12.7
Test Cross
Alleles are different forms of the same gene. Humans and other diploid organisms inherit two alleles of every gene, one from each parent.
An allele is…
A test cross is a technique used to determine the genotype of an organism expressing a dominant trait, such as purple flower color in pea plants.
Because a dominant allele is expressed over a recessive allele, organisms with the homozygous dominant genotype, containing two dominant alleles, appear the same as organisms with the heterozygous genotype, containing one dominant allele and one recessive allele.
A purple-flowered pea plant of unknown genotype is crossed with a white-flowered pea plant having a homozygous recessive genotype.
Because the genotype of the white-flowered parent is known, the offspring phenotypes reveal whether the unknown parent is homozygous dominant or heterozygous.
In one scenario, if all offspring from the test cross display the dominant phenotype, the unknown parent must be homozygous dominant for purple flowers.
All offspring are heterozygous and receive one recessive allele from the recessive parent and one dominant allele from the other parent.
In another scenario, if the offspring include equal numbers of dominant and recessive phenotypes, the unknown parent must be heterozygous.
All of the offspring still receive their recessive allele from the recessive parent.
Half of the offspring receive a dominant allele from the heterozygous parent and exhibit the dominant phenotype, while the other half receive a recessive allele and exhibit the recessive phenotype.
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Q1: Why can't you determine an organism's genotype just by looking at a dominant trait?
Two different genotypes produce the same dominant phenotype. A homozygous dominant organism (PP) and a heterozygous organism (Pp) both display the dominant trait because the dominant allele masks the recessive one. Since you cannot distinguish between them visually, a test cross is needed to reveal the unknown genotype.
Q2: What organism do you cross with in a test cross and why?
You cross the organism with unknown genotype with one displaying the recessive phenotype, which must be homozygous recessive (pp). This parent can only contribute recessive alleles to offspring, making the recessive parent's contribution predictable and allowing the unknown parent's genotype to be revealed through offspring phenotypes.
Q3: What do all-dominant offspring tell you about the unknown parent's genotype?
If all offspring display the dominant phenotype, the unknown parent must be homozygous dominant (PP). All offspring receive one recessive allele from the homozygous recessive parent and one dominant allele from the homozygous dominant parent, resulting in heterozygous (Pp) offspring that all express the dominant trait.
Q4: What does a 1:1 ratio of dominant to recessive offspring reveal?
A 1:1 ratio indicates the unknown parent is heterozygous (Pp). Half the offspring inherit the dominant allele and display the dominant phenotype, while the other half inherit two recessive alleles and display the recessive phenotype. This equal split reveals the heterozygous genotype of the unknown parent.
Q5: How does a test cross relate to monohybrid crosses?
A test cross is a specific type of cross used to determine unknown genotypes, while monohybrid crosses examine inheritance of a single trait. Test crosses use monogenic traits and monohybrid crosses principles to analyze offspring ratios and deduce parental genotypes based on predictable inheritance patterns.
Q6: Why is the recessive parent's genotype always known in a test cross?
Only one genotype produces a recessive phenotype: homozygous recessive (pp). An organism displaying a recessive trait must have two copies of the recessive allele because the recessive allele is only expressed when no dominant allele is present to mask it.
Q7: Can test crosses be used with traits showing multiple alleles or incomplete dominance?
Test crosses are designed for simple dominant-recessive traits where one allele completely masks the other. Traits with multiple alleles or incomplete dominance require different analytical approaches because the phenotypes do not follow the simple dominant-recessive pattern that test crosses depend on.