Edexcel A (SNAB) A Level Biology:復習筆記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.
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