Polydactyly

Key takeaways

Polydactyly belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Polydactyly to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Polydactyly from memory before moving on to harder problems.

Reference excerpt

Polydactyly (commonly known as sixth finger and extra finger) is a birth defect that results in extra fingers or toes. The hands are more commonly involved than the feet. Extra fingers may be painful, affect self-esteem, or result in clumsiness. Polydactyly is associated with at least 39 genetic mutations. It may either present alone or with other defects. Cases may run in families. The underlying mechanism involves an error in limb bud formation during early development. Diagnosis may occur before birth via prenatal ultrasound as early as nine weeks. X-rays may be useful after a child is a year old. The opposite is oligodactyly (fewer fingers or toes). Treatment varies from removal by cautery to more involved surgery. While putting a tight band around the base has been carried out, this is not typically recommended. If surgery is required, this is often done around two years of age. Occasionally multiple surgeries are required. Polydactyly is present in about 4 to 12 per 10,000 newborns. It is the most common defect of the hands and feet. In the United States, Black people are more commonly affected than white people. The term is from Greek πολύς (polys) 'many' and δάκτυλος (daktylos) 'finger'.

Signs and symptoms In humans/animals this condition can present itself on one or both hands or feet. The extra digit is usually a small piece of soft tissue that can be removed. Occasionally it contains bone without joints; rarely it may be a complete functioning digit. The extra digit is most common on the ulnar (little finger) side of the hand, less common on the radial (thumb) side, and very rarely within the middle three digits. These are respectively known as postaxial (little finger), preaxial (thumb), and central (ring, middle, index fingers) polydactyly. The extra digit is most commonly an abnormal fork in an existing digit, or it may rarely originate at the wrist as a normal digit does. Polydactyly can be divided into three major types, which are discussed below, which depend on the location of the additional digit. In 2019 it was found that in cases of polydactyly with a fully functional additional digit, muscles to control the extra digit may be duplicated, resulting in increased motor control that allows the patient to carry out certain tasks with one hand that would normally require two.

Ulnar or postaxial

This is the most common situation, in which the extra digit is on the ulnar side of the hand, thus the side of the little finger. This can also be called postaxial polydactyly. It can manifest itself very subtly, for instance only as a nubbin on the ulnar side of the little finger, or very distinctly, as a fully developed finger. Most commonly, the extra finger is rudimentary, consisting of an end phalanx with a nail, and connected to the hand with a small skin pedicle. Mostly one neurovascular bundle can be identified, with no tendons present in the extra digit. In case of a fully developed extra finger, the duplication usually presents itself at the level of the metacarpophalangeal joint. A triplication of the little finger is very rare. Ulnar polydactyly occurs ten times more often in African populations. The incidence in Caucasians is reported as 1 in 1,339 live births, compared with 1 in 143 live births in Africans. Ulnar polydactyly is also often part of a syndrome. In patients with African ancestry ulnar polydactyly mostly occurs isolated, whereas the presentation in Caucasians is often associated with a syndrome, though in a retrospective review, only 4 of 37 cases of ulnar polydactyly in Caucasians were syndromic.

Radial or preaxial

This is a less common situation, in which the affectation is on the side of the hand towards the thumb. Radial polydactyly refers to the presence of an extra digit (or extra digits) on the radial side of the hand. It is most frequent in Indian populations and it is the second most common congenital hand disorder. The incidence of radial polydactyly is reported as 1 in every 3,000 live births. The clinical features of radial polydactyly will depend upon the extent of duplication. Radial polydactyly varies from a barely visible radial skin tag to complete duplication. Thumb polydactyly varies from barely visible broadening of the distal phalanx to full duplication of the thumb including the first metacarpal. Radial polydactyly is frequently associated with several syndromes.

Central This is a rare condition, in which the extra digit is on the ring, middle or index finger. Of these fingers, the index finger is most often affected, whereas the ring finger is rarely affected. This type of polydactyly can be associated with syndactyly, cleft hand, and several other syndromes. Polysyndactyly presents various degrees of syndactyly affecting fingers three and four.

Causes

Polydactyly is associated with different mutations, either mutations in a gene itself or in a cis-regulatory element responsible for the expression of a specific gene. Mutations in Hoxa- or Hoxd clusters are reported leading to polydactyly. Interactions of Hoxd13 and GLI3 induce synpolydactyly, a combination of extra and consolidated digits. Other signal transduction pathways in this context are the Wnt signaling pathway or Notch. In the specific case of preaxial polydactyly (Hemingway mutant), a cis-acting mutation approximately 1Mb upstream of SHH gene has been implicated. Normally SHH is expressed in an organiser region, called the zone of polarizing activity (ZPA) on the posterior limb side. From there it diffuses anteriorly, laterally to the growth direction of the limb. In the mutant, smaller ectopic expression in a new organiser region is seen on the anterior side of the limb. This ectopic expression causes cell proliferation delivering the raw material for one or more new digits. Polydactyly can occur by itself, or more commonly, as one feature of a syndrome of congenital anomalies. When it occurs by itself, it is associated with autosomal dominant mutations in single genes, i.e. it is not a multifactorial trait. But mutation in a variety of genes can give rise to polydactyly. Typically the mutated gene is involved in developmental patterning, and a syndrome of congenital anomalies results, of which polydactyly is one feature or two. Polydactyly has been linked to the prenatal environment in a 2020 study showing a relationship to maternal PM10 pollution exposure in China. Types include:

… excerpt ends here. Continue reading the full article.

Illustrations

Polydactyly: Postaxial polydactyly — Postaxial polydactyly

Worked examples

Example 1 — a first encounter with Polydactyly

Start with the simplest possible case. Write down what Polydactyly claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Polydactyly before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Polydactyly ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Polydactyly

In research: Polydactyly appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Polydactyly in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Polydactyly is common in secondary-school and first-year university syllabi. It links to neighbouring topics Autosomal dominant disorders, Congenital disorders of musculoskeletal system, Fingers, so understanding it makes those chapters shorter.
In everyday life: Look for Polydactyly outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Polydactyly in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Polydactyly means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Polydactyly out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Polydactyly in simple terms?

Polydactyly (commonly known as sixth finger and extra finger) is a birth defect that results in extra fingers or toes. The hands are more commonly involved than the feet.

Why does Polydactyly matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Polydactyly?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Polydactyly.