Appunti di Genetica in inglese

G F E E F G

H B D H

! duplicated)

EFGH

of other genes.

! Figure 16.10

! Translocations.

a)—Nonreciprocal b)—Nonreciprocal c)—Reciprocal interchromosomal

intrachromosomal interchromosomal translocation

! translocation translocation

A A A M A M A M M A

! B D B N D N B N N B

E B C

C E C O O O P

! C D

D F D P F P C Q

B E

E E Q G O Q D R

C F

F F R H P R E

! Q

G G

G G F

H H

H H

R G

H

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4. Translocation !

A translocation is a chromosomal mutation in which there is a change in position of chromosome

segments and the gene sequences they contain to a different location in the genome: a

chromosome’s part moves.! !

No gain or loss of genetic material is involved in a translocation.

!

Translocation can be:!

- intra-chromosomal: changes within the same chromosome !

- inter-chromosomal: changes between two chromosomes.

Inter-chromosomal translocation can be:

—> reciprocal: an exchange of segments between the two chromosomes is involved;

—> non-reciprocal: if a one-way transfer is involved.!

!

I

n strains homozygous for a reciprocal translocation, meiosis takes place normally because all

chromosome pairs can pair properly, and crossing-over does not produce any abnormal

chromatids.!

!

In strains heterozygous for a reciprocal translocation, however, all homologous chromosome parts

pair as best they can. !

Since one set of normal chromosomes (1+2) and one set of

translocated chromosomes (1’+2’) are involved, the result is a cross-

like configuration in meiotic prophase I. !

These cross-like figures consist of four associated chromosomes, !

each partially homologous to two other chromosomes in the group.

!

Segregation at anaphase I may occur in three different ways:!

!

1. Alternate segregation (50%): alternate centromeres

migrate to the same pole:!

- 1+2 go to one pole of the cell;!

- 1’+2’ go to the other pole of the cell.!

In this case all the gametes have all the genetic material Alternate segregation (50%)

therefore all the gametes are vital and functional. !

!

!

!

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2. Adjacent-1 segregation (50%): adjacent non-homologous centromeres migrate to the same

According to Mendel this is the other possibility

pole:!

- 1+2’ go to one pole of the cell;! Adjacent-1 segregation (50%)

- 1’+2 go to the other pole of the cell;! Missing VW

In this case the four gametes are not vital and not functional Missing ABC

since the first two missed the genes VW while the other two

missed the genes ABC.! !

Adjacent-1 segregation occurs about as frequently as alternate segregation.

!

! Adjacent-2 segregation (very rare)

3. Adjacent-2 segregation (very very rare): different pairs of Homologous centromers migrate to the same pole

adjacent homologous centromeres migrate to the same pole:!

- 1+1’ go to one pole of the cell;!

- 2+2’ go to the other pole of the cell; !

!

In sum, of the six theoretically possible gametes, the two from alternate segregation are functional,

the two from adjacent-1 segregation usually are inviable (because of gene duplications and

!

deficiencies), and the two from adjacent-2 seldom occur and are inviable if they do.

!

Moreover, since there is equal possibly of having an adjacent-1 and an alternate segregation, it

can happen that half of our gametes are functional while the other half is not viable: when this

happens this condition is called “semisterility” or “half-sterility”. !

!

In general reciprocal translocations are diagnosed genetically by semisterility and by the apparent

linkage of genes known to be on separate chromosomes.!

!

In humans translocations are always carried in the

heterozygous state.!

For example let’s analyze Down syndrome.!

The Down syndrome is caused by an extra

chromosome 21 that failed to segregate from its

homolog during meiosis (95% of all cases).!

!

A less common type of Down syndrome is called

Robertsonian translocation. This can occur in a

family! Fusion between chromosome 21 and 14:

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translocation occurs between them in one of the parents. !

NB: therefore Down syndrome can be caused by:!

1. non-disjunction of chromosomes: that form of Down syndrome is not dependent from the

heritability because there is no portatore;!

2. translocation: it’s dependent on the heritability because there is a portatore.!

!

Burkitt’s lymphoma!

Translocations can also appear in cancer cells.!

The Burkitt’s lymphoma for example is due to a translocation between chromosomes 8 and 14: the

c-myc gene moves to the chromosome 8 to the chromosome 14 and therefore its regulation

changes. !

Once c-myc has moved, the Ig-Cu gene starts regulating his expression: c-myc is a proto-

oncogene and is normally inactive when located on the chromosome 8. !

By moving to the chromosome 14, c-myc becomes active and causes this tumor.!

!

This reciprocal translocation converts a proto-oncogene to an oncogene and causes the tumor. !

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Lessons 41+42!

CHROMOSOME MUTATIONS: changes in chromosomes number

Spontaneous changes in chromosome number are going on in all organisms during evolution.!

If you look at our dinner table you can see that many species that we eat today arose through

spontaneous changes in chromosome number.!

Also breeders used this process to improve productivity and other characters.!

!

There are two types of changes in chromosome numbers:!

- changes in the whole chromosomes sets (all duplicated);!

- changes in parts f the chromosome set (just one more chromosome).!

!

The number of chromosomes in a basic set is called monoploid number (x), it refers to the number

of types of chromosomes that you have.!

!

!

Organisms with a complete set of chromosomes or an exact

multiple of the monoploid number of chromosomes are called

!

“euploid”.

!

Eukaryotes normally carry:!

one chromosome set (haploids);!

• two chromosome sets (diploids).!

•

!

Those that have more sets are called polyploid.!

!

Chromosome mutations that result in variations in the number of

chromosome sets occur in nature, and the resulting organism or

cells are also euploid. !

Chromosome mutations resulting in variations in the number of

individual chromosomes are examples of aneuploidy. !

An aneuploid organism or cell has a chromosome number that is

not an exact multiple of the haploid set of chromosomes (one

chromosome less or one chromosome more).!

!

!

The haploid number (n) refers to the number of chromosomes in gametes.!

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!

TO SUM UP:!

Monoploid number (x) —> it refers to the number of types of chromosomes that you have;!

• Haploid number (n) —> it refers to the number of chromosomes in gametes.!

•

In humans:!

- x=23 —> 23 different types of chromosomes;!

- n=23 —> 23 chromosome in the gamete;!

In wheat:!

- x=7 —> 7 different types of chromosomes;!

- n=21 —> 21 chromosomes in the gametes, in total the chromosomes are 42 because there are

6 copies for each of the 7 chromosomes, in gametes the chromosomes are half —> 21.!

!

MODERN WHEAT!

It has 42 chromosomes. !

It is hexaploid: it has 6 sets of seven chromosomes, therefore for each of the 7 types of

chromosomes that wheat has, it has 6 copies.!

!

Since gametes contain just half of the chromosomes which are contained in normal cells, the

gametes of wheat contains 21 chromosomes: in gametes there are 3 copies for each of the 7

chromosomes.!

!

In wheat the monoploid number (number of types of chromosomes) is 7 —> x=7 because you

have 7 different types of chromosomes.!

In wheat the haploid number (number of chromosomes in gametes) is 21 —> n=21!

!

NB: If wheat would be diploid it would have 14 chromosomes !

!

MONOPLOIDS ORGANISMS!

They have just one set of chromosomes. Examples or monopolies organisms are bees, wasps and

ants. !

Males developed parthenogenetically: they development from unfertilized egg. !

Monoploids organisms are abnormal: they bypass meiosis and they produced gametes by mitosis.!

!

They are very important for plant breeding: !

- in diploids recessive genes have to be made homozygous in order to be seen in the phenotype;!

- if you have a monoploid organism then you don’t have problems with recessive and dominant

characters because if you go into the next generation these to alleles segregate: you have nay

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In diploids recessive genes have to be made homozygous

Favourable allele combinations are broken up by meiosis

Monoploids provide a way around this

one copy of each gene, therefore a monoploid

plant express exactly her genotype. As a result Cold treatment

the recessive character can always be seen in

the phenotype.!

!

Monoploids plant come fro pollen cells from angers

with are treated with cold: by that procedure small

embryos are created on the plate. These embryos are haploid (monoploid) and from them a plant

generates: Plant monoploids can be analysed for favourable traits and then the selected plants can

be made diploid. !

!

How can you endure diploid formation from a monoploid? By applying colchicine to meristematic

cells. In so doing a monoploid plant, which is sterile, can become a diploid plant, which is fertile

and therefore can produce gametes by meiosis.!

!

TRIPLOIDS ORGANISMS!

Autopolyploids organisms can arise spontaneously in nature or are constructed by geneticists by

crossing a 4x (tetraploid) with a 2x (diploid) because:!

- gametes of the tetraploid are 2n;!

- gametes of the diploid are n;!

Therefore the offspring will be 3n (triploid): a triploid is sterile because a triploid doesn’t balance his

gametes:!

- the first chromosome goes to one pole, the second chromosome goes to the other pole;!

- the third one can go on one pole or on the other: it’s random where it goes. !

!

As a result in one gametes will have 2 chromosomes while the other gamete will have 1

chromosome: therefore the gametes will be unbalanced and will have a chromosome number that

is intermediate between haploid and diploid: aneuploid. !

When gametes are not balanced, the organism is sterile.!

!

!

!

Generally:!

when you are considering a polyploid despair series (3n, 5n …) they are normally STERILE

•