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Define the term "limb polarity" to mean things such as writing handedness, golf handedness, throwing ball arm preference and natural boxing hand. I avoid using the term "handedness" because of its limiting perspective usually limited to writing. In reality, there are many different kinds of handedness so many limb polarities. Learnt polarity such as ability to kick with both left/right foot is not the polarity: it is the natural skills such as coordination that make you use the other limb better than the other."

I find studies about handedness focusing only on writing handedness red herring: the thing is that there are many-many different kinds of handedness as well as footnedness. When reserchers focus only on writing handedness, they miss the large picture because every person has a lot of different handedness. The fact that you write with your right hand does not mean that you are right-handed in hockey or in other activity or play football with your right leg. I extend handedness now also to limbs such as legs and thumbs by the term limb polarity as defined above.

The below twin study data, now published, I think raises new questions.

Helper questions

  1. What is the reason that the twins have the same handedness but their handedness differ in golf handedness? What determines limb polarity?

  2. Should researchers focus on limb polarity instead of writing handedness in their studies?

  3. Which genes determine the limb polarity in mammals?

  4. Is there some academic term already used for the term limb polarity?

Twin Study Data

Assume identical twins with totally-matching handedness profiles

  • writing: right VS right
  • ice-hockey: left VS left
  • football: right VS right
  • baseball: left VS left
  • tennis: right VS right
  • throwing ball: right VS right
  • golf: right VS left (ONLY DIFFERENCE known so far)

so limb polarity differs only in the last activity but otherwise the same. Why?

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This is is a cross-research topic between Embryology and Genetics. Some physiology too because those movement tracts are fine regulated. Not all mechanisms are known here. Your Finnish discussion about twin studies then again seems to be based on evidence based medicine and causality - skipping all genetic factors, all control mechanisms etc. I will try to answer on this after some weeks when I am on holiday. You can find some information about it in Embryology textbooks. –  Masi Jul 4 at 18:22
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See Ma K. Embryonic left-right separation mechanism allows confinement of mutation-induced phenotypes to one lateral body half of bilaterians. Am J Med Genet A. 2013 Dec;161A(12):3095-114. doi: 10.1002/ajmg.a.36188. Epub 2013 Oct 29. PubMed PMID: 24254848. for some possible mechanisms for limb-specific polarity –  JaakkoK Jul 7 at 9:56
    
The golf could differ purely because it is easier to get hold of right handed clubs - that's why my lefty brother plays right handed. The other examples don't use "handed" kit do they? (Maybe Hockey, but with it being one stick versus a whole bag of clubs that contributes to greater availability of kit - parents can buy their right handed first child a right-handed stick and second [left handed] child a left handed stick, but can afford to buy two sets of golf clubs) –  GriffinEvo Jul 7 at 12:02

1 Answer 1

I am trying to collect here perhaps related material.

Certain diseases and perhaps certain extraordinary skills may be attributed to certain polarities.

Diseases

  1. Mothers with "bilateral hereditary primary breast cancer" are more likely to carry it to offspring (I.).

Material

I. Ma K. Embryonic left-right separation mechanism allows confinement of mutation-induced phenotypes to one lateral body half of bilaterians. Am J Med Genet A. 2013 Dec;161A(12):3095-114. doi: 10.1002/ajmg.a.36188. Epub 2013 Oct 29. PubMed PMID: 24254848. [1].

(1) mutations and the mutation-induced phenotypes to be strictly confined to one lateral body half in animals and humans; (2) mothers with bilateral hereditary primary breast cancer to transmit their disease to their offspring at twofold of the rate compared to mothers with unilateral hereditary breast cancer; and (3) a mosaic embryo carrying genetic or epigenetic mutations to develop into either an individual with the mutation-induced phenotype confined unilaterally, or a pair of twins displaying complete, partial, or mirror-image discordance for the phenotype.

Key words: "bilateral cancer; discordant twins; embryogenesis; embryonic development; embryonic left-right separation mechanism; fluctuation asymmetry; gynandromorph; gynandromorphism; hemiatrophy; left-right asymmetry; unilateral cancer; unilateral diseases"

Attributions

[1] Professor JaakkoK mentioned this publication in a comment for some possible mechanisms.

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