Sex Determination and Sex-linked Inheritance: visual lesson, ISC Class 12 Biology

Lesson

Sex Determination, Mutation and Genetic Disorders

Part 1 — how sex is decided, and how genes on the sex chromosomes are inherited.

In this lesson you will

Explain sex determination in humans, birds, honey bees and grasshoppers
Explain sex-linked inheritance of eye and body colour in Drosophila
Work out the inheritance of haemophilia and colour blindness

Diagram

Four systems of sex determination

In some organisms the male makes two kinds of gametes (male heterogamety: XX–XY, XX–XO); in others the female does (female heterogamety: ZW–ZZ). Honey bees use haplodiploidy.

Diagram

Sex determination in humans

Human females are 44 + XX and males 44 + XY. All eggs carry X; half the sperms carry X and half Y. So each child has an equal chance of being a girl or a boy, and the sperm decides the sex.

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