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
• A haploid nucleus is a nucleus containing a single set of unpaired chromosomes, e.g. in gametes
• A 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

• 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

Meoisis

• 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: I^A, I^B, I^O (the base letter, I stands for immunnoglobulin)
• I^O is recessive, two I^O alleles must be present for the person to have type O blood
• I^A and I^B are co-dominant but both are dominant to I^O
• If a person receives an I^A allele and an I^B allele, their blood type is AB, in which characteristics of both A and B antigens are expressed
• Because I^O is dominated by both I^A and I^B; alleles, a person with blood group A could have the genotypes I^AI^O or I^AI^A
• This has implication when having children because, if both parents carry the I^O allele, a child could be born with the genotype I^OI^O (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