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AQA A Level Biology復習筆記7.1.4 Predicting Inheritance: Test Crosses

Category: A-level課程, 教材筆記, 福利干貨 Date: 2022年6月29日下午4:51

Predicting Inheritance: Test Crosses

A test cross can be used to?deduce the?genotype?of an unknown individual that is expressing a dominant?phenotype
The individual in question is crossed with an individual that is expressing the?recessive phenotype
This is because an individual with a recessive phenotype has a known genotype
The resulting phenotypes of the offspring provides sufficient information to suggest the genotype of the unknown individual

Results

For a monohybrid test cross:
- If?no?offspring exhibit the recessive phenotype then the unknown genotype is?homozygous dominant
- If?at least one?of the offspring exhibit the recessive phenotype then the unknown genotype is?heterozygous
For a dihybrid test cross:
- If no offspring exhibit the recessive phenotype for either gene then the unknown genotype is?homozygous dominant?for both genes
- If at least one of the offspring exhibit the recessive phenotype for one gene but not the other, then the unknown genotype is?heterozygous for one gene and homozygous dominant for the other
- If at least one of the offspring exhibit the recessive phenotype for both genes then the unknown genotype is?heterozygous for both?genes

Worked Example

Worked example: Test crosses

Rabbits have a single gene for ear length that has two alleles:
- D, a dominant allele that produces long ears
- d, a recessive allele that produces shorter ears
A breeder has a rabbit called Floppy that has long ears and they want to know the genotype of the rabbit
- There are two possibilities:?DD?or?Dd
The breeder crosses the long-eared rabbit with a short-eared rabbit
- A rabbit displaying the recessive short ear phenotype has to have the genotype?dd

Test cross possibility one table

Test-Cross-Possibility-One-Table

Predicted ratio of phenotypes of offspring – 1 long ears

Predicted ratio of genotypes of offspring – 1 Dd

Test cross possibility two table

Test-Cross-Possibility-Two-Table

Predicted ratio of phenotypes of offspring – 1 long ears : 1 short ears

Predicted ratio of genotypes of offspring – 1 Dd : 1 dd

The breeder identifies the different phenotypes present in the offspring
There is at least one offspring with the short ear phenotype
This tells the breeder that their rabbit Floppy has the genotype?Dd
If Floppy was genotype?DD?none of the offspring would have short ears

Exam Tip

Make sure before you start a test cross you think about the following: how many genes are there, how many alleles of each gene are there, which is the dominant allele, what type of dominance is it and is there linkage or epistasis between genes?

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