“Sausage rolls!” I hear him exclaim, sitting in his place at the table while indulging in his favourite ice-cream. His knowledge is limited, his speech is limited, his comprehension of life is limited – but his emotions are heightened. This is my 13-year-old brother, Sebbie; he has Down’s syndrome, a congenital disorder arising from a chromosome defect.

He may not seem as clever as the average child; however, his intelligence, though less apparent, is no less valuable. He is not able to conform to society’s expectations by taking exams and tests to demonstrate his intellect. But Albert Einstein said: “Everybody is a genius. But if you judge a fish by its ability to climb a tree, it will live its whole life believing that it is stupid.” He is clever, but cannot be part of “our” life because he is not clever in the “normal” way.

He is sitting opposite me at the table eating his pudding, while dancing to a “happy song”; he just has to physically express enjoyment (he loves his vanilla soft scoop). Internal rhythm is often talked about by musicians and is something to which I cannot relate; Sebbie’s rhythm, however, represents his effusive personality. Dancing is one of his greatest passions.

Looking at him from across the table, I think how far he has come, physically and mentally. He has recently learned to swim and is making progress towards adding numbers.

‘He can be tricky, perplexing and sometimes outrageous, but his sense of fun and love outweighs all else.’ Photograph: Oliver and Sebbie Shone

I will never forget the day Sebbie was born; it was my fourth birthday and he arrived so quickly I didn’t even have time to finish my cake before going to see him in the hospital. He was in the special care baby unit, surrounded by wires and lying in a tiny pod. Lots of doctors surrounded him and, young as I was, I realised that he was not a healthy child, but to me he was just my new little brother and was perfect. I did not understand, then, the overwhelming distress and horror that faced my family.

There were many big words flying around that I did not understand. Tetralogy of Fallot … atrial septal defect … open-heart surgery … My parents were alarmed, panicked; I had not seen them like this and I felt afraid too. This was the start of a long and terrifying journey of operations, hopes and fears. But nothing seems to dampen his enthusiasm and his quirky outlook on life.

It is hard to judge “normal” in this world. There is talk of testing for Down’s syndrome, selective abortion and eradicating this condition. While I understand that no disability is ideal, I think it would be desperately sad to lose these precious children and adults who bring so much light into their families’ lives. Gone would be my crazy brother who talks of pizzas, trains and skiing all in the same sentence; gone would be the adoration that he feels for me and gone would be that wonderful dancing and tuneless singing.

We often find ourselves laughing round the table at funny remarks and comments of his; he has always had the ability to generate laughter. His sense of humour is infectious; there have been many times that he has diffused a tense situation with his comical asides. It is hard to take life too seriously when Sebbie lifts up his jumper and tells me to “Put your belly away!” His sense of timing is impeccable.

His lack of inhibition means he is more likely than not to join another family at the park or beach while giving us a cheeky wave. He has no sense of embarrassment, shyness or social conventions; this can lead to some embarrassment from me, although most people’s reactions are positive.

I often think of his future and what it holds. Sebbie will never be able to live alone and will need constant care and supervision, which I plan to always be a part of. Although we love him, it is frightening to look ahead to a life that will never be independent. We not only worry about how he will cope in later life, but also the major operation that will probably face him before he is 18.

Sebbie’s current condition (although he is physically more than able) can lead to frustration and difficulty. Even the simplest tasks, such as putting on a coat, socks and shoes, can lead to disputes, especially as he is such a strong character. Growing up with a brother with Down’s syndrome has been a real challenge but I would not change him for the world. He can be tricky, perplexing and sometimes outrageous, but his sense of fun and love outweighs all else. There are adults with the condition who have learned to drive cars, are performing in drama groups and are holding down jobs.

My great uncle had the same condition and he was born exactly 100 years before Sebbie, in 1903. Thankfully times have changed since he was alive – he spent his life in an institution. Now disabled children have huge opportunities open to them and are achieving more than was ever thought possible. With our help I hope that Sebbie will grasp these opportunities. Who knows what he might achieve? I will always be at his side to help him along and I know the rewards I get back from him, in the form of love and friendship will make it worthwhile.

• Oliver originally wrote this piece as part of his GCSE English course last year