There are hundreds, if not thousands, of blockchains. The number will continue to grow as blockchain technology is adopted and adapted to new use cases. In time, there could be hundreds of thousands or millions of chains in the same way that there are so many computer networks in the world today.

There are different types of blockchains. There are public and private blockchains. There are blockchains that have only one coin or token in use. There are blockchains with lots of tokens on them. And there are chains with no coins or tokens.

It can sometimes be confusing as to which is which.

Public projects with their own 'native' coins usually have their own blockchain. Examples are Bitcoin, Ethereum, Litecoin, Dash, Zcash, Monero, Dogecoin, Decred, Horizen, DigiByte, Ravencoin, etc. These are all separate blockchains.

Many projects don't run on their own blockchain, but on another upon which they are built, usually on smart-contract platforms. ERC-20 tokens, for example, run on Ethereum. This includes BAT, Decentraland, ChainLink, DAI, USDC, Enjin, Aragon, etc. They all run on the public Ethereum blockchain.

The advantage of being a token on another chain is that you get all the security and stability benefits of the parent chain and ease of setup by following a standard.

In addition to Ethereum, other smart contract platforms include Ethereum Classic, Tezos, EOS, and Cardano. New tokens will emerge on these blockchains as projects and use cases are born that leverage the unique advantages of each chain.

There are also private chains that don't have a coin or token, but track information for particular business purposes. These are often built on Hyperledger or Corda, where the chain is only accessible to 'permissioned' users. We may never know how many of these exist as they are akin to (or used on) private networks that aren't exposed to the public.

Public blockchains have coins/tokens and mining/minting to incentivize participation in support of the network.

Private blockchains typically don't have coins or tokens because they're financed by corporations and their partners who run those chains in support of their core businesses.

Yes. As with anything, blockchains have advantages and disadvantages and should be used for appropriate purposes.

Depending on your perspective or usage, the following may be seen as drawbacks:

Some blockchains are 'slow'. Blockchains move (or process information) at a certain speed (and different blockchains move at different speeds). For example, the Bitcoin network processes blocks on average every 10 minutes. It is recommended that one wait for 6 confirmations (ie. blocks) before feeling confident that a transaction will not be 'reversed' due to a 51% attack. 6 confirmations * 10 minutes = 60 minutes. In a point-of-sale situation with a merchant accepting bitcoin (ie. a coffee shop), it's not practical to wait an hour after a customer has purchased to give them their merchandise. (This drawback can be overcome with second-layer solutions like Lightning Network, blockchains with faster confirmation times, or other technologies like masternodes with InstantSend and ChainLocks.)

Blockchains can only handle a certain amount of data/transactions per block. If a blockchain becomes overwhelmed with use, transactions back up and don't get processed in a timely manner, similar to bumper-to-bumper traffic stuck on a freeway. This leads to longer wait times and higher fees as users compete with each other to get their transactions processed sooner. (This happens frequently with both Bitcoin and Ethereum.)

Storage space. As blockchains age, they get bigger and bigger, requiring more and more disk space. At a certain scale, only those with huge amounts of disk space will be able to keep full records of blockchains. This decreases decentralization and potentially invites abuse or attack.

Energy usage. As Proof-of-Work blockchains grow, they require ever greater amounts of energy usage. Depending on the source of that energy, this can be detrimental if the source is causing harm or is at risk of running out (ie. coal plants, fossil fuels).

Privacy (or lack thereof). On public blockchains like Bitcoin, all transactions by all participants are available to be seen on the blockchain. While identities are pseudonymous on Bitcoin, if a pseudonymous identity is ever revealed, every transaction that person has ever made can be tracked. It's like having your bank statements open to the entire world. In cases where privacy is desired or required (not just for illegitimate purposes), this is unacceptable. In this case, blockchains with privacy technology should be used. On the flipside, when private blockchains are used, transactions cannot be traced. This may be seen as a drawback to authorities and governments who wish to track transactions.

One of the drawbacks is also one of blockchain's strengths. Immutability. Theoretically, once something's on a blockchain, it's there forever. One way this may be seen as a drawback is in the case of social networks built on blockchains. With traditional database-driven social networks, posts can be edited or deleted. With blockchain-driven social networks, posts can never be edited or deleted. Even with additional layers built on top of blockchains (like websites or apps) that could block, screen, or filter posts, anyone with a block explorer can look at the original post on the blockchain itself. So, if you ever change your mind or regret something you post on a blockchain-driven social network, you'll be unable to remove it.

A blockchain is immutable because (in simple terms):

The ledger is append only.
As opposed to a traditional database, which allows records to be inserted or edited, a blockchain only adds records to the end of the ledger.

Records are linked together and cryptographically sealed with unique 'fingerprints' such that any attempt to modify older, existing records (ie. blocks) results in different 'fingerprints' and is immediately detected and rejected by the network.

Said another way, a blockchain network reaches consensus on which blocks appear in which order. Any attempt to modify and broadcast different blocks in a different order is rejected by the network, assuming that the network is properly decentralized and not subject to a 51% attack.

A 51% attack is when a miner or group of miners obtain 51% or more of the network hash power, allowing them to rewrite blockchain history. This attack is used for (among other things) a double-spend, where the attacker executes a transaction on the current blockchain (for example, to purchase goods), then rewrites the blockchain with different blocks that don't contain that transaction. Thereby, according to the rewritten blockchain, the attacker never spent those funds, making them free to spend again.

This attack has occurred on several public blockchain networks, including Bitcoin Gold, Ethereum Classic, and Horizen (which has since implemented a mitigation strategy).

One mitigation strategy is ChainLocks, implemented by the Dash network, in which (in simple terms) a majority quorum of nodes agree upon the state of the blockchain such that attacking blocks are rejected.

Hi Jacques, im sorry i was not working with etherum but another method called Quantum digital numerical algorithm or QDNA which the chain would act very similar to genertation DNA. 

The system would allow their to be 3 main areas -

1)help with dna fountain technology for mass storage and the writeability method between computer and bio storage

2) authentification, traceability and ownership between generations of the blockchain movements (could be used in monetary or information)

3) along with an underpining encryption/transport capability to be done without data corruptability. 

Sorry I could not be of much help beyond this. My research only got up to looking at how I could turn my own DNA into an encryption method / digital signature 

Regards matthew 

My 2 cents about the push for digital dollars is that the current crisis has taught us how our paper money is a source of confusion and strain because it is analog. Take for example the recent stimulus money and how difficult it was to reach people on time not to mention some money that went to deceased people; here we have a pretty balkanized and inefficient system. The digital currency would be a better way of delivering money, but it will require a new architecture.

I believe to combat pandemics like the covid-19, the healthcare sector that would benefit from blockchain is patient identity. With identity established, there will be more transparency in matching patients and their records, leading to better interoperability with other datbases. Additionally, blockchain identity allows patients to travel digitally with their records anywhere, sharing them with providers, and volunteering them with clinicians researchers or drug manufacturers for medical research purposes. This would be a boon to our fight against a pandemic like the covid-19 because of effective tracing.

if the funds were never received then the transaction was incomplete and as such you shouldn’t pay any gas (funds) for that transaction. The said transfer funds should have returned to the senders wallet, which means you can send them again to a different wallet address if you wish, you only pay what is known as gas for the transfer if it completes, so you would pay for second transaction.

The age of a wallet address has no significance on the transaction costs.

i hope this answers your question, if not and you need further clarity please get back to me and I will endeavour to explain further

regards

Jack Ambler

+447864864005