In mirabilis jalapa, why flowers on white branches always produced white progeny?
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Answer:
One of the earliest and best known examples of cytoplasmic inheritance is that discovered by Correns in a variegated variety of the four-o'clock plant, Mirabilis jalapa. Variegated plants have some branches which carry normal green leaves, some branches with variegated leaves (mosaic of green and white patches) and some branches which have all white leaves.
Correns discovered that seed produced by flowers carried on the green branches gave progeny which were all normal green. It made no difference whether the phenotype of the branch which carried the flower used for pollen was green, white or variegated. Seed taken from white branches likewise gave all white progeny, regardless of the pollen donor phenotype. These of course died in the seedling stage. Seeds from flowers on variegated branches gave three kinds of progeny, green, white and variegated, in varying proportions; again regardless of the pollen donor phenotype. In other words, the phenotype of the progeny always resembled the female parent and the male made no contribution at all to the character. The effect is seen quite clearly in the difference which Correns found between reciprocal crosses.
The explanation for this unusual pattern of inheritance is that the genes concerned are located in the chloroplasts within the cytoplasm, not in the nucleus, and are therefore transmitted only through the female parent. In eukaryote organisms the zygote normally receives the bulk of its cytoplasm from the egg cell and the male gamete contributes little more than a nucleus. Any genes contained in the cell organelles of the cytoplasm will therefore show maternal inheritance. The leaf variegation is due to two kinds of chloroplasts: normal green ones and defective ones lacking in chlorophyll pigment. Chloroplasts are genetically autonomous (i.e., self-determining) and have their own system of heredity in the form of chloroplast chromosomes. These are small circular naked DNA molecules which carry genes controlling some aspects of chloroplast structure and function. A mutation in one of these genes, which affects the synthesis of chlorophyll as in Mirabilis, will therefore follow the chloroplast in its transmission and will not be inherited in the same way as a nuclear gene.
Inheritance of leaf variegation in Mirabilis jalapa. The character is controlled by cytoplasmic genes located in the chloroplast chromosome. Chloroplasts are self-perpetuating cell organelles and during sexual reproduction they are only transmitted through the cytoplasm of the egg cell, as undifferentiated protoplastids. They are not inherited through the pollen. The progeny of crosses therefore have the characters of the female parent and show maternal inheritance.
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