John has a short biography in HoP 1439-1509 (1725-6), which shows that he was MP for Nottinghamshire in 1449. This attributes him with a birthdate of 1405 and a marriage date of circa 1425, but these are probably both broad estimates (no specific primary reference is given for the dates). No parentage is given or guessed at, but it seems clear that he belonged to the branch of the Roos family based at Ingmanthorpe within the parish of Kirk Deighton, West Yorkshire, as alleged in the Vis. Notts entry; he appears frequently in West Yorkshire deeds, mostly relating to property at North Deighton, in the period from 1439 to 1458.
In particular, he seems to have been in dispute with the head of the Ingmanthorpe family, Sir Robert Roos (d 1451) , since in 1447 he submitted to arbitration over claims to the manors of Kirk Deighton, Ingmanthorpe, North Deighton and Steeton (inter alia; West Yorkshire Archives, WYL230/121). In Trinity term 11 Henry VI  Sir Robert and one "John Roos knight" were at loggerheads over the advowson of Kirk Deighton (Pedigrees from the Plea Rolls, 348) - certainly John of Laxton was only an esquire, so that case might not relate to him.
Vis. Notts is so muddled at this point that it is almost worthless. If we accept that the generations of John and his son Robert have simply been transposed, that pedigree would seek to make him the son of Sir Robert Roos of Ingmanthorpe, and the brother of "William Roos, Lord of Ingmanthorpe". I'm not sure there was a William holding Ingmanthorpe at this time period, while there were two Sir Roberts, grandfather and grandson, the first of whom died at a fairly advanced age in 1393, and the second of whom was living in 1393 and died about 1451 (Admon at York). The latter certainly had a younger brother John Roos [and a William, FWIW], named in the 1399 will of their father Thomas, but it isn't clear that he was the man who married Isabel Etton. The younger Sir Robert's aunt, Eleanor Roos, also left a will (York, 1438, printed in Testamenta Eboracensia, ii, 65) in which she names "John Roos, esquire" without expressing the nature of their relationship - possibly this is the Laxton man.
In the absence of clearer evidence, the relationship between the Ingmanthorpe and Laxton families remains unknown, although it seems highly likely that the latter was a cadet branch of the former.
Thanks to CP as kindly pointed out by John Higgins, we can likely place John Roos, the founder of the Laxton branch, within the Ingmanthorpe line; rather then being a cadet branch, it appears that the Laxton family was the senior line, largely disinherited in the late 1300s.
The key to this identification lies in the court cases between John Roos of Laxton and Sir Robert Roos of Ingmanthorpe, disputing various family manors and rights (1433-1447). We have a brief pedigree for Sir Robert, abstracted from the 1433 case:
1. Robert Roos, Lord of Ingmanthorpe
2. Thomas Roos, married Joan
3. Sir Robert Roos, plaintiff in 1433
1. is Sir Robert Roos was one of the witnesses in the famous heraldic case of Scrope v Grosvenor (1389), when he is said to be aged 76 (so born circa 1313). His will from 1393 survives at York, and is reproduced in Testamenta Eboracensia. The only son he named in it is 2. Thomas, who also left a will (1399) at York, in which he names his son 3. Robert - the plaintiff of 1433. This second Sir Robert succeeded his father Thomas in the Ingmanthorpe estates.
However, as CP makes clear, the older Sir Robert had an elder son, also named Robert, whom he appears to have disinherited. CP states that this Robert recovered the manors of North Deighton and Steeton in 1396. As "Robert Roos, elder son and heir of Robert Roos, knight, of Ingmanthorpe", he was dealing with property at North Deighton in 1398/9 (22 Richard II) [West Yorkshire Archives, WYL230/84, 86). He held the manor of North Deighton between 1397 and 1408 (West Yorkshire Archives, Court Rolls of Robert Roos for North Deighton, WYL230/1580). As we have seen, John Roos of Laxton afterwards held the manor of North Deighton. The 1433 and 1447 cases likely represent John's attempt to revisit the exclusion of Robert Roos from his wider inheritance, and therefore it seems reasonable to conclude that John was the son (or perhaps grandson) of Robert Roos, elder son of Sir Robert. Adding the further details from CP, we can propose the following descent:
A. Sir William de Ros of Helmsley, died circa 1264 (son of Robert de Ros and Isabel of Scotland)
B. Sir William de Ros of Ingmanthorpe (younger son), dead by 1310; married circa 1268 Eustache, daughter and heir of Ralph Fitz Hugh
C. Sir William de Ros of Ingmanthorpe, dead by 1334; married 1310 Isabel, daughter and coheir of Richard de Stetton
D. Sir Robert Roos of Ingmanthorpe, born circa 1313; died 1393
E. Robert Roos of North Deighton, ff 1407 - disinherited by his father
F. John Roos of North Deighton, married Isabel de Etton, heiress of Laxton