Railway Science (Biology-Reproduction in Plant)

Unisexual flowers - A flower that has either the male or female reproductive organs in separate flowers. Examples : Papaya, watermelon, Pumpkin, Pine etc. Bisexual Flowers - A flower that has both male and female reproductive organs in separate flowers. Examples : Lily, Rose, Sunflower, Tulip, Daffodil, Mustard, Brinjal, Hibiscus, Tomato, etc.

Plant fertilization is the process of fusion of male and female reproductive cells or gametes to produce a fertilized egg, which is a zygote. This zygote develops further and results in a flower and later fruit in the case of gymnosperms.
The process of fertilization in plants is facilitated by pollination, germination, etc. There are three types of fertilization processes (Porogamy, Chalazogamy, Mesogamy) based on the entrance site of the pollen tube into the ovule.

Plants having genotype TtWw produce 4 different kinds of gametes TW, Tw, tW, tw. One of the four gametes carry recessive alleles for both the characters (tw). Hence, the percentage of gametes with two recessive alleles is 1/4 × 100% = 25%. Genotype is the genetic arrangement that makes up the traits that an organism inherited from its parents. Phenotype refers to an individual's observable traits, such as height, eye color and blood type.

7 pairs of contrasting characters in pea plants studied by Mendel are Flower color (Purple or White), Pea shape (Round or Wrinkled), Pea color (Green or Yellow), Pod shape (Constricted or Inflated), Pod color (Green or Yellow), Plant size (Tall or Dwarf) and Position of flowers (Axial or Terminal).

Round and yellow seeds (RrYy) are produced in F1 generation, If a round, green seeded pea plant (RRyy) is crossed with wrinkled, yellow seeded pea plant, (rrYY). When this plant is subjected to self pollination, in the F2 generation, 9 Yellow round, 3 yellow wrinkled, 3 green round and 1 green wrinkled plants are formed. The dihybrid phenotypic ratio is 9 : 3 : 3 : 1.

Gregor Mendel conducted hybridisation experiments on garden peas for seven years (1856-1863) and proposed the laws of inheritance in living organisms. He conducted such artificial pollination/cross pollination experiments using several true-breeding pea lines. He selected 14 true-breeding pea plant varieties, as pairs which were similar except for one character with contrasting traits.

When a pure tall plant (TT) is crossed with the pure short plant (tt), then the progeny in the F1 generation will be hybrid (Tt). When the F1 generation is self-crossed (Tt), then in the F2 generation, the progeny produced will be tall homozygous (TT), Tall heterozygous (Tt), and dwarf homozygous in the ratio of 1:2:1. This type of cross between a single pair of contrasting characters is called a monohybrid cross. The tall and dwarf plants were produced in a ratio of 3:1 (75% tall plants and 25% dwarf plants). Mendel discovered the fundamental laws of inheritance.

Fertilisation - When two male gametes (pollen) mate with two female gametes (ovum), they produce a diploid zygote. Types of Fertilisation in Plants : Porogamy is the process of fertilization of a seed plant involving the passage of the pollen tube into the ovule by the micropyle.
Chalazogamy - Process of fertilization in which the pollen tube penetrates to the embryo sac through the tissue of the chalaza. Mesogamy - In this type of fertilization, the pollen tube enters the ovule through its middle part or through the integuments of the ovule.

Mendel experiments - Round and yellow seeds (RrYy) are produced in the F1 generation. If a round, green seeded pea plant (RRyy) is crossed with wrinkled, yellow seeded pea plant, (rrYY).
When this plant is subjected to self pollination, in the F2 generation, 9 Yellow round, 3 yellow wrinkled, 3 green round and 1 green wrinkled plants are formed. The dihybrid phenotypic ratio is 9 : 3 : 3 : 1. Mendel’s experiments show that the tall/short trait and the round seed/wrinkled seed trait are independently inherited.

Mendel’s laws of inheritance consist : Law of segregation (Second Law of Inheritance) - It states that during the production of gametes, two copies of each hereditary factor segregate so that offspring acquire one factor from each parent.
Law of Dominance - It states that Hybrid offspring will only inherit the dominant trait in the phenotype. The alleles that are suppressed are called the recessive traits while the alleles that determine the trait are known as the dominant traits.
Law of Independent Assortment - It states that a pair of traits segregate independently of another pair during gamete formation. Given by - Gregor Mendel. Year - 1865.