1st Term revision sheet Gr10 2025 ======.pdf

# Principles of inheritance and variation This topic explores the fundamental mechanisms by which traits are passed from parents to offspring and the resulting variations in populations [2](#page=2) [3](#page=3) [4](#page=4) [5](#page=5) [6](#page=6) [7](#page=7) [8](#page=8) [9](#page=9). ### 1.1 Genetic terminology and concepts * **Gene:** A length of DNA that codes for a protein [6](#page=6). * **Allele:** Different versions of a gene [7](#page=7) [8](#page=8). * **Genotype:** The genetic makeup of an organism, represented by the combination of alleles it possesses [8](#page=8). * **Homozygous:** Having two identical alleles for a particular gene (e.g., $AA$ or $aa$) [2](#page=2) [3](#page=3). * **Heterozygous:** Having two different alleles for a particular gene (e.g., $Aa$) [2](#page=2) [3](#page=3) [8](#page=8). * **Phenotype:** The observable characteristics or traits of an organism, resulting from its genotype and environmental influences [2](#page=2) [6](#page=6). * **Dominant allele:** An allele that is always expressed in the phenotype when present in the genotype [2](#page=2) [3](#page=3). * **Recessive allele:** An allele that is only expressed in the phenotype when the dominant allele is absent, meaning it is present in a homozygous state [2](#page=2) [3](#page=3). * **Pure-breeding:** An organism that is homozygous for a particular trait, meaning it will always pass on the same alleles for that trait to its offspring. This occurs when two homozygous individuals breed [2](#page=2). ### 1.2 Mechanisms of inheritance * **Punnett Square:** A diagram used to predict the genotypes of a particular cross or breeding experiment. It visually represents the possible combinations of alleles that offspring can inherit from their parents [7](#page=7). > **Example:** A Punnett square can be used to determine the probability of offspring inheriting a specific trait when parents have known genotypes, such as crossing $Tt$ with $Tt$ [2](#page=2). * **Pedigree Diagram:** A chart that shows the inheritance of genetic traits within a family over several generations. It uses standardized symbols to represent individuals and their relationships, helping to track the transmission of genetic disorders or other characteristics [9](#page=9). ### 1.3 Chromosomes and cell division in inheritance * **Chromosome:** A structure found in the nucleus of a cell, made of DNA tightly coiled around proteins, which carries genetic information [4](#page=4) [6](#page=6). * A human male has sex chromosomes XY [6](#page=6). * A human female has sex chromosomes XX [6](#page=6). * **Zygote:** The initial cell formed when a sperm cell fertilizes an egg cell [4](#page=4). * A sperm cell from a domestic cat contains 19 chromosomes. If it fertilizes an egg, the resulting zygote will be diploid with 38 chromosomes [4](#page=4). * **Meiosis:** A type of cell division that reduces the number of chromosomes by half, producing four haploid gametes (sex cells). It is essential for sexual reproduction and introduces genetic variation [5](#page=5). * **Mitosis:** A type of cell division that results in two daughter cells each having the same number and kind of chromosomes as the parent nucleus, typical of ordinary tissue growth [5](#page=5). * Mitosis produces genetically identical daughter cells [5](#page=5). * Functions of mitosis include growth, repair of damaged tissue, replacement of cells, and asexual reproduction [5](#page=5). ### 1.4 Genetic variation and disorders * **Variation:** Differences in the genetic makeup and observable traits among individuals within a population, which arise from processes like meiosis and fertilization [2](#page=2) [3](#page=3) [4](#page=4) [5](#page=5) [6](#page=6) [7](#page=7) [8](#page=8) [9](#page=9). * **Genetic disorders:** Conditions caused by abnormalities in an individual's genetic material. * **Down's syndrome:** A genetic disorder caused by a change in chromosomes, specifically the presence of an extra copy of chromosome 21 [6](#page=6). * **Sickle cell anaemia:** A genetic disorder caused by an abnormal form of haemoglobin due to a specific gene mutation [8](#page=8). * The gene for haemoglobin exists in two alternative forms: $H^A$ (normal) and $H^S$ (abnormal) [8](#page=8). * Individuals with sickle cell anaemia have the genotype $H^SH^S$ [8](#page=8). * Cells with abnormal haemoglobin can become sickle-shaped, leading to problems in the blood system, such as blockages and reduced oxygen transport [8](#page=8). * A child with sickle cell anaemia can be born to parents who do not have the disorder if both parents are heterozygous carriers ($H^AH^S$). The probability of a child inheriting sickle cell anaemia from two heterozygous parents is 25% [8](#page=8). * **Albinism:** A recessive genetic disorder characterized by a lack of pigment production [9](#page=9). * The allele for normal melanin production ($A$) is dominant to the allele for albinism ($a$) [9](#page=9). * Individuals with albinism have the genotype $aa$ [9](#page=9). * An unaffected person can be heterozygous ($Aa$) or homozygous dominant ($AA$) [9](#page=9). > **Tip:** Pedigree diagrams are crucial for understanding the inheritance patterns of recessive disorders like albinism [9](#page=9). ### 1.5 Sex-linked characteristics * **Sex-linked characteristic:** A trait that is controlled by a gene located on a sex chromosome (X or Y chromosome) [3](#page=3) [7](#page=7) [9](#page=9). * These conditions often occur more frequently in one sex than the other, typically men, because they have only one X chromosome [3](#page=3) [9](#page=9). * **Red-green colour blindness:** An example of a sex-linked characteristic controlled by a gene on the X chromosome [3](#page=3) [7](#page=7) [9](#page=9). * The alternative versions of this gene are $X^A$ (no colour blindness) and $X^a$ (colour blindness) [7](#page=7). * A male with colour blindness has the genotype $X^aY$ [7](#page=7). * A sex-linked characteristic cannot pass directly from father to son because sons inherit their Y chromosome from their father [3](#page=3) [9](#page=9). * **Inheritance of sex:** The chance of a baby being male or female is approximately 50%. A Punnett square can illustrate this, with male gametes carrying X or Y chromosomes and female gametes carrying X chromosomes [2](#page=2) [3](#page=3) [7](#page=7). ### 1.6 Blood groups * **ABO blood group system:** An example of multiple alleles and codominance [3](#page=3). > **Example:** A woman with blood group AB (genotype $I^A I^B$) has a child with a man who has blood group B and is heterozygous (genotype $I^B i$). The possible genotypes for the child are $I^A I^B$, $I^A i$, $I^B i$, and $ii$. The probability of the child having blood group B (genotypes $I^B I^B$ or $I^B i$) is 50% [3](#page=3). --- # Cell division and chromosome number This section explores the fundamental processes of cell division, mitosis and meiosis, detailing their roles, outcomes, and the resulting chromosome numbers in daughter cells, while differentiating between haploid and diploid cells. ### 2.1 Cell division processes Cell division is a fundamental biological process by which a parent cell divides into two or more daughter cells. These processes are crucial for growth, repair, reproduction, and genetic diversity [5](#page=5). ### 2.2 Mitosis Mitosis is a type of cell division that results in two daughter cells each having the same number and kind of chromosomes as the parent nucleus, typical of ordinary tissue growth [4](#page=4). #### 2.2.1 Functions of mitosis Mitosis serves several vital functions in multicellular organisms: * **Growth:** It increases the number of cells, leading to organismal growth [5](#page=5). * **Repair of damaged tissue:** Mitosis replaces damaged or worn-out cells, aiding in tissue repair [5](#page=5). * **Replacement of cells:** Many cells in the body have a limited lifespan and are continuously replaced through mitosis [5](#page=5). * **Asexual reproduction:** In some organisms, mitosis is the primary mode of asexual reproduction [5](#page=5). #### 2.2.2 Outcome of mitosis * The daughter cells produced by mitosis are genetically identical to the parent cell [5](#page=5). * Duplication of chromosomes occurs before mitosis [5](#page=5). * The number of chromosomes in a daughter cell is the same as in the parent cell [5](#page=5). > **Tip:** Mitosis ensures that each new cell receives a complete and identical set of genetic information from the parent cell. ### 2.3 Meiosis Meiosis is a specialized type of cell division that reduces the chromosome number by half, creating four haploid cells, each genetically distinct from the parent cell and from each other. This process is essential for sexual reproduction [4](#page=4) [5](#page=5). #### 2.3.1 Functions of meiosis The primary function of meiosis is the production of gametes (sex cells) for sexual reproduction [5](#page=5). #### 2.3.2 Outcome of meiosis * Meiosis results in the production of haploid cells [4](#page=4) [5](#page=5). * Meiosis is often referred to as 'reduction division' because the number of chromosomes is reduced [5](#page=5). > **Tip:** Meiosis is critical for maintaining the correct chromosome number across generations in sexually reproducing organisms. Without this reduction, the chromosome number would double with each fertilization event. ### 2.4 Chromosome number and cell types The number of chromosomes in a cell is a defining characteristic that distinguishes between different cell types and stages of the cell cycle. #### 2.4.1 Diploid cells * Diploid cells contain two complete sets of chromosomes, one inherited from each parent [4](#page=4). * Human body cells are diploid and typically contain 46 chromosomes [15](#page=15). * A zygote, formed by the fertilization of an egg by a sperm, is diploid. For example, if a sperm cell from a domestic cat contains 19 chromosomes and fertilizes an egg, the resulting zygote will be diploid with 38 chromosomes [4](#page=4). #### 2.4.2 Haploid cells * Haploid cells contain only one complete set of chromosomes. * Gametes, such as egg cells and sperm cells, are haploid [15](#page=15). * Human egg and sperm cells each contain 23 chromosomes [15](#page=15). > **Example:** A diploid human cell has 46 chromosomes ($2n = 46$). After meiosis, the resulting haploid gametes (sperm or egg) will each have 23 chromosomes ($n = 23$). Upon fertilization, the fusion of a sperm ($n$) and an egg ($n$) restores the diploid number in the zygote ($n+n = 2n = 46$). ### 2.5 Genes and alleles * A **gene** is defined as a length of DNA that codes for a protein [4](#page=4). * **Alleles** are different versions of a gene. For example, a gene for coat texture in guinea pigs might have different alleles that result in different textures [4](#page=4). --- # Human reproduction and development This topic explores the intricate processes of human reproduction and development, encompassing the structures of reproductive systems, the formation and function of gametes, hormonal regulation, fertilization, fetal growth, and the transmission of certain diseases. ### 3.1 The reproductive systems #### 3.1.1 Male reproductive system The male reproductive system is responsible for producing sperm cells and delivering them to the female reproductive tract. Key structures include [11](#page=11): * **Testis:** Produces sperm cells and male sex hormones like testosterone [11](#page=11). * **Scrotum:** A sac that holds the testes outside the body, regulating their temperature for optimal sperm production . * **Prostate gland:** Contributes fluid to the semen, which nourishes and aids in the motility of sperm . * **Penis:** The external organ responsible for delivering sperm into the female reproductive tract during sexual intercourse. #### 3.1.2 Female reproductive system The female reproductive system produces egg cells, receives sperm, facilitates fertilization, and supports fetal development. Key structures include [13](#page=13): * **Ovary:** Produces egg cells (ova) and female sex hormones like oestrogen and progesterone [11](#page=11). * **Oviduct (fallopian tube):** Connects the ovary to the uterus; fertilization typically occurs here [15](#page=15). * **Uterus:** A muscular organ where a fertilized egg implants and the fetus develops [13](#page=13) [14](#page=14). * **Cervix:** The lower, narrow part of the uterus that opens into the vagina. * **Vagina:** The muscular canal that connects the uterus to the outside of the body, serving as the birth canal and receiving the penis during intercourse. ### 3.2 Gametes Gametes are reproductive cells that fuse during fertilization to form a new organism. #### 3.2.1 Sperm cells (male gametes) Sperm cells are small, numerous, and motile. They possess adaptive features for their function [11](#page=11): * **Head:** Contains the nucleus with 23 chromosomes and the acrosome, which holds enzymes to penetrate the egg's jelly coat [11](#page=11) [13](#page=13). * **Midpiece:** Packed with mitochondria, providing energy for movement [11](#page=11). * **Tail (flagellum):** A whip-like structure that propels the sperm towards the egg [11](#page=11) . Sperm are produced in much greater numbers than egg cells because many are needed to fertilize a single egg, and the chance of any single sperm reaching the egg is small [14](#page=14). #### 3.2.2 Egg cells (female gametes) Egg cells are large, few, and non-motile. They contain 23 chromosomes and possess a jelly coat that sperm must penetrate. Unlike sperm, they do not have a flagellum for movement, nor an acrosome with digestive enzymes [10](#page=10) [11](#page=11) [15](#page=15) . > **Tip:** The large number of sperm and the immobility of the egg are crucial for ensuring successful fertilization by increasing the probability of encounter and selection. ### 3.3 Hormonal control of reproduction and pregnancy Hormones play a critical role in regulating the menstrual cycle, ovulation, pregnancy, and the development of secondary sexual characteristics. * **FSH (Follicle-Stimulating Hormone):** Stimulates the development of follicles in the ovary, which contain egg cells [10](#page=10) [11](#page=11). * **LH (Luteinizing Hormone):** Triggers ovulation, the release of an egg from the ovary [10](#page=10) [11](#page=11). * **Oestrogen:** Responsible for the development of female secondary sexual characteristics, such as breast development and widening of the hips. It also helps to maintain the uterine lining [11](#page=11). * **Progesterone:** Maintains the thickness of the uterine lining during pregnancy, preparing it for implantation and preventing menstruation. It is produced by the corpus luteum in the ovary and later by the placenta [11](#page=11) [14](#page=14). * **Testosterone:** The primary male sex hormone, responsible for the development of male secondary sexual characteristics and sperm production [14](#page=14). ### 3.4 Fertilization and fetal development #### 3.4.1 Fertilization Fertilization is the fusion of a sperm cell and an egg cell, typically occurring in the oviduct. The sperm's acrosome releases enzymes to digest the egg's jelly coat, allowing one sperm to penetrate and fuse with the egg, forming a zygote [15](#page=15). #### 3.4.2 Implantation and pregnancy The fertilized egg, now a zygote, travels to the uterus and implants in the uterine lining. This marks the beginning of pregnancy [13](#page=13). * **Placenta:** A vital organ that develops during pregnancy, facilitating the exchange of nutrients, oxygen, and waste products between the mother and the fetus. It is NOT a barrier to all substances, such as nicotine. The placenta also produces hormones like oestrogen and progesterone to maintain the pregnancy [10](#page=10). * **Fetal development:** The embryo and then fetus grow and develop within the uterus, supported by the placenta and amniotic fluid. Substances like dissolved nutrients and oxygen pass from the mother to the fetus, while waste products like carbon dioxide and urea pass from the fetus to the mother [14](#page=14). > **Example:** During pregnancy, the mother's body provides essential nutrients and oxygen to the developing fetus through the placenta. Simultaneously, the fetus's metabolic waste products are transferred to the mother's bloodstream for elimination. ### 3.5 Transmission of diseases: HIV Human Immunodeficiency Virus (HIV) is a virus that can be transmitted through various means. * **Transmission routes:** HIV is primarily transmitted through sexual contact, blood and needles, and from mother to child. It is NOT transmitted through airborne droplets, coughing, or sneezing [12](#page=12) [15](#page=15) . * **Effects of HIV:** HIV destroys lymphocytes, which are crucial white blood cells involved in the immune response and antibody production. This significantly weakens the immune system, making the individual susceptible to opportunistic infections . * **Treatment:** HIV is caused by a virus, not bacteria, and therefore cannot be treated with antibiotics. While there is no cure, treatments can manage the virus and its progression . > **Tip:** Understanding the specific modes of transmission for diseases like HIV is crucial for public health awareness and prevention strategies. --- ## Common mistakes to avoid - Review all topics thoroughly before exams - Pay attention to formulas and key definitions - Practice with examples provided in each section - Don't memorize without understanding the underlying concepts

| Term | Definition | |------|------------| | Allele | Different versions of the same gene that occupy the same position on a chromosome. | | Genotype | The genetic makeup of an organism, referring to the combination of alleles it possesses for a particular trait. | | Phenotype | The observable physical or biochemical characteristics of an organism, as determined by its genotype and environmental influences. | | Homozygous recessive | An individual having two identical recessive alleles for a particular gene, resulting in the expression of the recessive trait. | | Homozygous dominant | An individual having two identical dominant alleles for a particular gene, resulting in the expression of the dominant trait. | | Heterozygous | An individual having two different alleles for a particular gene. | | Dominant allele | An allele that is expressed in the phenotype when present in the genotype, even if only one copy is present. | | Recessive allele | An allele that is only expressed in the phenotype when two copies are present in the genotype, with no dominant allele masking its effect. | | Sex-linked characteristic | A trait in which the gene responsible is located on one of the sex chromosomes (X or Y). | | Gene | A specific sequence of nucleotides in DNA or RNA that is located usually on a chromosome and that is the functional unit of inheritance controlling the transmission and expression of one or more traits. | | Chromosome | A thread-like structure of nucleic acids and protein found in the nucleus of most living cells, carrying genetic information in the form of genes. | | Gamete | A mature haploid male or female germ cell that is able to unite with another in fertilization (such as a sperm or egg cell). | | Meiosis | A type of cell division that reduces the number of chromosomes in the parent cell by half and produces four gamete cells. | | Mitosis | A type of cell division that results in two daughter cells each having the same number and kind of chromosomes as the parent nucleus, typical of ordinary tissue growth. | | Haploid | A cell or organism having a single set of unpaired chromosomes. | | Diploid | A cell or organism containing two complete sets of chromosomes, one from each parent. | | Zygote | The diploid cell formed when a sperm cell fertilizes an egg cell. | | Fertilisation | The fusion of male and female gametes to form a zygote. | | Placenta | An organ that develops in the uterus during pregnancy, providing oxygen and nutrients to the growing baby and removing waste products from the baby's blood. | | Hormone | A chemical substance produced in the body that controls and regulates the activity of certain cells or organs. | | Ovulation | The release of an egg from the ovary. | | Flagellum | A long, whip-like filament that helps in the propulsion of microorganisms and sperm cells. | | Mitochondria | Organelles found in large numbers in most cells, in which the biochemical processes of respiration and energy production occur. | | Acrosome | A cap-like structure on the head of a sperm cell that contains enzymes necessary to penetrate the egg. | | Implantation | The process by which a fertilized egg cell (blastocyst) attaches to the wall of the uterus. | | HIV (Human Immunodeficiency Virus) | A virus that attacks the body's immune system, specifically targeting CD4 cells (T cells), which are crucial for fighting off infections. | | Lymphocytes | A type of white blood cell that is part of the body's immune system. They help the body fight infection and other diseases. |