Accumulation Of Variation During Reproduction

Inheritance from the previous generation provides the offspring with a fundamental body design, along with subtle variations. When this new generation reproduces, it also passes on its inherited traits and variations, along with any newly created variations from DNA copying errors in its own reproductive process.

In asexual reproduction (like bacterial fission), the offspring are very similar, with only minor variations arising from DNA copying inaccuracies. If a bacterium divides into two, and those two divide again, the four resulting bacteria will be almost identical, with minimal differences due to slight copying errors.

In contrast, sexual reproduction generates significantly greater diversity. Because it involves combining genetic material from two individuals, the offspring inherit a mix of variations already present in the parents, plus new variations. This leads to a more rapid accumulation of diverse traits within a population.

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The Importance Of Variation

Variation plays a crucial role in the survival of species, especially in changing environments. Organisms are generally well-suited to their specific ecological niche (environment). However, environments are not static and can change due to various factors (temperature shifts, climate change, geological events, etc.).

If all individuals in a population were identical, a drastic environmental change to which they were not suited could wipe out the entire population. However, if variations are present among individuals, some variants might possess traits that allow them to tolerate or survive the new conditions (e.g., resistance to heat, ability to utilize a different food source). These surviving individuals can then reproduce, passing on their advantageous variations, and the population can adapt to the changed environment. Thus, variation is vital for the long-term survival and adaptability of a species, even if some variations may not be beneficial or even harmful to an individual in a stable environment.

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Heredity

Despite variations, the fundamental outcome of reproduction is the generation of new individuals with a similar body design to the parents. The rules governing how traits and characteristics are passed down from parents to offspring are determined by **heredity**. Heredity ensures that offspring inherit the basic features of their species while also exhibiting unique variations.

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Inherited Traits

**Inherited traits** are characteristics or features that are passed down from parents to their offspring through genes. While a child shares basic features with its parents (e.g., being human), it is not an exact copy; variations exist within the human population. Simple examples of inherited traits in humans include eye colour, hair colour and texture, height, and also features like attached or free earlobes.

Studying the inheritance patterns of specific traits within families or populations helps to understand the rules of heredity.

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Rules For The Inheritance Of Traits – Mendel’s Contributions

The fundamental rules of inheritance were worked out by **Gregor Johann Mendel** through experiments with garden pea plants (Pisum sativum) in the 19th century. Mendel studied the inheritance of specific **contrasting traits** (e.g., tall/short plants, round/wrinkled seeds, violet/white flowers). His methodical approach, involving crossing plants with different traits and counting the number of progeny exhibiting each trait across generations, allowed him to deduce the patterns of inheritance.

Mendel's experiments showed that:

• When crossing a tall pea plant with a short pea plant (parental generation, P), the first generation offspring (F1 progeny) were all tall. No intermediate heights were observed. This suggested that one trait (tallness) was expressed, while the other (shortness) was not.

• When the F1 tall plants were self-pollinated, the second generation offspring (F2 progeny) were not all tall. About three-quarters were tall, and one-quarter were short. This indicated that the 'shortness' trait was still present (inherited) in the F1 generation, even though it was not expressed. It reappeared in the F2 generation.

Based on these results, Mendel proposed that each trait is controlled by two 'factors' (now called **genes**). Sexually reproducing organisms inherit one copy of each gene from each parent. These gene copies (alleles) can be identical or different.

• A trait is called **dominant** if a single copy of the gene is sufficient for it to be expressed (e.g., the gene for tallness 'T').
• A trait is called **recessive** if two copies of the gene are required for it to be expressed (e.g., the gene for shortness 't'). If a dominant allele is present, the recessive allele's effect is masked.

In the tall/short example, the gene combinations (genotypes) are TT (homozygous tall), Tt (heterozygous tall), and tt (homozygous short). Both TT and Tt plants are tall (phenotype), while only tt plants are short. Thus, tallness (T) is dominant over shortness (t).

Mendel also studied the inheritance of two traits simultaneously (dihybrid crosses), such as seed shape (round/wrinkled) and seed colour (yellow/green). When crossing a tall plant with round seeds (dominant traits) and a short plant with wrinkled seeds (recessive traits), the F1 generation had all tall plants with round seeds.

When the F1 plants (tall with round seeds) were self-pollinated, the F2 generation showed not only the original combinations (tall-round, short-wrinkled) but also **new combinations** (tall-wrinkled, short-round). This led Mendel to conclude that traits are inherited **independently** of each other. The inheritance of seed shape is independent of the inheritance of seed colour.

This principle of independent assortment holds true for genes located on different chromosomes. Genes on the same chromosome may be linked.

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How Do These Traits Get Expressed?

Genes, which are segments of DNA, are the information source for making **proteins**. Proteins perform various functions in the cell and body, ultimately controlling the traits we observe.

For example, plant height can depend on the amount of a specific plant hormone. The production of this hormone is controlled by enzymes, which are proteins. The gene for a particular enzyme provides the instructions for making that enzyme. If the gene functions efficiently, enough enzyme is made, leading to sufficient hormone production and a tall plant. If the gene has an alteration (mutation) that makes the enzyme less efficient, less hormone is produced, resulting in a short plant. Thus, genes control traits by controlling the production of proteins.

In sexually reproducing organisms, offspring inherit two copies of each gene, one from each parent. These two copies are located on homologous chromosomes, one maternal and one paternal. For this mechanism to work and maintain the correct amount of DNA across generations, the germ cells (gametes - sperm and egg) must contain only a single set of genes (one copy of each chromosome).

This is achieved through **meiosis**, a special type of cell division that reduces the chromosome number by half during gamete formation. When a male gamete (with one set of chromosomes) and a female gamete (with one set of chromosomes) combine during fertilisation, they form a zygote with the normal diploid number of chromosomes (two sets), one set from each parent. This restores the species' DNA content and ensures genetic stability while allowing for the combination of traits from two parents.

While the rules of inheritance apply to asexually reproducing organisms as well, their inheritance mechanisms are simpler (passing on a copy of a single set of genes), resulting in less variation compared to sexual reproduction.

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Sex Determination

One crucial trait determined by inheritance in many species is sex. Different species have different mechanisms for sex determination. Some are influenced by environmental factors (e.g., temperature during egg incubation in some reptiles), while others are purely genetic.

In **human beings**, sex is largely **genetically determined** by the sex chromosomes. Humans have 23 pairs of chromosomes: 22 pairs of autosomes and one pair of sex chromosomes.

• Females have two X chromosomes (XX).
• Males have one X chromosome and one Y chromosome (XY).

During gamete formation (meiosis):

• Females produce eggs that all contain one X chromosome.
• Males produce sperms of two types: half contain an X chromosome, and half contain a Y chromosome.

The sex of the child is determined at the time of fertilization, depending on which type of sperm fertilizes the egg:

• If a sperm carrying an **X chromosome** fertilizes the egg (which always carries X), the zygote will have XX chromosomes, developing into a **female child**.
• If a sperm carrying a **Y chromosome** fertilizes the egg, the zygote will have XY chromosomes, developing into a **male child**.

This means the father determines the sex of the child. Statistically, there is an equal chance (50%) of a child being male or female, as the probability of an X sperm or a Y sperm fertilizing the egg is roughly equal.

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Page No. 129

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Page No. 133

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