How is sex decided?

The idea

Species determine sex differently: in some reptiles the temperature at which fertilised eggs are kept decides it, snails can change sex, and in human beings sex is largely genetically determined.

Boy or girl, male or female

Many living things have males and females. When a new individual is born, something decides whether it is male or female.

Do all species use the same strategy to decide sex?

• No, different species use very different strategies
• Yes, genes decide it in every species
• Yes, temperature decides it in every species

Three strategies

Open every tab.

Many ways

Some animals depend on the environment, as some reptiles do with temperature. Snails can change sex. In human beings genes largely decide.

Different species determine sex differently. In some reptiles, the temperature at which fertilised eggs are kept decides it. Snails can change sex. In human beings, sex is largely genetically determined.

Notes

Species decide sex differently: temperature in some reptiles, change of sex in snails, largely genetic in human beings.

Check yourself

In a few reptiles, what decides whether the animal in a fertilised egg is male or female?

Snails can change sex. What does this indicate?

In human beings, the sex of the individual is…

Do all species decide the sex of a newborn in the same way?

• The chromosome the egg gets from the father. In these reptiles the temperature at which fertilised eggs are kept decides the sex.
• The temperature at which the eggs are kept — correct. Yes!
• The colour of the egg. The book names the temperature at which the fertilised eggs are kept as the cue.
• Sex is genetically determined in snails. If individuals can change sex, sex is not genetically determined.
• Sex in snails is decided by the temperature of the eggs. The book mentions egg temperature for a few reptiles. For snails it says individuals can change sex.
• Sex is not genetically determined in snails — correct. Yes!
• largely genetically determined — correct. Yes!
• decided by the temperature of the eggs. That is how it is decided in a few reptiles, not in human beings.
• changed by the individual, as in snails. Individual snails can change sex. In human beings sex is largely genetically determined.
• Yes, they all use temperature. Temperature decides sex only in some animals such as a few reptiles.
• No, different species use very different strategies — correct. Yes!
• Yes, they all use genes. Sex is genetically determined in human beings, but not in snails, which can change sex.