Biology - Test crosses
How can the genotype  of an organism for a particular gene be worked
out if all we see is the phenotype? If the organism shows the
characteristic resulting from the recessive  allele  we know that it
must be homozygous  recessive.  For an organism showing the dominant
characteristic in its phenotype,  how do we know whether its genotype is
homozygous dominant  or heterozygous? 
The organism of known genotype for the characteristic, that is homozygous
recessive, is crossed with the 'unknown'. If any of the offspring show the
characteristic due to the recessive allele, then the 'unknown' must be
heterozygous for that characteristic.
If none of the offspring show this phenotype does it necessarily mean the
'unknown' is homozygous?
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