Sexual Reproduction | Biology for GCSE/IGCSE - Year 11 PDF Download

Gametes & Zygotes

Sexual Reproduction

• Sexual reproduction involves the merging of two sex cells' nuclei to create a fertilized egg cell known as a zygote, leading to the birth of genetically distinct offspring.
  • This process results in the combination of genetic material from two distinct parents, ensuring diversity in the offspring.
• Fertilization describes the union of gamete nuclei, where each gamete originates from different parents, contributing to genetic variation in the offspring.
  • The fusion of gamete nuclei during fertilization is crucial for the genetic diversity seen in offspring.

Gametes

• Gametes: Gametes are specialized sex cells, such as sperm and ovum in animals and pollen nucleus and ovum in plants.
• Chromosome Difference: Unlike normal body cells, gametes have half the number of chromosomes, termed as a haploid nucleus. Normal human body cells have 46 chromosomes, while each gamete contains 23 chromosomes.
• Zygote Formation: When male and female gametes fuse during fertilization, they form a zygote, which is a fertilized egg cell. The zygote comprises the complete set of 46 chromosomes, with half inherited from each parent, resulting in a diploid nucleus.

Haploid & Diploid Cells

• Gamete Nuclei: Haploid cells have half the chromosomes of normal body cells. In humans, this equals 23 chromosomes.
• Zygote Nuclei: Diploid cells have the same chromosome count as normal body cells, which is 23 pairs in humans. The zygote maintains its diploid state as it develops into a fetus and embryo during pregnancy.

Advantages & Disadvantages of Sexual Reproduction

• When crop plants reproduce sexually, variation is increased. This can lead to the production of genetic variants that are better adapted to cope with changes in weather conditions or that yield significantly higher harvests. Example: A crop plant may produce offspring that are more resistant to drought due to increased genetic variation.
• However, this increased variation can also result in offspring that are less successful than the parent plant in terms of growth or harvest quality. Example: The genetic variability may lead to some plants being less productive in terms of crop yield compared to the parent plant.