Edexcel A (SNAB) A Level Biology:复习笔记2.4.5 Monohybrid Crosses

Monohybrid Crosses

  • Monohybrid inheritance looks at how the alleles for a single gene are passed on from one generation to the next
  • Known information about the genotypes, phenotypes and the process of meiosis are used to make predictions about the phenotypes of offspring that would result from specific breeding pairs
  • When two individuals sexually reproduce there is an equal chance of either allele from their homologous pair making it into their gametes and subsequently the nucleus of the zygote
    • This means there is an equal chance of the zygote inheriting either allele from their parent
  • Genetic diagrams are often used to present this information in a clear and precise manner so that predictions can be made
    • These diagrams include a characteristic table called a Punnett square
  • The predicted genotypes that genetic diagrams produce are all based on chance
    • There is no way to predict which gametes will fuse so sometimes the observed or real-life results can differ from the predictions

Genetic crosses can be shown by a Punnet square

Worked Example

Worked example: Genetic diagram

  • One of the genes for the coat colour of horses has the following two alleles:
    • B, a dominant allele produces a black coat when present
    • b, a recessive allele produces a chestnut coat when present in a homozygous individual
  • In this example a heterozygous male is crossed with heterozygous female

Parental phenotype: Black coat x Black coat

Parental genotypes: Bb x Bb

Parental gametes: B or b x B or b

Monohybrid Punnett square with Heterozygotes Table

  • Predicted ratio of phenotypes in offspring – 3 black coat : 1 chestnut coat
  • Predicted ratio of genotypes in offspring – 1 BB : 2 Bb : 1 bb

Exam Tip

You need to be able to interpret genetic diagrams so take some time to get familiar with different sorts of diagrams such as pedigree charts and Punnet squares.