Heredity and Evolution — Notes

Accumulation of Variation During Reproduction

Variation refers to the differences that exist between individuals of the same species. These differences can be small, like variations in height or eye colour, or significant, like resistance to a disease. Reproduction is the process by which new individuals are produced.

Inheritance from the previous generation provides the blueprint for the new individual. This means that offspring resemble their parents and inherit their basic body design. However, reproduction is not a perfect copy-paste process.

Inheritance from the previous generation

New individuals receive genetic information from their parents. This inheritance ensures that the offspring possess the fundamental characteristics of their species, such as the number of limbs, the presence of a backbone, or the general body plan. For example, a human baby inherits the basic human body design from its parents.

However, inheritance is not simply about receiving identical copies of traits. Subtle differences, or variations, are also passed down. These variations, combined with newly created differences in the offspring, contribute to the diversity within a population.

Accumulation of Variation During Reproduction

Reproduction is the engine for generating and accumulating variations within a population over generations.

• Asexual Reproduction: Even in asexual reproduction, where a single parent produces genetically identical offspring, small inaccuracies in DNA copying can lead to minor variations. These variations are usually very subtle and do not significantly alter the offspring's overall appearance or function. Imagine a bacterium dividing; the two new bacteria will be almost identical, with only minuscule differences.

• Sexual Reproduction: Sexual reproduction, involving two parents, amplifies the creation of variation.

• It involves the fusion of gametes (sperm and egg) from two individuals. Each gamete carries a unique combination of genes from each parent.

• When these gametes fuse, the resulting zygote inherits a mix of genes from both parents, creating a new combination of traits.

• This process, along with the inherent variations within each parent's gametes, leads to offspring that are distinct from both parents and from each other.

• The diversity generated by sexual reproduction is greater than that from asexual reproduction.

The continuous process of reproduction, generation after generation, allows these variations to accumulate. While some variations might be minor and have little impact, others can be more significant.

Remember

While asexual reproduction produces variations primarily due to minor DNA copying errors, sexual reproduction generates much greater diversity through the combination of genes from two parents and inherent variations in gametes.

Exam-smart

Questions might present scenarios where a population shows high diversity or very little diversity. You would need to relate this to the mode of reproduction (sexual vs. asexual) and infer that sexual reproduction leads to greater accumulation of variations over time. For instance, a field of identical sugarcane plants suggests asexual reproduction or selective breeding, while a population of diverse animals points to sexual reproduction.

Selection of variants by environmental factors

Environmental factors act as selective pressures, determining which variations within a population are more likely to survive and reproduce. This process, known as natural selection, means that individuals with traits better suited to their environment have a higher chance of passing those advantageous traits to their offspring. Over generations, this leads to the gradual evolution of a species as beneficial variations become more common.

For example, consider a population of bacteria. If the environment suddenly becomes much hotter, heat-resistant bacteria will survive and multiply, while those that are not heat-resistant will die. The trait of heat resistance, which was a variation in the original population, becomes advantageous. The surviving bacteria reproduce, and the next generation will have a higher proportion of heat-resistant individuals.

This is a fundamental mechanism of evolution:

• Variation: The raw material for evolution, arising from genetic changes.
• Environmental pressure: Factors like climate, predators, food availability, or disease that favor certain variations.
• Differential survival and reproduction: Individuals with advantageous variations are more likely to survive and reproduce.
• Inheritance: Advantageous traits are passed on to offspring.
• Adaptation: Over time, the population becomes better suited to its environment.

Remember

Natural selection doesn't create new traits; it acts upon existing variations. The environment "selects" the fittest individuals based on their inherited traits.

Exam-smart

A question might present a scenario with a population and an environmental change (e.g., drought, introduction of a new predator). You'll need to identify which pre-existing variation would be most beneficial for survival and explain how it would become more common in the population over time due to natural selection.

HEREDITY

Inherited Traits

Inherited traits are characteristics passed down from parents to their offspring. These traits are determined by genes, which are segments of DNA. DNA carries the genetic information that dictates various features of an organism, from physical appearance to predisposition to certain conditions. In sexually reproducing organisms, an individual receives two copies of each gene, one from each parent.

Rules for the Inheritance of Traits – Mendel’s Contributions

Gregor Johann Mendel, an Austrian monk, laid the foundation for our understanding of heredity through his experiments with pea plants. He meticulously studied contrasting visible characters, such as seed shape (round vs. wrinkled), plant height (tall vs. short), and flower colour (violet vs. white).

Mendel's key contributions include:

• Quantitative Analysis: He was the first to use mathematical and statistical methods to analyse inheritance patterns, by counting individuals exhibiting specific traits in each generation. This approach allowed him to derive precise laws of inheritance.
• Laws of Inheritance: His experiments led to the formulation of fundamental principles, which are now known as Mendel's Laws. These laws explain how traits are transmitted from one generation to the next.

Mendel's Experimental Approach:

1. He selected pea plants with distinct, contrasting traits.
2. He performed controlled crosses between these plants.
3. He observed and counted the traits present in the progeny (offspring) of each generation.

Example: Crossing a tall pea plant with a short pea plant.

• First Generation (F1): All plants were tall. This indicated that one trait masked the other.
• Second Generation (F2): When F1 plants self-pollinated, the F2 generation showed a mix of tall and short plants, with approximately one-quarter being short. This revealed that both traits (tallness and shortness) were present in the F1 generation, but only tallness was expressed.

Inheritance of Traits Over Two Generations

Mendel's experiments clearly demonstrated how traits are inherited across generations. He tracked specific characteristics through the first filial (F1) and second filial (F2) generations.

• F1 Generation: When parents with contrasting traits are crossed, the F1 generation displays only one of the parental traits. For example, crossing pure-bred tall and short pea plants results in all tall plants in the F1 generation. This shows that one trait, the dominant trait, masks the expression of the other, the recessive trait.
• F2 Generation: When F1 individuals self-pollinate, the F2 generation shows both parental traits, typically in a specific ratio. In the tall/short plant example, the F2 generation usually exhibits a ratio of approximately 3 tall plants to 1 short plant. This ratio reflects the segregation of the two inherited factors (genes) during gamete formation.

The genotypic ratio in the F2 generation for a monohybrid cross is typically 1:2:1 (e.g., TT:Tt:tt), where TT and Tt represent tall plants, and tt represents short plants. This ratio further explains the phenotypic ratio of 3:1 (tall:short).

Dominant and Recessive Traits

In sexually reproducing organisms, for any given trait, there are two copies of the gene, one inherited from each parent. These two copies, called alleles, may be identical or different.

• Dominant Trait: If the two alleles for a trait are different, the allele that expresses itself and masks the expression of the other allele is called the dominant trait. In Mendel's experiments, tallness in pea plants was dominant over shortness.
• Recessive Trait: The allele whose expression is masked by the dominant allele is called the recessive trait. Shortness in pea plants was recessive to tallness.

A trait is expressed in an individual based on the combination of alleles they possess:

• Homozygous Dominant: Two dominant alleles (e.g., TT). The dominant trait is expressed.
• Heterozygous: One dominant and one recessive allele (e.g., Tt). The dominant trait is expressed.
• Homozygous Recessive: Two recessive alleles (e.g., tt). The recessive trait is expressed.

Independent Inheritance of Two Separate Traits

Mendel also investigated the inheritance of two different traits simultaneously. This is known as a dihybrid cross.

Experiment: Crossing a pea plant with tallness and round seeds (dominant for both) with a plant with shortness and wrinkled seeds (recessive for both).

• F1 Generation: All plants were tall with round seeds, indicating that tallness and roundness were dominant.
• F2 Generation: When F1 plants self-pollinated, the F2 generation showed not only the original combinations (tall, round and short, wrinkled) but also new combinations: tall with wrinkled seeds, and short with round seeds.

This led to the Law of Independent Assortment, which states that the alleles for different traits segregate independently of each other during gamete formation. This means that the inheritance of one trait does not influence the inheritance of another trait, provided the genes for these traits are on different chromosomes.

In a dihybrid cross, the phenotypic ratio in the F2 generation is typically 9:3:3:1 for the four possible combinations of traits.

How Do These Traits Get Expressed?

Traits are expressed through the action of proteins, which are synthesized based on the information encoded in genes.

1. DNA and Genes: A gene is a specific segment of DNA that contains the instructions for making a particular protein.
2. Proteins and Traits: Proteins perform a vast array of functions in the body. Many of these proteins act as enzymes that catalyze biochemical reactions.
3. Mechanism of Expression: For example, the height of a pea plant is influenced by plant hormones that promote growth. The production of these hormones depends on the efficiency of specific enzymes. A gene that codes for an efficient enzyme will lead to the production of sufficient hormone, resulting in a tall plant. If the gene has a mutation that makes the enzyme less efficient, less hormone is produced, leading to a shorter plant. Thus, genes control traits by dictating the synthesis of proteins that carry out specific functions.

During sexual reproduction, each parent contributes a set of chromosomes (and thus genes) to the offspring. Germ cells (sperm and egg) are haploid, meaning they contain only one set of chromosomes. When a sperm and egg fuse during fertilization, the resulting zygote is diploid, restoring the full complement of chromosomes and ensuring the inheritance of traits from both parents.

Sex Determination

The mechanism by which the sex of an individual is determined varies across species.

• Environmental Factors: In some reptiles, the temperature during incubation determines the sex of the developing offspring.
• Genetic Factors: In many species, including humans, sex is primarily determined by genes.

Sex Determination in Human Beings

In humans, the sex of an individual is determined by a specific pair of chromosomes called sex chromosomes.

• Females have two identical sex chromosomes, denoted as XX.
• Males have two different sex chromosomes, denoted as XY.

When gametes (sperm and egg) are formed, each carries one sex chromosome:

• Eggs always carry an X chromosome.
• Sperm can carry either an X or a Y chromosome.

The sex of the child is determined by the type of sperm that fertilizes the egg:

• If a sperm carrying an X chromosome fertilizes the egg (which also carries X), the resulting zygote will be XX, developing into a female.
• If a sperm carrying a Y chromosome fertilizes the egg (which carries X), the resulting zygote will be XY, developing into a male.

Therefore, it is the father who determines the sex of the child, as he contributes either the X or the Y chromosome.

Remember

In humans, the sex chromosomes are X and Y. Females are XX, and males are XY. The Y chromosome carries the gene that initiates male development.

Exam-smart

Case study questions might describe a scenario of inheritance with specific traits and ask to identify dominant/recessive traits or predict offspring genotypes/phenotypes. For sex determination, expect questions about how the X and Y chromosomes lead to male or female offspring.