Graduate Medical School Admissions Test (GAMSAT) Section III: Biological Sciences Syllabus

State Mendel's Law of Independent Assortment.

Alleles of different genes segregate independently of one another during gamete formation, provided the genes are on different chromosomes (or far apart on the same chromosome).

In a monohybrid cross $Aa \times Aa$, what are the genotypic and phenotypic ratios of the offspring?

Genotypic ratio $1\,AA : 2\,Aa : 1\,aa$; phenotypic ratio $3$ dominant $: 1$ recessive.

What phenotypic ratio is expected in the $F_2$ generation of a dihybrid cross between two heterozygotes ($AaBb \times AaBb$)?

A $9:3:3:1$ phenotypic ratio.

Define genotype, phenotype, homozygous, and heterozygous.

Genotype: an organism's allele combination. Phenotype: its observable traits. Homozygous: two identical alleles ($AA$ or $aa$). Heterozygous: two different alleles ($Aa$).

Distinguish mitosis from meiosis in terms of products and chromosome number.

Mitosis produces $2$ genetically identical diploid ($2n$) daughter cells for growth/repair. Meiosis produces $4$ genetically distinct haploid ($n$) gametes via two divisions.

Name the four phases of mitosis in order and a key event of each.

Prophase (chromatin condenses, spindle forms), metaphase (chromosomes align at the equator), anaphase (sister chromatids separate to poles), telophase (nuclear envelopes reform, chromosomes decondense).

What two events in meiosis I generate genetic variation?

Crossing over (recombination) between homologous chromosomes during prophase I, and independent assortment of homologous pairs at metaphase I.

Define a chromosome, chromatid, and homologous chromosomes.

A chromosome is a condensed DNA-protein structure. Sister chromatids are the two identical copies of a replicated chromosome joined at the centromere. Homologous chromosomes are a maternal-paternal pair carrying the same genes at the same loci.

Explain incomplete dominance and codominance with an example each.

Incomplete dominance: heterozygote shows a blended intermediate phenotype (e.g. red $\times$ white snapdragons $\to$ pink). Codominance: both alleles are fully expressed (e.g. AB blood group expresses both A and B antigens).

How is sex-linked (X-linked) recessive inheritance expressed differently in males and females?

Males (XY) are hemizygous, so a single recessive allele on the X is expressed (e.g. haemophilia, colour blindness); females (XX) must be homozygous recessive to show the trait and can be carriers.

What are multiple alleles, illustrated by the ABO blood group system?

A gene with more than two allelic forms in a population. ABO has three alleles: $I^A$ and $I^B$ (codominant) and $i$ (recessive), giving blood groups A, B, AB, and O.

Define a gene mutation and distinguish substitution, insertion, and deletion.

A mutation is a change in DNA base sequence. Substitution swaps one base for another; insertion adds bases; deletion removes bases. Insertions/deletions not in multiples of three cause a frameshift.

Compare silent, missense, and nonsense point mutations.

Silent: codon still codes for the same amino acid (no change). Missense: codes for a different amino acid. Nonsense: creates a premature stop codon, truncating the protein.

What is the difference between germline and somatic mutations?

Germline mutations occur in gametes and are heritable, passing to offspring; somatic mutations occur in body cells, are not inherited, but can cause conditions such as cancer.

State the Hardy-Weinberg allele and genotype frequency equations.

For two alleles, $p + q = 1$ and $$p^{2} + 2pq + q^{2} = 1$$ where $p^{2}$ = homozygous dominant, $2pq$ = heterozygous, and $q^{2}$ = homozygous recessive frequencies.

List the five conditions required for Hardy-Weinberg equilibrium.

No mutation, no natural selection, no gene flow (migration), random mating, and a very large (effectively infinite) population so no genetic drift occurs.

In a population, $16\%$ of individuals show a recessive phenotype. What are the allele frequencies $p$ and $q$?

$q^{2} = 0.16$, so $q = \sqrt{0.16} = 0.4$ and $p = 1 - q = 0.6$.

Define genetic drift and gene flow as mechanisms of evolution.

Genetic drift is random change in allele frequencies between generations (strong in small populations); gene flow is the transfer of alleles between populations through migration and interbreeding.

Trace the path of blood through the human heart and double circulation.

Deoxygenated blood: body $\to$ vena cava $\to$ right atrium $\to$ right ventricle $\to$ pulmonary artery $\to$ lungs. Oxygenated: lungs $\to$ pulmonary vein $\to$ left atrium $\to$ left ventricle $\to$ aorta $\to$ body.

Compare arteries, veins, and capillaries by structure and function.

Arteries: thick muscular/elastic walls carrying blood at high pressure away from the heart. Veins: thinner walls with valves returning blood at low pressure. Capillaries: one-cell-thick walls for exchange of gases, nutrients, and waste.

Describe gas exchange at the alveoli.

$\ce{O2}$ diffuses from alveolar air (high partial pressure) into the blood, while $\ce{CO2}$ diffuses from blood into the alveoli to be exhaled, driven by partial pressure gradients across the thin, moist, large surface area.

Outline the structure and function of a neuron and the direction of nerve impulse flow.

Dendrites receive signals; the cell body integrates them; the axon conducts the action potential away to axon terminals. Impulses travel dendrite $\to$ cell body $\to$ axon $\to$ synapse.

What are the approximate resting membrane potential and the ion movements that generate an action potential?

Resting potential $\approx -70\,\text{mV}$. Depolarisation: $\ce{Na+}$ rushes in; repolarisation: $\ce{K+}$ flows out; the $\ce{Na+/K+}$ pump restores the gradient (3 $\ce{Na+}$ out, 2 $\ce{K+}$ in per ATP).

Contrast the nervous and endocrine systems as communication systems.

Nervous system: fast, short-lived electrical/chemical signals via neurons, targeting specific cells. Endocrine system: slower, longer-lasting hormonal signals via the blood, with widespread effects.

Outline the main regions of the digestive tract and one function of each.

Mouth (mechanical/chemical digestion, amylase), stomach (acid and pepsin digest protein), small intestine (enzymatic digestion and absorption of nutrients), large intestine (water and ion absorption), with liver/pancreas as accessory organs.

Describe the function of the nephron in the excretory system.

The nephron filters blood (glomerulus), then reabsorbs water, glucose, and ions and secretes wastes along the tubule, producing urine to excrete nitrogenous waste (urea) and regulate water/electrolyte balance.

Define homeostasis and describe negative feedback.

Homeostasis is maintaining a stable internal environment despite external change. Negative feedback detects a deviation from a set point and triggers a response that reverses it, restoring balance (e.g. thermoregulation).

How do insulin and glucagon regulate blood glucose?

When blood glucose is high, the pancreas releases insulin, promoting glucose uptake and glycogen storage. When low, it releases glucagon, stimulating glycogen breakdown to raise glucose, an antagonistic negative-feedback pair.

Distinguish innate (non-specific) from adaptive (specific) immunity.

Innate immunity is rapid, non-specific (skin, phagocytes, inflammation) with no memory. Adaptive immunity is slower, specific to antigens via B and T lymphocytes, and produces immunological memory.

Compare the roles of B cells and T cells in the adaptive immune response.

B cells mediate humoral immunity by producing antibodies against extracellular pathogens. T cells mediate cell-mediated immunity: helper T cells coordinate the response, and cytotoxic T cells kill infected cells.

Summarise the menstrual cycle hormones and the events of fertilisation.

FSH stimulates follicle growth; rising oestrogen triggers an LH surge causing ovulation (~day 14); the corpus luteum secretes progesterone to maintain the endometrium. Fertilisation, the fusion of haploid sperm and egg, restores the diploid ($2n$) zygote in the oviduct.

State Darwin's theory of evolution by natural selection and define a species.

Heritable variation exists; organisms overproduce offspring competing for limited resources; better-adapted individuals survive and reproduce ('survival of the fittest'), so favourable alleles increase over generations. A species is a group of organisms that can interbreed to produce fertile offspring.

Define and order the ecological hierarchy: population, community, ecosystem.

Population: members of one species in an area. Community: all interacting populations of different species. Ecosystem: the community plus its abiotic (non-living) environment, linked by energy flow and nutrient cycling.

Why does only about $10\%$ of energy transfer between trophic levels, and what shape does an energy pyramid take?

Roughly $10\%$ of energy passes to the next trophic level because the rest is lost as heat (respiration), in undigested waste, and uneaten material; this gives an upright pyramid of energy that narrows toward higher levels.

Classify microorganisms causing disease and give the basic structural difference between bacteria and viruses.

Pathogens include bacteria, viruses, fungi, and protozoa. Bacteria are living cells (prokaryotic, with their own metabolism, killed by antibiotics); viruses are non-cellular, are obligate intracellular parasites needing a host to replicate, and are unaffected by antibiotics.