BRAINCAKEChi-Squared Test- Dihybrid Crosses
For a dihybrid cross there are usually four phenotype categories, so there are 4 − 1 = 3 degrees of freedom, and the critical value at p = 0.05 is 7.82.
Worked example: two Drosophila heterozygous for two genes on different chromosomes are crossed. The expected ratio is 9 : 3 : 3 : 1. The 160 offspring are 94 wild type, 26 with vestigial wings only, 32 with ebony body only and 8 with both.
Null hypothesis: there is no significant difference between the observed numbers and those expected from a 9 : 3 : 3 : 1 ratio.
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Key terms in this lesson
- degrees of freedom
- The number of categories in a statistical test minus one.
- critical value
- The value of a test statistic, such as chi-squared, that must be reached or exceeded for a difference to be significant.
- null hypothesis
- A statement that there is no significant difference between observed and expected results, and any difference is due to chance.
More in Genetic Information
- Drosophila
- Probability
- Chi-Squared Test
- Chi-Squared Test - Monohybrid Crosses
- Gene Linkage
- Identifying Linked Genes
- Chromosome Mapping
- Sex-Linkage
All 30 lessons in Genetic Information · All Edexcel A-level Biology topics