Molecular Biology Online Tutorials

The Chromosome Theory of Inheritance

The notion that a chromosome carries the gene is the basis of the Chromosome Theory of Inheritance. This new theory appeared in the early part of the 20th century when scientists began to accpet that Mendel's observations about genes were true. Defined, the theory states that chromosomes are actually linear "strings" of genes that are located at specific sties along the chromosome. It can be summed up with these main points:

The unique place on the gene is called the Locus.

Diploid organisms such as humans have two copies of all chromosomes

Sex chromosomes (X,Y) are haploid and contain only one copy of a gene

Genes can be the same for both chromosomes, called homozygous

Genes can be different on each chromosome, called heterozygous

The term genotype is used to describe molecular genetics (with respect to genes)

The term phenotype is used to describe how an organism LOOKS becasue of their genes

Phenotypes can be wither Wild-type (typically dominant) or Mutant (typically recessive)

The chromosome is conatined inside the nucleus and is a physical entity which contains all genetic information is the form of DNA. The DNA is almost always tightly packed around a histone protein. When the cells receives a signal that anew protein needs to be made, the the gene that codes for that specific protein, is unwound from the histone so the gene can be replicated and made into the desired functional protein.

Watch the following videos on how DNA gets packaged into tightly wound chromatin and replicated.

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