Inheritance and variation

To answer this, translate the wording into genotypes. The parents are described as pure-breeding parents, which means each parent is homozygous (both alleles match). Because they have contrasting traits, one parent must carry two dominant alleles (for example, $AA$) while the other carries two recessive alleles (for example, $aa$). This is a key idea in Topic: Evolution - Inheritance and variation.

To see what the $F_1$ offspring can be, cross the two parents ($AA\times aa$). The $AA$ parent can only contribute $A$ gametes, and the $aa$ parent can only contribute $a$ gametes.

$\begin{array}{ccc}& A& A\\ a& Aa& Aa\\ a& Aa& Aa\end{array}$

Every box shows the same genotype: $Aa$.

The Punnett square shows that all possible offspring outcomes are $Aa$, so $100\%$ of the $F_1$ generation has one dominant allele and one recessive allele. Having two different alleles makes them heterozygous. Because they come from two different true-breeding lines, they are also described as hybrids.

Using the result $Aa$ for every offspring:

• Option B is wrong because $Aa$ is heterozygous, not homozygous like $AA$ or $aa$.
• Option C is wrong because the dominant allele $A$ is present in every offspring, so they will not all display the recessive trait.
• Option D is wrong because a $3:1$ phenotype ratio is expected in the $F_2$ generation (from $Aa\times Aa$), not in the $F_1$ from $AA\times aa$.

Since every $F_1$ offspring has genotype $Aa$, the correct description is that they are all heterozygous hybrids.

In genetics, pure-breeding (true-breeding) individuals are homozygous for a trait. With contrasting traits, one parent must be homozygous dominant (use $AA$) and the other must be homozygous recessive (use $aa$). This fits the core ideas in Evolution: Inheritance and variation.

In the cross $AA\times aa$, each parent can only make one type of gamete:

1. The $AA$ parent can only pass on $A$.
2. The $aa$ parent can only pass on $a$.

So every $F_1$ offspring must receive $A$ from one parent and $a$ from the other, giving $Aa$.

Because every $F_1$ individual is $Aa$, they do not have matching alleles. That means they are heterozygous, so the option claiming all offspring are homozygous cannot be correct.

Mendel’s Law of Dominance states that in a heterozygote like $Aa$, the dominant allele determines the visible trait. Therefore, all $F_1$ offspring will show the dominant phenotype rather than the recessive one.

A $3:1$ phenotypic ratio is typical when two heterozygous $F_1$ individuals are crossed to produce $F_2$. In the $F_1$ from $AA\times aa$, all offspring are the same genotype and will look the same, so a $3:1$ ratio does not apply.

All $F_1$ offspring are $Aa$, meaning they are heterozygous hybrids. This is exactly what you expect from a monohybrid cross between two pure-breeding parents with different traits.

Start by converting the description into alleles. Pure-breeding means homozygous, and contrasting traits means one parent is $AA$ while the other is $aa$.

So the cross is:

$AA\times aa$

A $3:1$ phenotype ratio is associated with the $F_2$ generation (from $Aa\times Aa$). It does not occur in the $F_1$ when the starting parents are pure-breeding opposites.

To be homozygous ($AA$ or $aa$), an offspring must inherit the same allele from both parents. But here, one parent can only give $A$ and the other can only give $a$, so the $F_1$ cannot be homozygous.

Every $F_1$ individual receives a dominant allele $A$. That dominant allele determines the phenotype in a heterozygote, so the recessive trait will not be shown by all $F_1$ offspring.

After ruling out the other statements, the only outcome that fits $AA\times aa$ is that every $F_1$ offspring is $Aa$. This means they are heterozygous and can be called hybrids.

Gregor Mendel is widely known as the “father of genetics.” What made his work stand out was that he didn’t just describe traits—he carefully counted outcomes and used maths to interpret them. This question is asking specifically why using a large sample size helped his pea-plant investigations succeed.

This links directly to Topic: Inheritance and variation, where we explore how characteristics are transmitted from parents to offspring and why patterns sometimes look clearer with more data.

Genetic inheritance involves randomness. During meiosis and fertilisation, which alleles end up together in one offspring is influenced by chance.

A key statistics idea here is the Law of Large Numbers: as you collect more results, the overall pattern tends to move closer to the true expected average. In Mendel’s case, counting more offspring meant his observed numbers were more likely to match the predicted genetic ratios (such as the well-known $3:1$ outcome in many monohybrid crosses).

Now consider why the other statements don’t explain Mendel’s success:

• Option A: Mendel kept things controlled by mainly working with one organism—the garden pea (Pisum sativum)—rather than switching among many species.
• Option B: The number of organisms you count doesn’t stop mutations; mutations are biological events that can happen regardless of sample size.
• Option C: Mendel worked in the 1800s, long before DNA was identified as the genetic material and long before its structure was described. He could not directly observe DNA.

By using very large numbers of pea plants, Mendel reduced the effect of random variation that can distort results in small trials. With thousands of offspring, unusual chance outcomes “even out,” making the underlying ratios easier to see.

It’s similar to flipping a coin: with only a few flips, you might get an uneven result by luck, but with 1,000 flips you’d expect something close to $50:50$. Mendel applied the same idea—large counts made the statistical ratios clear and dependable.

Mendel earned his reputation because he investigated inheritance with careful planning and detailed counting. To find the best answer here, we can use Process of Elimination, checking each option against what we know about Mendel’s experiments and basic scientific reasoning.

Mendel didn’t use large sample sizes so he could test many species at once. In fact, he deliberately focused on one main species—the garden pea (Pisum sativum)—so that conditions stayed consistent. The large numbers were used to strengthen patterns within that one species, not to compare across different organisms.

A bigger sample does not block mutations. Mutations are random changes in DNA. Growing more plants won’t prevent them; if anything, increasing the number of individuals could make it more likely you notice a rare mutation simply because you are observing more offspring.

Mendel’s studies took place in the mid-1800s. DNA’s structure wasn’t worked out until 1953. Mendel suggested invisible “factors” that controlled traits (what we now call genes), but he could not directly view DNA or even know its molecular structure.

After removing the incorrect statements, the best explanation left is about statistical reliability. With thousands of offspring, random chance has less influence on the overall pattern, so the results more closely match predicted ratios—like the classic $3:1$ phenotype ratio seen in the $F_2$ generation of a monohybrid cross.

We are being asked to judge the idea that phenotype depends only on genotype, and that the environment has zero effect. In genetics we separate an organism’s DNA information (genotype) from the traits we can measure or see (phenotype). This fits the unit on inheritance and variation where we discuss why individuals with similar genes can still look different.

A common way to summarize trait expression is:

$P=G+E$

Where:

• $P$ means phenotype (the characteristics you observe).
• $G$ means genotype (the alleles/DNA an organism has).
• $E$ means environment (factors such as food, temperature, light, training, or disease).

This reminds us that DNA sets up possibilities, but conditions around the organism can change how strongly genes are expressed and what the final trait looks like.

Choice A supports the claim by saying DNA gives a complete set of instructions that doesn’t depend on outside conditions. That doesn’t match $P=G+E$. For instance, two genetically identical cuttings (clones) can end up different sizes if one receives more light or better soil nutrients than the other.

Choice B says the environment can affect phenotype only if it changes the DNA sequence. That’s not true. Many environmental effects change traits without altering the DNA at all—for example, diet can influence body mass and growth, and sunlight can affect skin tanning, even though the underlying DNA sequence stays the same.

Choice D goes too far the other way, claiming genes don’t matter. But genotype clearly sets major limits and directs development (for example, genes strongly influence traits like blood type). The environment can shift outcomes, but it cannot replace the genetic information completely.

Choice C fits the $P=G+E$ model. The original statement is inaccurate because phenotype is produced by genes interacting with environmental conditions, not by genotype alone.

The claim is: “Phenotype is controlled only by genotype, with no environmental impact.”

Key terms:

• Genotype: the allele combination/DNA instructions (for example, $BB$ or $Bb$).
• Phenotype: the measurable or visible result (for example, flower color, height).

Saying the environment has no effect is an absolute statement, so we can test it by removing options that are too extreme.

Choice A agrees with the statement by treating genotype as the entire story. But real organisms show that conditions can change how traits develop. For example, two plants with the same genes related to growth can end up different heights if one grows with plenty of light and the other is kept in low light.

Choice B argues that the environment influences phenotype only by first causing mutations. Although some environmental factors can damage DNA (such as UV radiation), many environmental effects happen without changing the DNA sequence—nutrition, temperature, and exercise can all shift traits directly.

Choice D claims genes don’t matter and the environment determines traits entirely. That cannot be correct because genotype sets the basic biological plan and the range of possible outcomes. No environment can turn one species’ genetic starting point into another species’ body plan.

With the extreme positions eliminated, the best evaluation is that phenotype comes from both genotype and environment interacting. This is often summarized as:

$P=G+E$

Where:

• $P$ = Phenotype
• $G$ = Genotype
• $E$ = Environment

A polygenic trait is a feature produced by the combined (additive) influence of two or more genes.

Compared with single-gene (monogenic) traits, polygenic traits usually show continuous variation—many possible outcomes across a spectrum (such as a wide range of heights or skin tones) instead of neat “either/or” groups.

The ABO blood group is determined by one gene locus (the $I$ gene) that has three alleles ($I^A$, $I^B$, and $i$). Since one gene location controls it, it is a monogenic trait, not a polygenic one.

Red-green color blindness is an X-linked characteristic caused by a change in a single gene on the X chromosome. Because it relies on only one gene, it does not match the definition of a polygenic trait.

Albinism is most often inherited as an autosomal recessive condition due to a mutation in one gene (for example, a gene involved in making the enzyme tyrosinase). It is typically a clear-cut trait: a person is affected or not affected.

In IB Biology, height is commonly used as the key example of polygenic inheritance.

Many genes contribute to how tall someone becomes, so height doesn’t sort people into simple boxes. Instead, it produces continuous variation in populations and often forms a bell-shaped curve when you graph lots of people’s heights.

We need to pick a polygenic trait.

In genetics, traits are often grouped into:

1. Monogenic (single-gene) traits: controlled by one gene locus and usually seen as distinct categories (you have it or you don’t).
2. Polygenic traits: determined by the additive effect of multiple genes, and they usually show continuous variation (a broad range of outcomes, often like a bell curve).

Start with Option A. The ABO blood group system is controlled by a single gene locus (the $I$ gene), even though there are three alleles ($I^A$, $I^B$, and $i$). Because one gene locus determines it, it is monogenic rather than polygenic.

Next, Option B. Red–green colour blindness is sex-linked and results from a mutation in one gene on the X chromosome. Since a single gene is responsible, it is a monogenic trait.

Now look at Option D. Albinism (such as oculocutaneous albinism) is usually inherited as an autosomal recessive condition and is caused by a mutation in one gene affecting melanin production (for example, the $TYR$ gene). This is another single-gene inheritance example.

After ruling out the monogenic traits, we are left with Option C.

Height cannot be sorted into a few fixed categories; instead, it varies widely across humans. This happens because height is influenced by the combined (additive) effects of hundreds of genes, and it is also affected by environmental factors such as diet and health. That makes height a strong example of a polygenic trait.

Using Analysis of Variation Type, we separate traits by the kind of variation they show:

• Monogenic traits depend on one gene and tend to show discrete variation, meaning clear categories (like “present” or “absent”).
• Polygenic traits depend on multiple genes whose effects add up, leading to continuous variation—many possible values across a range, often forming a bell-shaped (normal) distribution.

Consider Option A: ABO blood group. People fall into A, B, AB, or O—there aren’t halfway blood types. That clear set of categories is discrete variation, and it is controlled by a single gene with multiple alleles ($I^A$, $I^B$, and $i$).

Now look at Option B: Red-green color blindness and Option D: Albinism.

• Red-green color blindness is X-linked and controlled by one gene; a person is typically classified as affected or not.
• Albinism is usually recessive and results from a single gene affecting melanin production.

Because both are usually described in distinct groups (affected vs. unaffected), they show discrete variation and are not polygenic.

Now consider Option C: Height. If you measured everyone in a year group, you wouldn’t get just two or four categories—you’d get many different measurements from very short to very tall.

When those measurements are graphed, they commonly form a bell-shaped curve. That pattern strongly suggests a polygenic trait, since many genes (and environmental influences) contribute to the final phenotype.

By focusing on variation type, we can rule out the discrete traits (Blood group, Color blindness, and Albinism) and confirm that height shows the continuous variation expected for polygenic inheritance.