Hybridisation

Concept:
Polyembryony is the phenomenon in which more than one embryo develops within a single seed. It is commonly observed in certain plant species such as Citrus and Mango. Polyembryony can arise in different ways depending on the origin of embryos:

• True polyembryony: Multiple embryos develop from a single fertilized egg (zygote).
• Cleavage polyembryony: The zygote divides into several embryos.
• Adventive polyembryony: Embryos arise from somatic tissues of the ovule such as the nucellus or integuments, in addition to the normal zygotic embryo.

In plants like Citrus, embryos develop from the diploid nucellar cells surrounding the embryo sac along with the fertilized zygote. These additional embryos are genetically identical to the parent plant because they originate from somatic tissue.
Step 1: Understanding the origin of embryos in Citrus.
In Citrus seeds, apart from the zygotic embryo formed by fertilization, several embryos arise from diploid nucellar cells present in the ovule.
Step 2: Identifying the type of polyembryony.
Since these extra embryos arise from somatic cells (nucellus) rather than from the zygote, the phenomenon is called:
Step 3: Eliminating incorrect options.

• (A) Adventive Embryos arise from nucellus or integuments (Correct)
• (B) Cleavage Zygote splits into multiple embryos
• (C) True Multiple embryos formed from fertilized egg
• (D) Defective Not a recognized biological term for polyembryony

Final Answer:

Concept: Hybridization depends on the steric number: The steric number determines the hybridization.

Step 1: Determine the bonding in . Sulfur forms six sigma bonds with six fluorine atoms.

Step 2: Find the steric number.

Step 3: Determine the hybridization. For steric number : Thus,

Concept: Hybridization is the mixing of atomic orbitals of similar energy to form new equivalent hybrid orbitals. The type of hybridization depends on the number of regions of electron density (sigma bonds or lone pairs) around the central atom.

Step 1: Determine the bonding in . The structure of carbon dioxide is: Carbon forms two double bonds with two oxygen atoms.

Step 2: Count regions of electron density. Each double bond contains one sigma bond. Therefore, carbon has:
• 2 sigma bonds
• 0 lone pairs Total regions of electron density .

Step 3: Determine hybridization. Two regions of electron density correspond to hybridization and a linear molecular geometry with bond angle . Thus, the hybridization of carbon in is .

Concept: Hybridization describes the mixing of atomic orbitals to form equivalent hybrid orbitals. In the ethyne molecule ( ), each carbon atom forms a triple bond with another carbon atom and a single bond with hydrogen.

Step 1: Determine the bonding around carbon. Each carbon forms:
• One -bond with hydrogen
• One -bond with the other carbon
• Two -bonds between the carbon atoms

Step 2: Count the regions of electron density. Each carbon has two regions of electron density (two -bonds).

Step 3: Identify the hybridization. Two regions of electron density correspond to hybridization, giving a linear geometry with a bond angle of .

Concept: A dihybrid cross involves the inheritance of two different traits simultaneously. According to Mendel’s Law of Independent Assortment, the alleles of different genes assort independently during gamete formation. Consider two traits:
• Seed shape: (Round) dominant over (Wrinkled)
• Seed color: (Yellow) dominant over (Green) A typical dihybrid cross is:

Step 1: Determine the possible gametes. Each parent can produce four types of gametes:

Step 2: Form the Punnett square combinations. Combining the four gametes from each parent gives possible offspring combinations.

Step 3: Determine the phenotypes. The resulting phenotypes are:
• Round Yellow
• Round Green
• Wrinkled Yellow
• Wrinkled Green

Step 4: Write the phenotypic ratio.

Concept: Hybridization describes the mixing of atomic orbitals to form equivalent hybrid orbitals. The type of hybridization depends on the number of electron domains around the atom.

Step 1: Write the structure of ethyne. Each carbon atom forms:
• One sigma bond with hydrogen
• One sigma bond with another carbon
• Two pi bonds in the triple bond

Step 2: Determine hybridization. Each carbon has two sigma bonds. Thus, the hybridization is .

Step 3: Determine geometry. The geometry of -hybridized carbon is linear with bond angle: Hence, carbon in ethyne has hybridization and linear geometry.

Concept:
A dihybrid cross involves two pairs of contrasting traits. According to Mendel’s law of independent assortment, alleles of different genes assort independently during gamete formation.

Step 1: Consider the parental cross.
Example:

Step 2: F generation.
All offspring are heterozygous:

Step 3: F generation.
Self-cross: Gametes formed:

Step 4: Phenotypic ratio.
On combining these gametes, four phenotypes appear in the ratio:

Step 5: Interpretation.
• 9: Both dominant traits
• 3: First dominant, second recessive
• 3: First recessive, second dominant
• 1: Both recessive