Test
Practice Test — 20 Questions
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