radio checked blinks in js

I want to make a code if you put your resident registration number(7 numbers) in the text box and press the result button, automatically checks the gender according to the first number(1,3 = check male/ 2,4 = check female). But if I press the result button, the radio check will blink once and disappear. What should I do?

<form action="#" name="idForm">
            <fieldset>
                <legend>gender check</legend>
                <input type="text" name="idNumber" maxlength="7" id="num">
                <label for="num"> : 7 numbers</label>
                <button name="result">result</button>
                <input type="radio" id="male" name="gender">
                <label for="male">man</label>
                <input type="radio" id="female" name="gender">
                <label for="female">woman</label>
            </fieldset>
        </form>
        <script>
            var idNumber = document.idForm.idNumber;
            var result = document.idForm.result;

            var male = document.getElementById('male');
            var female = document.getElementById('female');

            result.onclick = function(){
                var v = idNumber.value;
                if(v.length == 7){
                    if(v.charAt(0) == 1 || v.charAt(0) == 3){
                        male.checked=true;
                    }else if(v.charAt(0) == 2 || v.charAt(0) == 4){
                        female.checked=true;
                    }else{
                        alert('! : 7 numbers');
                    }
                }else{
                    alert('! : 7 numbers');
                }
            };
        </script>