Cell differentiation

• Each chromosome has several genes, each of which codes for a particular allele. Each gene is found at a particular place on the chromosome and this location is known as the locus of the gene. Each gene has at least two different forms, known as alleles
• Some features are determined by multiple alleles. This means that there are more than two variants. No matter how many alleles, any diploid individual will only inherit two of them
• Sex linkage is the exception of random inheritance of genes. In sex linkage, certain characteristics are inherited on the sex chromosomes, and so it makes a difference if you are male or female. Some characteristics inherited on a single gene are always associated with other characteristic, also carried on single genes
• Most traits in a living organism are determined by several or many several genes. They are polygenic. Characteristics like eye colour, weight and intelligence are determined by several different genes at different loci, and interactions with the environment and more variety
• Digenic (dihybrid) crosses are breeding experiments involving the inheritance of two pairs of contrasting characteristics at the same time.

  
• Sometimes the ratios of dihybrid crosses are not what you expected. This is because:

  • small sample size
  • experimental error
  • the process is random so something unexpected could have happened
  • the genes are linked
• With completely unlinked genes, two genes are found on separate chromosomes and approximately equal numbers of gametes are formed which contain either the parental combinations of alleles or a different combination of alleles (recombined types)
• Genes for different character uses which are found on the same chromosome are linked. They are inherited as if they were a single gene. When genes are closely linked, recombination events will rarely separate them during meiosis
• The tightness of the linkage of a pair of genes is related to how close together the linked genes are on the chromosomes. Genes that are very close together are less likely to be split by meiosis than genes that are further apart
• If two or more genes are positioned very close together on a chromosome, they may be so tightly linked that they are never split during meiosis, and so the gametes formed will always be of the parental types
• Cell differentiation occurs when unspecialised cells switch different genes on or off as needed to become specialised cells.
• Epigenetic modifications ensure that a wide range of very specific proteins are made within a cell as it differentiates for a specific function. They can be made in response to internal or external stimuli. Once they occur, they are passed on when the cell divides by mitosis
• The process of cell differentiation is:

  1. Chemical stimulus (e.g demethylation)/transcription factor
  2. certain genes are activated or inactivated
  3. mRNA produced from these genes
  4. transactions of mRNA to form polypeptide/proteins
  5. permanent modification of cell