Heredity-Notes

Accumulation of variation during reproduction refers to the gradual increase of small differences in the genetic material that occur when organisms reproduce. These differences, or variations, are introduced in different ways depending on whether reproduction is asexual or sexual. In asexual reproduction, since there is only one parent involved and no mixing of genes, the offspring are mostly identical to the parent. However, small variations can still arise due to errors during DNA copying or mutations. These minor changes accumulate slowly over generations but are generally limited in extent.

In sexual reproduction, variation occurs more extensively because the offspring inherit a unique combination of genes from two parents. During the formation of gametes (sperm and egg cells) through meiosis, processes like crossing over (where homologous chromosomes exchange parts) and independent assortment (random distribution of chromosomes) create new gene combinations. When two gametes fuse during fertilization, the resulting offspring has a genetic makeup that is different from both parents, leading to a wide range of variations in traits. This accumulation of variation is passed from generation to generation and is vital for the genetic diversity within populations.

These accumulated variations are important for the survival and evolution of species because they provide the raw material for natural selection. Variations that give certain individuals an advantage in their environment increase their chances of survival and reproduction, allowing beneficial traits to be passed on more frequently. Thus, variation accumulated during reproduction helps organisms adapt to changing environments and enhances the overall fitness of populations.

To summarize:

• Asexual reproduction results in limited variation due to mutations during DNA replication.
• Sexual reproduction creates extensive variation through genetic recombination, independent assortment, and fertilization.
• Accumulated genetic variations contribute to diversity, adaptation, and evolution of species. This explains why no two individuals are exactly alike and why species can evolve over time through the gradual build-up of genetic differences across generations.

Inherited traits are characteristics or features that are passed down from parents to their offspring through genes. These traits are determined by the genetic material present in the DNA and are present in an organism from birth.
Examples of inherited traits in humans include

• eye colour,
• hair colour,
• blood group,
• attached or
• free earlobes, and
• the ability to roll the tongue.

Inherited traits differ from acquired traits, which are developed during an organism's lifetime due to environmental influences or personal experiences and are not transmitted to the next generation. For instance, skills like riding a bicycle or scars from injuries are acquired traits and cannot be inherited genetically.

Difference between Inherited and Acquired Traits

Gregor Mendel, known as the “Father of Genetics,” devised the foundational rules for the inheritance of traits after conducting systematic experiments with pea plants. His contributions dramatically changed our understanding of how characteristics are transmitted from parents to offspring.

Mendel selected pea plants (Pisum sativum) because they exhibit clear, contrasting traits (such as tall/short, round/wrinkled seeds, and purple/white flowers). He crossed plants with different traits and studied the distribution of these traits over multiple generations, carefully counting and analyzing thousands of plants. Through his patient observations, Mendel discovered predictable patterns in how traits are inherited.

Mendel’s Rules for Inheritance:

• 1. Law of Dominance:
  When two different alleles for a trait come together in an organism (such as T for tallness and t for shortness), only one (the dominant allele) expresses itself in the organism’s appearance. The recessive allele remains hidden but is not lost. For example, in a cross between tall (TT) and short (tt) pea plants, all offspring in the first generation (F₁) show the tall trait, as tallness dominates over shortness.
• 2. Law of Segregation:
  Every trait is controlled by two alleles, one from each parent. During the formation of gametes (sperm or egg cells), these alleles segregate (separate) so that each gamete carries only one allele for each trait. When fertilization occurs, the offspring inherits one allele from each parent, restoring the pair. This law explains why recessive traits can reappear in the next generation.
• 3. Law of Independent Assortment:
  When considering multiple traits (like seed shape and seed colour), Mendel noticed that the inheritance of one trait does not affect the inheritance of another. The alleles for different traits are passed on independently of each other, which leads to new combinations in the progeny.

Mendel’s Experiments and Insights:

• Mendel’s methodical cross-pollination experiments produced large data sets, allowing him to establish mathematical ratios for inheritance (such as the 3:1 ratio for dominant and recessive traits in the second filial, F₂ generation).
• He proved that traits do not blend but are inherited as distinct units (genes).
• Mendel’s conclusions apply to all sexually reproducing organisms and form the basis of modern genetics.

Mendel’s pioneering work laid out clear, logical rules for the inheritance of traits, empowering future scientists to explore genes, inheritance patterns, and genetic diseases. His laws remain fundamental for studying heredity in the NCERT Class X Science curriculum.

Traits get expressed through the information carried in genes, which are specific segments of DNA present in the chromosomes of an organism. These genes produce proteins by a two-step biological process called transcription and translation. The proteins produced may act as enzymes or hormones, which directly influence the formation, development, and functioning of various physical and biochemical characteristics visible in an organism. This visible expression of traits is known as the phenotype.

The way traits are expressed depends on the interaction between alleles, which are different forms of a gene. Some alleles are dominant, which means their traits appear in the organism whenever present, while others are recessive and only appear if two copies are inherited (one from each parent). In heterozygous organisms having one dominant and one recessive allele, the dominant trait is usually expressed. Besides dominance, there are cases like incomplete dominance, where the phenotype is a blend of both alleles, and codominance, where both alleles show expression simultaneously.

Additionally, the environment can influence how genes are expressed, making the process dynamic and complex. Expression can be controlled at multiple levels in the cells, and sometimes multiple genes contribute to a single trait. Therefore, trait expression results from the combined effect of genetic instructions and environmental factors, leading to the unique characteristics of an individual organism.

In summary, traits get expressed as proteins coded by genes, influenced by dominant-recessive relationships between alleles and modulated by cellular and environmental factors, which together determine an organism's observable characteristics.

• Variations arising during the process of reproduction can be inherited.
• These variations may lead to increased survival of the individuals.
• Sexually reproducing individuals have two copies of genes for the same trait. If the copies are not identical, the trait that gets expressed is called the dominant trait and the other is called the recessive trait.
• Traits in one individual may be inherited separately, giving rise to new combinations of traits in the offspring of sexual reproduction.
• Sex is determined by different factors in various species. In human beings, the sex of the child depends on whether the paternal chromosome is X (for girls) or Y (for boys).