Lesson 1 of 6 / Reproduction in humans
Sexual reproduction combines genetic information
What event forms a zygote, and why are offspring genetically varied?
In this lesson: Define sexual reproduction using gamete nuclear fusion, a zygote and genetically dissimilar offspring.
About 4 min
The key ideaSexual reproduction involves fusion of male and female gamete nuclei to form a zygote and produces genetically dissimilar offspring.
A gamete is a reproductive cell. In humans the male gamete is a sperm and the female gamete is an egg; the zygote is the cell formed after fertilisation.
Explore the idea
What forms the zygote?
A sperm and an egg are gametes. Each contains a nucleus with genetic information.
Original sequence model, not to scale or a timed record. The cell-division image represents an early stage; implantation involves a larger developing ball of cells.
Explanation
Sexual reproduction involves the fusion of the nuclei of male and female gametes. In humans, a sperm nucleus fuses with an egg nucleus. This event is fertilisation, and the resulting cell is a zygote.
Each gamete contributes genetic information. Combining contributions gives the zygote a new combination rather than an identical copy of either parent. Sexual reproduction therefore produces genetically dissimilar offspring.
Contact between two cells is not the complete definition of fertilisation: the key event is nuclear fusion. An egg being released from an ovary is ovulation, a different event that can occur without fertilisation.
After a zygote forms, it can divide repeatedly as development begins. These later divisions increase cell number; they are distinct from the fusion of two gamete nuclei that first formed the zygote.
Step by step
- 1
Identify the gametes
In humans, sperm and egg supply the male and female nuclei.
- 2
Name the event
Nuclear fusion is fertilisation.
- 3
State the result
A zygote forms with a combination of genetic information.
Worked example
Correct the definition
A student defines sexual reproduction as an egg leaving an ovary. What essential event and result are missing?
One way to explain it
The definition needs fusion of male and female gamete nuclei to form a zygote, and the production of genetically dissimilar offspring. Egg release is ovulation, not fertilisation.
Why this answer works
- Distinguish release from fusion.
- Name the resulting zygote.
- Include genetic difference in the definition.
Is this true? "A zygote is an unfertilised egg waiting to be released."
A zygote forms after male and female gamete nuclei fuse. An unfertilised egg is still a gamete.