Gregor Johann Mendel• Austrian Monk, born in what is now Czech

Republic in 1822

• Son of peasant farmer, studiedTheology and was ordainedpriest Order St. Augustine.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 for34 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– 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 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 F1generation.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 Assortment

Based on the pea results, Mendel postulated the 3. Principle of Independent Assortment :

“Members of one gene pair segregate “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 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!

Coming Next:

• Incomplete dominance• Human blood types• Complex traits• Sex-linked traits• Sex-linked traits

Dominanza incompleta o codominanza

Quando nell’eterozigote i due alleli si esprimono en trambi in egual misura e l’espressione di ciascun allele è riconosc ibile a livello fenotipico.

Esempio: gruppi sanguigni sistema ABO

Genotipo FenotipoGenotipo Fenotipo

IA – IA Gruppo A

IA – iIB – IB Gruppo B

IB – iIA – IB Gruppo AB

i - i Gruppo 0

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 qu ella dei geni

autosomici in quanto si osserva differenza tra gli incroci re ciproci, cioè

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

dalla madre.

Punnett Square for Sex Determination

Reginald Punnett (1875-1967) developed this device to explain sex determination. He 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)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 DominanteNeurofibromatosiCorea di Huntington

Autosomica RecessivaTalassemieFalcemiaFibrosi cisticaFenilchetonuriaFenilchetonuria

X-linkedDistrofia muscolareCecità ai coloriFavismoX-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 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 MitocondrialeGenoma MitocondrialeGenoma MitocondrialeGenoma Mitocondriale

16.600 bp16.600 bp16.600 bp16.600 bp37 geni37 geni37 geni37 geni

Eredità mitondriale Eredità mitondriale Eredità mitondriale Eredità mitondriale

oooo

Eredità maternaEredità maternaEredità maternaEredità materna

LEBER HEREDITARY OPTIC NEUROPATHY; LEBER HEREDITARY OPTIC NEUROPATHY; LEBER HEREDITARY OPTIC NEUROPATHY; LEBER HEREDITARY OPTIC NEUROPATHY; LHON LHON LHON 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)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 CFIF 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:for cf and have the disease:

C C C c

C c c c

C c

C

c

C C = normalC c = carrier, no symptomsc c = has cystic fibrosis

Neuropsychiatric diseases caused by expansionof trinucleotide repeats

• Myotonic dystrophy• Fragile X syndrome• Spinal and bulbar muscular atrophy (Kennedy’s)• Huntington’s disease

MicrosatellitesMicrosatellites

• short regions of repeating DNA sequence in the geno me(because their G+C content is usually higher or low erthan the average for the genome they frequently app earto band at a different buoyant density in CsCl grad ientsand hence are called “satellites”)

• microsatellites are often comprised of “trinucleotid e repeats”

X fragile(309550)

Frequenza : 1/4000 maschi.

Ereditarietà : Legata al cromosoma X. Malattia causata da mutazionedinamica.

Genetica: Nel 1991 è stato identificato il gene responsabile. Lamutazione è caratterizzata dall’amplificazione di un tratto di DNAmutazione è caratterizzata dall’amplificazione di un tratto di DNAcostituito da una specifica sequenza ripetuta (CGG). Nei soggettinormali è presente un numero di ripetizioni variabili da 6 a 55.Esistono due differenti tipi di mutazione: la premutazione (56-200) ela mutazione completa (>200). La probabilità di espansione aumentacon le dimensioni della premutazione e quindi con il passare dellegenerazioni (Paradosso di Sherman).

Diagnosi : La diagnosi molecolare (Southern blot) permette diindividuare 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 proteicoGenetica : 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 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 a re usually hybridized against Southern blots of genomic DNA cut with SacI I, EagI or BssHII to identify clustering of rare-cutter sites (Figure 10 .22).

Island-rescue PCR. This is a way of isolating CpG i sland sequences from YACs by amplifying sequences between islands and ne ighbouring AluYACs by amplifying sequences between islands and ne ighbouring Alurepeats.

HybridizationA genomic DNA clone can be hybridized against a Nor thern 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 assa y (see Figure 10.23). It relies on the observation that the vast majority of mammal ian 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 seq uences may indicate a gene (see Section 20.1.4)

Gene searching algorithms. A variety of computer pr ograms have been developed to search sequences for exons and other g ene-associated motifs (see Figure 10.25 and Section 20.1.4 ).

Human Genome Project

1990 – 2001 – ………..

Studio delle malattie genetiche

Diagnosi

Cura

Prevenzione