Course 22.2Lesson 2 of 3
Sex is decided in different ways in different animals. This lesson covers the four systems that an exam asks about: XX and XY in humans and the fruit fly, XX and XO in some insects, ZZ and ZW in birds, and the haplo-diploid honey bee. You find which parent decides the sex and why the ratio is near 1 : 1.
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A human baby is a boy or a girl. Which chromosome decides this, and which parent passes it on? The answer is different for a bird and for a bee. This lesson takes the main systems one by one.
A human body cell has 46 chromosomes. Autosomes are the 44 chromosomes, 22 pairs, that are the same in men and women. The last pair is the sex chromosomes, and it differs. In 1891 Henking noticed in some insects that one chromosome went to half of the sperm and not to the other half. He called it the X body. It was later found to be a sex chromosome.
A sex that makes only one kind of gamete is homogametic. A sex that makes two kinds is heterogametic.
In humans and in the fruit fly, the female has two similar chromosomes, XX. The male has one X and a smaller chromosome called Y, XY.
If an X sperm fertilises the egg, the child is a girl (XX). If a Y sperm fertilises it, the child is a boy (XY). The father decides the sex. The chance is one half each time, so about as many boys as girls are born.
A-Level and IB add that a gene on the Y chromosome, called SRY, switches on the development of the testis. NCERT and the NTA syllabus do not require the name.
In many insects, such as the grasshopper, the male has only one X and no Y chromosome. This is XO. The female is XX. Half the sperm carry an X and half carry no sex chromosome. The XO system is in NCERT, but the NTA syllabus names only humans, birds and the honey bee, so it is not needed for NEET.
In birds, the system is the other way round. The male has two similar chromosomes, ZZ. The female has two different ones, ZW. The letters Z and W are used so that nobody mixes them up with X and Y.
All sperm carry one Z. Half the eggs carry Z and half carry W. A Z egg gives a male, and a W egg gives a female. In birds the mother decides the sex.
In the honey bee, the sex depends on the number of sets of chromosomes. This is haplo-diploidy.
The queen mates once and stores the sperm. She can fertilise an egg or not, as it passes out. A drone makes his sperm by mitosis, not by meiosis, because his cells already have only one set. So all the sperm of one drone are alike.
A drone has no father, and so he has no sons, but he has a grandfather and he can have grandsons.
You should see that every egg that the queen does not fertilise becomes a drone. If she fertilises all the eggs, there are no drones.
The program finds the sex of the children in the human and the bird systems with exact fractions, and it counts the bees of a colony.
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
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