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Monohybrid crosses

Monohybrid crosses. By the end of this lesson:. You should be able to:. Know what a monohybrid cross Know how to draw out a monohybrid cross Predict the proportions of phenotypes of the F 2 offspring in a monohybrid cross Know why there is a difference between

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Monohybrid crosses

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  1. Monohybrid crosses

  2. By the end of this lesson: • You should be able to: • Know what a monohybrid cross • Know how to draw out a monohybrid cross • Predict the proportions of phenotypes of the F2 offspring in a monohybrid cross • Know why there is a difference between observed and predicted ratios in monohybrid crosses

  3. F1 and F2 generations • When 2 parents are crossed they produce offspring. • We refer to these offspring as the First Filial generation (F1) • When we do genetics experiments, we often take individuals from the F1 generation, and cross them • Their offspring are called the Second Filial generation (F2 )

  4. X Parents F1 generation Then we cross two members of the F1 generation

  5. F1 generation X F2 generation

  6. Monohybrid Crosses • In standard grade we have to work out theoretical crosses. • When we do this we only consider one characteristic at a time • This is called a monohybrid cross

  7. An example • The gene for coat colour in mice has 2 alleles • Brown which is dominant • White which is recessive • What GENOTYPE would a HOMOZYGOUS brown mouse have? • BB • What GENOTYPE would a HOMOZYGOUS white mouse have? • bb

  8. Brown mouse White mouse Parents phenotype X BB bb Parents genotype All b All B Gametes F1 genotype All Bb F1 phenotype All heterozygous brown Now we cross 2 individuals from the F1 generation

  9. Brown mouse Brown mouse F1 phenotype X Bb Bb F1 genotype B or b B or b Gametes F2 genotype Aargghh, how do we work this out????? Use a punnett square

  10. Punnett square Gametes from one parent Gametes from other parent

  11. Results! • So all the possible F2 generation genotypes are • BB (Brown) • Bb (Brown) • Bb (Brown) • bb (white) • The F2phenotypic ratio would be • 3 brown : 1 white • The F2genotypic ratio would be • 1 BB: 2 Bb : 1 bb • 1:2:1

  12. Does this happen in nature? • It is important to remember that when we are working out ratios, that it is only probable as fertilisation is a random process, and the true ratio is a matter of chance.

  13. Work • Insert and complete the “Genetics- Monohybrid Cross” sheet from your diagram pack.

  14. Work • Calculate the F2 generation phenotypic and genotypic ratios of a cross between these flies- homozygous straight- winged and curved-winged fruit fly • Straight wing is dominant

  15. Work • Once you are confident that you understand how to write out a monohybrid cross, ask your teacher for a question sheet. • Remember to set our your answers in the correct format, as you have been shown!

  16. By the end of this lesson: • Can you do the following? • Do you know what a monohybrid cross is? • Can you draw out a monohybrid cross? • Can you predict the proportions of phenotypes of the F2 offspring in a monohybrid cross? • Explain why there could be a difference between the observed and predicted ratio

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