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Gregor Johann Mendel

• Austrian Monk, born in what is now Czech

Republic in 1822

• Son of peasant farmer, studied

Theology and was ordained

priest Order St. Augustine.

• Went to the university of Vienna, where he

studied botany and learned the Scientific Method

• Worked with pure lines of peas for eight years

• Pea experiments had been conducted centuries

earlier in England, but were poorly interpreted

• Conducted pea research between 1856 and

1863

• In 1866 he published Experiments in Plant

Hybridization, (Versuche über Pflanzen-

Hybriden) in which he

established his three

Principles of Inheritance

• Work was largely ignored for

34 years, until 1900, when

3 independent botanists

rediscovered Mendel‟s work.

(De Vries, von Tschermak & Correns)

• Mendel was the first biologist to use

mathematics to explain his results

quantitatively.

• Mendel predicted

– The concept of genes

– That genes occur in pairs

– That one gene of each pair is present in the

gametes

Genetics terms you need to

know:• Gene – a unit of heredity;

a section of DNA sequence encoding a single protein

• Genome – the entire set of genes in an organism

• Alleles – two genes that occupy the same position on homologous chromosomes and that cover the same trait (like „flavors‟ of a trait).

• Locus – a fixed location on a strand of DNA where a gene or one of its alleles is located.

• Homozygous – having identical genes (one

from each parent) for a particular

characteristic.

• Heterozygous – having two different genes

for a particular characteristic.

• Dominant – the allele of a gene that masks

or suppresses the expression of an alternate

allele; the trait appears in the heterozygous

condition.

• Recessive – an allele that is masked by a

dominant allele; does not appear in the

heterozygous condition, only in

homozygous.

• Genotype – the genetic makeup of an

organisms

• Phenotype – the physical appearance

of an organism (Genotype + environment)

• Monohybrid cross: a genetic cross

involving a single pair of genes (one trait);

parents differ by a single trait.

• P = Parental generation

• F1 = First filial generation; offspring from a

genetic cross.

• F2 = Second filial generation of a genetic

cross

Mendel‟s Principles

• 1. Principle of Dominance:

One allele masked another, one allele

was dominant over the other in the F1

generation.

• 2. Principle of Segregation:

When gametes are formed, the pairs

of hereditary factors (genes) become

separated, so that each sex cell

(egg/sperm) receives only one kind of

gene.

Principle of Independent

AssortmentBased on the pea results, Mendel postulated

the

3. Principle of Independent Assortment:

“Members of one gene pair segregate

independently from other gene pairs during

gamete formation”

Genes get shuffled – these many combinations

are one of the advantages of sexual

reproduction

A Warning on Assortment

• Today we know independent assortment works only if the genes lie on different chromosomes

• If two genes lie on the same chromosome, they will be transmitted together

• Mendel looked at seven traits he reported as independently assorted. His peas had seven pairs of chromosomes. Historians say he likely threw away data that did not fit his hypotheses!

Modifiche ed estensioni alle ipotesi di

Mendel

• Dominanza incompleta (gli alleli dominanti non

mascherano completamente quelli recessivi)

• Codominanza (entrambi gli alleli sono evidenziabili

negli eterozigoti)

• Geni associati

• Alleli multipli

• Eredità legata al sesso

• Fenotipi complessi

La frequenza di ricombinazione è stata usata per la costruzione di mappe genetiche

A e B = 9.6%

A e C = 5%

C e B = 5%

Il gene codifica per un enzima responsabile

della produzione di un pigmento rosso.

Effetto dosaggio importante!!!!

Teoricamente la dominanza completa non esiste

Dominanza incompleta o codominanza

Quando nell’eterozigote i due alleli si esprimono entrambi in egual

misura e l’espressione di ciascun allele è riconoscibile a livello

fenotipico.

Esempio: gruppi sanguigni sistema ABO

Genotipo Fenotipo

IA – IA Gruppo A

IA – i

IB – IB Gruppo B

IB – i

IA – IB Gruppo AB

i - i Gruppo 0

Pleiotropia: la mutazione di un singolo gene produce diversi effetti

sull‟individuo

Penetranza: la probabilità che un allele si esprima negli individui che lo

possiedono;

Penetranza completa: se il 100% degli individui portanti un determinato allele

manifestano il fenotipo corrispondente;

Penetranza ridotta o incompleta: se la frequenza di espressione è inferiore al

100%.

Espressività: grado di manifestazione del carattere;

Espressività uniforme: il carattere fenotipico è uguale in tutti gli individui

Espressività variabile: manifestazione fenotipica differente in individui con lo

stesso genotipo;

:

Geni localizzati sul cromosoma X

La trasmissione ereditaria dei geni X-linked è diversa da quella dei geni

autosomici in quanto si osserva differenza tra gli incroci reciproci, cioè

la F1 è diversa a seconda che un carattere sia trasmesso dal padre o

dalla madre.

Punnett Square for Sex Determination

Reginald Punnett

(1875-1967) developed

this device to explain

sex determination. He

explored sex-linked

coloration in chickens.

Female gametes across top

Male gametes along side

Inattivazione del cromosoma X nelle cellule di mammifero

Modelli di Ereditarietà

AutosomicaIl gene la cui mutazione è responsabile dell‟insorgenza del fenotipo è

localizzato sugli autosomi

X-linkedIl gene è localizzato sul cromosoma X (differente l‟espressione nei due

sessi)

Y-linkedIl gene è localizzato sul cromosoma Y (Eredità paterna)

MitocodrialeIl fenotipo è determinato da geni localizzati nel genoma mitocondriale

(Eredità materna)

Modelli di Ereditarietà

Dominante L‟eterozigote manifesta il fenotipo (guadagno di funzione)

RecessivoSoltanto l‟omozigote manifesta il fenotipo (perdita di funzione)

Autosomica DominanteNeurofibromatosi

Corea di Huntington

Autosomica RecessivaTalassemie

Falcemia

Fibrosi cistica

Fenilchetonuria

X-linkedDistrofia muscolare

Cecità ai colori

Favismo

X-fragile

Year Disease MIM n Location Gene Chromosome abnormality

1986 Duchenne muscular

dystrophy

310200 Xp21.3 DMD (a) del(X)(p21.3)

(b) t(X;21)(p21.3:p13)

Retinoblastoma 180200 13q14 RB del(13)(q13.1q14.5)

1989 Cystic fibrosis 219700 7q31 CFTR None

1990 Neurofibromatosis 1 162200 17q11.2 NF1 Balanced translocations

t(1;17)(p34.3:q11.2)

t(17;22)(q11.2:q11.2)

Wilms' tumor 194070 11p13 WT1 del(11)(p14p13)

1991 Aniridia 106210 11p13 PAX6 t(4;11)(q22;p13)

del(11)(p13)

Familial polyposis coli 175100 5q21 APC del(5)(q15q22)

Fragile-X syndrome 309550 Xq27.3 FMR1 FRAXA fragile site

Myotonic dystrophy 160900 19q13.3 DMPK None

1993 Huntington's disease 143100 4p16 HD None

Tuberous sclerosis 2 191092 16p13 TSC2 Microdeletions in candidate

region

von Hippel-Lindau disease 193300 3p25 VHL Microdeletions in candidate

region

1994 Achondroplasia 100800 4p16 FGFR3 None

Early-onset breast/ovarian

cancer

113705 17q21 BRCA1 None

Polycystic kidney disease 173900 16p13.3 PKD1 t(16;22) (p13.3;q11.21)

601313

1995 Spinal muscular atrophy 253300 5q13 SMN1 None

600354

•(A) Autosomal dominant;

•(B) autosomal recessive;

•(C) X-linked recessive;

•(D) X-linked dominant;

•(E) Y-linked.

04_02.jpg

Genoma Mitocondriale

16.600 bp

37 geni

Eredità mitondriale

o

Eredità materna

LEBER HEREDITARY OPTIC NEUROPATHY; LHON

Human case: CF• Mendel‟s Principles of Heredity apply

universally to all organisms.

• Cystic Fibrosis: a lethal genetic disease

affecting Caucasians.

• Caused by mutant recessive gene carried by

1 in 20 people of European descent (12M)

• One in 400 Caucasian couples will be both

carriers of CF – 1 in 4 children will have it.

• CF disease affects transport

in tissues – mucus is accumulated

in lungs, causing infections.

Inheritance pattern of CF

IF two parents carry the recessive gene of

Cystic Fibrosis (c), that is, they are

heterozygous (C c), one in four of their

children is expected to be homozygous

for cf and have the disease:

C C C c

C c c c

C c

C

c

C C = normal

C c = carrier, no symptoms

c c = has cystic fibrosis

Neuropsychiatric diseases caused by expansion

of trinucleotide repeats

• Myotonic dystrophy

• Fragile X syndrome

• Spinal and bulbar muscular atrophy (Kennedy’s)

• Huntington’s disease

Microsatellites

• short regions of repeating DNA sequence in the genome

(because their G+C content is usually higher or lower

than the average for the genome they frequently appear

to band at a different buoyant density in CsCl gradients

and hence are called “satellites”)

• microsatellites are often comprised of “trinucleotide repeats”

X fragile

(309550)Frequenza: 1/4000 maschi.

Ereditarietà: Legata al cromosoma X. Malattia causata da mutazione

dinamica.

Genetica: Nel 1991 è stato identificato il gene responsabile. La

mutazione è caratterizzata dall‟amplificazione di un tratto di DNA

costituito da una specifica sequenza ripetuta (CGG). Nei soggetti

normali è presente un numero di ripetizioni variabili da 6 a 55.

Esistono due differenti tipi di mutazione: la premutazione (56-200) e

la mutazione completa (>200). La probabilità di espansione aumenta

con le dimensioni della premutazione e quindi con il passare delle

generazioni (Paradosso di Sherman).

Diagnosi: La diagnosi molecolare (Southern blot) permette di

individuare anche gli individui con la premutazione.

Malattia di Huntington

(143100)

Frequenza: 5-10/100.000 nati vivi

Ereditarietà: autosomica dominante. Malattia causata da mutazione dinamica

Genetica: Il gene responsabile della malattia ed il suo prodotto proteicosono stati identificati. Il gene definito Intersting Transcript (IT-15), èlocalizzato sul braccio corto del cromosoma 4 (4p16.3). La malattia èassociata all‟amplificazione patologica di una specifica sequenza ripetuta(CAG) nell‟allele mutato. Nella popolazione normale la tripletta è ripetuta10-30 volte. Nei pazienti affetti il numero di ripetizioni varia da 36 a più di100. Un numero intermedio di espansioni 30-35 volte, è considerato unapremutazione.

Diagnosi: Il test genetico si basa sulla determinazione del numero diespansione della tripletta.

This database is a catalog of human genes and genetic disorders authored and

edited by Dr. Victor A. McKusick and his colleagues at Johns Hopkins and

elsewhere,

and developed for the World Wide Web by NCBI, the National Center for

Biotechnology Information.

The database contains textual information and references.

It also contains copious links to MEDLINE and sequence records in the Entrez

system, and links to additional related resources at NCBI and elsewhere.

http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=OMIM

Commonly used methods for identifying genes in cloned DNA

Method Comments

Zoo blotting

A DNA clone is hybridized at reduced hybridization stringency against a

Southern blot of genomic DNA samples from a variety of animal species, a

zoo blot.

Depends on coding DNA being more strongly conserved in evolution than

non-coding DNA (Figure 10.21).

CpG island identification

Many vertebrate genes have associated CpG islands, hypomethylated GC-

rich sequences usually having multiple rare-cutter restriction sites ( Cross

and Bird, 1995).

Identification by restriction mapping. DNA clones are usually hybridized

against Southern blots of genomic DNA cut with SacII, EagI or BssHII to

identify clustering of rare-cutter sites (Figure 10.22).

Island-rescue PCR. This is a way of isolating CpG island sequences from

YACs by amplifying sequences between islands and neighbouring Alu

repeats.

Hybridization

A genomic DNA clone can be hybridized against a Northern to mRNA/cDNA

blot of mRNA from a panel of culture cell lines, or against appropriate cDNA

libraries.

Exon trapping

This is essentially an artificial RNA splicing assay (see Figure 10.23). It relies

on the observation that the vast majority of mammalian genes contain

multiple exons which need to be spliced together at the RNA level.

cDNA selection or captureThese techniques involve repeated purification of a subset of genomic DNA

clones which hybridize to a given cDNA population (see Figure 10.24).

Computer analysis of DNA

sequence

Homology searches. Any DNA sequence obtained from a genomic clone can

be compared against all other sequences in sequence data-bases. Significant

homology to known coding DNA or gene-associated sequences may indicate

a gene (see Section 20.1.4)

Gene searching algorithms. A variety of computer programs have been

developed to search sequences for exons and other gene-associated motifs

(see Figure 10.25 and Section 20.1.4).

Human Genome Project

1990 – 2001 – ………..

Studio delle malattie genetiche

Diagnosi

Cura

Prevenzione