Inheritance in Biology

Inheritance is the process by which traits are passed from parents to offspring through genes. These genes are segments of DNA located on chromosomes, and they contain instructions for building proteins that shape an organism’s characteristics.

Key points:

• Genes come in pairs — one from each parent.

• Different versions of a gene are called alleles.

• Some alleles are dominant (expressed even if only one copy is present), while others are recessive (expressed only when two copies are present).

• Traits can be simple (controlled by one gene) or complex (influenced by many genes).

Genes and the Environment

Traits don’t come only from genes. Many characteristics result from an interaction between genes and the environment.

Examples of gene–environment interaction:

• Height — genes set a potential range, but nutrition determines how fully that potential is reached.

• Skin color — genes determine pigment production, but sunlight affects actual pigmentation.

• Behavior and intelligence — shaped by genetic tendencies and also by experiences, education, diet, and social factors.

• Health risks — genes might predispose someone to a condition (like diabetes), but lifestyle can strongly influence whether it appears.

Important idea:

Genes provide the blueprint; the environment influences how that blueprint is expressed.

Meiosis

Meiosis is a type of cell division that forms sex cells (sperm and eggs). Each sex cell receives half the usual number of chromosomes. This ensures that chromosome numbers remain stable across generations.

During meiosis:

• Crossing over occurs, where paired chromosomes exchange DNA segments.

• Independent assortment randomly distributes chromosomes into sex cells.

Both increase variation.

Fertilization

When a sperm cell and an egg cell unite, their chromosomes combine. This restores the full chromosome number and creates a new combination of inherited traits.

Types of Inheritance

Mendelian inheritance

Traits controlled by one gene with dominant and recessive alleles.Example: flower color in pea plants.

Polygenic inheritance

Traits controlled by several genes, producing a range of outcomes.Example: height and skin color.

Codominance

Both alleles are fully expressed.Example: AB blood type.

Incomplete dominance

Neither allele is completely dominant, so the trait appears blended.Example: pink flowers from red and white parents.

Sex-linked inheritance

Traits controlled by genes on the sex chromosomes.Example: color blindness.

Mutations

Mutations are changes in the DNA sequence. They can be harmful, neutral, or beneficial. Mutations contribute to variation within populations and can lead to evolutionary change.

DNA

DNA, or deoxyribonucleic acid, is the molecule that stores all genetic information in living organisms. It carries the instructions needed for growth, development, and functioning.

Important points about DNA:

1. DNA has a double helix structure made of two strands.

2. Each strand contains nucleotides with bases: adenine (A), thymine (T), cytosine (C), and guanine (G).

3. A pairs with T, and C pairs with G.

4. DNA sequences form genes.

5. DNA is packaged into chromosomes inside the nucleus.