Genetics Fundamentals

• Reproduction is the process of creating new organisms.
• Individuals within a species share similarities, maintaining continuity with slight differences
• Heredity causes similarities between organisms.
• Variations cause differences between organisms.

Heredity

• Heredity is the transmission of genetically based characteristics from parent to offspring.
• All organisms share common ancestors.
• Heredity ensures the continuation of relationships between successive generations.
• Traits pass from generation to generation, maintaining consistent features.

Traits

• A trait is a transferable characteristic from parent to offspring.
• Traits continue features from one generation to the next, present in a zygote.
• The zygote develops into a specific type of organism.

Genetics

• Genetics studies the transmission of body features, similarities, and differences from parents to offspring.
• Genetics studies heredity and variation.
• The term "Genetics" was created by Bateson in 1905.
• Gregor Johann Mendel is known as the 'Father of Genetics'.

Variation

• Variation refers to the differences in characteristics between parent and offspring.
• No two individuals in a species are exactly alike.

Types of Variation

• Somatic Variation: Occurs in body cells, developed during an individual's lifetime like cutting tails in dogs, swimming, dancing etc.
  • It is not inherited or transmitted, and cannot undergo direct evolution; also known as acquired traits.
• Gametic Variation: Takes place in gametes/reproductive cells and is inherited and transmitted, it can undergo direct evolution, for example, human height or skin color.
  • It is also known as inherited traits.

Accumulation of Variation during Reproduction

• Variation happens during both sexual and asexual reproduction.
• Successive generations accumulate variations.
• Variations become noticeable if they continue to be inherited.

Variations in Asexual Reproduction

• Variations are less common.
• Variations arise from minor inaccuracies in DNA copying.

Variations in Sexual Reproduction

• Variations are more prominent.
• Variations result from independent assortment, crossing over during meiosis, random union of gametes during fertilization and mutation.

Importance of Variation

• Advantages vary among individuals depending on the nature of variations.
• Genetic variation allows natural selection to act on it.
• Increased survival chances are the main advantage of variation in a species in a changing environment.
• Survival and reproductive success relate to inherited traits functioning contextually.
• An individual’s survival depends if genes producing a particular trait are adapted. – Free or attached ear lobes are variants in human populations.

Organic Evolution

• Variation leads to organic evolution.
• Evolution is a steady change of forms over time.
• Evolution creates complex body designs, even as simpler ones thrive.

Carriers of Heredity

• Gametes carry heredity, transmitting traits from parent to offspring.
• Gametes are the link between generations.

Chromosomes

• Chromosomes in a cell's nucleus carry hereditary traits, appearing as rod-like structures in dividing cells.
• Chromatin fibers form a network in the nucleus of undividing cells.
• Chromosomes consist of 40% DNA and 60% histone proteins.
• DNA (Deoxyribonucleic acid) and histone proteins make up chromosomes carry genetic code.
• DNA is a double helix from 2 polynucleotide strands discovered by James D. Watson and Francis H.C. Crick.
• DNA contains nucleotides composed of nitrogenous bases (Adenine, Guanine, Cytosine, Thymine), a pentose sugar, and a phosphate group.
• Bases pair complementarily: Adenine bonds with Thymine, Cytosine with Guanine.

Number of Chromosomes

• The number of chromosomes is constant within a species.
• Haploid cells have a single set of chromosomes.
• Diploid cells contain two sets of all types of chromosomes.
• Examples of chromosome numbers:
  • Dog and Fowl have 78.
  • Humans have 46.
  • Mice have 40.
  • Gorillas have 48.
  • Crayfish have 200.
  • Ascaris has 2.
• An individual’s characters and complexity do not correlate with the number of chromosomes.

Homologous Chromosomes

• Two identical chromosomes in a pair share shape, size, structure, and genes for the same characteristics.
• Each parent contributes one homologous chromosome to the offspring.

Genes

• Genes are specific segments of DNA on chromosomes.
• Genes are the units of heredity and responsible for inheritance.
• Genes contain the expression of characteristics, with different traits formed by different genes, carrying the genetic code for each characteristic.

Genetic Code Contents

• Genes contain genetic code for protein synthesis, where proteins control specific traits.
• Inheritance of a tallness gene instructs the plant or animal to produce growth hormone.
• The complexity and character depend on the genes present.

Alleles

• Alleles are pairs of genes that code for particular traits.
• Alleles are different forms of the same gene.
• Alleles occupy the same position on homologous chromosomes, affecting, in two different ways, the same character.
• Instances include height, where one allele codes for tallness and one for shortness.

Expressing Alleles

• Homozygous alleles are identical pairs, like tallness (TT) or dwarfness (tt) in true-breeding pea plants.
• Heterozygous alleles are hybrid pairs expressing contrasting characters (Tt).

Transferring Traits

• Genetic information is carried within chromosomes in the cell nucleus which consist of DNA.
• Genes contained in the DNA are units of heredity and are responsible for inheritance.
• Genes carry codes for characteristics, such as height and facial features.
• Inheritance is equally contributed by both parents, each providing genetic material.
• Traits have two factors, one from each parent.
• DNA provides the source for making proteins in the cell, sections of it are called a gene for that protein.
• Plant height depends on the amount of secreted hormone which is also affected by synthesis of proteins (enzymes) produced from cellular DNA by cellular DNA.

Terminologies

• Locus: The position on a chromosome where an allele for a trait is located.
• Dominant Gene: Expresses itself even with a contrasting trait.
  • Heterozygous (Tt).
  • Dominant traits use capital letters.
• Recessive Gene: Only expressed when present on both chromosomes as homozygous alleles (tt).
  • Uses small letters.
• Genotype: Gene description represented by letter pairs.
  • Letters signify traits inherited from parents.
  • Dominant genes get capital letters, and recessive genes get small letters.
  • There are three genotype possibilities, such as HH, Hh, or hh for height.
• Phenotype: Visible characteristic expression.
  • Examples are eye & hair color.
• First Filial Generation: The offspring from crossed true-breeding plants (F1 generation).
• Second Filial Generation: The offspring from crossing first filial generation plants.
  • This is known as the F2 generation
• Punnett Square: Graphical representation to calculate offspring genotype probability in genetic cross.
  • Gametes on top row and left columns.
  • Combinations in boxes.
• Genome: The complete set of genes in a cell or organism.

Mendel's Work

• Gregor Johann Mendel (1822-1884) studied plant breeding and hybridization, proposing inheritance laws.

Mendel's Plant Selection

• Plant Selected by Mendel include Pisum sativum (garden pea).
• 14 true-breeding pea plants were used as pairs similar except for one contrasting character.
• True breeding plants undergo continuous self-pollination, showing stable trait inheritance across generations
• Mendel was known as the Father of Genetics.

Contrasting Characters

• Mendel studied seven pairs of contrasting characters in garden peas:
  • Seed shape (round vs. wrinkled)
  • Seed color (yellow vs. green)
  • Flower color (violet vs. white)
  • Pod shape (full vs. constricted)
  • Pod color (green vs. yellow)
  • Flower position (axial vs. terminal)
  • Stem height (tall vs. dwarf)

Why Garden Pea?

• Garden peas were chosen because:
  • They have contrasting traits.
  • True breeding varieties are available.
  • They have short reproductive spans.
  • Bisexual flowers normally self-pollinate but can be prevented easily.
  • The flower size allows for easy cross-pollination.
  • Plants produce many seeds.

Mendel's Experiments

• Mendel crossed pollinated plants to study each character individually.

Inheritance of One Gene

• Mendel cross-bred tall and dwarf plants in hybridization.
• The first hybrid generation (F1) produced only tall plants.
• Repeated experiments showed F1 progeny resembled one parent, and the other trait was unseen.

Law of Dominance

• The dominant allele expresses itself, while the recessive is unexpressed when a contrasting allele is present.
• Self-pollination of the F1 generation resulted in 75% tall and 25% dwarf plants (3:1 ratio) in the F2 generation, expressing the unseen trait.

Monohybrid Cross

• Crossing pure breeding pea plants produced F1 offspring which showed one parent's dominant trait.
• Self-fertilization of F1 plants resulted in both parental traits in a 3:1 ratio in the F2 generation.
• F2 traits were identical to parental traits without mixing.
• Dwarf plants were possible from F1 tall plants because:
  • Dwarfness separated/segregated from tallness.
  • Tallness didn't blend with or influence dwarfness.
• Dominant individuals can be homozygous (TT) or heterozygous (Tt).
• Recessive plants are always homozygous (tt).

Law of Segregation

• Mendel made the Law of Segregation based on the Monohybrid cross.
• Two genes controlling a character separate without influencing each other during gamete formation, so each gamete receives one gene, ensuring purity.

Laws of Independent Assortment

• Genes controlling different characteristics assort independently, without influencing each other during gamete formation.

Dihybrid Cross

• The law of independent assortment was explained using the dihybrid cross.
• Considered inheritance of two different characters simultaneously.
• Cross of Round Yellow seeds and Wrinkled Green seeds.
• The F1 generation produced round and Yellow seeds.
• The F2 generation had four combinations:
  • 9 Round Yellow
  • 3 Round Green
  • 3 Wrinkled Yellow
  • 1 Wrinkled green

Human Blood Group Inheritance

• Human Blood Group Inheritance: the ABO blood groups are controlled by genes denoted by A, B, and O(i).
• Genes A and B show no dominance (codominant).
• Both genes are dominant over the O gene.
• Hence, blood group depends on gene types present, e.g.

Blood Group Genes

• Blood group A: AA or AO
• Blood group B: BB or BO
• Blood group AB: AB
• Blood group O: OO

Rh Factor

• The Rhesus monkey factor is a protein on blood cells.
• People are either Rh-positive (have the protein) or Rh-negative (do not have the protein).
• Mostly matters when a mother is Rh-negative and the child is Rh-positive

Determining Sex

• Human sex is genetically determined.
• There are 23 pairs of chromosomes, 22 are similar and known as autosomes.
• The last pair is determines genetics:
  • sex chromosome which is XY in males.
  • XX in females.
• Males produces two types of sperms: X and Y.
• Females produce one type of egg: X.
• If:
  • X sperm fertilizes egg, baby is female.
  • Y sperm fertilizes egg, baby is male.
• The ovum has equal chance of being fertilized by X or Y sperm.
• Sex is determined at fertilization.

Sex Determination Factors

• In some animals, sex is determined by:
  • Reptiles: temperature during egg incubation.
  • Turtles: high temperature results in female offspring.
  • Lizards: high temperature results in male offspring.
  • Snails species can change sex.

Test your knowledge of basic genetics concepts. Questions cover phenotype, Punnett squares, Mendel's experiments, nucleotides, and chromosome numbers. Determine your understanding of heredity and genetic traits.