Genetics
Asexual Reproduction -
Asexual reproduction is a type of reproduction that does not involve the fusion of gametes (sex cells). Instead, a single organism produces offspring that are genetically identical to itself—essentially clones. This process occurs through mechanisms like binary fission, budding, fragmentation, or vegetative propagation, depending on the organism.
Since there's no mixing of genetic material, the offspring inherit 100% of their DNA from one parent. While this allows for rapid population growth, it also means less genetic variation, which can make populations more vulnerable to changes in the environment.
Common in bacteria, some plants, fungi, and simple animals like hydras and starfish, asexual reproduction is efficient but limits adaptability compared to sexual reproduction.
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Binary Fission:
Binary fission is a simple form of asexual reproduction, mainly seen in prokaryotes like bacteria. In this process, the single-celled organism duplicates its DNA, then splits into two identical cells. Each new cell receives an exact copy of the original genetic material, making them clones of the parent.
- Prokaryote Bacteria
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Spores:
Spores are reproductive cells produced by some fungi, algae, and plants in asexual reproduction. They are typically haploid and can develop into a new organism without fertilization. Spores are often resistant to harsh conditions and can survive until they find a suitable environment to grow, where they divide and produce genetically identical offspring.
Both binary fission and spore formation allow organisms to reproduce quickly and efficiently without the need for a mate, but result in little genetic variation.
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Mitosis is a type of cell division that produces two genetically identical daughter cells from a single parent cell. It's used for growth, repair, and asexual reproduction in multicellular organisms.
Mitosis has five main phases:
1. Prophase:
* Chromosomes condense and become visible.
* The nuclear membrane breaks down.
* Spindle fibers start to form.
2. Metaphase:
* Chromosomes line up at the cell’s equator.
* Spindle fibers attach to the centromeres of the chromosomes.
3. Anaphase:
* Sister chromatids are pulled apart to opposite poles of the cell.
4. Telophase:
* Chromosomes reach the poles and decondense.
* Nuclear membranes reform around each set of chromosomes.
5. Cytokinesis (not officially a phase of mitosis but happens afterward):
* The cytoplasm divides, forming two separate, identical cells.
Mitosis ensures that each new cell has the same number of chromosomes as the original, maintaining genetic continuity.
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Sexual Reproduction -
Sexual reproduction involves the fusion of two gametes—one from each parent (typically a sperm and an egg). This process results in offspring that are genetically unique, as they inherit a mix of DNA from both parents.
It involves meiosis, a type of cell division that reduces the chromosome number by half, creating haploid gametes. When fertilization occurs, the resulting zygote has a complete set of chromosomes—half from each parent.
Sexual reproduction introduces genetic variation through processes like independent assortment, crossing over, and random fertilization, which enhances a population's ability to adapt to changing environments. It's common in animals, most plants, and many fungi.
Homologous Pairs:
Homologous pairs are matching pairs of chromosomes—one from each parent—that have the same genes in the same order, but may have different versions (alleles). Humans have 23 homologous pairs (46 chromosomes total), including one pair of sex chromosomes (XX or XY).
Diploid Cells (2n):
Diploid cells have two sets of chromosomes—one from each parent. This means they contain homologous pairs. In humans, most body cells (somatic cells) are diploid, with 46 chromosomes (23 pairs).
Haploid Cells (n):
Haploid cells have only one set of chromosomes, so they contain no homologous pairs. These are typically gametes (sperm and egg cells). In humans, haploid cells have 23 chromosomes. During fertilization, two haploid cells combine to form a diploid zygote.
These terms are central to understanding how genetic information is organized and passed on in both asexual and sexual reproduction.
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Meiosis is a type of cell division that produces haploid gametes (sperm and egg cells) from a diploid parent cell. It reduces the chromosome number by half and introduces genetic variation. Meiosis is essential for sexual reproduction.
It consists of two divisions: Meiosis I and Meiosis II, each with 4 phases:
Meiosis I (reduction division — homologous chromosomes separate):
1. Prophase I:
* Chromosomes condense and pair up as homologous pairs.
* Crossing over occurs—segments of DNA are exchanged between chromosomes.
* Nuclear membrane breaks down, spindle forms.
2. Metaphase I:
* Homologous pairs line up at the cell's equator.
3. Anaphase I:
* Homologous chromosomes are pulled apart to opposite ends (sister chromatids stay together).
4. Telophase I and Cytokinesis:
* Two haploid cells form, each with half the number of chromosomes.
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Meiosis II (like mitosis — sister chromatids separate):
1. Prophase II:
* New spindles form in each haploid cell.
2. Metaphase II:
* Chromosomes line up at the equator (single file).
3. Anaphase II:
* Sister chromatids are separated and pulled to opposite ends.
4. Telophase II and Cytokinesis:
* Four genetically unique haploid cells are produced.
Key Point: Meiosis results in 4 haploid cells with genetic variation, unlike mitosis, which produces 2 identical diploid cells.
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🔹 Meiosis I – Reduction Division
* Purpose: Reduces chromosome number from diploid (2n) to haploid (n).
* What separates: Homologous chromosome pairs.
* Crossing over: Occurs in Prophase I, introducing genetic variation.
* Result: Two haploid cells, each with chromosomes still made of sister chromatids.
* Key event: Genetic recombination and independent assortment for variation.
🔹 Meiosis II – Division Like Mitosis
* Purpose: Separates sister chromatids.
* What separates: Individual chromatids within each chromosome.
* Crossing over: Does not occur again.
* Result: Four genetically unique haploid cells (gametes).
* Key event: Ensures each gamete has a single copy of each chromosome.
✅ Summary:
Feature
Meiosis I
Meiosis II
Chromosomes separated
Homologous pairs
Sister chromatids
Chromosome number change
Diploid → Haploid
Haploid → Haploid
Crossing over
Yes, in Prophase I
No
Number of cells made
2 haploid cells
4 haploid cells total
Type of division
Reductional
Equational (like mitosis)
Together, Meiosis I and II create genetically diverse gametes, essential for sexual reproduction.
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Inheritance
Inheritance is the process by which genetic information is passed from parents to offspring. Traits (like eye color or blood type) are inherited through genes carried on chromosomes.
Alleles
Alleles are different versions of the same gene. For example, a gene for eye color might have a brown allele and a blue allele. You inherit one allele from each parent for each gene.
Genotype
A genotype is the genetic makeup of an organism—the combination of alleles it has for a particular gene (e.g., BB, Bb, or bb for eye color). It determines potential traits.
Phenotype
A phenotype is the physical expression or observable trait resulting from the genotype (e.g., brown eyes). It can also be influenced by the environment.
Homozygous
An individual is homozygous when they have two identical alleles for a gene (e.g., BB or bb). This often leads to a clear, consistent trait being expressed.
Heterozygous
An individual is heterozygous when they have two different alleles for a gene (e.g., Bb). Usually, the dominant allele determines the phenotype, while the recessive one is hidden.
These terms are fundamental to understanding how traits are inherited and expressed in organisms.
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A Punnett square is a simple grid used in genetics to predict the possible genotypes and phenotypes of offspring from a particular cross or mating.
It shows how alleles from each parent combine during fertilization.
How it works:
* Each parent's alleles are written along the top and side of the grid.
* The boxes are filled in by combining one allele from each parent.
* The results show the probable genetic outcomes and their ratios or percentages.
Example:
If one parent is Bb and the other is bb:
B
b
b
Bb
bb
b
Bb
bb
Results:
* Genotypes: 2 Bb, 2 bb
* Phenotypes (if B = brown eyes, b = blue eyes):
* 50% brown eyes
* 50% blue eyes
Punnett squares help visualize Mendelian inheritance and are useful for predicting the likelihood of dominant and recessive traits in offspring.
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The Year 10 Science syllabus in NSW (Australia)—especially focusing on Genetics and Biology—here’s a list of likely topics and themes that would appear in a topic test:
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🔬 1. Cell Division
* Mitosis: phases, purpose, and outcome.
* Meiosis: phases, purpose in sexual reproduction, and how it leads to variation.
* Differences between mitosis and meiosis.
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🧬 2. DNA and Genetics
* Structure of DNA (double helix, base pairs: A-T, C-G).
* Genes, chromosomes, and DNA: how they're related.
* Homologous chromosomes, diploid vs haploid cells.
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🧫 3. Asexual vs Sexual Reproduction
* Definitions and examples.
* Binary fission, spore formation, budding (asexual).
* Advantages/disadvantages of both methods.
* How they affect genetic variation.
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🧪 4. Inheritance and Punnett Squares
* Mendelian inheritance (dominant and recessive traits).
* Alleles, genotype, phenotype, homozygous, heterozygous.
* Using and interpreting Punnett squares.
* Predicting probabilities and ratios of traits.
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🌱 5. Genetic Variation and Evolution
* How meiosis, mutations, and sexual reproduction cause variation.
* Natural selection and the role of genetic diversity.
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🧠 6. Key Vocabulary and Application
* Terms like mutation, trait, zygote, gamete.
* Applying knowledge to real-life examples (e.g., inheritance of eye colour, genetic disorders).
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📝 Types of Questions You Might See
* Multiple choice: e.g., “Which of the following is true about meiosis?”
* Short answer: explaining terms, comparing processes.
* Diagrams: labeling phases of mitosis/meiosis, Punnett square problems.
* Longer response: explaining how traits are inherited or why variation is important.
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