the effect of will be only felt few generation later , lesser giving birth to lesser girls when all other things are equal means 2.1 will not work .
For example If you start with a population of 100 (50 couples )and every next gen there are 9 boys to 1 girl and fertility ratio is 2 then your third gen will have only 20 kids as there will be only 20 women in the second gen . The fourth gen will have only 4 kids and there may not a 5 gen.
Either balance has to be fixed or replacement ratio of women has to be 1 not all kids , to sustain a 1:10 ratio of gender means 10 kids per female so there will be 1 girl kid
For example If you start with a population of 100 (50 couples )and every next gen there are 9 boys to 1 girl and fertility ratio is 2 then your third gen will have only 20 kids as there will be only 20 women in the second gen . The fourth gen will have only 4 kids and there may not a 5 gen.
Either balance has to be fixed or replacement ratio of women has to be 1 not all kids , to sustain a 1:10 ratio of gender means 10 kids per female so there will be 1 girl kid