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First exams 2025

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Identifying Recombinants (HL) (HL IB Biology)

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Identifying Recombinants

  • Genetic diagrams involving autosomal linkage often predict solely parental type offspring (offspring that have the same combination of characteristics as their parents)
  • However in reality recombinant offspring (offspring that have a different combination of characteristics to their parents) are often produced
    • This is due to the crossing over that occurs during meiosis
    • The crossing over and exchanging of genetic material breaks the linkage between the genes and recombines the characteristics of the parents

Crossing over of bivalents diagram

Genetic Variation Crossing Over

The process of crossing-over results in recombinant phenotypes that can differ from the parental phenotype.
  • The frequency of recombinants within a population will nearly always be less than that of non-recombinants
    • Crossing over is random and chiasmata form at different locations with each meiotic division
  •  Recombination frequency between two linked genes is greater when genes are further apart on the same chromosome
    • There are more possible locations for a chiasma to form between the genes

Identifying recombinants using test crosses

  • Test crosses are often used to determine unknown genotypes
  • Similarly, they can be used to identify recombinant phenotypes in offspring
  • An individual is crossed with a homozygous recessive individual (for both traits)
    • If any of the offspring possess a non-parental phenotype then they are labelled as recombinants
      • These individuals have new allele combinations due to the process of crossing over during meiosis leading to the exchange of genetic material between chromosomes

Drawing a Punnett square to show dihybrid inheritance of linked genes

  • A number of sweet pea plants were generated by crossing double-homozygous dominant plants (PL)(PL) with double-homozygous recessive plants (pl)(pl) to produce a 100% heterozygous F1 generation (PL)(pl) as expected
  • Members of this generation then interbreed to produce the F2 generation
  • Alleles:
    • P = purple flowers, dominant to p = red flowers
    • L = long seeds, dominant to l = round seeds

Possible Gametes Table

t_eXWW5F_f1-recombinant-gametes

 F2 Punnet Square Showing Possible Genotypesf2-genotypes

  • According to Mendelian ratios and the Punnett square, the F2 generation should follow the typical 9:3:3:1 ratio
  • However, in reality, the frequency of recombinant gametes will be much lower than that of parental gametes
    • This affects the resulting offspring phenotypes, with fewer recombinant phenotypes occurring than expected

Expected vs Predicted Phenotypes Table

expected-vs-predicted-phenotype-table

Observations

  • More of the F2 offspring than expected showed the parental phenotypes
  • Fewer plants with recombinant phenotypes were produced than the 9:3:3:1 ratio would suggest
  • The actual ratios found were referred to as 'non-Mendelian' as they didn't follow Mendel’s pattern
  • However, this was not zero; some recombinants were still being produced

Possible Theories to Explain These Findings

  • At the time, it was known that many genes were carried on a few chromosomes
  • The idea that certain genes share the same chromosome was being developed by many scientists
  • This suggested that genes could be inherited together, not by the law of independent assortment as put forward by Mendel
  • The idea of linkage of genes was developed to explain the non-Mendelian ratios
    • The frequency of recombinant phenotypes is lower because crossing over is a random process and the chiasmata do not always form in the same place for each meiotic division
    • The frequency of recombinant gametes also depends on the closeness of linkage between the two genes
      • Genes located close together on a chromosome are less likely to be separated by crossing over
      • So recombinants of those two genes will be less frequent
  • Thomas Hunt Morgan later provided proof of linkage to explain non-Mendelian ratios in his experimentation with fruit flies (Drosophila melanogaster)

Examiner Tip

Remember to distinguish between sex linkage and autosomal linkage. The explanation of non-Mendelian ratios falls into the domain of autosomal linkage for IB.

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Emma

Author: Emma

Expertise: Biology

Prior to working at SME, Emma was a Biology teacher for 5 years. During those years she taught three different GCSE exam boards and two A-Level exam boards, gaining a wide range of teaching expertise in the subject. Emma particularly enjoys learning about ecology and conservation. Emma is passionate about making her students achieve the highest possible grades in their exams by creating amazing revision resources!