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* Inheritance (heredity): The transmission of genes from parents to offspring. * Gene: - Unit of information about heritable trait passed from parents to offspring. - Each gene has a specific location (locus) on a chromosome. * Alleles: - Alternate forms of genes. - Designated by letters. * Dominant = Capital E, A, S, T, …. The expression of dominant alleles masks the effect of recessive alleles in heterozygous individual. * Recessive = small e, a, s, t, …. Fully expressed only in homozygous recessive individuals. * Genotype: Genes makeup of an individual for a particular traits. (EE , ee , SS , ss , ….). * Phenotype: Visible expression of the genotype or the trait. (e.g. unattached ear, attached ear, brown eye,….). * Recessive allele: Must be received from both parents to bring about recessive phenotype. * Only one dominant: Only one parent is enough to express the dominant phenotype. * Genotype: Heterozygous Ee (2 identical alleles). * Phenotype: Unattached ear lobe. ----------------------------------------------------------------------------------------* Genotype: Homozygous dominant EE (2 identical alleles). * Phenotype: Unattached ear lobe. ----------------------------------------------------------------------------------------* Genotype: Homozygous recessive ee (identical alleles). * Phenotype: Attached ear lobe. ----------------------------------------------------------------------------------------* True breeding lineage: Offspring of genetic crosses inherits a pair of identical alleles EE generation after generation. * Hybrid offspring: Offspring of a genetic cross inherits a pair of non identical allele for a trait (Ss, Aa, Ee,….). (Monohybrid Inheritance) Cross between 2 parents that are homozygous dominant (EE) & homozygous recessive (ee) of a single trait. * In F1, after fertilization (gamete E with e), all individuals will be heterozygous dominant (Ee). * In F2, *Ratio: 75% : 25% 3 : 1 3/4 : 1/4 Dominant : Recessive *Genotype: EE or ee : ee *Phenotype: unattached ear : attached ear * E e E EE Ee e Ee ee * Punnet Square: Used to determine the phenotypic & genotypic ratio among the offspring when all possible sperms are given an equal chance to fertilize all. * E e E EE Ee e Ee ee * Test Cross: * If individual that has dominant phenotype for a definite trait but its genotype is unknown (EE or Ee). * Crossed to known homozygous recessive individual (ee). * If individual was homozygous dominant (EE), the cross with homozygous recessive individual will give dominant phenotype of the trait. * If individual was heterozygous (Ee) then the cross with homozygous recessive individual will give 50 % dominant phenotype & 50 % recessive phenotype for this trait. * Dihybrid inheritance: Mendle selected true – breeding plants that are different in 2 traits (flower colour & plant height). * Purple tall Dominant AABB * White dwarf Recessive aabb F1 offspring inherits 2 gene pairs each consisting of nonidentical alleles (AaBb). * The result in F2 will be: Gametes AB Ab aB ab Ratio 9/16 3/16 3/16 1/16 AB AABB AABb AaBB AaBb Ab AABb AAbb AaBb Aabb aB AaBB AaBb aaBB aaBb ab AaBb Aabb aaBb aabb Genotype AABB, AaBB, AABb, AaBb AAbb, Aabb aaBB, aaBb aabb Phenotype Purple tall Purple dwarf White tall White dwarf * Dominance relation: - Mendle studied traits having clearly dominant or recessive forms. - Is the expression of all traits like this No.? 1. Incomplete dominance: - One allele of a pair isn’t fully dominant over its partner. - Heterozygous phenotype is an intermediate between the 2 homozygous phenotypes. e.g. 1: Colour of snapdragon flower.. Red RR Pink RR1 White R1R1 * R1 R R1 R1R1 R1R R R1R RR Cross between 2 pink result in: 1 Red 2 Pink 1 White e.g. 2: Hair of caucasians.. Curly hair HH Straight hair HH1 Wavy hair H1H1 (Multiple alleles) - The trait is controlled by multiple alleles but individual has only 2 alleles. - Alleles are equally expressed at the same time in heterozygous individuals. e.g. ABO type: * 3 oeesesi elt res ioes sfs lfrtle res fest rof s le laO type. * These alleles determine the presence or absence of antigens on the RBCs. Genotype IAIA, IAi IBIB, IBi IAIB ii Phenotype Type A Type B Type AB Type O IA A antigen on the RBCs. IB B antigen on the RBCs. i neither A or B antigen on the RBCs. AB both A & B antigen on the RBCs. * Alleles IA & IB are dominant over i & are fally expressed when they present together (codominance). Mating between AB & AB can produce A,AB & B with a ratio of 1 : 2 : 1 respectively. * Type AB is known as universal acceptor & type O is known as universal donor, why? Rh factor: is inherited separately from ABO type. - Rh + ve has a particular antigen. - Rh – ve antigen is absent. (Continuous Variation) - e.g. skin colour, height of man, eye colour). - Skin colour is controlled by poly genes (2 or more alleles) in which all dominant (capital letters) add equally to the phenotype. - The individual has a copy of all allelic pair. - If it is assumed that skin colour is controlled by 2 pairs of alleles & each capital letter contributes pigments to the skin. Genotype AABB AABb, AaBB AaBb, AAbb, aaBB Aabb, aaBb aabb Phenotype Very dark Dark Medium brown Light Very light - When very dark person reproduce with very light person, children may have medium brown skin. - If 2 people with medium brown skin reproduce together, the children may range in skin colour from very dark to very light. Multiple effects of a single genes (Pleiotropy): Expression of alleles at a single location on a chromosome may have positive or negative effects on 2 or more traits. Pleiotropy: Pleio = more - Tropy = to change. e.g. sickle - cell anemia. * HbA: normal haemoglobin molecule normal blood cells. * Hbs: mutant (abnormal) allele, abnormal Hb molecules, abnormal RBCs (sickle - shaped). * HbAHbA: normal. * HbAHbs: show few symptoms. * HbsHbs: abnormal sickle - shaped RBCs. - abnormal sickle - shaped RBCs may lead to rapture of blood capillaries. These may result in: * Oxygen transport affected. * Cells become starved for oxygen. * Another phenotypes may appear like mental retardation, kidney failure, jaundice,…. Sex - linked traits: Human male & female have 23 pairs of chromosomes, 22 pairs of autosomal chromosomes (non sex chromosomes) & one pair allosomes (sex chromosomes). * Sex chromosomes are xx in female & xy in males. - Inheritance of sex? Or (sex deter mination). * Sex - linked traits are found on x chromosome not y. Very few alleles have been found on y chromosome since it is much smaller than x chromosome. e.g. haemophilia & red - green colour blindness. XB: normal. Xb: haemophilia. Genotype XB XB XB Xb Xb Xb XB Y XbY Phenotype female normal female normal, carrier female haemophilic male normal male haemophilic Carrier: * Individuals appear normal but can pass alleles for genetic disorder. * If XB Xb mates XB Y the result will be? - Alleles for sex - linked traits pass from the father to his daughters & from the mother to her sons. Chromosomal abnormalities: Mutation: A heritable change in the DNA including alternation in chromosome structure or number & also alternation in a gene due to hange in DNA composition. Mutagen: An environmental agent that increases the change of a mutation. e.g. radiation, chemicals & viruses. A. Change in chromosomal structure: 1. Deletion: Loss of chromosome region one or more genes may be lost. 2. Inversion: Part of chromosome becomes oriented in the reverse direction without any loss. 3. Translocation: Broken part of chromosome become attached to a non homologous chromosome. 4. Duplication: Normal chromosome have gene sequences that are repeated several times. Deletion of duplication occur between 2 homologous chromosomes. B. Change in chromosomal number: This change in number may occur in autosomal or sex chromosomes. *Occur in the form of polily which may be: 1. Euploidy: variation in chromosome number by whole set. - Polyploidy: individuals have 3 or more of each type of chromosomes Triploidy 3n, tetraploidy 4n. 2. Aneoploidy: Addition or deletion of one or more chromosome from a diploid set. Monosomy (2n – 1): Single copy of one type of chromosome is present. Trisomy (2n + 1): 3 copies of the same kind of chromosome are present. * An abnormal chromosomal makeup in an individual can be due to non disjunction. 1. Non disjunction during meiosis I: Both members of homologous pairs go into the same gamete. 2. Non disjunction during meiosis II: Sister chromatids fail to separate. So, both daughter chromosomes go into the same gamete. Human autosomal abnormalities: Down Syndrome: Chromosome number: 47 chromosomes are present in the cell instead of 46. Trisomy (2n + 1): In which chromosome 21 is represented by 3 copies. Cause: * Non disjunction during meiosis. * Egg with 24 chromosome (n + 1) is fertilized with a normal sperm (n) to give zygote (2n + 1). Characters: - Affects males & female. - Short neck. - Drooping eyelids. - Open mouth & protruded tongue. - Moon - face (similar to Mongolian face). Hence the name Mongolism. - Mentally retarded. Sex - chromosome abnormalities: Result from non disjunction of sex chromosome. 1. Klinefelter’s Syndrome: Chromosome number: Individuals (males) have 47 chromosomes; 44 autosomes + XXY (44 + XXY). Cause: Abnormal egg with 24 chromosomes containing XX is fertilized with a normal sperm Y. Characters: - Affects males only. - Sterile males. - Breasts like those of females. - Tall arm & legs. - Absence of body hair. - Some mental retardation. 2. Turner’s Syndrome (44 + X0): Chromosome number: Individuals (females) have 45 chromosomes (monosomy); 44 autosomes + X (44 + X0). Cause: * Abnormal egg with 22 chromosomes is fertilized with a normal sperm X. Characters: - Affects females only. - Sterile females. - Short. - Poorly developed ovaries & sex organs. - Broad chest with poorly developed breast. - Not mentally retarded as other syndromes.