Biology · Principles of Inheritance and Variation · NEET
A test cross is when you take an organism that shows the dominant trait, but you do not know its genotype, and you cross it with a homozygous recessive parent. For example, a tall pea plant (which could be TT or Tt) is crossed with a dwarf plant (tt). The children (progeny) then tell you the hidden genotype of the tall plant.
No, you cannot tell the genotype just by looking. A TT plant and a Tt plant both look tall, because T is dominant and hides the recessive t. So the phenotype (outside look) is the same, but the genotype (inside gene make-up) is different. A test cross is the tool that reveals which one it is.
Always with the homozygous recessive parent (like tt or vv). This is important. The recessive parent can only give recessive gametes (all t). So the offspring's appearance is decided fully by the gamete that comes from the unknown parent. That is why the recessive parent works like a clean testing screen.
Two possible results: (1) If ALL offspring show the dominant trait, the unknown parent was homozygous (TT). (2) If offspring appear in a 1:1 ratio (half dominant, half recessive), the unknown parent was heterozygous (Tt). So a 1:1 ratio is the signal for heterozygous.
The heterozygous parent (Tt) makes two kinds of gametes: half carry T and half carry t. The recessive parent (tt) makes only t gametes. So T meets t to give Tt (dominant), and t meets t to give tt (recessive). That gives 1 dominant : 1 recessive, which is 1:1.
A back cross is crossing the F1 offspring with ANY one of its two parents (dominant or recessive). A test cross is a special back cross where you cross specifically with the HOMOZYGOUS RECESSIVE parent to find the genotype. So every test cross is a back cross, but not every back cross is a test cross.
Yes. In a dihybrid test cross, an F1 that is heterozygous for two genes is crossed with the double homozygous recessive. If the genes are on different chromosomes, you expect a 1:1:1:1 ratio. If more parental-type offspring appear than recombinant-type, it means the two genes are linked (on the same chromosome).
In a plant, black seed color (BB/Bb) is dominant over white seed color (bb). In order to find out the genotype of the black seed plant, which of the following genotype will you cross it with?
A heterozygous pea plant with violet flowers was crossed with a homozygous pea plant with white flowers. Violet is dominant over white. Which one of the following represents the expected combinations among 40 progenies formed?
Match List I with List II. A. Two or more alternative forms of a gene; B. Cross of F1 progeny with homozygous recessive parent; C. Cross of F1 progeny with any of the parents; D. Number of chromosome sets in plant. List II: I. Back cross; II. Ploidy; III. Allele; IV. Test cross.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
No. A 1:1 ratio comes only when the unknown parent is heterozygous (Tt x tt). If the unknown parent is homozygous (TT x tt), then all offspring show the dominant trait, so there is no split. The result itself is what tells you the genotype.
Mendel used it. To find the genotype of a tall F2 pea plant, he crossed it with a dwarf plant (homozygous recessive) instead of self-crossing it. He called this a test cross.
You can, but it is slower and less clear. Self-crossing a Tt plant gives a 3:1 ratio, and you need many offspring to be sure. A test cross gives a clean, easy 1:1 (or all-dominant) result that is quicker to read.
No. A back cross means crossing offspring with either parent. It becomes a test cross only when the parent used is the homozygous recessive one. So a test cross is a special type of back cross.
