A soft hyphen, or SHY, is the Unicode character U+00AD. It marks a preferred optional division inside a word and normally has no visible shape when that division is unused. Selecting it invokes the language-appropriate presentation for an intraword break, which can involve more than adding one hyphen.^[ref]

For example, a suitable renderer can show ex followed by SHY and ample as example on one line, or as ex- and ample on successive lines. The visible hyphen then belongs to the chosen layout. In languages with spelling changes at a division, the layout implementation may also need to modify the divided parts.^[ref]

A soft hyphen supplies a hyphenation opportunity; it does not force the line breaking algorithm to use it. The algorithm can retain the unbroken word when its typesetting line width and paragraph cost permit that choice.

A discretionary break describes an optional line breaking choice whose printed material may depend on whether the break is selected. In TeX, \discretionary{pre}{post}{none} supplies material before a chosen break, after it, or in the unbroken case.^[ref]

For a purely mechanical example, ab\discretionary{-}{X}{Y}cd can produce abYcd without the break, or ab- followed on the next line by Xcd when it is selected. The arbitrary strings make all three branches visible. A common hyphenation case simply inserts a hyphen before the break and supplies no material afterward.

Each alternative has its own geometry. A typesetting box used only in the pre-break branch must contribute its width when that branch is tested. The natural line width, typesetting glue and typesetting penalty calculation must therefore use the selected representation. Treating the choice merely as an invisible marker could miss the width of the inserted hyphen.

An HTML element is a structural component of an HTML document with a defined name, attributes and semantics. Paragraphs, headings and controls are examples. An HTML tag is source syntax that can identify the start or end of an element; the element itself is the object represented in the parsed Document Object Model.^[ref]

In <p>Hello <em>world</em>.</p>, the paragraph contains an emphasis element as well as text. The word world is inside the emphasis element, while Hello and the final period are other text in the paragraph. These parent-child relationships make the structure useful for navigation, editing and matching CSS selectors.

Not every element is written with an explicit start/end pair. For example, the line-break element br is a void element and has no end tag in HTML syntax. Conversely, two source strings can produce equivalent document structure because parsing accounts for omitted tags and other specified rules. Counting written tags is therefore not a reliable substitute for inspecting elements.^[ref]

An HTML element's semantic role and its display are connected through browser behavior but remain different descriptions. A paragraph can wrap across several visual lines, and a CSS property can change its appearance without replacing the paragraph element. Choosing structural elements for their meaning allows the same content to remain intelligible under different styles.

An HTML attribute supplies information about an HTML element, usually through a name and a value in its start tag. Its meaning is defined for the relevant element and attribute. The source notation is converted into attribute data on the element in the Document Object Model.^[ref]

For example, <p id="notice" class="warning">Save</p> gives the paragraph the identifier notice and the class warning. CSS selectors #notice and .warning can then match it for different reasons. The text Save is element content, rather than another attribute value.

Some attributes use presence as their boolean signal. For an applicable HTML boolean attribute, disabled and disabled="false" both express presence; the latter spelling does not make it false. Removing the attribute expresses absence. Treating every value as a programming-language boolean would therefore misread the document.^[ref]

Attribute data and live interface properties are related by specified reflection rules, not by a universal identity. For some controls, an attribute describes a default while the current state can change through interaction. A JavaScript property access must consequently be interpreted using that interface's definition. This is particularly useful when debugging why the original source and the state a user sees no longer look identical.

Standard gravity is the conventional reference accelerationexactly. It supplies a fixed reference for unit conversions and for expressing an acceleration as a multiple of $g_n$. It is distinct from the locally measured gravitational acceleration, which depends on location and height.^[ref]

For example, $2g_n=19.61330\mathrm{m}{\mathrm{s}}^{-2}$. If a vertical force on a mass of $2\mathrm{kg}$ is calculated using this reference, $F=m{g}_n=19.61330\mathrm{N}$. The same calculation using a measured local value $g_{\mathrm{local}}$ gives $F=m{g}_{\mathrm{local}}$ instead. The reference value's exactness does not remove measurement uncertainty from a real force or from the actual local gravitational field.

One can measure local gravitational acceleration by tracking a body in free fall with an optical interferometer, taking $z$ positive downward and fitting its vertical position to $z(t)=z_0+v_{0}t+\frac{1}{2}g{t}^2$ after instrumental corrections. Such an experiment estimates $g$ at the apparatus; it does not determine the already stipulated number $g_n$.^[ref]

A typographic ligature combines two or more written forms into a single glyph. A familiar example is an “fi” shape joining the end of “f” to the following “i”. Its use can improve spacing or avoid a collision between the two shapes.

An OpenType shaping system can first map the separate text characters “f” and “i” to their default glyph indices, then apply a glyph substitution that replaces that pair by one ligature glyph index. The stored text need not be replaced by a special ligature character. Selection and copying can therefore preserve the original two-character sequence even though the display contains one glyph.^[ref]

A typographic ligature also need not be produced merely by moving two glyphs closer together: substitution changes the selected form, whereas positioning changes where selected forms are drawn.

Glyph substitution replaces selected glyph indices by other glyph indices while laying out text. In OpenType, the GSUB table specifies these replacements after the default character-to-glyph index mapping. A substitution can replace one glyph by another, several by a typographic ligature, or one by several; its application can depend on surrounding glyphs.^[ref]

For a hypothetical computer font, suppose f and i initially select indices17 and23. A matching ligature rule could replace the pair by index91. These numbers belong to that particular computer font; they do not change the stored Unicode sequence into code point91. The example also shows why substitution differs from kerning: replacing the pair selects a new shape, whereas adjusting the pair's positions can retain both original shapes.

Lookup order matters when one substitution creates or removes the input pattern for a later substitution. Text shaping must therefore interpret the applicable OpenType rules, rather than independently choosing a default glyph for each text character.

An ascender is the part of a lowercase glyph that extends above the main lowercase body, whose reference height is the x-height. Familiar examples occur in “b”, “d” and “k”. It is a feature of a particular drawn form, rather than a promise that every glyph in the font reaches the same height.^[ref]

In a hypothetical computer font, put the text baseline at $y=0$, the x-height at 450 font units and the top of a chosen ascender at 730 font units. That part extends 280 font units above the lowercase reference. With 1000 font units per em and a scale of 20 pixels per em, the corresponding ideal heights above the text baseline are 9 and 14.6 pixels.

This local geometry is distinct from the global font ascent used by a text-layout system. That metric can also accommodate accents or other tall forms. Glyph positioning must preserve the chosen text baseline while allowing these different bounding boxes; simply aligning the tops of all glyphs would change the intended alignment.

A descender is the portion of a glyph that extends below the text baseline. Lowercase “p”, “q” and “y” commonly have one; the exact form depends on the typeface. A descender describes the visible shape relative to the writing reference, rather than the distance through which the logical pen position advances.^[ref]

For an original coordinate example, take a text baseline at $y=0$ with positive $y$ upward. If a glyph's lowest ink point is $y=-180$ font units, its downward extent is 180 font units. At a scale of 20 pixels per em with 1000 font units per em, this becomes 3.6 pixels before grid fitting. The extent is positive even though the lowest coordinate is negative.

The font descent is a related global layout metric, with sign conventions that depend on the format or API. It need not equal the downward extent of each individual descender. Line height and neighbouring ink extents together determine whether forms on successive lines collide; the nominal font size alone does not decide that.

The x-height is the height of the main lowercase body of a typeface, conventionally associated with the letter “x”. It excludes the extra extent of ascenders and descenders; the cap height describes a different reference. Small optical overshoots mean that not every lowercase outline without an ascender has exactly the same ink height.^[ref]

Write $r$ for the x-height as a fraction of an em. At nominal scale $p$ pixels per em, its ideal scaled height is $rp$ pixels, before grid fitting. Equal font sizes do not force equal $r$, so changing the typeface can alter the apparent lowercase size.

To preserve a target height $h$ while switching to a design with ratio $r_2>0$, choose $p_2=h/r_2$. If the original ratio was $r_1$ at scale $p_1$, this becomesFor a hypothetical switch from $r_1=0.5$ at 16 pixels per em to $r_2=0.4$, the matching scale is 20 pixels per em. Both lowercase bodies then have ideal height 8 pixels. This is the geometry behind metric-based font-size adjustment when a fallback font replaces another.^[ref]

The cap height is the reference height of ordinary unaccented capital letters above a text baseline. It describes uppercase letter design, while the x-height concerns the main lowercase body. A font ascent can reserve more space for accents or other tall forms than the cap height alone requires.

OpenType records a nominal cap height in the sCapHeight field of its OS/2 metrics table, in font units. The metric is an approximation for the uppercase design, so taking the largest ink bounding box among every uppercase glyph need not give the intended value.^[ref]

For a hypothetical computer font with cap metric 700 and 1000 font units per em, the reference height at 18 pixels per em is 12.6 pixels before grid fitting. An ascent metric of 800 font units would instead reserve 14.4 pixels above the text baseline. These describe different geometric features of the same design.

Aligning a capital heading against an icon can therefore require choosing which reference matters: the text baseline, the nominal cap height, or a measured ink bounding box. Keeping those quantities separate makes the alignment rule reproducible.

Gene therapy uses delivery, replacement or modification of genetic material to change cellular function for a therapeutic purpose. It includes gene addition as well as some forms of genome editing; changing the inherited germline is not required. The delivered sequence, delivery system and target cell type are distinct components of the intervention. ^[ref]

An experimental example introduced a long-wavelength opsin into adult dichromatic squirrel monkeys. Behavioural colour-discrimination measurements changed after treatment, linking genetic delivery to a functional sensory outcome in that model. This was an adult primate experiment, not a demonstration that the same intervention works for every human visual disorder. ^[ref]

Genome editing deliberately changes the DNA sequence at selected sites in a genome. Changes can include a small insertion or deletion, replacement of a sequence, or introduction of a specific base change. This differs from merely measuring gene expression or transiently changing it without altering the underlying DNA.

In a common CRISPR-Cas9 approach, a guide RNA directs a nuclease to a matching DNA region, and cellular DNA repair determines the resulting sequence change. Repair can disrupt a gene or use a supplied template to introduce a designed change. Le Cong and colleagues demonstrated programmable targeting of endogenous mammalian sites and simultaneous editing at several sites in 2013.^[ref]

A targeted cut is therefore an intermediate operation, not proof of the intended edit. DNA sequencing of the resulting alleles distinguishes the planned change from unintended repair products.

Parthenogenesis is development of offspring from an egg without fertilization. It can be a regular reproductive mode or occur facultatively in a lineage that also reproduces sexually. The term describes the absence of fertilization; it does not by itself specify chromosome number or guarantee offspring genetically identical to the mother.^[ref]

A simple genetic example shows the distinction from cloning. Suppose a diploid mother has alleles $A$ and $a$ at one locus. If a haploid egg carrying $A$ restores diploidy by copying its own chromosome set, the offspring is $AA$, not the mother's $Aa$. An egg carrying $a$ would give $aa$. This illustrates one possible mechanism, not every route of parthenogenesis.

The 2006 Komodo-dragon study tested offspring from females isolated from males using microsatellite markers. The offspring carried maternal alleles but were homozygous at the tested loci and were not identical clones. Comparisons with potential fathers and the distribution of alleles across the clutch supported parthenogenesis rather than ordinary fertilization. One female later produced offspring sexually, showing that the observed reproductive mode was facultative.^[ref]

An isolated female producing eggs is therefore suggestive but not sufficient evidence: prior mating and sperm storage can be alternative explanations in suitable species. Developmental observations and genetic parentage evidence answer different parts of the question.

Genetic redundancy occurs when overlapping functions of different genes allow one to compensate for loss of another under specified conditions. A normal-looking single mutant can therefore conceal an important contribution to a process.

For a toy model, let $a,b\in \{0,1\}$ indicate whether either of two sufficient functions is present. The outputis $1$ for $(a,b)=(1,1),(1,0),(0,1)$ and $0$ only for $(0,0)$. This is a Boolean disjunction model of compensation, not a claim that all biological effects are binary.

The SHATTERPROOF genes give an experimental example. Individual SHATTERPROOF1 or SHATTERPROOF2 disruptions did not produce the novel opening defect detected in the double mutant, connecting overlapping regulatory functions to fruit dehiscence.^[ref]

The conclusion is conditional on the measured phenotype and environment. Compensation for opening a fruit does not establish identical gene expression, identical DNA sequences or interchangeability in every tissue.

INDEHISCENT, abbreviated IND, is an Arabidopsis thaliana regulatory gene whose encoded transcription factor helps specify the fruit tissues required for opening and seed dispersal. The gene name and its protein product are related uses of the name; an INDEHISCENT mutant is an altered organism, not a separate anatomical layer. A 2004 study identified IND and its role in patterning dispersal-related fruit cell types.^[ref]

The distinction between growth and identity is experimentally useful. The 2004 work showed that the action of FRUITFULL included preventing misplaced IND activity, rather than simply acting as a direct fruit-elongation switch.^[ref] A malformed short fruit can therefore reflect a change in tissue identity rather than failure of a generic growth process.

A 2016 study compared normal fruits with INDEHISCENT mutants and found an early defect in the cell divisions that form the dehiscence zone. It connected the factor to auxin transport and response during early development.^[ref] Together these experiments motivate a regulatory explanation: local gene expression changes developmental signals, those signals influence cell behaviour, and the resulting tissue arrangement makes a separation layer possible.

FRUITFULL, also called FUL or AGL8, is an Arabidopsis thaliana developmental regulatory gene encoding a MADS-box transcription factor. In the fruit, it helps establish normal valve tissue identity. The gene, its protein product and a mutant plant's phenotype are different levels of description.

The 1998 study of the ful-1 mutant identified an inserted enhancer-trap element disrupting the gene. A reporter marked valve tissue, and mutant valve cells failed to elongate and differentiate normally.^[ref] Comparing local expression and cell anatomy supplies more mechanistic information than noting only that a mature fruit is short.

A later genetic analysis clarified that the effect need not mean that FRUITFULL directly drives every elongation step. Its regulatory role includes preventing misplaced INDEHISCENT activity.^[ref] In a causal comparison, defective growth machinery and altered tissue identity can both produce a short organ. Perturbing the regulatory pathway and measuring which identity develops helps distinguish those explanations. The combined history connects gene expression, cell differentiation, fruit dehiscence and seed dispersal without treating final organ length as a direct assay of a single molecular function.

An alkyl group is a monovalent group obtained formally by removing one hydrogen atom from an acyclic saturated hydrocarbon. The general composition is ${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2n+1}$: examples include $\mathrm{C}{\mathrm{H}}_3-$ and $\mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{H}}_2-$. The dash denotes an attachment chemical bond, rather than necessarily an unpaired electron.^[ref]

“Alkyl” specifies a structural fragment. The corresponding free radical, carbocation and carbanion have different electronic states and reactivities. Branched alkyl groups can also share an elemental formula while giving different connectivity and steric environments around the attachment site.

The specific five-carbon prenyl group is the alkenyl group 3-methylbut-2-en-1-yl, with connectivityHere $\mathrm{R}$ is the rest of the attached molecule; the prenyl group itself has composition ${\mathrm{C}}_5{\mathrm{H}}_9$. The ChEBI structural record uses SMILES *CC=C(C)C, with the asterisk marking the attachment point.^[ref]

The count is instructive: the attachment-end $\mathrm{C}{\mathrm{H}}_2$ contributes one carbon and two hydrogen atoms; $\mathrm{CH}$ contributes one and one; the next double-bonded carbon contributes no hydrogen; and the two methyl groups contribute two carbon and six hydrogen atoms. Together these give five carbon and nine hydrogen atoms. The double bond is one bond away from the attachment atom, connecting the prenyl group with the broader allyl group pattern.

Attaching the group through oxygen gives $\mathrm{R}-\mathrm{O}-\mathrm{C}{\mathrm{H}}_2-\mathrm{CH}=\mathrm{C}(\mathrm{C}{\mathrm{H}}_3{)}_2$, a prenyl ether; attachment through another atom gives a different linkage. The same carbon fragment therefore does not specify the whole functional group environment. In biological prenylation, the terminology can also encompass longer isoprenoid-derived attachments, so the particular transferred group must be stated.

An alkenyl group is a monovalent carbon–hydrogen substituent derived from an alkene by removing one hydrogen atom. The attachment can be at a double-bonded carbon or elsewhere in the carbon skeleton. For an acyclic parent with one double bond, the group composition is ${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2n-1}$.^[ref]

For example, a vinyl group attaches as $\mathrm{R}-\mathrm{CH}=\mathrm{C}{\mathrm{H}}_2$ and contributes ${\mathrm{C}}_2{\mathrm{H}}_3$, whereas an allyl group attaches as $\mathrm{R}-\mathrm{C}{\mathrm{H}}_2-\mathrm{CH}=\mathrm{C}{\mathrm{H}}_2$ and contributes ${\mathrm{C}}_3{\mathrm{H}}_5$. The attachment atom in the first example belongs to the double bond; in the second it is adjacent to the double bond. A prenyl group is another alkenyl group, with five carbon atoms.^[ref]

The missing hydrogen identifies an attachment site in a larger molecule; it does not mean that the completed attached group must contain an unpaired electron. Formulae alone also do not specify connectivity. For $n=3$, the composition ${\mathrm{C}}_3{\mathrm{H}}_5$ does not by itself tell where the attachment or double bond lies. A structural formula supplies information that the atom count omits.