Sexual Reproduction in Humans

Sexual Reproduction in Humans

The process

  • Sexual reproduction in humans involves the fusion of two haploid gametes (sperm and egg) creating a diploid zygote. This process is known as fertilisation.
  • The male gamete, or sperm, is produced in the testes through a process called spermatogenesis.
  • The female gamete, or egg, is produced in the ovaries through a process known as oogenesis.

Genetic Variation

  • One of the key benefits of sexual reproduction is the generation of genetic variation.
  • This variation arises from two main sources - independent assortment of chromosomes during meiosis and crossing over of homologous chromosomes.
  • Random fertilisation also leads to genetic variation, as any sperm can potentially fertilize any egg.

Meiosis and Independent Assortment

  • Chromosomes form homologous pairs in the first division of meiosis. How these pairs align at the metaphase plate is random and leads to different combinations in the resulting gametes - this is known as independent assortment.
  • There are 2^23 possible combinations in humans due to independent assortment, leading to millions of potential genetic combinations.

Crossing Over

  • Crossing over takes place during prophase I of meiosis where corresponding segments of homologous chromosomes are exchanged.
  • Crossing over results in recombination of genetic material, further increasing genetic variation.

Disorders of Sexual Reproduction

  • Errors during meiosis can lead to aneuploidy, a condition in which the offspring has an abnormal number of chromosomes.
  • Examples include Down’s syndrome (trisomy 21) and Turner’s Syndrome (monosomy X).

The role of Genes and the Environment

  • Although genes provide the blueprint, the environment in which an individual grows and develops can significantly influence the phenotype.
  • This represents the concept of genotype-environment interaction and is crucial in the study of inheritance and variation.

DNA and Inheritance

  • The DNA molecule is made up of two chains of nucleotides that are arranged in a double helix structure.
  • DNA is copied through a process called DNA replication, allowing genetic information to be passed down to offspring.
  • The base-pairing rule, where adenine pairs with thymine and guanine pairs with cytosine, is crucial for accurate replication.

Genes and Alleles

  • A gene is a segment of DNA that codes for a specific trait, whereas an allele is a variation of a gene.
  • Alleles can be dominant, where only one copy is needed for the trait to be expressed, or recessive, where two copies are needed.
  • Homozygous individuals have two copies of the same allele, whereas heterozygous individuals have two different alleles of a gene.

Evolution and Natural Selection

  • Natural selection is the process by which individuals with characteristics that are advantageous for the environment survive and reproduce more successfully, influencing the genetic makeup of future generations.
  • Over time, these changes can lead to the emergence of new species, a process known as speciation.
  • This forms the basis of Darwin’s theory of evolution.