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Inheritance is the transmission of genetic information from generation to generation

Chromosomes, Genes, and Proteins

  • A chromosome is a thread-like structure of DNA, carrying genetic information in the form of genes
  • A gene is a length of DNA that codes for a protein
  • An allele is a version of a gene
  • 23 pairs of chromosome are present in each human cell; one pair is the sex chromosome which determines the sex of the individual
  • Males have XY, and females have XX
  • Each chromosome is made up of a large number of gene coding for the formation of different proteins which give us our characteristics
  • DNA controls cell function by controlling the production of proteins (some of which are enzymes), antibodies and receptors for neurotransmitters
  • Each nucleotide carries one of four bases (A, T, C, or G)
  • A string of nucleotides therefore holds a sequence of bases
  • This sequence forms a code, which instructs the cell to make particular proteins
  • Proteins are made from amino acids linked together
  • The type and sequence of the amino acids joined together will determine the kind of protein formed

The Manufacture of Protein in Cells

  • DNA molecules remain in the nucleus, but the proteins they carry the codes for are needed elsewhere in the cell
  • A molecules called messenger RNA (mRNA) is used to transfer the information from the nucleus
  • It is much smaller than a DNA molecule and is made up of only one strand
  • Another difference is that mRNA molecules contain slightly different bases (A, C, G, and U)
  • Base U is uracil. It attaches to the DNA base A
  • To pass on the protein code, the double helix of DNA unwinds to expose the chain of bases
  • One strand acts as a template. A messenger RNA molecule is formed along part of this strand, made up of a chain of nucleotides with complementary bases to a section of the DNA strand
  • The mRNA molecule carrying the protein code then passes out of the nucleus, through a nuclear pore in the membrane
  • Once in the cytoplasm, it attaches itself to a ribosome
  • Ribosomes make proteins
  • The mRNA molecules instructs the ribosome to put together a chain of amino acids in a specific sequence, thus making a protein
  • Other mRNA molecules will carry codes for different proteins

Gene Expression

  • Body cells do not have the same requirements for proteins
  • For example, the function of some cells in the stomach is to make the protein, pepsin. Bone marrow cells make the protein hemoglobin, but do not need digestive enzymes
  • Specialized cells all contain the same genes in their nuclei, but only the genes needed to code for specific proteins are switched on (expressed)
  • This enables the cell to make only the proteins it needs to fulfill its function
  • haploid nucleus is a nucleus containing a single set of unpaired chromosomes, e.g. in gametes
  • diploid nucleus is a nucleus containing two sets of chromosomes, e.g. in body cells. In a diploid cell, there is a pair of each type of chromosome and in a human diploid cell, there are 23 pairs


  • Mitosis is a nuclear division giving rise to genetically identical cells
  • Mitosis is the way in which any cell (plant or animal) divides when an organism is:
    • growing,
    • repairing a damaged part of its body,
    • replacing worn out cells
  • The exact duplication of chromosomes occurs before mitosis
  • During mitosis, the copies of chromosomes separate, maintaining the chromosome number

Stem Cells

  • Stem cells are those cells in the body that have retained their power of division.
  • Examples are the basal cells of the skin, which keep dividing to make new skin cells
  • Stem cells can produce only one type of tissue; epidermis, blood, muscle, nerves, etc.
  • Stem cells are unspecialized cells that divide by mitosis to produce daughter cells that can become specialized for specific functions


  • Meiosis is a reduction division in which the chromosome number is halved from diploid to haploid resulting in genetically different cells
  • Meiosis is involved in the production of gametes
  • During meiosis, the new cells get a mixture of homologous chromosomes from father and mother
  • A sperm cell could contain a chromosome 1 from the father and a chromosome 2 from the mother
  • There are all sorts of combination
  • Gametes are genetically different from the parent cells
  • Meiosis produces genetic variation
  • When ova are formed in a woman, all the ova will carry an X chromosome
  • When sperm are formed in a man, half the sperm will carry an X chromosome and the other half, a Y chromosome

Monohybrid Inheritance

  • Genotype is the genetic make-up of an organism in terms of the alleles present
  • Phenotype is the observable features of an organism
  • Homozygous means having two identical alleles of a particular gene. Two identical homozygous individuals that breed together will be pure-breeding
  • Heterozygous means having two different alleles of a particular gene. A heterozygous individual will not be pure-breeding
  • Dominant is an allele that is expressed if it is present
  • Recessive is an allele that is only expressed when there is no dominant allele of the gene present

Genetic Diagram Predicting the Results of Monohybrid Crosses

Photo Credit: DocBrown.info

Explain How to Use a Test Cross to Identify an Unknown Genotype?

  • Use Punnett squares in crosses which result in more than one genotype to work out and show the possible different genotypes

Inheritance of ABO Blood Groups – An Examples of Co-dominance

  • In humans, there are 4 blood types (phenotypes): A, B, AB, and O
  • Blood type is controlled by 3 alleles: IA, IB, IO (the base letter, I stands for immunnoglobulin)
  • IO is recessive, two IO alleles must be present for the person to have type O blood
  • IA and IB are co-dominant but both are dominant to IO
  • If a person receives an IA allele and an IB allele, their blood type is AB, in which characteristics of both A and B antigens are expressed
  • Because IO is dominated by both IA and IB; alleles, a person with blood group A could have the genotypes IAIO or IAIA
  • This has implication when having children because, if both parents carry the IO allele, a child could be born with the genotype IOIO (blood group O), eben though neither of the parents have this phenotype
  • A sex-linked characteristic is a characteristic in which the gene responsible is located on a sex chromosome and this makes the characteristic more common in one sex than in the other