Conduct practical investigations to predict variations in the genotype of offspring by modelling meiosis, including the crossing over of homologous chromosomes, fertilisation and mutations (ACSBL084)

Variations due to Fertilization, Crossing Over of Chromosomes and Mutations:

• During the formation of zygote, the single celled zygote receives 50% trait from each parent. Thus, the genotype of the zygote is a “mixture” of that of the parents and thus, the zygote, when develops into a complete organism shows variant characteristics.
• Crossing over during meiosis cell division also accounts for genetic variation, because due to the swapping of genetic material during crossing over, the chromatids held together by the centromere are no longer identical. Homologous chromosomes, one inherited from each parent pair along their lengths, gene by gene. Breaks occur along the chromosomes, and they re-join, trading some of their genes. The chromosomes now have genes in a unique combination.
• Mutations (alterations in gene due to a number of external factors including chemicals and environmental radiations) have a great amount of contribution in genotypic variation because:
  • Mutations can introduce new character which means new genotype
  • Mutations can alter certain traits by absolutely silencing a particular gene thus producing genotypic variation
  • Mutations can also produce different products from similarly sequenced mRNA transcripts

Example of genotypic variation due to crossing over:

• Taking the example of Drosophila melanogaster
  • In Drosophila, two pairs of characters are involved in a dihybrid cross. Mutant fly having recessive characters of pink eyes (r) and curled wings (s) is crossed with a wild fly having dominant characters of red eyes (R) and straight wings (S).
  • In this cross, all the progeny of F1 hybrid shows red eyes and straight wings having genotype RrSs.
  • Now a female from these F1 hybrids is crossed with a double recessive male of P generation which is known as a test cross. This F1 hybrid female will produce four types of gametes.
  • When F1 female gametes mated with single type of male gametes, the F2 generation consists of 49% flies with red eyes and straight wings, 49% with pink eyes and curled wings, 1% with red eyes and curled wings and 1% with pink eyes and straight wings.
  • This result in F2 generation shows that the two types are non-cross overs which combine to form 98% and two types of new combinations or re-combinations of the remaining 2% are produced due to crossing over.
  • This experiment shows that both the genes for each allelic pair are situated in the same chromosome. They connected together in 98% gametes having no chromosomal interchange but in the remaining 2% gametes there is interchange between their non-sister chromatids of the homologous chromosomes. This interchange of segments of chromatids occurs due to crossing over.

Example of genotypic variation due to mutation:

• Sickle Cell Anaemia:
  • Sickle-Cell Anaemia is an autosomal recessive disorder that affects 1 in 500 African Americans.
  • The single replacement of the sixth amino acid in the beta-globin, glutamic acid, with valine results in deformed red blood cells.
  • These sickle-shaped cells cannot carry nearly as much oxygen as normal red blood cells and they get caught more easily in the capillaries, cutting off blood supply to vital organs.

Extract from HSC Biology Stage 6 Syllabus. © 2017 Board of Studies NSW.