How Traits Are Expressed, and Sex Determination: Quick Revision

A brief note on How Traits Are Expressed, and Sex Determination. Read it before a test. Try each question at the end before you open its answer.

From gene to trait

• DNA is the information source for making proteins.
• A gene is a section of DNA for one protein.
• An enzyme is a protein. In a pea plant, one enzyme makes a growth hormone.
• Efficient enzyme, much hormone: a tall plant. Poor enzyme, little hormone: a short plant.
• A body cell has two copies of every gene, one from each parent. So both parents shape each trait.

Chromosomes and inheritance

• Genes lie on chromosomes, which come in pairs in body cells.
• Human body cells: $46$ chromosomes, $23$ pairs.
• A gamete carries one chromosome of each pair: $23$ in humans.
• Fertilisation gives the full number again, so it stays constant.
• Genes on different chromosomes are passed on independently, which gives new mixtures of traits.

How sex is determined

• Environment: in some reptiles the temperature of the eggs decides; some snails change sex.
• Humans: $22$ matched pairs and one pair of sex chromosomes.
• Women are XX; men are XY, with a short Y.
• Every egg carries X. Half the sperm carry X, half carry Y.
• X sperm: a girl (XX). Y sperm: a boy (XY). The father's sperm decides.
• Each birth has an equal chance of a girl or a boy.

A worked example

A body cell of an onion has $16$ chromosomes. How many does its gamete carry, and how many does the zygote have?

1. The gamete. One chromosome of each pair: $\dfrac{16}{2} = 8$.
2. The zygote. One gamete from each parent: $8 + 8 = 16$.
3. The point. The number returns to $16$, so it stays constant.

Practice with new numbers

A program to try

The program prints the pairs and the gamete number for three organisms.

Mistakes to avoid

• A gene is not the trait. It is information that works through a protein.
• A gamete does not carry full pairs. It carries one of each pair.
• The mother does not decide the sex. All her eggs carry X.
• Earlier births do not change the chance. It is one half each time.
• Not all animals use genes to decide sex. Some reptiles use temperature.

Check yourself

1. Name the kind of protein that speeds up the making of a growth hormone.

2. How many chromosomes does a human egg carry?

3. Which sex chromosomes does a woman have?

4. Why is the chance of a girl one half at every birth?

5. Why does a child of a tall parent and a short parent get genes from both?