# Concept 9 Specialized chromosomes determine gender.

### Cross pure-bred pea plants to identify dominant flower color.

HI! Let's try this problem. Dave and Dana have two boys. If they decide to have another child, will they have a boy or girl? Since they already have two boys, their next child will likely be a boy. That is incorrect. Since they already have two boys, their next child will likely be a girl. That is incorrect. The sex of the child is not dependent on the sex of the previous children. That is correct. The sex of each child is determined at the moment of conception, and depends on the contribution of male sperm carrying either an X or Y chromosome. Let's build a Punnett square and calculate the chance that this next child will be a boy or a girl. Dana will make eggs that have: one X chromosome. That is correct. one Y chromosome. That is incorrect. two X chromosomes. That is incorrect. one X or one Y chromosome. That is incorrect. two Y chromosomes. That is incorrect. Dana will make eggs that have one X chromosome. Dave will make sperm that have: one X chromosome. That is incorrect. one Y chromosome. That is incorrect. two X chromosomes. That is incorrect. one X or one Y chromosome. That is correct. two Y chromosomes. That is incorrect. Dave will make sperm that have one X or one Y chromosome. When fertilization occurs, how many will be boys and how many will be girls? All will be boys. That is incorrect. Half will be boys, half will be girls. That is correct. All will be girls. That is incorrect. There will three boys for every girl. That is incorrect. There will be three girls for every boy. That is incorrect. When fertilization occurs, half the progeny will be girls (XX) and half will be boys (XY).