17. Inheritance

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17.1 Chromosomes, genes and proteins

A chromosome is a long, thread-like structure made of DNA. The DNA carries genetic information in sections called genes: cell → nucleus → chromosome → DNA → gene.

A chromosome therefore contains many genes at particular positions. A gene is part of a DNA molecule; it is not a separate substance attached to the chromosome.

DNA is the material, a chromosome is one organised DNA structure, and genes are information-carrying lengths within that DNA.

A gene is a length of DNA that codes for a protein.

Its sequence provides the information needed to place amino acids in the order for that protein. Different proteins can have different roles, such as enzymes or membrane carriers.

A gene does not directly code for a whole characteristic. It codes for a protein, and the protein can contribute to the characteristic.

An allele is an alternative form of a gene.

The gene names the inherited instruction—for example, a gene affecting a particular protein. Alleles are different versions of that same instruction and may have different DNA base sequences.

Two alleles are not two unrelated genes. They are alternative forms at the same gene position on corresponding chromosomes.

Human females usually have XX sex chromosomes and males usually have XY. Every egg carries X, while sperm carry either X or Y.

	sperm X	sperm Y
egg X	XX (female)	XY (male)
egg X	XX (female)	XY (male)

Fertilisation therefore gives an expected 1:1 ratio: a sperm carrying X produces XX, while a sperm carrying Y produces XY. The sperm from the father determines which combination forms.

The 1:1 ratio is a probability across many fertilisations, not a guarantee that every family has equal numbers of girls and boys.

The sequence of bases in a gene determines the sequence of amino acids used to make a specific protein.

gene base sequence → information copied into mRNA → amino acids assembled in a particular order → specific protein

It is the order of bases—not merely the number of bases—that carries the information. Detailed nucleotide structure is outside this syllabus objective.

Different sequences of amino acids cause protein molecules to fold into different shapes. The amino-acid order affects the attractions and bonds within the chain, so it determines the final three-dimensional shape.

Shape controls function. For example, an enzyme needs an active site with a complementary shape to its substrate, and a receptor needs a binding site that fits its signalling molecule.

A changed amino-acid sequence may change a protein's shape and function; it does not simply make the same protein shorter or longer.

DNA controls cell function by controlling which proteins the cell produces. A gene's base sequence specifies an amino-acid sequence; that sequence gives a protein its shape, and the shape enables its function.

Protein type	How it affects cell function
enzyme	catalyses a particular reaction
membrane carrier	moves a particular substance across a membrane
neurotransmitter receptor	binds a particular neurotransmitter and allows a response

DNA controls these functions through protein production; the DNA molecule does not itself catalyse reactions or carry substances across the cell membrane.

A cell makes a protein by copying a gene's information into messenger RNA (mRNA), then using a ribosome to assemble amino acids in the specified order.

The gene coding for the protein remains in the nucleus.
An mRNA copy of the gene is made in the nucleus.
The mRNA moves into the cytoplasm and passes through a ribosome.
The ribosome assembles amino acids into a protein.
The sequence of bases in mRNA determines the specific amino-acid sequence.

mRNA carries the copied information; the ribosome is the assembly site. The DNA stays protected in the nucleus while its information is used in the cytoplasm.

At this level, the detailed molecular stages called transcription and translation are not required.

Most body cells in an organism contain the same genes, but they do not use all of them. A particular cell expresses only the genes needed to make the proteins for its function; many other genes are not expressed.

A neurone and a phagocyte contain the same set of genes, but they make different combinations of proteins. This produces different structures and specialised functions.

Specialised cells usually differ because different genes are switched on or off—not because each cell type has a completely different set of genes.

A haploid nucleus contains a single set of chromosomes. The symbol n represents the haploid number.

Human gametes are haploid: an egg or sperm nucleus contains 23 chromosomes, one chromosome of each type.

Haploid means one complete set, not one chromosome in total and not one chromosome pair.

A diploid nucleus contains two sets of chromosomes. The symbol 2n represents the diploid number.

One set was inherited from the mother and one from the father. A zygote and most body cells are diploid.

Diploid means two complete sets, not simply any nucleus containing two chromosomes.

In a diploid cell, there is a pair of each type of chromosome: one member of each pair came from each parent. The two chromosomes in a pair carry the same types of genes at corresponding positions, though their alleles may differ.

A human diploid cell has 23 pairs, so it has 46 chromosomes in total. A human haploid gamete has one chromosome from each pair, so it has 23 chromosomes.

‘23 pairs’ means 46 individual chromosomes. It is different from the haploid number of 23 individual chromosomes.

17.2 Mitosis

Mitosis is nuclear division that gives rise to genetically identical cells.

The copied genetic information is shared equally, so each daughter nucleus receives the same chromosome set and genetic information as the parent nucleus.

Mitosis describes nuclear division. Cell division usually follows, but detailed names and appearances of mitotic stages are not required here.

Mitosis produces genetically identical cells, so it can increase cell number without changing the genetic instructions passed to the new cells.

Role	Why identical new cells are useful
growth	increases the number of cells in an organism
repair of damaged tissues	supplies cells to rebuild the damaged region
replacement	replaces worn-out or dead cells with the same cell type
asexual reproduction	produces offspring from one parent without gamete fusion

Production of gametes is a role of meiosis, not mitosis. Growth by mitosis increases cell number; it does not simply make every existing cell larger.

Before mitosis begins, every chromosome is replicated exactly. This makes a copy of the DNA in each chromosome, so the DNA mass in the cell doubles before nuclear division.

The two copies are needed because they will later be separated into different daughter nuclei. Exact replication preserves the same genetic information in both products.

Replication happens before mitosis, not during the separation itself. Doubling DNA does not double the chromosome number: each replicated chromosome consists of two copies awaiting separation.

During mitosis, the copies of each chromosome separate and move into different daughter nuclei. Each daughter nucleus therefore receives one complete copy of every chromosome.

The chromosome number is maintained: if the parent nucleus has 16 chromosomes, each daughter nucleus also has 16—not 8 or 32.

The DNA was copied beforehand so it can be divided between two nuclei. Mitosis separates copies; it does not halve the chromosome number as meiosis does.

Stem cells are unspecialised cells that divide by mitosis to produce daughter cells that can become specialised for particular functions.

stem cell → mitosis → genetically identical daughter cells → some cells specialise by expressing the genes needed for a particular cell type

In an embryo, stem cells supply cells for growth and the formation of different tissues. In tissues such as skin or stomach lining, they can produce replacements for cells that are lost or damaged.

A stem cell is defined by being unspecialised and able to produce cells that can specialise. It is not already a specialised cell, and it divides by mitosis rather than meiosis.

17.3 Meiosis

Meiosis is involved in producing gametes for sexual reproduction. In humans, it produces sperm in the testes and eggs in the ovaries; in flowering plants, it is involved in producing the cells that lead to pollen and egg cells.

Gametes must contain one set of chromosomes so that fusion of two gamete nuclei at fertilisation restores two sets in the zygote.

Meiosis produces cells for sexual reproduction. Mitosis produces genetically identical cells for growth, repair, replacement and asexual reproduction.

Meiosis is reduction division: a diploid cell divides to produce haploid cells whose chromosome number is half that of the parent cell. The cells produced are genetically different from one another.

For example, if a diploid parent cell has 24 chromosomes, each haploid product has 12. In humans, the change is from 46 chromosomes in a diploid cell to 23 in a gamete.

Halving the number prevents chromosome number from doubling in every generation. Genetic differences among gametes contribute to variation after fertilisation.

Reduction refers to chromosome number changing from diploid to haploid—not chromosomes becoming smaller. The detailed stages of meiosis are not required for this objective.

17.4 Monohybrid inheritance

Inheritance is the transmission of genetic information from generation to generation.

Parents pass alleles to offspring in their gametes. At fertilisation, the offspring receives one allele of each gene from each parent, creating its genotype.

Inheritance refers to genetic information passed between generations, not features acquired during an organism's lifetime.

Genotype is an organism's genetic make-up, described in terms of the alleles it has for a gene.

If a gene has alleles T and t, possible diploid genotypes are TT, Tt and tt. The two symbols represent the two alleles, one inherited from each parent.

Genotype names the allele combination, not the visible feature. Tt is a genotype; tall is a phenotype.

Phenotype means the observable features of an organism.

Phenotype can result from the genotype and, for some features, environmental effects. In a simple dominant–recessive example, TT and Tt may both give the tall phenotype while tt gives dwarf.

A phenotype is what can be observed or measured; it is not the pair of allele symbols carried by the organism.

An organism is homozygous for a gene when it has two identical alleles of that gene.

With alleles B and b, BB is homozygous dominant and bb is homozygous recessive. Both contain a matching pair.

Homozygous does not always mean dominant: both BB and bb are homozygous.

Two identical homozygous individuals that breed together are pure-breeding for that gene because each parent can pass on only one allele type.

BB × BB produces only BB offspring; bb × bb produces only bb offspring. Under the same conditions, the relevant phenotype therefore remains consistent across offspring.

Pure-breeding refers to the specified gene or characteristic. It does not mean the individuals are genetically identical at every gene.

An organism is heterozygous for a gene when it has two different alleles of that gene.

For alleles T and t, the heterozygous genotype is Tt. It can make gametes carrying T and gametes carrying t.

Tt is one genotype containing two different alleles; it is not a mixture of the two parental phenotypes.

A heterozygous individual is not pure-breeding because it can pass either of its two different alleles to its offspring.

A Tt parent makes T and t gametes. When suitable parents are crossed, this can produce offspring with different genotypes and may reveal more than one phenotype.

Showing a dominant phenotype does not prove an individual is pure-breeding: both TT and Tt can show it, but only TT is homozygous dominant.

A dominant allele is expressed in the phenotype whenever it is present in the genotype.

If T is dominant, both TT and Tt express the dominant phenotype. One copy of T is sufficient for expression in this simple monohybrid model.

Dominant does not mean common, stronger or better. It describes expression in a heterozygous genotype.

A recessive allele is expressed only when no dominant allele of the same gene is present.

If t is recessive to T, the recessive phenotype appears in tt. A Tt individual carries t but shows the dominant phenotype.

A recessive allele is not absent from a heterozygote; it is present but its phenotype is not expressed while the dominant allele is present.

A pedigree traces a characteristic through a family. Squares represent males, circles represent females, a horizontal line joins parents, and vertical lines lead to offspring. Shading is defined by the diagram's key.

Read the key before assigning a phenotype.
Follow relationships one generation at a time.
Use parent–offspring combinations to test possible genotypes.
Mark only genotypes forced by the evidence; keep alternatives when the pedigree does not decide.

Shading does not automatically mean dominant, recessive or diseased. Its meaning and the inheritance model must come from the key and family evidence.

For a monohybrid cross: 1. choose allele symbols and state dominance; 2. write parental phenotypes and genotypes; 3. list each parent's gametes; 4. combine one allele from each parent; 5. translate offspring genotypes into phenotypes; 6. state the phenotypic ratio.

Cross	Offspring phenotypic ratio
Tt × tt	1 dominant : 1 recessive
Tt × Tt	3 dominant : 1 recessive

These ratios are expected probabilities across many offspring, not a guarantee for a small family. This Core objective is limited to 1:1 and 3:1 phenotypic ratios.

A Punnett square shows every possible combination of parental gamete alleles when a cross can produce more than one genotype.

Put one parent's gametes across the top and the other parent's down the side. Fill each cell by combining its row and column alleles. For Tt × Tt, the cells are TT, Tt, Tt and tt, giving a 1:2:1 genotype ratio.

Each square shows a possible fertilisation, not four actual children. Repeated cells indicate a higher probability of that genotype.

A test cross identifies whether an individual with a dominant phenotype is homozygous dominant or heterozygous by crossing it with a homozygous recessive individual.

Unknown parent	Cross with tt	Expected offspring
TT	TT × tt	all Tt, all dominant phenotype
Tt	Tt × tt	about 1 dominant : 1 recessive

A large number of offspring makes the inference more reliable. A small set containing only dominant offspring does not prove the unknown parent is TT.

Codominance occurs when both alleles in a heterozygous organism contribute to its phenotype.

If red-hair and white-hair alleles are codominant, a heterozygote can show both red and white hairs. Neither allele is hidden by the other.

Codominance means both allele effects contribute. It is not the same as a dominant allele completely masking a recessive allele.

The ABO blood-group gene has three alleles: Iᴬ, Iᴮ and Iᵒ. Iᴬ and Iᴮ are codominant with each other, while Iᵒ is recessive to both.

Phenotype	Possible genotype(s)
A	IᴬIᴬ or IᴬIᵒ
B	IᴮIᴮ or IᴮIᵒ
AB	IᴬIᴮ
O	IᵒIᵒ

For IᴬIᵒ × IᴮIᵒ, the possible offspring are IᴬIᴮ, IᴬIᵒ, IᴮIᵒ and IᵒIᵒ: blood groups AB, A, B and O.

A and B describe phenotypes, while Iᴬ and Iᴮ are alleles. Blood group O requires two Iᵒ alleles.

A sex-linked characteristic is controlled by a gene located on a sex chromosome, making the characteristic more common in one sex than the other.

For an X-linked recessive allele, a male has only one X chromosome, so one recessive allele on that X is expressed. A female usually needs the recessive allele on both X chromosomes to show the characteristic; a heterozygous female is a carrier.

Sex-linked does not mean carried only by males or located automatically on the Y chromosome. The chromosome location must be stated.

Red-green colour blindness is an example of sex linkage. The gene is on the X chromosome, and the colour-blindness allele is recessive.

Using Xᴮ for normal colour vision and Xᵇ for colour blindness: XᵇY is a colour-blind male; XᴮXᵇ is a female carrier with normal colour vision; XᵇXᵇ is a colour-blind female.

A father gives his Y chromosome, not his X, to a son. Therefore an X-linked colour-blindness allele cannot pass directly from father to son.

Use the same genetic-diagram sequence—parental genotypes, gametes, offspring genotypes, phenotypes and ratio—but preserve codominant or sex-chromosome notation throughout.

Cross type	Essential notation move
codominance	keep both allele superscripts, because both may contribute to the heterozygous phenotype
sex linkage	attach each allele to X and include Y as a separate male gamete

Codominance: CᴿCᵂ × CᴿCᵂ gives 1 red : 2 codominant : 1 white. X-linked recessive: XᴮXᵇ × XᴮY gives daughters XᴮXᴮ or XᴮXᵇ and sons XᴮY or XᵇY, so one quarter of all predicted offspring are colour-blind sons.

Calculate phenotype probabilities only after separating genotypes correctly. For sex-linked crosses, a probability among all offspring is different from a probability conditional on the child being male or female.