Genes and Inheritance Chapter Notes | Year 9 Science IGCSE (Cambridge) PDF Download

Chromosomes, Genes, and DNA

Genetic Blueprint

Chromosomes

• Chromosomes are thread-like structures found in the nucleus of every cell. They play a crucial role in controlling the activities of the cell.
• Chromosomes were first discovered in the 19th century when advancements in microscopy allowed scientists to observe cell structures more clearly.
• These structures are visible under a light microscope only during cell division and require special stains to be seen clearly.
• In dividing plant cells, stained chromosomes can appear dark red, making other cell structures less visible.
• The common fruit fly, known as Drosophila melanogaster , has 8 chromosomes that are notably thick, referred to as giant chromosomes, which resemble long, striped threads.
• Different species have varying numbers of chromosomes. For instance, human cells contain 46 chromosomes, while mango tree cells have 40 chromosomes.
• Chromosomes are numbered by length, starting with chromosome 1, the longest, and continuing in descending order.
• Human cells have two of each type of chromosome, forming pairs. This is evident in photographs where chromosomes are arranged by size.
• Just before a cell divides, each chromosome makes a copy of itself, and the two copies remain joined together, giving the chromosome a cross-like appearance.

Genes

• Genes are the basic units of heredity found within each chromosome. A single chromosome can contain hundreds of genes, all arranged in a specific order along its length.
• Each gene is responsible for controlling a particular characteristic or trait of an organism. For instance, a gene might determine the colour of a person's eyes or the shape of a plant's leaves.
• To give you an idea of how many genes are on different chromosomes, chromosome 1 in humans has about 2,000 genes, while the shorter chromosome 15 has around 600 genes.
• Scientists are still figuring out what all the genes do, but they do know some of their specific jobs. For example, two genes on chromosome 15 help decide what colour a person's eyes will be.
• Genes can come in different versions, which is why people and animals can have different traits. For example, there are different versions of the eye colour gene that can make someone have blue eyes or brown eyes.

DNA

• Chromosomes are composed of a substance called DNA, with each chromosome consisting of one long molecule of DNA. Since genes are segments of chromosomes, they are also made of DNA.
• The structure of a DNA molecule is known as a double helix, which resembles a twisted ladder. On average, a single gene contains about 2,500 twists in its DNA.
• The double helix structure of DNA is too small to be visible using the microscopes typically used to observe chromosomes.
• DNA was identified in the 1950s, and researchers have since discovered that the DNA within genes governs the traits of organisms.
• DNA holds a complete set of instructions necessary for constructing a functioning cell and, ultimately, an entire organism.

In the next section, we will delve into the concept of gametes and their significance in the process of inheritance.

Gametes

Fertilization Process

Gametes are special cells in humans that have 23 chromosomes, which are needed for sexual reproduction. These cells are responsible for passing on genetic information from parents to their children.

Sperm Cells: These are the male gametes. Sperm cells are much smaller than egg cells and have a few key features:

• Nucleus: Contains 23 chromosomes.
• Cytoplasm: Very small amount.
• Tail: Helps the sperm swim to the egg.

Egg Cells: These are the female gametes. Egg cells are larger than sperm cells and have their own set of important features:

• Size: About the size of a pinhead, much larger than sperm.
• Nucleus: Contains 23 chromosomes.
• Cytoplasm: Contains food reserves.
• Cell Surface Membrane: Protects the contents of the egg.

Unlike sperm, egg cells cannot move on their own and rely on the food reserves in their cytoplasm for early development after fertilization. Fertilization is the process where a sperm cell and an egg cell unite to form a new cell called a zygote. This happens when the nuclei of the sperm and egg combine, bringing together their genetic material. Here’s how it works:

• Chromosome Contribution: The zygote ends up with 46 chromosomes in total, with 23 coming from the sperm and 23 from the egg. This restores the complete set of chromosomes necessary for human development.
• Initial Steps: During fertilization, the sperm enters the egg, and their nuclei fuse together. This marks the beginning of the development of a new organism.
• Cell Division: After fertilization, the zygote undergoes several rounds of cell division. This process is crucial as it leads to the formation of all the different cells that make up a human body.

Boy or Girl?

When a sperm fertilizes an egg, the combination of chromosomes determines the sex of the baby. Here’s how it works:

• Chromosome Contribution: Each parent contributes one set of chromosomes, making a total of 46 chromosomes in the new human. However, mitochondrial DNA, which is a small part of the genetic material, is passed down only from the mother.
• Sex Chromosomes: These chromosomes determine the sex of the baby. If the baby inherits two X chromosomes (one from each parent), it will be female (XX). If it inherits one X and one Y chromosome (X from the mother and Y from the father), it will be male (XY).
• Size Difference: The X and Y chromosomes are different in size. The X chromosome is larger, while the Y chromosome is much smaller. This difference can be seen in images taken with a scanning electron microscope.
• Egg and Sperm Contribution: All egg cells carry one X chromosome. Sperm cells can carry either an X or a Y chromosome. This is why the sex of the baby depends on which sperm fertilizes the egg. If an X-carrying sperm fertilizes the egg, the baby will be XX (female). If a Y-carrying sperm fertilizes the egg, the baby will be XY (male).
• Research Findings: Studies have shown that the chances of having a boy or a girl are roughly equal. However, there are slight variations in these chances due to different biological factors.

Inheritance is the process by which DNA, organized into chromosomes containing genes, is passed from parents to their offspring. Genes carry the instructions for various traits and characteristics.

Sex Inheritance involves determining the sex of a baby based on the X or Y chromosomes inherited from the parents. The combination of these chromosomes decides whether the baby will be male or female.

Trait Inheritance refers to the passing down of other characteristics from parents to their children through the genes located on the chromosomes. These traits can include physical features, behaviors, and other inherited qualities.

Understanding Variation in Organisms

Genetic Variation

Introduction to Variation

• Organisms from different species, such as horses and donkeys or lions and tigers, exhibit clear differences in their appearance.
• Within the same species, individual organisms show variations, which are the differences found among members of the same species, including humans and other animals like goats.
• Despite being part of the same species, individuals can vary significantly in their traits, highlighting the diversity within a species.

Role of DNA in Variation

• Variation within a species is primarily determined by the DNA present in chromosomes, which is inherited from parents.
• An organism's life begins when a sperm cell fertilizes an egg cell, with each cell containing chromosomes that hold genes made of DNA.
• Genes that affect specific traits, like horn development in goats, come in different versions with slightly different DNA sequences.
• For instance, different gene versions in goats can lead to long horns, short horns, no horns, or variations in horn shape, such as curved or straight horns.
• Sperm and egg cells carry various combinations of these gene versions, influencing traits like height, coat color, or tail shape.
• During fertilization, the union of a sperm and egg cell forms a zygote with a unique DNA combination, distinct from its parents and siblings.
• Genetic differences among individuals in a species arise from variations in genes due to mutations, genetic recombination, and environmental factors.
• These genetic differences can lead to visible similarities and differences, as seen in families where individuals share some DNA but each possesses a unique genetic makeup.

Bacteria and Antibiotics

Evolutionary Adaptation

• Bacteria are tiny living things that can only be seen under a special microscope. They are so small that you could fit about a thousand of them on the head of a pin. Most bacteria are not harmful to humans; in fact, some are helpful and necessary for our bodies. However, there are also harmful bacteria that can make us sick.
• Antibiotics are powerful medicines used to kill harmful bacteria and treat infections. They are very effective against bacterial infections, but they do not work on infections caused by viruses, such as the common cold or flu.
• Some people mistakenly think that antibiotics can help with viral infections, which is not true. Taking antibiotics when they are not needed can lead to serious problems. It can allow harmful bacteria to become resistant to the antibiotics, making it harder to treat infections in the future. This is a big concern for doctors and healthcare professionals.
• Antibiotic resistance happens when bacteria change and become strong enough to survive even when antibiotics are used. This can happen if antibiotics are used too much, or if they are not used properly. For example, if a patient does not finish their course of antibiotics, some bacteria might survive and become resistant.
• To prevent antibiotic resistance, it is important to use antibiotics only when they are really needed and to follow the doctor’s instructions carefully. This way, we can keep antibiotics working effectively for everyone.

Strong evidence for natural selection

Adaptive Evolution

• Bacteria and Antibiotic Resistance :
  • Genetic Variation: In a population of bacteria, there is genetic variation. By chance, some bacteria have genes that make them resistant to antibiotics.
  • Introduction of Antibiotics: When an antibiotic is introduced, it kills the bacteria that do not have the resistance gene.
  • Survival and Reproduction: The bacteria with the resistance gene survive and reproduce.
  • Increase in Resistance: Over time, the population of bacteria becomes mostly resistant to the antibiotic.
• Pepper Moths in England:
  • Initial Variation: Peppered moths have either pale or dark wings.
  • Camouflage and Predation: Pale moths are better camouflaged on lichen-covered tree bark, making them less visible to birds.
  • Industrial Impact: Before the Industrial Revolution, pale moths were more common. However, during the Industrial Revolution, smoke from factories killed lichens and darkened tree trunks.
  • Shift in Population: Dark moths became more common because they blended in better with the darkened tree trunks, while pale moths became more visible and were eaten more often.
  • Reversal of Trends: Today, with reduced pollution, lichens have grown back, improving the camouflage of pale moths once again.

Extinction

Impact of Environmental Changes on Species Survival

Silent Extinction

• Environmental changes can significantly impact a species' ability to survive.
• For instance, the peppered moth case demonstrates how shifts in tree trunk colour affected moth populations.
• If the proportion of dark and pale moths had not adjusted when tree trunks darkened, the pale moths would likely have faced increased predation, leading to a drastic decline.
• A failure to adapt to environmental changes could push a population to the brink of extinction, potentially resulting in the complete disappearance of the species.
• Scientists are concerned that climate change may trigger environmental shifts that could threaten the extinction of many species unable to adapt rapidly enough.