10.2.4 Skills: 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

• 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 were then interbred to produce the F₂ generation
• Alleles:
  • P = purple flowers, dominant to p = red flowers
  • L = long seeds, dominant to l = round seeds

• 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

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)