Qormaata galuumsa Baayoloojii, bara 2016 A.L.I (2024)

Gene flow is the transfer of alleles from one population to another. It happens when individuals move into a population (immigration) or out of it (emigration) and then breed. That is exactly what option D describes.

Options A and C fail because gene flow does change allele frequencies: the arriving alleles shift the balance of the gene pool. Option B fails because evolution has no goal, so no process drives a population towards perfection.

Textbook: Biology Grade 12, Unit 4 Evolution, section 4.1.8 Gene flow (immigration and emigration) (page 223).

The nervous system has two divisions. The central nervous system is the brain and the spinal cord. The peripheral nervous system is every nerve outside them: afferent (sensory) nerves carrying signals to the brain and spinal cord, and efferent (motor) nerves, including somatic motor nerves, carrying commands out to muscles and glands.

Only option C lists nerves alone. The other options each include the brain or the spinal cord, which belong to the central nervous system.

Textbook: Biology Grade 12, Unit 5 Human body system, section 5.1.4 Types of the nervous system (page 254).

The cell body holds the nucleus and most organelles, including the mitochondria. It keeps the neuron alive, maintains its structure, and supplies the energy the cell needs. So option B is the correct pair.

The others are mismatched: the axon carries impulses away from the cell body, dendrites receive signals from other neurons, and the myelin sheath insulates the axon and speeds up transmission rather than reducing it.

Textbook: Biology Grade 12, Unit 5 Human body system, section 5.1.1 Neurons and their functions (page 240).

A nerve impulse (action potential) is an electrical event. Sodium and potassium ions sit unevenly on the two sides of the axon membrane, so there is a charge difference across it. When a stimulus lets sodium ions rush in, that charge difference suddenly reverses, and the reversal sweeps along the axon as the impulse. So option A is correct.

Option C is wrong because a perfectly balanced charge means no impulse at all. Neurotransmitters (options B and D) act only at the synapse between neurons, not inside the axon.

Textbook: Biology Grade 12, Unit 5 Human body system, section 5.1.2 The nerve impulse and transmission (page 246).

A synapse is the meeting point between two neurons: the axon ending of one neuron and the dendrite of the next. At this junction a chemical messenger (a neurotransmitter) carries the signal across a tiny gap, so messages can pass from neuron to neuron. That matches option B.

Option A is the job of the myelin sheath, not the synapse. Option C is wrong because the action potential travels along the axon, and option D is wrong because a synapse passes the signal in one direction only.

Textbook: Biology Grade 12, Unit 5 Human body system, section 5.1.2 The nerve impulse and transmission (page 246).

Red blood cells with no A or B antigen, together with both anti-A and anti-B antibodies in the plasma, describe blood type O. A person with type O can donate to anyone, but can receive red cells only from another type O donor.

If this friend received type A, B, or AB blood, his anti-A and anti-B antibodies would attack the donor cells and make them clump, which is dangerous. So option B is the only safe choice.

Textbook: Biology Grade 10, Unit 5 Human Biology (ABO blood groups and transfusion).