There is a couple patterns I see with dyslexic spellings of initial blends. One of them is the use of <chr> when a word begins as <tr> and the other common one is using <jr> for words with an initial <dr>. Some experts might state that the child has not mastered phonemic awareness and I used to believe this. I would spend endless hours with them to separate the sound before the <r>. I now know that I was missing something valuable related to these English spellings and pronunciations. I realize that these children actually have excellent hearing and excellent discrimination of phones.
In the case of an initial word that begins with the blend <tr>, we blend it so quickly in connected speech that we often say <truck> as /tʃɹʌk/, although most people “think” that they say /tɹʌk/. Only when we say the word in isolation and really focus on the enunciation the <t> separate from the <r> does it actually sound like /tɹʌk/. The only way to understand the difference is to say a sentence with <truck> in it: The red truck is big. One may not fully agree with me, but when a student writes <truck> as <chruck>, they are able to distinguish the phone [tʃ] before the [ɹ]. Therefore they are translating the phoneme [tʃ] into <ch> as that is what most children are taught to do- translate phonemes into graphemes.
Upon studying word patters of <tr> and <chr>, I will share with you a secret. It’s actually no secret, because anyone can study what I have. All the words that I was able to find that had an initial <chr>, the <ch> grapheme was represented with a /k/ phoneme. Some common examples were <chrome>, <Christmas>, and <chronic>. I was unable to find any English words that started as <chr> that was pronounced [tʃɹ]. Therefore, whether or not it is argued that a child has or has not mastered phonemic awareness, a student can be informed that if they have an initial blend with the phones [tʃɹ] that it will never be spelled <chr>, as <chr> is reserved for /kɹ/.
When I encountered a substitution of <jr> for when the initial spelling was <dr>, I started to wonder if there was a similar constraint. This is another common “misspelling” that I see in dyslexic students. I researched words with an initial <jr> and there just isn’t any English words with an initial <jr>. Zero. One may ask, what about <jr.>? This is an abbreviation and it is still pronounced /ʤuːnjər/ in full. Again this child is reproducing the phones that we actually say in connected speech. Saying a word alone, one may separate the [d] and the [ɹ] when one says <dry>, but in connected speech it will be pronounced more like [ʤɹaɪ ]. Say, “The river is dry.” Most likely it was said [ʤɹaɪ]. Again, whether or not one agrees with the pronunciation, a student can be taught that the spelling will never be <jr> because English just doesn’t use that spelling sequence initially.
The next question is why. Why do we not have words that start are <jr> or [tʃɹ]? The answer relates to a linguistic term phonotactics. Let me break down this word: phon(e) + o + tactic+s. The base <phon(e)> relates to sound produced by something with a mouth and lungs. The base <tactic> relates to arranging. So, <phonotactics> is about how we arrange our mouth/toungue to make phonemes. Why is this important? English (and all languages actually) has phonotactic constraints. There are certain consonant phones that we just don’t put together. When the movie, “Frozen” came out, I remember a long discussion with a student about the character Sven. She swore the character’s name was “Spen”. The reason is that we don’t typically have <sv> letter sequence in English. There is the loan word <svelte> but that’s about it. So this child’s brain interpreted the /sv/ phones into what makes “sense” and phonotactically possible in English, “thinking” it was “Spen”. It was a processing issue, not a hearing issue for an monolingual English speaker. If English allowed words to be spelled <jr> along with <tr> then we could more easily distinguish these phones. The phones [ʤɹ] is not phonologically distinctive from [tɹ] as there is only one way to spell both of these pronunciations, <tr>. Likewise the phones [tʃɹ] can only be realized in spelling with the graphemes<tr>. Phonotactics explains a lot about not only how we pronounce words, but can also help us to eliminate spelling patterns English doesn’t use. Thus it makes spelling a bit more easy to understand. I hope this explanation can help a student to understand the spelling patterns of <dr> and <tr> a little more easily.