Lecture 12 - Genes and Chromosomes

All living things pass on their genetic heritage by a common process.

Genes and Chromosomes. Lecture Outline 12

1. Genetic Linkage groups

Since there are many more genes than chromosomes, many genes are linked. The number of linkage groups is equal to the number of pairs of chromosomes.

Mosquito	= 3	Cat	= 17
Drosophila	= 4	Human	= 23
Mendel’s pea plant	= 7	Dogs	= 39
Cabbage	= 9	Goldfish	= 47

In Drosophila , B and b, the genes for black and gray body color, and W and w, the genes for long and short wings, are on the same chromosome. If (B/W b/w) flies are crossed with (b/w b/w) flies, only two phenotypes should be produced. (Note: Linked genes are written together with a slash, i.e. B/W and b/w).

	B/W	b/w
b/w	B/W b/w (50%)	b/w b/w(50%)

Contrary to expectation, one gets results suggesting that there has been an exchange between the homologous chromosomes.

	B/W	b/w	B/w	b/W
b/w	B/W b/w (41%)	b/w b/w (41%)	B/w b/w (9%)	b/W b/w (9%)

2. Chiasmata - formation in meiosis basis for genetic crossing over.

Microscopic observation of chiasmata formation between homologous chromosomes during meiosis I is physical evidence of genetic crossing over. This greatly increases the diversity of gametes. In humans, without crossing over the variety of gametes is 2²³ or 8 million, with crossing over the number is astronomical!

3. Mapping genes from crossover rates.

    The frequency of crossing over between two genes depends on their relative distance from one another. The frequency of crossing over between linked genes can be used to construct a chromosome map.

4. X-linked genes.

    Genes on the X chromosome are "sex linked". The white eye mutation in Drosophila is sex linked. A cross between a white-eyed mutant and a red eyed, wild type produces red eyed hybrids. Crosses between the F₁ hybrids gives a ratio of 3 red to 1 white eyed F₂ , however, the white-eyed flies were all males. T.H. Morgan, in 1910, correctly identified this as an X-linked mutant.
    Sex-linked defects in humans: Color blindness, hemophilia, and Duchenne’s muscular dystrophy.

5. Nondisjunction.

If segregation of homologous chromosomes does not occur in meiosis I, gametes with both homologous chromosomes will be formed.

Autosomal:
   Trisomy 21: Down Syndrome (Frequency increases with age of mother.)
    Trisomy 13 and 18: Cleft palate (13) and mental retardation (13 and 18).

X Chromosome
   Nondisjunction of X-chromosomes leads to two kinds of eggs, XX and no X,
    at fertilization with normal sperm (X or Y) results in the following:
                XXX Triple-X females                         XXY Kleinfelter males
                      X Turner syndrome females      Y Nonviable

Detection by amniocentesis.
    Culture white blood cells in presence of colchicine, which inhibits mitotic spindle formation. Chromosomes blocked at metaphase. Swell cells and stain.

6. Modifications in chromosome structure.

A variety of chromosomal aberrations. some which have important evolutionary significance.

Type of Alteration Possible favorable effects Possible harmful effects

Deletion Usually detrimental. Possible elimination of bad genes Loss of critical genes; disrupts chromosome meiosis separation

Duplication Material for evolution of new members of protein family Interferes with chromosome separation. May disrupt gene.

Inversion Increase genetic diversity by changing gene positions Reduces fertility; loss of control of gene expression.

Translocation Enormous genetic changes may generate evolutionary advances May activate cancer genes; reduce fertility; loss of genes

    Historical note: In 1900 three botanists, Hugo De Vries, Erich von Tschermak and Carl Correns, simultaneously rediscovered Mendel’s laws and Mendel’s publication. In 1902, Walter Sutton compared behavior of chromosome and genes.