One X or two X

In the picture

1. 1The badge sits on the left adult’s notched long chromosome, a changed X.
2. 2The badge sits on the right adult’s short chromosome, a Y.
3. 3The badge sits on the boy’s notched long chromosome, his only X.
4. 4The badge sits on the girl’s plain long chromosome, her second X.

Examples

• Telling red and green sweets apart can be hard.
• Matching red and green socks can be difficult.

Quiz: One X or two X (Age 13)

1. 1

   Why is red-green color blindness more common in boys than in girls?

   • Because girls have two X chromosomes, so a changed X is always repaired before it can cause the condition.
   • Because boys have only one X chromosome, so one changed copy can cause it.
   • Because boys inherit a changed X chromosome from their father more often than girls do.
2. 2

   For a girl to inherit red-green color blindness, what usually must be true?

   • She must inherit a changed Y chromosome from her father.
   • She must inherit one changed X chromosome from her father.
   • She must inherit a changed X chromosome from each parent.
3. 3

   From which parent does a boy inherit the X chromosome that can carry red-green color blindness?

   • His mother, because she gives him an X while his father gives a Y.
   • His father, because he gives him an X while his mother gives a Y.
   • Both parents, because he gets one X from each.

The answers, each with its explanation, are in the app. This lesson has left the free week, so its quiz is part of the subscription.