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Sexual reproduction and genetic variation

Review your understanding of sexual reproduction and genetic variation in this free article aligned to NGSS standards.

Key points

  • Many sexually reproducing organisms are diploid. Diploid organisms have two sets of chromosomes in each cell. These chromosomes are organized into homologous pairs.
  • In order to reproduce, diploid organisms produce gametes in the form of egg or sperm cells. These gametes are haploid. Haploid gametes have only one set of chromosomes.
  • Sexual reproduction happens via fertilization. Fertilization is the fusion of gametes from two parents. Fertilization leads to new, diploid offspring.
  • Sexual reproduction results in genetic variation, or genetic differences, between parents and offspring. Offspring inherit one set of chromosomes from each parent. So, an offspring has a mixture of chromosomes (and alleles) from its two parents.
  • Sexual reproduction also results in genetic variation among siblings. When a parent forms a gamete, only one chromosome from each homologous pair is included at random. So, an offspring might inherit a different combination of chromosomes (and alleles) compared to its siblings.
The left side of a diagram shows a cell belonging to Parent 1 with two chromosomes inside. One chromosome is pink and the other is yellow. The chromosomes are labeled “a pair of homologous chromosomes”. Two arrows point away from the parent cell, toward cells labeled “possible gametes”. One of the gametes contains a pink chromosome and the other contains a yellow chromosome. The right side of the diagram shows a cell belonging to Parent 2 with two chromosomes inside. One chromosome is light blue and the other is dark blue. The chromosomes are labeled “a pair of homologous chromosomes”. Two arrows point away from the parent cell, toward cells labeled “possible gametes”. One of the gametes contains a light blue chromosome and the other contains a dark blue chromosome. Underneath the gametes is a dotted line. Below the dotted line are four cells in a row labeled “possible offspring”. The cell on the far left contains one pink and one dark blue chromosome. The next cell to the right contains one pink and one light blue chromosome. The next cell to the right contains one yellow and one dark blue chromosome. The cell on the far right contains one yellow and one light blue chromosome.
A diagram showing how sexual reproduction leads to genetic variation. Offspring have a mixture of chromosomes from both parents. Offspring can also inherit a different combination of chromosomes compared to their siblings. Image created with Biorender.com.

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