Why Is a Test Cross Important for Determining Genetic Makeup?
Test Cross and Back Cross
Any organism is nothing but a by-product of its genetic makeup and environment. If you want to learn about it in detail, then you have to know the genetic vocabulary, terms, and concepts. This article talks about the test cross and how it helps recognize the genotype. It gets performed between an organism exhibiting the dominant trait and another organism displaying the recessive trait. Gregor Mendel, also famous as the father of modern genetics, is the founder of test cross. Before you understand the difference between back cross and test cross, you must know what a test cross is? This article takes you through it all, so sit back and keep reading till the end.
What are Genotype and Phenotype?
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In biology, a gene refers to a section of DNA that encodes a trait. The particular arrangement of nucleotides (each composed of a phosphate group, sugar, and a base) in a gene can vary from copies of the same gene. Thus, a gene can occur in various forms across organisms. Those various forms are nothing but alleles. The precisely fixed position on the chromosome that contains a specific gene is a locus.
A diploid organism has two copies of a single allele or one copy of two different alleles from their parents. When the individual inherits two similar alleles, it means they have a homozygous genotype at that locus. But, if they have two different alleles, then it refers to a heterozygous genotype at that locus. A subsequent combination of alleles possessed by an individual for a specific gene is their genotype.
Phenotype refers to a total of an organism's observable characteristics. The primary difference between genotype and phenotype is that genotype gets inherited from the parents of an organism. And phenotype is the opposite. The phenotype, however, does get influenced by genotype and not vice versa. Factors like epigenetic modifications, environment and lifestyle do affect the phenotype.
What is a Test Cross?
A test cross is a method of exploring the genotype of an organism. Earlier, the test cross got used as an experimental mating test to figure out the alleles present in the genotype. During the test cross, the F1 hybrid gets crossed back with the recessive parent.
In simple words, the test cross is the cross of an organism having an unknown dominant genotype with an organism that is homozygous recessive for that trait. Test cross can help you determine if the individual getting tested is homozygous dominant or heterozygous dominant.
What is a Back Cross?
Backcross refers to the mating of hybrid organisms with one of its parents or an organism that is genetically equivalent to the parent. In other words, breeding of F1 hybrids with either of the two parents gets referred to as a backcross.
The backcross is helpful in genetics studies to isolate particular characteristics in a related group of animals or plants. In animal breeding, backcross also gets called a top cross. It gets widely used in horticulture, animal breeding, and for the production of gene knockout organisms.
Back cross example – particular crop plant hybrids get backcrossed with wild species to recover their helpful traits like high yield, disease resistance, etc.
The fundamental difference between the test cross and backcross is that the test cross gets used to discriminate against the genotype of a phenotypically dominant individual. And backcross gets used to recover an elite genotype from a parent carrying it.
Difference Between Back Cross and Test Cross
FAQs on How Does a Test Cross Help Identify Genotype?
1. What is a test cross in genetics, and what is its primary purpose?
A test cross is a specific type of genetic cross where an organism expressing a dominant phenotype (but having an unknown genotype) is bred with another organism that is homozygous recessive for the same trait. The primary purpose of a test cross is to determine whether the dominant organism is homozygous dominant (e.g., TT) or heterozygous (e.g., Tt) for that trait by analysing the phenotypes of the offspring.
2. How does a test cross help in identifying the genotype of an organism?
A test cross identifies an unknown genotype by observing the phenotypes of the progeny. The logic is as follows:
- Scenario 1: If the unknown parent is homozygous dominant (e.g., AA), all offspring will receive a dominant allele from it. When crossed with a homozygous recessive parent (aa), all offspring will have a heterozygous genotype (Aa) and will display the dominant phenotype.
- Scenario 2: If the unknown parent is heterozygous (e.g., Aa), it will produce two types of gametes (A and a). When crossed with a homozygous recessive parent (aa), approximately half the offspring will be heterozygous (Aa) showing the dominant phenotype, and the other half will be homozygous recessive (aa) showing the recessive phenotype. This results in a 1:1 phenotypic ratio.
Therefore, the appearance of any recessive offspring proves the unknown parent was heterozygous.
3. Can you explain a test cross with a classic example from Mendel's pea plants?
Certainly. Let's consider the trait for flower colour in pea plants, where purple (P) is dominant over white (p). If we have a purple-flowered plant, its genotype could be either PP (homozygous dominant) or Pp (heterozygous). To determine its genotype, we perform a test cross with a white-flowered plant, which must have the genotype pp.
- If the purple parent is PP: The cross is PP x pp. All offspring will have the genotype Pp and will be purple-flowered.
- If the purple parent is Pp: The cross is Pp x pp. The offspring will have genotypes Pp and pp in a 1:1 ratio, resulting in a 1:1 phenotypic ratio of purple flowers to white flowers.
4. What is the key difference between a test cross and a back cross?
A back cross is a general term for crossing an F1 generation hybrid with one of its parents (either the dominant or the recessive one). A test cross is a more specific type of back cross. In a test cross, the F1 hybrid is crossed specifically with the homozygous recessive parent. Therefore, every test cross is a back cross, but not every back cross is a test cross.
5. Why must the parent used for a test cross always be homozygous recessive?
The parent must be homozygous recessive because it only contributes recessive alleles to the offspring. This ensures that the phenotype of the offspring is a direct reflection of the alleles contributed by the unknown dominant parent. If a homozygous dominant or heterozygous parent were used instead, its dominant alleles would mask the recessive alleles from the tested parent, making it impossible to determine its genotype from the offspring's appearance.
6. What do the different resulting ratios in a test cross signify?
The phenotypic ratio of the offspring is the key indicator of the unknown parent's genotype:
- A result where 100% of the offspring display the dominant phenotype signifies that the unknown parent is homozygous dominant.
- A result showing a 1:1 ratio of dominant to recessive phenotypes signifies that the unknown parent is heterozygous.
7. Can a test cross be used for a dihybrid cross as well?
Yes, a test cross is a very powerful tool for dihybrid crosses. For instance, to find the genotype of a pea plant that is Round and Yellow (phenotype), its genotype could be RRYY, RRYy, RrYY, or RrYy. By crossing it with a homozygous recessive plant (wrinkled and green, rryy), the phenotypic ratios of the offspring will reveal the exact genotype of the parent. A 1:1:1:1 ratio in the offspring, for example, would confirm the parent was heterozygous for both traits (RrYy).
