Chromosome theory

Chromosome theory:

Although Gregor Mendel had discovered the principles on how traits were passed from generation to generation, the mechanism involved was still unknown.

• His work was forgotten until approximately 1900
• In 1902, Walter Sutton, after comparing chromosome migration data with Mendel's data concluded that Factors (genes) are located on the cells' chromosomes.
• This is known as the Chromosome theory of heredity.

Chromosome theory of heredity

This theory has enormous implications on gene behavior.
The fact that certain traits do not follow the predicted rules can be explained by this theory through the concept of Gene Linkage, Crossing over & Sex Linkage

• Gene Linkage refers to the fact that more than one gene is located on each chromosome.
  • As this chromosome segregates during meiosis, all genes on it segregate with it, and not independently, as the law of independent assortment would dictate.
  • Certain groups which tend to stay together during meiosis are referred to as Linkage groups.
• Crossing over refers to the process which occurs during the first meiotic division.
  • Segments of homologous overlap, break & rejoin.
  • Results in chromosomes with different gene combinations. These are called recombinants.
  • Genes which tend to remain together during meiosis are known as linkage groups.
  • Typically the closer 2 genes are physically located to each other on a chromosome, the less frequently they segregate from each other during meiosis. This allows geneticists to locate genes on a particular chromosome through the process of gene mapping.

• Sex Linkage refers to the fact that unlike the autosomes, the sex chromosomes differ in structure from each other. This has great implications.
  • Since a male of a species has an XY assortment of chromosomes, many traits which are recessive on the X chromosome will be expressed since the Y chromosome may not have the corresponding dominant gene on the Y chromosome.
  • Since females have an XX arrangement of chromosomes, recessive traits can be masked by the dominant genes located on the homologous X chromosome.

• This phenomenon can be seen with humans when comparing incedences of color blindness and hemophilia in humans. The frequency of both is much greater for males than females. Because the genes for both are located on the X chromosome, and are recessive.
  • If a female receives the affected X chromosome, it is masked by the normal one. That individual is unaffected, but is known as a carrier.
  • If a male receives the defective gene, he will be affected since he lacks the corresponding dominant gene on his Y chromosome.

Mutations

• There are typically 2 types of mutations: Chromosomal Mutations and Gene mutations.
• Chromosomal Mutations occur when there is a change in number and/or structure of the chromosomes themselves.

There are 5 types:

1. Deletion - When a part of a chromosome is lost
2. Duplication - a part of a chromosome is repeated
3. Inversion - a segment of the chromosome gets oriented in the opposite direction
4. Translocation - When a segment from one chromosome breaks and reattaches to another chromosome.
5. Nondisjunction - Occurs when chromosomes fail to separate properly during meiosis. This can result in missing chromosomes, or to many chromosomes (Polyploidy).

• Usually fatal in animals and therefor humans, although some like trisomy 21, are not fatal, but result in serious abnormalities.
• Can result in hardier varieties in plants.

 

• Gene mutations refer to errors in the nucleutide sequence if a DNA strand.
  • If a substitution error (a nucleutide is simply replaced by another) occurs, usually only 1 amino acid is affected. This is usually non-fatal. However some human genetic disorders such as sickle-cell anemia, are caused by this type of mutation.
  • If, however, a nucleotide is inserted, or deleted from a DNA strand, a frameshift mutation occurs. Since the 3 BP codon sequences are all skewed 1 position, ALL the amino acids from that point on will be affected. These are typically more serious than substitution errors.