How do Organisms Reproduce? · Lesson 8 of 14
Sexual Reproduction
“Two sets of genetic information meet and variation gets a powerful boost.”
• Compare sexual and asexual reproduction. • Explain how sexual reproduction increases variation. • Explain the chromosome-number problem. • Describe meiosis as formation of half-set gametes. • Explain how fertilisation restores chromosome number. • Relate gamete specialisation to reproductive roles.
Asexual reproduction can produce many offspring quickly, but most variation comes only from copying differences. Sexual reproduction combines hereditary information from two individuals, creating new combinations in one generation.
Sexual reproduction is reproduction involving fusion of male and female gametes, usually contributed by two parents, to form a zygote.
Why Sexual Reproduction Produces More Variation
Each parent has accumulated a different set of variations across earlier generations. Combining DNA from two parents brings these existing differences together in new arrangements. The offspring is not a simple copy of either parent.
The Chromosome-Number Problem
If ordinary body cells with full chromosome sets fused, the amount of DNA would double in every generation. Organisms avoid this by producing specialised germ cells with half the usual chromosome number. Meiosis creates these gametes. During fertilisation, two half sets unite and restore the normal chromosome number in the zygote.
Meiosis is a specialised cell division that produces germ cells with half the chromosome number of ordinary body cells.
Fertilisation is the fusion of male and female gametes to form a zygote.
Specialisation Of Gametes
In many simple organisms, gametes may look similar. In more complex organisms, the female gamete is usually larger and contains stored material that supports early development, while the male gamete is smaller and motile. These different roles are associated with specialised male and female reproductive organs.
| Feature | Asexual reproduction | Sexual reproduction |
|---|---|---|
| Parents involved | One | Usually two |
| Gamete fusion | Absent | Present |
| Main source of variation | DNA-copying differences | Copying differences plus recombination of parental DNA |
| Speed and simplicity | Often faster and simpler | Requires gametes and fertilisation |
| Offspring | Usually closely resemble parent | Contain new combinations from both parents |
Problem
A species has 20 chromosomes in body cells. How many should each gamete contain?
- 1.Body cells contain the full double set: 20.
- 2.Meiosis halves the chromosome number.
- 3.20 ÷ 2 = 10.
- 4.Each gamete should contain 10 chromosomes.
Problem
Gametes each contain 12 chromosomes. Find the zygote number.
- 1.Each gamete contributes one half set of 12.
- 2.Fertilisation combines the two sets.
- 3.12 + 12 = 24.
- 4.The zygote contains 24 chromosomes.
Problem
A student says two 24-chromosome body cells should fuse to form every offspring. Identify the problem.
- 1.Fusion would produce a 48-chromosome cell.
- 2.Repeating this each generation would double chromosome number continually.
- 3.Specialised meiosis must first reduce each gamete to 12.
- 4.Fusion of 12 and 12 restores 24, maintaining the species number.
An offspring does not normally receive a complete double chromosome set from each parent. Each gamete contributes one reduced set.
Quiz
What does meiosis produce?
Why must chromosome number be halved?
What forms when gametes fuse?
Why does sexual reproduction increase variation?
Which gamete is commonly motile?
Practice Problems
- Compare sexual and asexual reproduction.
- Explain the chromosome-number problem and its solution.
- A body cell has 32 chromosomes. Calculate gamete and zygote numbers.
- Explain how two parents create new combinations without inventing entirely new genes.
- Relate the size and movement of male and female gametes to their roles.
Key Takeaways
• Sexual reproduction combines hereditary information from two parents. • New parental combinations increase variation. • Meiosis halves chromosome number in gametes. • Fertilisation combines gametes and forms a zygote. • The normal chromosome number is restored at fertilisation. • Female gametes commonly contain stored material. • Male gametes are commonly smaller and motile. • Asexual reproduction lacks gamete fusion.