Rep. Johh Young, whose 22 years in Congress were marred by an alleged sex scandal in 1976, was defeated in a primary runoff election Saturday, making him the eighth Texas Democrat to vacate a House seat at the end of the current term.

Young's loss, along with six retirements and an earlier primary defeat of Rep. Dale Milford, means that at least one-third of the 24-member Texas House delegation - the fifth largest and once among the most senior and powerful - will be occupied by newcomers when the 96th Congress convenes in January.

And with Republican chances better where there is no incumbent Democrat. GOP officials have hopes for significant gains this November both in Texas and elsewhere in the nation. But their efforts to capitalize on the Democratic turnover could be hampered by party infighting, as illustrated by Saturday's victory of George W. Bush in the Republican runoff for the 19th Congressional District - the seat being vacated after 44 years by Appropriations Committee Chairman George H. Mahon.

Bush is the son of George H. W. Bush, President Ford's Central Intelligence Agency director and former envoy to China and GOP national chairman. The younger Bush defeated former Odessa mayor Jim Reese, who had an endorsement from Ronald Reagan and $3,000 from a Reagan-related political action fund. Bush got 6,737 votes to Reese's 5,322, according to unofficial totals.

Texas Republicans were bitterly divided in 1976's presidential primary here, in which Reagan beat Ford. Asked yesterday whether he would campaign for the younger Bush, Reese said he is undecided as yet on any election role. Asked if he would support his party's nominee, Reese said in an interview: "I haven't been thinking along those lines."

Bush will face Democrat Kent Hance in November in 1976. Republican Reese garnered 45 percent of the vote against Mahon.

Also victorious in Saturday's runoff was Democratic state Rep. Mickey Leland who beat state Rep. Anthony Hall. A black social activist, Leland faces no Republican opposition in November and in effect was elected to succeed Rep. Barbara Jordan, who is retiring from the House. Leland, 33, has been active in various antiwar and social causes, including the establishment of a free neighborhood health clinic. He is a pharmacy graduate of Texas Southern University, he enrolled, he says, after being denied admission to the University of Houston for racial reasons.

Young lost his renomination bid to state. Rep. Joe Wyatt, of Bloomington, by 26,379 votes to 28,072 in complete but unofficial returns. Wyatt had led in the May 6 primary with 42.6 percent of the vote. Young himself was the man issue - Wyatt saying the incumbent was out of touch with the district's conservative leanings and attacking his voting his attendence and Young's conversion of $25,000 in campaign funds to personal use.

Young had cited the public works plums obtained for the district, his seniority and the fight to prevent closing of the Corpus Christi Naval Air Station.

During the campaign, Wyatt didn't concentrate on the 1976 allegation by a former aide to Young that the congressman overpaid her in exchange for sexual favors. Young has denied the allegation. His wife, who stood by him through the scandal, committed suicide last summer.

Besides Jordan and Mahon, House retirements from Texas also include Democrats Omar Burleson, W.R. Poage, Olin Teague and Bob Krueger, who is running for the Senate.

In th 1976 congressional elections, Republicans nationwide won 3.4 per cent of their races against Democratics incumbents and 7.8 per cent against nonincumbent Democrats, thus giving hope to Republicans to cash in on the heavy congressional turnover.

All told, there were six congressional runoffs in Texas Saturday, with the Bush-Reese contest the only GOP one. Complete but unofficial returns showed this lineup for November:

6th District - Phil Gramm (D) vs. Wes Mowery (R), for Teague's seat.

11th District - Marvin Leath (D) vs. Jack Burgess (R), Poage's seat.

14th District - Joe Wyatt (D) vs. Joy Yates (R), for Young's seat.

17th District - Charles Stenholm (D) vs. Bill Fisher (R), for Burieson's seat.

18th District - Mickey Leland (D), no Republican opponent for Jordan's seat.

19th District - Kent Hance (D) vs. George W. Bush (R), for Mahon's seat.