SS3 Biology Second Term Comprehensive Study Notes

HEREDITY AND GENETICS: AN INTRODUCTION

• Genetic is the study of heredity and variation in living things.

• HEREDITY OR INHERITANCE: Defined as the transmission and expression of characteristics or traits in an organism from parents to offspring.

• VARIATION: Defined as the differences which exist between parents and offspring as well as among the offsprings of the same species.

CHARACTERS OR TRAITS TRANSMITTED IN MAN AND PLANTS

• It is only those traits that constitute the genetic makeup of the parents that can be transmitted and expressed in the offspring.

• Transmissible Traits in Animals (Man):     - Colour of the skin, colour of eyes, colour of the hairs, and hair texture.     - Size of body stature, shape of the head, shape of the ears, shape of the mouth and lips, and shape of the nose.     - Length of the hands and legs, and length of the neck.     - Blood grouping, baldness, tongue rolling, hemophilia, voice, intelligence, composure, and attitude.     - Sickle cell anaemia.

• Transmissible Traits in Plants:     - Colour and shape of the leaves and shoot.     - Seed size and shape.     - Colour of the flowers.     - Size of the fruit and pigmentation.

MECHANISMS OF TRAIT TRANSMISSION

• Only characters controlled by genes can be transmitted.

• Diploid Organisms: Possess two sets of chromosomes referred to as homologues. These organisms have two copies of each gene, with each copy occupying identical locations or loci on the homologous chromosomes.

• Gamete Production: Produced by meiosis in reproductive organs. A male individual produces sperm, and a female individual produces egg cells or ova.

• Meiosis: During this process, the number of chromosomes in a cell is halved. Gametes are therefore haploid ($n$), containing one set of chromosomes and only one copy of each gene.

• Fertilization: During sexual reproduction, the gametes of a male and female fuse to form a zygote. Each zygote is diploid ($2n$) as it receives one set of chromosomes/genes from each parent.

• Genotype: The set of genes an organism inherits during fertilization. It remains constant throughout the life span.

• Phenotype: The physical appearance or observable features of an organism, determined by its genotype and the environment in which it lives.

BASIC GENETIC CONCEPTS

• GENE: The physical unit of inheritance transmitted from one generation to another, responsible for controlling the development of characters in new organisms.

• CHROMOSOMES: Strands of genetic material recognizable during cell division found in the nucleus. They consist of DNA (deoxyribonucleic acid) and protein.

• CHARACTER OR TRAIT: Inheritable attributes or features possessed by an organism (e.g., height or size).

• ALLELOMORPHS: Pairs of genes at a locus that control contrasting characters. A pair of allelomorphs is called an allelic pair; each member is an allele of the other.

• PHENOTYPE: The sum total of all observable features (physical, physiological, and behavioral) such as height, weight, and skin colour.

• GENOTYPE: The sum total of genes inherited from both parents or the genetic constitution of an individual, including both dominant and recessive traits.

• DOMINANT CHARACTER: A trait expressed in an offspring even when two individuals with contrasting characters are crossed.

• RECESSIVE CHARACTER: A trait from one parent that is masked or does not produce its effect in the presence of a dominant gene. Shortness is typically recessive, while tallness is dominant.

• HOMOZYGOUS: An individual with identical alleles for a particular trait (e.g., $TT$ or $tt$).

• HETEROZYGOUS: An individual having two different members of a pair of genes controlling a pair of contrasting alleles located at the same position on a pair of chromosomes (e.g., $Tt$ for tallness or $Rr$ for a plant).

• GAMETE: A single cell formed as a result of the union of a male gamete with a female gamete (zygote precursor).

• FILIAL GENERATION: Offspring of parents make up the filial generation ($F_1$, $F_2$, and $F_3$ for the first, second, and third generations).

• HYBRID: An offspring from a cross between parents that are genetically different but of the same species.

• HYBRIDIZATION: The crossing of plants or animals with contrasting characters.

• MONOHYBRIDIZATION: Involves crossing two organisms with one pair of contrasting characters.

• DIHYBRID CROSS: A cross involving the inheritance pattern of two characters simultaneously.

• LOCUS: The specific site or location of a gene on a chromosome.

• HAPLOID: An organism or cell with one set of chromosomes ($n$).

• DIPLOID: An organism or cell with two sets of chromosomes ($2n$) in the body cell.

• MUTATION: A change in the genetic makeup of an organism resulting in a new, inheritable characteristic.

• BACKCROSS: Mating an $F_1$ individual with an individual that has the parental genotype.

• PURE BREED: An individual that is homozygous for a particular trait.

• CO-DOMINANCE: A situation where the phenotype of the heterozygote exhibits properties of both parent traits (e.g., ABO blood group system where A and B are both dominant over O, resulting in AB).

• SEX-LINKED CHARACTER: Characters carried by genes located on the sex chromosomes (usually the X-chromosome). These are expressed in males even if recessive. In females, they are expressed only if both X-chromosomes carry the recessive gene (e.g., colour blindness, hemophilia, baldness).

MENDEL’S WORKS IN GENETICS

• Gregor Mendel (1822 - 1884): An Austrian monk known as the "father of Genetics" for his foundational work on heredity and variation.

• Experimental Subject: Mendel utilized the garden pea, Pisum sativum.

• Reasons for using the Pea:     - Peas are usually self-pollinating and easy to cross-pollinate manually.     - They possess a very short life span compared to animals and other plants.

• Mendel's Procedure:     1. He planted tall plants for generations to ensure they were pure-breeding (producing only tall offspring).     2. He planted short plants for generations to ensure they were pure-breeding (producing only short offspring).     3. He crossed tall plants with short pea plants. He collected pollen from the tall plants (male) and pollinated the stigma of short plants (female), and vice versa.     4. The seeds produced from this cross resulted in only tall plants, which he called the first filial generation ($F_1$).     5. He then allowed the $F_1$ plants to self-pollinate. The resulting seeds produced both tall and short plants in a ratio of $3:1$. This was the second filial generation ($F_2$).

• Summary of Monohybrid Cross (Tallness vs. Shortness):     - Parents: $TT$ (Tall) $\times$ $tt$ (Short)     - Gametes: $T$, $T$ and $t$, $t$     - $F_1$ Generation: All offspring are $Tt$ (Tall)     - $F_1$ Cross: $Tt$ $\times$ $Tt$     - $F_2$ Generation Phenotypic Ratio: $3$ Tall : $1$ Short     - $F_2$ Generation Genotypic Ratio: $1$ $TT$ : $2$ $Tt$ : $1$ $tt$

• Mendelian Traits Studied: Height/length, colour of seeds, and surface of seed coat.

MENDELIAN LAWS OF INHERITANCE

1. First Law: Law of Segregation: States that genes are responsible for the development of individuals and are independently transmitted from one generation to another without undergoing alteration.

2. Second Law: Law of Independent Assortment: States that each character behaves as a separate unit, and pairs of alleles for a given character distribute themselves into gametes independently of other allelic pairs.

SEX DETERMINATION AND LINKED CHARACTERS

• Sex Determination: In humans, sex is determined by the 23rd pair of chromosomes.     - Females: Have a pair of similar chromosomes ($XX$). Each egg contains an $X$ chromosome.     - Males: Have contrasting chromosomes ($XY$). Half of the sperms contain an $X$ chromosome, and half contain a $Y$ chromosome.     - Outcome: If an $X$-sperm fertilizes an $X$-egg, the offspring is female ($XX$). If a $Y$-sperm fertilizes an $X$-egg, the offspring is male ($XY$).

• Sex-Linked Characters: Genes found on the sex chromosomes (usually X). Examples include:     - Colour Blindness: Sufferers cannot distinguish between certain colors (e.g., red and green). It is recessive and X-linked. The $Y$ chromosome does not carry it.         - $X^C X^C$: Normal female         - $X^C Y$: Normal male         - $X^C X^c$: Carrier female         - $X^c Y$: Colour blind male     - Hemophilia: A disease where blood clotting proteins (fibrin) are not produced, leading to serious bleeding.     - Baldness: An inherited sex-linked trait.

BLOOD GROUPS AND GENETIC DISORDERS

• Sickle Cell Anaemia: A genetic disorder where red blood cells change from round to a sickle shape due to low oxygen concentration, causing pain in bones and joints. It is caused by the recessive gene $s$ ($Hb^S$) vs the normal $A$ ($Hb^A$).     - Carrier: $Hb^A Hb^S$     - Sufferer: $Hb^S Hb^S$

• Rhesus Factor: An agglutinogen in red blood cells. Present = Rhesus positive ($Rh^+$); Absent = Rhesus negative ($Rh^-$).     - Compatibility:         - Father $Rh^+$ and Mother $Rh^+$ = Compatible.         - Father $Rh^-$ and Mother $Rh^-$ = Compatible.         - Father $Rh^-$ and Mother $Rh^+$ = Compatible.         - Father $Rh^+$ and Mother $Rh^-$ = Incompatible.

• ABO Blood Group (Co-dominance): Determined by alleles $T^A$ (antigen A), $T^B$ (antigen B), and $T^o$ (no antigen). $T^A$ and $T^B$ are dominant over $T^o$.     - Blood Group A: Genotypes $T^A T^A$ or $T^A T^o$     - Blood Group B: Genotypes $T^B T^B$ or $T^B T^o$     - Blood Group AB: Genotype $T^A T^B$     - Blood Group O: Genotype $T^o T^o$

DIHYBRID INHERITANCE IN PEA PLANTS

• Traits: Round seeds ($R$) dominant over wrinkled ($r$); Yellow seeds ($Y$) dominant over green ($y$).

• Cross: Pure round yellow ($RR YY$) $\times$ pure wrinkled green ($rr yy$).

• $F_1$: All round yellow ($RrYy$).

• $F_2$ Phenotypic Ratio (9:3:3:1):     - $9$ Round and yellow     - $3$ Round and green     - $3$ Wrinkled and yellow     - $1$ Wrinkled and green

CHROMOSOME: THE BASIS OF HEREDITY

• Definition: Thread-like structures in the nucleus carrying hereditary information. Visible only during nuclear division in homologous pairs.

• Count: Human somatic cells have $46$ chromosomes ($23$ pairs, diploid $2N$). Sex cells (sperm/ovum) have $23$ chromosomes (haploid $N$).

• Structure: Each chromosome replicates to produce twin chromatids connected by a centromere.

• Chemical Composition:     - Proteins: Combined with nucleic acids.     - DNA (Deoxyribonucleic acid): The primary hereditary material making up the gene.     - RNA (Ribonucleic acid): Found in some viruses.

• DNA Nucleotides: Composed of a deoxyribose sugar ($s$), phosphate group ($p$), and nitrogenous bases:     - Purines: Adenine ($A$) and Guanine ($G$).     - Pyrimidines: Cytosine ($C$) and Thymine ($T$).

• Double Helix: Two chains coiled like a spring. Base pairing: $A-T$ and $G-C$.

• Role of Chromosomes:     - Gamete Formation: Homologous pairs separate during meiosis to form haploid daughter cells.     - Crossing Over: Occurs during the prophase of meiosis. Exchange of genetic material between chromatids leads to new combinations of alleles (gene reshuffling).

PROBABILITY IN GENETICS

• Formula: $\text{Probability} = \frac{\text{Number of times an event occurs}}{\text{Total number of trials}}$.

• Probability ranges from $0$ to $1$.

• Principles:     1. The result of one trial does not affect subsequent trials.     2. The chance of two independent events occurring simultaneously is the product of their individual chances.

VARIATION IN POPULATION

• Definition: Differences occurring between individuals of the same species.

• Causes of Variation:     1. Genotypic Variation: Resulting from new genetic combinations during sexual reproduction or mutation. These are inheritable.     2. Environmental Factors: Effects of climate, food supply, and social interaction. These are not inheritable (acquired).

• Types of Variation:     1. Morphological (Continuous) Variation: Deals with physical appearance. Features show a smooth transition between extremes (e.g., height, body shape, skin/hair colour, fingerprints).     2. Physiological (Discontinuous) Variation: Related to body function. Features fall into well-defined classes with no intermediates (e.g., blood groups A, B, AB, O; tongue rolling; sex; ability to taste Phenylthiocarbamide/PTC; behavioral patterns).

• Applications of Variation:     - Crime Detection: Fingerprinting (Whorl, arch, loop, and double whorl patterns) and DNA finger printing (detecting rapists/criminals via tissues or fluid).     - Blood Transfusion: Ensuring compatibility between donor and recipient (Group O is universal donor, AB is universal recipient).     - Paternity Determination: Using blood group tests and DNA finger printing to identify biological parents.

EVOLUTION: THE HISTORY OF LIFE

• Theory of Divine Creation: States all living things were created by God within six days.

• Theory of Organic Evolution: Proposes the first living organism formed from molecules like methane, ammonia, hydrogen, and water vapour under ultraviolet radiation and electrical lighting approximately two billion years ago.

• Lamarck’s Theory of Evolution (Jean Baptist Lamarck, 1801):     1. Theory of Need: Organisms develop new parts based on environmental needs (e.g., snakes becoming elongated to crawl through narrow places).     2. Theory of Use and Disuse: Organs that are used become well-developed; those not used disappear (e.g., snake legs disappearing as they hindered crawling).     3. Inheritance of Acquired Characteristics: Variations acquired during a lifetime are passed to the next generation (e.g., giraffes stretching necks to reach trees).

• Darwin’s Theory of Natural Selection (Charles Darwin, 1859):     - Overpopulation: More offspring are produced than can survive.     - Struggle for Existence: Constant competition for resources.     - Variation: No two individuals are exactly alike.     - Survival of the Fittest: Those with adaptive characteristics survive.     - Inheritance of Adaptive Traits: Successful traits are transmitted to offspring.     - Speciation: Accumulation of variations leads to new species.

• Types of Evolution:     - Divergent: Related species become different due to different environmental niches (e.g., insect mouthparts).     - Convergent: Unrelated species develop similar parts due to common needs (e.g., wings in birds vs. bats).

EVIDENCES OF EVOLUTION

• Cytology (Biochemical Similarities): Similarities in DNA, ATP, cytochromes, and organelles like mitochondria.

• Serological tests: Comparison of blood proteins (e.g., human blood is closer to chimpanzees than other animals).

• Molecular Records: Hereditary information stored in DNA across all organisms.

• Paleontology: Fossil records preserved in sedimentary rocks.

• Comparative Anatomy: Similarities in pentadactyl limbs, heart, and brain structures.

• Embryology: Embryos of different vertebrates (e.g., humans and rabbits) look very similar in early stages (e.g., tail, skin pouches).

• Vestigial Organs: Rudimentary organs that are non-functional in humans but useful in ancestors (e.g., appendix, coccyx/caudal vertebrae, posterior auricular muscles of the ears).

• Mutation: Sudden change in DNA structure providing raw materials for evolution. Types include gene (point) mutation and chromosome mutation. Causes include X-rays, UV light, and cosmic radiation.

QUESTIONS & DISCUSSION

• Percentage of Universal Donor Offspring: If a man with heterozygous B ($T^B T^o$) marries a woman with heterozygous A ($T^A T^o$), what percentage of their children will be universal donors (Blood Group O)? (Answer: $25\%$, genotype $T^o T^o$).

• Identification of Inherited Alleles: The alternative form of a gene representing a contrasting character is called an allele.

• Sickle Cell Carrier Genotypes: If parents are carriers, their genotypes are $Hb^A Hb^S$ and $Hb^A Hb^S$.

• Sex Distribution Probability: What are the chances of the next baby being a boy if there is already one boy and two girls? (Answer: $50\%$, as every fertilization is an independent event).

• Blood Group Data Analysis: Out of $250$ people, group A = $8.0\%$, B = $14.0\%$, AB = $32.8\%$, and O = $45.2\%$.     - Calculate Co-dominant blood group individuals: Individuals with AB = $0.328 \times 250 = 82$.     - Potential Donors for Blood Group B: Group B and Group O can donate to group B. Total % = $14.0 + 45.2 = 59.2\%$. Total individuals = $0.592 \times 250 = 148$.

• Insect Classification: A social insect example is the Honey bee.

• Disease Linkage: Haemophilia is a sex-linked disease mainly affecting males.

• DNA Nucleotides: Uracil is NOT found in the helical structure of DNA (it is found in RNA).