Unit 4:  The genetic revolution                                                       1   2   3   4  

1.2. Chromosome theory of inheritance

In 1908, two researches separately, W.S. Sutton in United States and T. Boveri in Germany, proposed the idea that Mendel’s hereditary factors are located in chromosomes.

They concluded it after observe the parallelism between the behaviour of chromosomes during the gametes formation and fertilisation, and the inheritance of the genetic traits. This idea is the basis of the chromosome theory of inheritance.

In 1915, Thomas H. Morgan thanks to his works with fruit fly (Drosophila melanogaster) confirmed this theory which was completed by other investigators in the following years.

The theory can be summarised into:

• The hereditary factors (genes) are located on chromosomes.

• Each gene occupies a fix position (locus) on a determine chromosome.

• Genes are disposed in line on the chromosomes.

• There are two types of chromosomes:

- Heterochromosomes or sexual chromosomes.

They are equal in females (XX)

They are different in males (XY)

- Autosomes:They are the rest of chromosomes and they are equal two by two.

• Two genes can be inherited:

- Together, when they are on the same chormome (linked genes)

- Separately, when they are on different chromosomes (independent genes)

1.7. The following sketch shows two cells, each one with four chromosomes, from which gametes are formed. In the picture, the chromosomes are shown and the position on them, of the alleles A, a, B, b and R, r. (A determines dark hair, and a, blonde hair, B determines straight hair and b, curly hair; R determines dark skin and r, fair skin)

a)   If we cross these individuals, which will the genotype of the offspring be?

b)  Which of these genes are inherited independently? And linked?

c)   Why linked genes do not inherit independently?