Gregor Mendel Gregor Mendel Brno, AustriaBrno, Austria

One of the first people to One of the first people to examine the inheritance of traits examine the inheritance of traits across generations by:across generations by:

Carrying out many experiments -Carrying out many experiments -breeding of pea plantsbreeding of pea plants• Examining the traits of the Examining the traits of the

offspring and parentsoffspring and parents• Proposing a model of inheritanceProposing a model of inheritance• 8 years (1856-1864)8 years (1856-1864)

Why Pea plants?Why Pea plants? There were many There were many

pea plants in the pea plants in the monastery monastery garden garden

Many offspring Many offspring produced in just produced in just one crossone cross

Fairly quick to Fairly quick to reproduce- don’t reproduce- don’t take too long to take too long to maturemature

How he did itHow he did it Although not a Although not a

professional scientist, his professional scientist, his methods were very methods were very scientific:scientific:• Investigated one or two Investigated one or two

traits at a time by traits at a time by deliberately breeding deliberately breeding selected plantsselected plants

• Always started with pure-Always started with pure-breeds for the traits he was breeds for the traits he was investigatinginvestigating

• Careful records of parents in Careful records of parents in crossescrosses

• Used statistics to analyse Used statistics to analyse resultsresults

TraitsTraits Although he Although he

examined one examined one at a time, at a time, there were 7 there were 7 different traits different traits that Mendel that Mendel examined over examined over the 8 years of the 8 years of his scientific his scientific workwork

Cross-breedingCross-breeding Transferred Transferred

pollen from pollen from one pure-one pure-breed plant to breed plant to anotheranother

Observed Observed offspring traits offspring traits beforebefore

Interbred the Interbred the first first generation generation offspring offspring

For ExampleFor Example Mendel crossed: pure-bred yellow seed Mendel crossed: pure-bred yellow seed

plant x pure-bred green seed plant and plant x pure-bred green seed plant and obtained all yellow seed offspringobtained all yellow seed offspring

So the factor that caused the yellow So the factor that caused the yellow colour was more powerful than the colour was more powerful than the factor for green colourfactor for green colour

Mendel said that the yellow factor Mendel said that the yellow factor "dominated" the green factor"dominated" the green factor

We now recognise that these factors are We now recognise that these factors are alleles of genes with slightly different alleles of genes with slightly different instructions on homologous instructions on homologous chromosomes and that one allele is chromosomes and that one allele is dominant to the otherdominant to the other

The experiment continuesThe experiment continues Mendel then allowed the first generation to Mendel then allowed the first generation to

self pollinate:self pollinate:

Gametes Y y

Y YY Yy

y Yy yy

Phenotypic ratio: 3/4 yellow : 1/4 green or 3 yellow : 1 green

The FindingsThe Findings

Gregor observed the following:Gregor observed the following:1.1. Each trait is controlled by a pair of Each trait is controlled by a pair of

inherited factors (alleles)inherited factors (alleles)2.2. For each trait individual plants For each trait individual plants

had two factors which could be had two factors which could be identical or different (homo or identical or different (homo or heterozygous). Gregor called heterozygous). Gregor called these pure-bred and hybrid.these pure-bred and hybrid.

3.3. Each factor was a discrete particle Each factor was a discrete particle that retained its identify across that retained its identify across generations (didn’t get diluted or generations (didn’t get diluted or blended)blended)

The FindingsThe Findings

4.4. The trait shown in the 1The trait shown in the 1stst generation hybrid plants were generation hybrid plants were assumed to be assumed to be dominantdominant while the while the hidden trait was hidden trait was recessiverecessive

5.5. During gamete formation, each During gamete formation, each pair of factors separated to pair of factors separated to different gametes – one factor per different gametes – one factor per gametegamete

6.6. In separating, the factors within In separating, the factors within the pair behaved independently of the pair behaved independently of each othereach other

The findingsThe findings

7.7. Results of crosses that were Results of crosses that were repeated revealed the same repeated revealed the same results, regardless of which plant results, regardless of which plant was used as the male parent and was used as the male parent and which was used as the female which was used as the female parentparent

Mendel’s LawsMendel’s Laws From these findings, Gregor From these findings, Gregor

devised two laws of inheritance:devised two laws of inheritance:• The principle of segregation of The principle of segregation of

alleles:alleles: That alleles of parents are separated That alleles of parents are separated

into different gametesinto different gametes

• The principle of independent The principle of independent assortment:assortment:

That alleles behave independently of That alleles behave independently of one another when separating into one another when separating into allelesalleles

Rejected & RediscoveredRejected & Rediscovered

When Mendel presented his When Mendel presented his findings to the scientific community findings to the scientific community in Brno in 1865-6 they were largely in Brno in 1865-6 they were largely ignored ignored

It was not until the early 1900s It was not until the early 1900s that they were rediscovered by 3 that they were rediscovered by 3 geneticists working independently geneticists working independently of one anotherof one another

Not so simple…Not so simple…

It was found that Mendel’s laws of It was found that Mendel’s laws of inheritance were largely correct inheritance were largely correct and seemed to apply to many traits and seemed to apply to many traits across many different speciesacross many different species

However, in the early 1900s two However, in the early 1900s two geneticists discovered that some geneticists discovered that some traits can be inherited together, traits can be inherited together, going against Mendel’s laws of going against Mendel’s laws of independent assortment (linkage)independent assortment (linkage)

ChromosomesChromosomes

In 1902, the structure of In 1902, the structure of chromosomes were first chromosomes were first discovered by an American discovered by an American scientistscientist

He assumed that these He assumed that these chromosomes housed the chromosomes housed the genetic factors proposed by genetic factors proposed by MendelMendel

DNA structureDNA structure The 3D structure of DNA was The 3D structure of DNA was

proposed by Watson and Crick in proposed by Watson and Crick in 19531953

What does DNA stand for?What does DNA stand for?

DeDe oxy-riboseoxy-ribose NucleicNucleic Acid Acid

Type of monosaccha

ride

Found in the nucleus (eukaryote) or nucleoid (prokaryote)

of cells

As in acids and bases

DNA can also be written as Deoxyribonucleic Acid

Building blocksBuilding blocks

A single unit of DNA is called a A single unit of DNA is called a nucleotide it is made up of:nucleotide it is made up of:• A Sugar Molecule (Deoxy-ribose)A Sugar Molecule (Deoxy-ribose)• A Phosphate molecule A Phosphate molecule • A Nitrogenous BaseA Nitrogenous Base

P

Building blocksBuilding blocks

P

P

P

P

P

The nucleotides are The nucleotides are bound together via bound together via covalent bonds covalent bonds between the between the phosphate groups, phosphate groups, forming a strand of forming a strand of DNADNA

These covalent These covalent bonds are very bonds are very strong and difficult strong and difficult to breakto break

The phosphate and The phosphate and sugar ‘backbone’ sugar ‘backbone’ are on the outer are on the outer part of the double part of the double helixhelix

DNADNA

DNA is made up of 2 strands.

The strands are held together by weak bonds between the nitrogen bases which face inwards

The bonding between the strands is made up of hydrogen bonds which are so weak that the two

strands will easily separate at 100˚C

http://academy.d20.co.edu/kadets/lundberg/dnapic2.html

http://www.tokyo-med.ac.jp/genet/picts/dna.jpg

There are four types of nitrogen bases:

Cytosine (C), Guanine (G), Adenine (A) and Thymine (T)

What pairs with C?

What pairs with A?

Complementary Base PairsComplementary Base Pairs

http://academy.d20.co.edu/kadets/lundberg/dnapic2.html

http://www.tokyo-med.ac.jp/genet/picts/dna.jpg

There are four types of nitrogen bases:

Cytosine (C), Guanine (G), Adenine (A) and Thymine (T)

Cytosine pairs with Guanine These are called

Adenine pairs with thymine complementary base pairs

DNADNA

The two strands of DNA run in opposite directions to one another and are said to be

‘anti-parallel’

Once the 2 strands of DNA are bound together, the strands coil to form a helical

shape

This is why DNA is often called a double helix

In this diagram, the 2 DNA strands have backbones that are shown in blue and red

The nitrogen bases are shown in yellowhttp://academy.d20.co.edu/kadets/lundberg/dnapic2.html

http://www.tokyo-med.ac.jp/genet/picts/dna.jpg

The significance of The significance of complementary base pairingcomplementary base pairing

Complementary base pairing means Complementary base pairing means that DNA can act as a template for its that DNA can act as a template for its own replicationown replication• If you know the sequence of one chain, If you know the sequence of one chain,

you can infer the sequence of the otheryou can infer the sequence of the other Contains genetic instructions for Contains genetic instructions for

protein productionprotein production DNA strands can dissociate and then DNA strands can dissociate and then

reassociate when heated and cooled reassociate when heated and cooled respectivelyrespectively

The Gene CodeThe Gene Code The order of the nitrogenous bases A, The order of the nitrogenous bases A,

T, G & C is very important. T, G & C is very important. The sequence provides cells with The sequence provides cells with

templates for the production of every templates for the production of every protein in the bodyprotein in the body

These proteins play many different These proteins play many different roles in the body roles in the body

Each segment of DNA which Each segment of DNA which determines the structure of one protein determines the structure of one protein is called a is called a genegene