inheritance & variation

• Fertilization occurs when sperm meets with the egg inside the fallopian tube.
• Genetic basis of inheritance:

  • Mendel and others provided an idea of inheritance patterns
  • 'Factors' represent the genetic basis of inheritance
  • Structure of genetic material and structural basis of genotype and phenotype conversion became focus of attention in biology for the next century
  • Major contributions to molecular biology from Watson, Crick, Nirenberg, Khorana, Kornbergs, Benzer, Monod, Brenner
• Structure and function of DNA and the story and theory of evolution examined and explained
• Mendel's Laws of Inheritance:

  • Mendel conducted hybridisation experiments on garden peas for seven years (1856-1863)
  • Statistical analysis and mathematical logic applied to problems in biology for the first time
  • Mendel investigated characters in the garden pea plant with two opposing traits
  • Genes are the units of inheritance, containing information required to express a particular trait
  • Alleles are slightly different forms of the same gene
  • Genotype (TT, Tt, tt) and phenotype (tall, dwarf) relationship
  • Dominant and recessive factors
  • Monohybrid cross and Punnett Square
• The F1 plant of genotype Tt produces gametes of the genotype T and t in equal proportion when self-pollinated
• When fertilisation takes place, pollen grains of genotype T have a 50% chance to pollinate eggs of genotype T and t, while pollen grains of genotype t have a 50% chance to pollinate eggs of genotype T and t
• Resultant zygotes can be of the genotypes TT, Tt, or tt
• Phenotypic character seen in F1 plants of genotype Tt is 'tall'
• In F2, 3/4 of the plants are tall, with some being TT and others being Tt
• It is not possible to distinguish between plants with genotypes TT and Tt externally
• Within the genotypic pair Tt, only one character 'T' tall is expressed
• Character T or 'tall' is said to dominate over the other allele t or 'dwarf' character
• Phenotypic ratio in F2 is 3/4 tall : 1/4 tt, with a genotypic ratio of 1:2:1
• Law of Dominance:

  • Characters are controlled by discrete units called factors
  • Factors occur in pairs
  • In a dissimilar pair of factors, one member of the pair dominates the other
• Law of Segregation:

  • Alleles do not show blending
  • Both characters are recovered in the F2 generation
  • Factors or alleles of a pair segregate from each other during gamete formation
• Incomplete Dominance:

  • F1 phenotype may not resemble either parent
  • Example: flower colour inheritance in dog flower (snapdragon)
• Co-dominance:

  • F1 generation resembles both parents
  • Example: ABO blood grouping in humans
• Multiple alleles:

  • More than two alleles governing the same character
  • Example: ABO blood grouping in humans
• Inheritance of Two Genes:

  • Yellow colour is dominant over green
  • Round shape is dominant over wrinkled
• In a dihybrid cross, the phenotypes round, yellow; wrinkled, yellow; round, green; and wrinkled, green appeared in the ratio 9:3:3:1
• Mendel's Law of Independent Assortment states that when two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters
• During meiosis, chromosomes and genes occur in pairs and segregate independently of each other
• Chromosomal Theory of Inheritance states that the pairing and separation of a pair of chromosomes leads to the segregation of a pair of factors they carry
• Linkage refers to the physical association of genes on a chromosome, while recombination describes the generation of non-parental gene combinations
• Polygenic traits are controlled by three or more genes and are influenced by the environment
• In polygenic inheritance, the phenotype reflects the contribution of each allele, where the effect of each allele is additive
• Pleiotropic genes exhibit multiple phenotypic expressions, where a single gene can affect multiple traits
• Pleiotropic gene: a gene that exhibits multiple phenotypic expressions
• Pleiotropy is caused by the effect of a gene on metabolic pathways contributing to different phenotypes
• Example: phenylketonuria disease in humans caused by a mutation in the gene for phenyl alanine hydroxylase
• Sex determination mechanisms:

  • Insects: Henking observed an X body in spermatogenesis, leading to the concept of X-chromosome
  • XO type: all eggs have an additional X-chromosome, fertilization by X-chromosome sperm results in females, by non-X sperm in males
  • XY type: males have XY chromosomes, females have XX chromosomes
  • Birds: females have ZW chromosomes, males have ZZ chromosomes
• Humans sex determination:

  • XY type: males have XY chromosomes, females have XX chromosomes
  • Males produce 2 types of gametes: X-chromosome and Y-chromosome
  • Females produce ovum with X-chromosome
  • Fertilization with X-chromosome sperm results in females, with Y-chromosome sperm results in males
• Mutation:

  • Results in alteration of DNA sequences and changes in genotype and phenotype
  • Point mutation: change in a single base pair of DNA
  • Example: sickle cell anemia caused by point mutation
  • Mutagens: chemical and physical factors inducing mutations, e.g., UV radiation
• Genetic disorders:

  • Mendelian disorders: determined by alteration in a single gene
  • Examples: Haemophilia, Cystic fibrosis, Sickle-cell anemia, Colour blindness, Phenylketonuria, Thalassemia
  • Mendelian disorders may be dominant or recessive
  • Pedigree analysis used to trace inheritance patterns of Mendelian disorders
• Haemophilia:

  • Affects a single protein involved in blood clotting, causing non-stop bleeding from simple cuts
  • Heterozygous females (carriers) can transmit the disease to sons
  • Female haemophiliacs are extremely rare, requiring a carrier mother and haemophilic father
  • Queen Victoria's family pedigree shows haemophilic descendants as she was a carrier
• Sickle-cell anaemia:

  • Autosomal recessive trait transmitted when both parents are carriers
  • Controlled by alleles Hb A and Hb S
  • Homozygous individuals for Hb S show the diseased phenotype
  • Heterozygous individuals are carriers with 50% probability of transmitting the gene
• Phenylketonuria:

  • Inherited as an autosomal recessive trait
  • Affected individuals lack an enzyme converting phenylalanine into tyrosine
  • Accumulation of phenylalanine leads to mental retardation
  • Phenylpyruvic acid and derivatives are excreted through urine
• Thalassemia:

  • Autosomal recessive blood disease transmitted when both parents are carriers
  • Mutation or deletion causes reduced synthesis of globin chains in haemoglobin
  • Classified based on affected globin chain: α Thalassemia affects α globin chain, β Thalassemia affects β globin chain
  • α Thalassemia controlled by genes HBA1 and HBA2, β Thalassemia controlled by gene HBB
• Chromosomal Disorders:

  • Caused by absence, excess, or abnormal arrangement of chromosomes
  • Aneuploidy results from failure of chromatid segregation, e.g., Down's syndrome with extra chromosome 21
  • Turner's syndrome results from loss of an X chromosome
  • Polyploidy results from failure of cytokinesis after cell division
• Down's Syndrome:

  • Genetic disorder with an extra copy of chromosome 21 (trisomy of 21)
  • Short stature, small round head, furrowed tongue, and mental retardation