CH13 Hereditary

• Gregor Mendel


  Father of genetics
• Mendel's three key observations

  • Each trait is controlled by a pair of genes that separate during gamete formation
  • Inheritance of one trait is not dependent on the inheritance of another
  • Organisms with alternate forms of a gene, known as alleles, will express the form that is dominant
• The production of non-identical offspring and spontaneous mutation of genes create variation of traits among organisms in a population
• When faced with diseases or competing for resources, individuals with favourable traits survive and reproduce, passing on these favourable traits to their offspring
• This process of natural selection eventually leads to evolution
• Gene


  • A unit of inheritance
  • Made of a sequence of nucleotides as part of a DNA molecule
  • Codes for a single polypeptide
• Allele


  • Alternative forms of a gene
  • Occupies the same genetic locus (same position on a chromosome)
  • Alleles code for different variations of the same phenotypic trait
• Alleles for fur colour in rabbits

  • One allele codes for black fur, one codes for brown fur
• Alleles for blood type in humans

  • IA, IB, and IO
• Homologous chromosomes


  • A pair of chromosomes of which one is paternal, and the other maternal in origin
  • They are of same shape and length, same centromere position and contain the same genes occupying the same genetic loci (same positions on a chromosome)
• Genotype


  Genetic make-up of an individual
• Phenotype


  Observable trait of an individual
• Homozygous


  • An individual having identical alleles of a single gene
  • Individual could be homozygous dominant (possessing only copies of the dominant allele) or homozygous recessive (possessing only copies of the recessive allele)
• Heterozygous


  An individual having different alleles of a single gene
• Complete dominance


  • An allele that has complete dominance over other alleles suppresses the effect of the other (recessive) alleles
  • The dominant allele is fully expressed and manifested in the phenotype of homozygous dominant individuals and heterozygous individuals
• Co-dominance


  An allele that is codominant with another allele of a gene will be equally expressed and manifested in the phenotype of a heterozygote
• Recessive


  • An allele that is recessive to other alleles is suppressed by the effects of the other (dominant) alleles
  • The recessive allele is only expressed and manifested in the phenotype of homozygous recessive individuals
• Predicting results of simple crosses

  Use terms: homozygous, heterozygous, F1 generation, F2 generation
• Solving problems involving monohybrid inheritance

  Use genetic diagrams
• Expected ratios are calculated probabilities based on a theoretical model
• Observed ratios are due to random chance because fusion of gametes in fertilisation is random
• As sample size increases, observed ratio will become closer to expected ratio
• ABO blood group


  • IA, IB, and IO are the gene alleles
  • IA and IB are co-dominant
  • IO is recessive to both IA and IB
• ABO blood group phenotypes and genotypes


  • A: IAIA or IAIO
  • B: IBIB or IBIO
  • AB: IAIB
  • O: IOIO
• Determination of sex in humans

  1. Females have 22 pairs of autosomes and 2 X chromosomes

  2. Males have 22 pairs of autosomes, 1 X chromosome and 1 Y chromosome

  3. If sperm with X fertilises ovum, offspring is female

  4. If sperm with Y fertilises ovum, offspring is male
• Phenotypic ratio of male : female = 1 : 1
• Mutation


  A random change in the nucleotide sequence of a gene
• Sickle cell anaemia


  • Mutation in haemoglobin A gene on chromosome 11 results in haemoglobin S that polymerises, causing red blood cells to become sickle shaped
• Down syndrome


  • Trisomy in chromosome 21 results in stunted growth and mental impairment
• Factors that increase mutation rate

  • Ionising radiation (e.g. X-ray, UV)
  • Chemical mutagens (e.g. tar, formaldehyde)
• Continuous variation


  • Phenotypes have intermediate forms and show gradation from one extreme to another
  • Phenotypes affected by environment
  • Phenotype controlled by many genes
  • Genes show additive effect
• Discontinuous variation


  • Phenotypes are distinct from one another and can be easily sorted into categories
  • Phenotypes not affected by environment
  • Phenotype controlled by one or few genes
  • Genes do not show additive effect
• Variation and competition lead to differential survival of, and reproduction by, those organisms best fitted to the environment
• Mechanism of evolution by natural selection


  1. Random gene mutation and sexual reproduction results in genetic variation

  2. Environmental factor exerts selection pressure, advantaged individuals survive and reproduce more

  3. Proportion of population with advantaged allele increases over generations, leading to evolution
• Natural selection for warfarin resistance in rats

  • Warfarin poisoning selects for warfarin resistant rats, which survive and reproduce more
• Natural selection for thick fur coats in rabbits

  • Temperature decrease selects for thick coated rabbits, which survive and reproduce more
• Artificial selection


  Animal or plant breeders select organisms with desirable characteristics to reproduce and pass on favourable alleles to offspring
• Artificial selection in plants


  • Pollinate flowers of soybeans with high oil content
• Artificial selection in animals


  • Breed cattle that produce large amounts of milk