Overview of reproduction and genetic variation
Inheritance, variation and evolution • Reproduction
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Sexual reproduction and gametes
Sexual reproduction involves the fusion of a male gamete and a female gamete. In animals, the male gamete is sperm and the female gamete is an ovum (egg). In flowering plants, the male gamete is pollen and the female gamete is an ovule that contains the egg cell. Fertilisation joins two haploid gametes to form a diploid zygote that develops into a new organism. Gametes form by meiosis, a specialised cell division that halves the chromosome number so that gametes are haploid. Meiosis occurs in the sex organs (ovaries and testes in mammals) and produces gametes that are genetically different from each other.
How sexual reproduction creates variation
Meiosis introduces variation through events such as independent assortment of chromosomes and DNA crossover, where chromosome sections swap between paired chromosomes. These processes produce gametes that each carry a unique combination of alleles. When two different gametes fuse at fertilisation, the offspring inherits a mixture of genetic information from both parents, increasing genetic diversity in the population. Genetic variation provides raw material for natural selection and for populations to respond to environmental change. Offspring from sexual reproduction are not clones of their parents because each contains a unique combination of parental genes.
Asexual reproduction and clones
Asexual reproduction involves a single parent and no fusion of gametes. Offspring arise by mitosis or other non-meiotic processes and are genetically identical to the parent, producing clones. Examples include binary fission in bacteria, vegetative reproduction in plants (runners, bulbs, tubers), and some forms of spore production. Environmental factors can cause phenotypic differences among clones even though the genetic information is the same. Asexual reproduction does not generate new allele combinations because no mixing of parental genomes occurs.
Advantages and disadvantages of sexual reproduction
Advantage: Sexual reproduction produces genetic variation among offspring, which increases the chance that some individuals survive environmental changes or new diseases. Variation supports natural selection and long-term adaptability. Disadvantage: Sexual reproduction often requires more time, energy and two parents, and usually produces fewer offspring per reproductive event. Finding mates and producing gametes can limit rapid population increase compared with asexual methods.
Advantages and disadvantages of asexual reproduction
Advantage: Asexual reproduction enables rapid population increase and efficient colonisation because a single organism can produce many offspring quickly without needing a mate. Processes such as binary fission can generate large numbers in short timescales. Disadvantage: Asexual reproduction produces genetically identical offspring, so the entire population may be vulnerable to the same diseases or environmental changes. Low genetic variation reduces the ability to adapt over time. Selective breeding and inbreeding can further reduce gene pool diversity.
Sex determination in humans (XX and XY)
Human sex is determined by the 23rd pair of chromosomes. All ova carry an X chromosome. Sperm carry either an X or a Y chromosome. Fertilisation of an X ovum by an X sperm produces an XX genotype (female). Fertilisation of an X ovum by a Y sperm produces an XY genotype (male). The father’s sperm therefore determines the genetic sex of the offspring. Approximately equal numbers of X- and Y-bearing sperm lead to roughly equal numbers of male and female births. The terms haploid and diploid describe gamete and body cell chromosome numbers respectively: gametes are haploid (half the full set) and the zygote is diploid (full chromosome set restored at fertilisation).
Key notes
Important points to keep in mind