D3.2 Inheritance

Question 1

D3.2.1 Production of haploid gametes in parents and their fusion to form a diploid zygote as the means of inheritance

Answer 1

In sexually reproducing eukaryotes, inheritance happens through the production of haploid gametes in the parents and their fusion to form a diploid zygote. This core pattern is shared by almost all eukaryotes that have a sexual life cycle, whether they are animals, plants, fungi, or protists. Those with a sexual life cycle switch between diploid and haploid stages: they produce haploid gametes by meiosis, which then fuse to form a diploid zygote, repeating the pattern in each generation.

A haploid cell has one set of chromosomes (symbolised as n).
In humans, gametes (sperm and egg) are haploid and contain 23 chromosomes each, instead of the 46 found in body (somatic) cells.

Gametes are the specialised sex cells (sperm in males, eggs/oocytes in females) that fuse during fertilisation. They are haploid and carry genetic material (DNA) from one parent to the next generation.

“parents” means the adult organisms that produce gametes.
Each parent contributes one gamete (and therefore one set of chromosomes) to form the zygote.

Meiosis is a type of cell division that produces haploid gametes from diploid germ‑line cells in the gonads (testes or ovaries). It involves one round of DNA replication followed by two rounds of division, reducing the chromosome number by half and shuffling alleles (through crossing over and independent assortment).

A diploid cell has two sets of chromosomes (symbolised as 2n).
In humans, body cells are diploid and have 23 pairs of chromosomes (46 total), including two copies of each autosomal gene.

A zygote is the single cell formed when two haploid gametes fuse during fertilisation. It is diploid and contains genetic material from both parents, and it undergoes mitosis to develop into a multicellular organism.

“Fusion” refers to the joining of the nuclei of two gametes (e.g., sperm nucleus and egg nucleus). This fusion restores the diploid chromosome number and combines alleles from both parents.

An autosomal gene is a gene located on an autosome (a non‑sex chromosome). In diploid cells, there are two alleles for each autosomal gene, one on each homologous chromosome, one inherited from each parent.

Inheritance means the passing of genes and alleles from parents to offspring via gametes and fertilisation. Because gametes are haploid and fuse randomly, offspring inherit combinations of alleles that can differ from both parents and from each other.

The syllabus line is mainly checking that you understand:
- A diploid somatic cell has two copies of each gene (one on each homologous chromosome), and
- Gametes are haploid, so they have only one copy of each gene, and
- Fusion of gametes at fertilisation restores the diploid state.
*In other words, they want you to know that you inherit one allele for a gene from each parent.

Each parent starts with two copies of each gene in their diploid cells. Through meiosis, they produce haploid gametes that carry only one copy of each gene (so the chromosome number, and hence the number of gene copies, is halved). When two gametes fuse at fertilisation, the resulting zygote has two copies of each gene again—one from each parent—restoring the diploid state.

Diploid → haploid → diploid
The parent’s body cells are diploid (2 copies of each gene).
Meiosis produces haploid gametes (1 copy of each gene).
Fertilisation fuses two haploid gametes to form a diploid zygote (2 copies of each gene again).

Question 2

D3.2.2 Methods for conducting genetic crosses in flowering plants

Answer 2

to understand how genetic crosses are set up in flowering plants and how we track inheritance across generations.

the meiosis process is the same in flowering plants as in humans, even though the structures and life cycle look different. Same meiosis → same haploid gametes → same diploid zygote, just packaged in flowers and pollen instead of testes/ovaries.

Diploid → haploid → diploid in plants where the parent plants are diploid; their flower organs produce haploid gametes.
Pollen carries the male gametes, and the female gametes are inside ovules in the ovary. Pollination brings pollen to the stigma, so sperm cells can fertilise the egg and form a diploid zygote, which develops into a seed.

The P generation (parental generation) is the first pair of plants crossed. Their offspring are the F1 generation (first filial).
When F1 plants are crossed (often with each other), the next offspring are the F2 generation.

A Punnett grid is a simple diagram that shows all possible combinations of gametes from two parents. Each box in the grid represents a possible genotype of the offspring, helping you predict ratios of traits in F1 and F2.

*Self‑pollination and self‑fertilisation - In plants like peas, each flower has both male (anthers) and female (ovary) organs, so one plant can self‑pollinate (pollen lands on its own stigma). This leads to self‑fertilisation, which is useful for producing true‑breeding lines and controlled crosses.

Why genetic crosses matter - Genetic crosses are used widely in plant breeding to combine desired traits (e.g., disease resistance, flower colour, yield) and create new varieties of crops or ornamental plants.

By following P → F1 → F2 and using Punnett grids, breeders can predict and select which crosses will give the traits they want.

Question 3

D3.2.3 Genotype as the combination of alleles inherited by an organism

Answer 3

Genotype = combination of alleles
Genotype is the set of alleles an organism has for a particular gene (or genes). For example, in a pea‑plant height gene, the genotype could be TT, Tt, or tt, where each letter represents one allele.

this is hidden information in the DNA and is usually found through genetic sampling (e.g., DNA sequencing, PCR, or pedigree analysis), not by just looking.

Homozygous vs heterozygous - Homozygous means an organism has two identical alleles for a gene (e.g., TT or tt). Heterozygous means it has two different alleles (e.g., Tt).

Genes vs alleles - A gene is a section of DNA that codes for a particular trait (e.g., a gene for flower colour). An allele is a specific version of that gene (for example, an allele for purple flowers vs an allele for white flowers).

So, a genotype describes which two alleles (same or different) an organism inherited for a given gene, and whether it is homozygous or heterozygous for that gene.

Question 4

D3.2.4 Phenotype as the observable traits of an organism resulting from genotype and environmental factors

Answer 4

The phenotype is the observable expression of the genotype, as it appears in the organism’s traits.

Phenotype is the observable characteristics of an organism, such as height, eye colour, or flower colour. It results from the interaction between genotype (genes/alleles) and environmental influences.

Traits mainly due to genotype:
These are traits that are determined almost entirely by genes and change very little with environment. Examples in humans: Blood group (ABO system) – controlled by specific alleles; environment does not change it. Eye colour (in simple cases) – mainly decided by inherited alleles, though lighting can affect appearance.

Traits mainly due to environment: These are traits that are strongly shaped by surroundings, nutrition, lifestyle, or experience, even though the underlying genes are present.
Examples in humans: Scars – from injury; not written in the DNA. Tanned skin – caused by UV exposure; a person’s genes set the potential for tanning, but the tan itself is environmental.

Traits from genotype & environment interaction: most traits are shaped by both genes and environment. Examples in humans: Height – genes set a range, but nutrition, illness, and hormones during growth determine final height. Body mass / obesity risk – some genotypes increase susceptibility, but diet and exercise strongly influence whether obesity develops. Skin cancer risk – certain genotypes (e.g., fair‑skin alleles) increase risk, but UV exposure (environment) is essential for the disease to occur.

Question 5

D3.2.5 Effects of dominant and recessive alleles on phenotype

Answer 5

Dominant vs recessive alleles:
A dominant allele produces its phenotype even when only one copy is present.
A recessive allele only shows its phenotype when no dominant allele is present (i.e., when the genotype is homozygous recessive).

Why homozygous‑dominant and heterozygous give the same phenotype: Homozygous‑dominant (e.g., AA):
Two dominant alleles → dominant trait is fully expressed.
Heterozygous (e.g., Aa): One dominant and one recessive allele → the dominant allele masks the recessive one, so the phenotype is the same as AA.

A recessive allele is only expressed in the phenotype when the genotype is homozygous recessive (aa), meaning there is no dominant allele present to mask it.

Question 6

D3.2.6 Phenotypic plasticity as the capacity to develop traits suited to the environment experienced by an organism, by varying patterns of gene expression

Answer 6

Phenotypic plasticity means an organism can change its traits (its phenotype) in response to the environment, even though its genotype stays the same.

It is the ability to develop different phenotypes from the same genotype, depending on environmental conditions (e.g., temperature, light, nutrition, water). The changes come from different patterns of gene expression (turning some genes “on” or “off” more or less), not from mutations in the DNA sequence.

*The DNA sequence (genotype) does not change during phenotypic plasticity. Only which genes are expressed and how strongly they are expressed changes, which alters the observable traits.

Phenotypic plasticity means the same genotype expresses different traits in different environments (e.g., same plant genotype → thicker vs thinner leaves, or tanning in the same person). The genotype doesn’t change, but gene expression and environment alter the visible trait. A recessive allele can still be present in the genotype during plasticity, but the environment isn’t “making the recessive allele suddenly appear”; it’s changing how genes are expressed in general.

Many plastic changes are reversible: if the environment changes back, the trait can change back too.

Question 7

D3.2.7 Phenylketonuria as an example of a human disease due to a recessive allele

Answer 7

Phenylketonuria (PKU) is a recessive genetic disorder that affects how the body handles the amino acid phenylalanine.

PKU is caused by a mutation in the PAH gene, which is located on an autosome (chromosome 12), or non-sex gene. Because the gene is autosomal, PKU affects males and females equally—it does not depend on whether you are XY or XX.

“Autosomal recessive” means:
You must inherit two recessive mutant alleles (one from each parent) to have the disease.
If you have only one mutant allele, you are a healthy carrier (you don’t have PKU).
This pattern explains why:
Two unaffected parents can have a child with PKU (if both are carriers). The chance of PKU in their children follows a predictable 25% pattern in a typical carrier–carrier cross.

PKU is caused by a mutation in an autosomal gene that codes for the enzyme phenylalanine hydroxylase. This enzyme normally converts phenylalanine (an amino acid found in protein‑rich foods) into tyrosine (another important amino acid).
When the enzyme is defective or missing, phenylalanine builds up to toxic levels in the blood and brain, which can cause intellectual disability and other neurological problems if not treated.

How it is managed
Early newborn screening can detect high phenylalanine levels.
Treatment is mainly a strict low‑phenylalanine diet (low‑protein foods, special formula) to prevent the harmful buildup and allow normal brain development

*there are also autosomal dominant diseases, meaning the gene is on an autosome (not a sex chromosome), so males and females are affected equally, and “dominant” means that one copy of the mutant allele is enough to cause the disease; the person is usually heterozygous (Aa).
Therefore, an affected person often has one affected parent, and each child of that person has about a 50% chance of inheriting the disease‑causing allele.
some well‑known examples include: Huntington’s disease (neurodegenerative disorder with movement and cognitive problems); Achondroplasia (a common form of dwarfism).

Contrast with autosomal recessive (like PKU)
Autosomal recessive: need two mutant alleles (aa) to have the disease; carriers with one allele (Aa) are unaffected.
Autosomal dominant: one mutant allele (Aa) is enough to cause disease; there is often no “silent carrier” phase in the same way.

Question 8

D3.2.8 Single-nucleotide polymorphisms and multiple alleles in gene pools

Answer 8

genetic variation is stored in the gene pool and how individuals “sample” it.

The gene pool is the total set of all alleles for a gene in a population, or the total collection of all alleles for all genes in an interbreeding population.

For many genes, there can be more than two alleles in the gene pool (multiple alleles).
(The human ABO blood‑group gene has three main alleles in the gene pool: Iᴬ, Iᴮ, and i. A person can only have two of these alleles (e.g., IᴬIᴬ, Iᴬi, IᴬIᴮ, etc.), but the whole population carries all three.)

A single‑nucleotide polymorphism (SNP) is a difference in one DNA base at a specific position between individuals (e.g., an A vs a G at one point in a gene). SNPs are a common source of multiple alleles; each different SNP variant can count as a different allele of that gene in the gene pool.

Even if the gene pool holds many alleles (including many SNPs), a diploid individual can only inherit two alleles for that gene (one from each parent). This is why people can be homozygous (two identical alleles) or heterozygous (two different alleles), but never have three or more for one autosomal gene.

example: Siblings share the same gene pool (the same parents = the same set of possible alleles). But each sibling inherits a different combination of two alleles for the blood‑type gene, so they can have different blood types.

Question 9

D3.2.9 ABO blood groups as an example of multiple alleles

Answer 9

For the ABO gene, there are three main alleles in the gene pool:
Iᴬ (allele for A antigen),
Iᴮ (allele for B antigen),
i (allele for no antigen, “O”).
Even though there are three alleles in the population, each person is diploid, so they can only inherit two alleles (one from each parent).
Iᴬ and Iᴮ are co‑dominant to each other: when both are present, both A and B antigens are made → blood type AB.
i is recessive to both: it only shows as blood type O when the genotype is ii (no Iᴬ or Iᴮ).

The combination of Iᴬ, Iᴮ, and i determines the four blood‑group phenotypes (A, B, AB, O).
A = IᴬIᴬ or Iᴬi
B = IᴮIᴮ or Iᴮi
AB = IᴬIᴮ
O = ii

Question 10

D.3.2.10 Incomplete dominance and codominance

Answer 10

A heterozygote is an organism that has two different alleles (one dominant and one recessive) for a gene (e.g., Rr, not RR or rr).
In standard dominance, the dominant allele “wins” and the phenotype is just the dominant one (e.g., Rr = red, same as RR).

“intermediate phenotype” – with incomplete dominance, the heterozygote does not look like either parent; instead it looks in‑between.

*four o’clock flower (Mirabilis jalapa) flower gene where:
Allele R = red flower
Allele r = white flower
Then:
RR → red flowers
rr → white flowers
Rr → pink flowers
So the heterozygote (Rr) has an intermediate phenotype: not pure red, not pure white, but something in the middle (pink).
- at the phenotypic level, you see a blended or in‑between trait in the hybrid.

In codominance, the heterozygote shows both parental phenotypes at once, not a blend.
Example: AB blood type (ABO system)
Alleles Iᴬ and Iᴮ are codominant.
Genotype IᴬIᴮ makes both A antigens and B antigens on the red blood cells → blood type AB.
So in the heterozygote you don’t see an “in‑between” antigen; instead you see both A and B expressed together.

Incomplete dominance: heterozygote = intermediate (e.g., red + white → pink).
Codominance: heterozygote = dual expression (e.g., A and B alleles → AB blood type with both A and B antigens).

***Codominance and incomplete dominance are both features of heterozygotes, not of homozygotes.

Question 11

D3.2.11 Sex determination in humans and inheritance of genes on sex chromosomes

Answer 11

the sex chromosome present in the sperm (X or Y) determines whether the zygote becomes genetically male (XY) or female (XX), and this then triggers the development of male‑typical or female‑typical physical traits.

In humans, females are XX and males are XY. All egg cells carry one X chromosome, but sperm can carry either an X or a Y chromosome.
Egg (X) + sperm (X) → XX zygote → develops typical female characteristics.
Egg (X) + sperm (Y) → XY zygote → develops typical male characteristics.

SRY gene on the Y chromosome:
The Y chromosome contains a key gene called SRY (Sex‑determining Region Y), which acts as a “master switch.” If SRY is present and active, it triggers the development of testes and other male‑typical structures; without it, the default pathway is towards female‑typical development.

The X chromosome is much larger than the Y and carries hundreds of genes (for many proteins and traits). The Y chromosome is small and carries far fewer genes, mainly those involved in male sex determination and sperm production. This means:
Females (XX) have two copies of each X‑linked gene.
Males (XY) have only one copy of each X‑linked gene and one copy of each Y‑linked gene.

Because males have only one X chromosome, X‑linked recessive conditions (like haemophilia or red–green colour blindness) are more common in males: A male needs only one recessive allele on his X chromosome to show the disease. A female usually needs two recessive alleles to show it; otherwise she is an unaffected (or less affected) carrier (the chance that a female will be affected is very low, because a female needs two mutant X‑linked alleles, one from each parent, to express the disease where two mutant X chromosomes is a low‑probability event), and most of the time, if she has only one mutant allele, she is a carrier and not affected or only very mildly affected.

For X‑linked dominant conditions, the pattern is different from X‑linked recessive, and males and females can both be affected, though the details change depending on who inherits the allele. Here, the disease‑causing allele is on the X chromosome, and one copy is enough to cause the disorder (like a dominant allele). This means, for affected male (XY), his single X has the mutant allele → he has the disease. For affected female (XX), only one X needs the mutant allele for her to be affected (she can be either heterozygous or homozygous).
If the father has the X‑linked dominant condition, he passes his X chromosome with the mutant allele to all his daughters → all daughters are affected. He passes his Y chromosome to his sons → sons are unaffected (they never get his X).
If the mother has the X‑linked dominant condition, each child (son or daughter) has a 50% chance of inheriting the mutant X and being affected.

How this differs from X‑linked recessive – In X‑linked recessive, males are affected much more often; females often need two mutant alleles. In X‑linked dominant, both males and females can be affected. There is no male‑only pattern; in fact, no male‑to‑male transmission is a hallmark (fathers never pass an X‑linked trait to sons).

Question 12

D3.2.12 Haemophilia as an example of a sex-linked genetic disorder

Answer 12

Haemophilia (A or B) is a classic example of an X‑linked recessive, sex‑linked genetic disorder.
Haemophilia is a bleeding disorder in which blood clotting is impaired because of a mutation in a gene for a clotting factor (factor VIII in haemophilia A, factor IX in haemophilia B).
The gene is located on the X chromosome, so it is sex‑linked and follows X‑linked recessive inheritance.

you use superscript letters on an uppercase X to show the alleles:
Let Xᴴ = X chromosome with the normal (dominant) allele (health‑producing).
Let Xʰ = X chromosome with the mutant (recessive) allele causing haemophilia.

Unaffected male: XᴴY (has the normal X allele).
Affected male: XʰY (has the mutant X allele; no second X to mask it).
Unaffected female (carrier): XᴴXʰ (one normal, one mutant; usually healthy or mildly affected).
Affected female (rare): XʰXʰ (two mutant alleles, low‑probability situation).

*Males are mainly affected because they have only one X chromosome, therefore only one allele for any X‑linked gene.
The Y chromosome carries different genes and no matching copy of most X‑linked genes. If that single X carries a recessive disease‑causing allele (Xʰ), there is no second X with a normal allele to “mask” it – Males literally cannot mask an X‑linked recessive disorder, and that’s why they usually show it if they inherit the mutant allele.

Question 13

D3.2.13 Pedigree charts to deduce patterns of inheritance of genetic disorders

Answer 13

A pedigree is a family tree where individuals are shown as symbols (squares = males, circles = females), and shaded symbols = affected by the disorder.
By tracing who has the disease and who doesn’t across generations, you can work out whether the trait is: autosomal dominant, autosomal recessive, X‑linked recessive, X‑linked dominant.

You look at some cases (a few people in the pedigree) and see a pattern (for example, lots of affected males and no male‑to‑male transmission).
From that, you draw a general conclusion: “This is probably X‑linked recessive.”
This is theory‑building from observations—scientists often use induction when they base a general inheritance pattern on part of the pedigree.
OR
You start with a general rule (e.g., “This is autosomal recessive”).
Then you apply that rule to specific individuals and deduce their genotypes (e.g., “This unaffected parent must be a heterozygous carrier”).
Deduction goes from general → specific.

1. For short‑answer and multiple‑choice (Paper 1)
   You are usually expected to deduce (work out) the mode of inheritance or genotypes step‑by‑step from the pedigree.
   Example:
   “Two unaffected parents → affected child” → you deduce “this must be recessive, and the parents must be carriers.”
   You don’t need to say “inductive” or “deductive” in Paper 1; you just use the logic.

For NOS (Nature of Science) or “evaluate/explain” questions (often Paper 2 or IA‑style questions), you may be asked explicitly about inductive vs deductive reasoning.
Inductive: Using a few people in the pedigree to infer a general pattern (e.g., “many affected males, no male‑to‑male transmission → probably X‑linked recessive”).
Deductive: Starting from that general pattern and working out specific genotypes for named individuals.
inductive = general pattern from specific cases,
deductive = specific genotype conclusions from that general rule.

Genetic basis for prohibiting marriage between close relatives:
Close relatives (like first cousins) are more likely to share the same recessive alleles from common ancestors.
If both parents carry the same recessive mutant allele, their children have a higher chance of being homozygous recessive and showing a recessive disorder.
So, in many societies, laws or customs prohibit or discourage close‑relative marriage to reduce the risk of rare recessive genetic disorders being expressed in offspring.

Question 14

D3.2.14 Continuous variation due to polygenic inheritance and/or environmental factors

Answer 14

Continuous variation = a trait with a smooth range of values; you can’t put most people into a small set of categories.
Example: skin colour (light → many shades of intermediate → dark) or height.
Skin colour = continuous variable (polygenic + environment).

Discrete (categorical) variation = a trait with a limited number of clear categories.
Example: ABO blood group (only A, B, AB, or O; you can list them all).
ABO blood group = discrete variable (controlled by one gene with multiple alleles).

Human skin colour is largely due to the amount of the pigment melanin in the skin.
Many genes (estimates range from several to dozens) each add a small amount of melanin production, so total skin darkness is additive (polygenic inheritance).
Environment (especially UV exposure / sun tanning) can also change skin colour, adding to continuous variation.
For a graph:
X‑axis: “degree of skin darkness”;
Y‑axis: number of people → you get a bell‑shaped curve (normal distribution), not separate bars.

*Role of environment:
Even with the same genetic “settings”, someone who lives in a sunny area may have darker skin than a genetically similar person living in a cloudy area.

This means: Polygenic genes define a person’s baseline skin colour; Environment can shift it around that baseline → more continuous spread in the population.

Because continuous traits form a distribution, scientists use measures of central tendency to describe the “average”:
- Mean = arithmetic average (sum of all values ÷ number of individuals).
- Median = middle value when data are ordered from low to high.
- Mode = most frequent value (peak of the distribution).
For a class measuring skin colour (e.g., via a colour scale), you would:
Collect all scores,
Put them on a number line,
Work out the mean, median, and mode to summarise where most people lie.

Question 15

D3.2.15 Box-and-whisker plots to represent data for a continuous variable such as student height

Answer 15

A box‑and‑whisker plot (or box plot) is a way to show the spread and central tendency of a continuous variable, such as student height.

Six things a box‑and‑whisker plot shows
For a continuous variable like height in a class, the box plot displays:

Minimum – the smallest height value (excluding outliers).

First quartile (Q₁ / lower quartile) – the height below which 25% of the students fall.

Median (Q₂) – the middle height; 50% of students are shorter, 50% are taller.

Third quartile (Q₃ / upper quartile) – the height above which 25% of the students fall.

Maximum – the largest height value (excluding outliers).

Outliers – individual data points that lie far outside the main cluster.

How outliers are defined:
Interquartile range (IQR) = Q₃ – Q₁.
A value is treated as an outlier if:
It is less than Q₁ – 1.5 × IQR,
or greater than Q₃ + 1.5 × IQR.
These outliers are then plotted as individual dots or asterisks beyond the whiskers.

What the box and whiskers represent:
The box (between Q₁ and Q₃) shows the middle 50% of the data (the “core” range of student heights).
The whiskers extend from the box to the minimum and maximum values that are not outliers.
The line inside the box is the median; it shows where the centre of the distribution lies.

Why this is useful for student height
Student height is a continuous variable with a distribution (shortest → tallest).

A box‑and‑whisker plot compresses all the data into one clear picture, so you can quickly see:
How spread out the heights are,
Where the middle 50% lie,
Whether there are any unusually short or tall students (outliers).