mandel genetics

7/30/2019 mandel genetics

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Biologi Asas

Basic Principle of Genetics:

Mandels Genetics


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Mandels Genetics

Plants & animals have been selectivelybreeded to produce more useful hybrids .

A hit or miss process since the actual

mechanisms governing inheritance wereunknown.

Knowledge of these genetic mechanisms

finally came as a result of careful laboratorybreeding experiments carried out over the lastcentury & a half.


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The invention of better microscopes (1890's)

allowed biologists to discover the basic facts

of cell division & sexual reproduction.

The focus of genetics research then shifted to

understanding what really happens in the

transmission of hereditary traits from parentsto children.

A number of hypotheses were suggested to

explain heredity, but Gregor Mendel, was theonly one who got it more or less right.


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Gregor Johann Mendel (July 20, 1822 January6, 1884) was an Austrian scientist, the founder ofthe new science of genetics.

Mendel demonstrated that the inheritance ofcertain traits in pea plants follows particularpatterns, now referred to as the lawsof Mendelian inheritance.

His ideas had been published in 1866 but largelywent unrecognized until 1900, which was longafter his death.

Although the significance of Mendel's work was

not recognized until the turn of the 20th century,the independent rediscovery of these lawsformed the foundation of the modern science ofgenetics.


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Gregor J. Mendel


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Through the selective cross-breeding of

28,000+ common pea plants over manygenerations between 1856 & 1863, Mendel

discovered that certain traits show up in

offspring without any blending of parentcharacteristics.

For instance, the pea flowers are either purple

or white. Intermediate colors do not appear in the

offspring of cross-pollinated pea plants.


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Mendel observed seven traits that are easilyrecognized and apparently only occur in oneof two forms:

1. flower color is purple or white

2. flower position is axil or terminal

3. stem length is long or short

4. seed shape is round or wrinkle

5. seed color is yellow or green

6. pod shape is inflated or constricted

7. pod color is yellow or green


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This observation that these traits do not show upin offspring plants with intermediate forms was

critically important because the leading theory inbiology at the time was that inherited traits blendfrom generation to generation.

Most of the leading scientists in the 19th centuryaccepted this "blending theory.

Charles Darwin proposed another equally wrongtheory known as "pangenesis" .

This held that hereditary "particles" in our bodiesare affected by the things we do during ourlifetime.


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These modified particles were thought to

migrate via blood to the reproductive cells &

subsequently could be inherited by the next

generation.

This was essentially a variation of Lamarck's

incorrect idea of the "inheritance of acquiredcharacteristics."


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Mendel picked common garden pea plants forthe focus of his research because they can be

grown easily in large numbers & theirreproduction can be manipulated.

Pea plants have both male & female reproductiveorgans.

As a result, they can either self-pollinatethemselves or cross-pollinate with another plant.

In his experiments, Mendel was able toselectively cross-pollinate purebred plants with

particular traits & observe the outcome overmany generations.

This was the basis for his conclusions about thenature of genetic inheritance.


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In cross-pollinating plants that either produce yellow

or green pea seeds exclusively, Mendel found that the

first offspring generation (f1) always has yellow seeds.However, the following generation (f2) consistently

has a 3:1 ratio of yellow to green.


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This 3:1 ratio occurs in later generations as well.

Mendel realized that this was the key to understanding

the basic mechanisms of inheritance.


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Mendel came to three important conclusionsfrom these experimental results:

1. that the inheritance of each trait isdetermined by "units" or "factors" that arepassed on to descendents unchanged (these

units are now called genes )2. that an individual inherits one such unit from

each parent for each trait

3. that a trait may not show up in an individualbut can still be passed on to the nextgeneration.


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The starting parent plants were homozygous forpea seed color.

Each had two identical forms (or alleles ) of thegene for this trait--2 yellows or 2 greens.

An allele is an alternative form of a gene (onemember of a pair) that is located at a specificposition on a specific chromosome.

These DNA codings determine distinct traits thatcan be passed on from parents to offspring.

Genes are segments of DNA locatedon chromosomes which contain the codes for theproduction of specific proteins.


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The plants in the f1 generation wereall heterozygous - each had inherited two

different alleles--one from each parent plant.

It becomes clearer when look at the actual

genetic makeup (genotype ), of the pea plants

instead of only the observable physical

characteristics (phenotype).


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Note that each of the f1 generation plants (shown

below) inherited a Y allele from one parent & a G allele

from the other.

When the f1 plants breed, they have an equal chance

of passing on either Y or G alleles to each offspring.


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With all of the 7 pea plant traits that Mendelexamined, 1 form appeared dominant over

the other, which is to say it masked thepresence of the other allele.

For example, when the genotype for pea seed

color is YG (heterozygous), the phenotype isyellow.

However, the dominant yellow allele does notalter the recessive green one in any way.

Both alleles can be passed on to the nextgeneration unchanged.


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Mendel's observations from these

experiments can be summarized in 2

principles:

1. the principle of segregation

2. the principle of independent assortment


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According to the principle of segregation, for

any particular trait, the pair of alleles of eachparent separate & only one allele passes from

each parent on to an offspring.

Which allele in a parent's pair of alleles isinherited is a matter of chance.

We now know that this segregation of alleles

occurs during the process of sex cell formation(i.e., meiosis ).


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Segregation of alleles in the production of sex

cells


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According to the principle of independentassortment, different pairs of alleles are

passed to offspring independently of eachother.

The result is that new combinations of genes

present in neither parent are possible. For example, a pea plant's inheritance of the

ability to produce purple flowers instead ofwhite ones does not make it more likely that it

will also inherit the ability to produce yellowpea seeds in contrast to green ones.


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Likewise, the principle of independent

assortment explains why the humaninheritance of a particular eye color does not

increase or decrease the likelihood of having 6

fingers on each hand.

Today, we know this is due to the fact that the

genes for independently assorted traits are

located on different chromosomes.


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Probability of Inheritance

The value of studying genetics is in

understanding how we can predict the

likelihood of inheriting particular traits.

This can help plant & animal breeders in

developing varieties that have more desirable

qualities.

It can also help people explain & predict

patterns of inheritance in family lines.


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One of the easiest ways to calculate themathematical probability of inheriting a specifictrait was invented by an early 20th centuryEnglish geneticist named Reginald Punnett .

His technique employs what we now calla Punnett square.

This is a simple graphical way of discovering all ofthe potential combinations ofgenotypes that canoccur in children, given the genotypes of theirparents.

It also shows us the odds of each of the offspringgenotypes occurring.


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Setting up & using a Punnett square is quite

simple once you understand how it works.

You begin by drawing a grid of perpendicular

lines:


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Next, you put the genotype of one parent across

the top and that of the other parent down the

left side.

For example, if parent pea plant genotypes were

YY and GG respectively, the setup would be:


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Note that only one letter goes in each box for

the parents. It does not matter which parent is on the side

or the top of the Punnett square.

Next, all you have to do is fill in the boxes bycopying the row & column-head letters acrossor down into the empty squares.

This gives us the predicted frequency of all of

the potential genotypes among the offspringeach time reproduction occurs.


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In this example, 100% of the offspring will

likely be heterozygous (YG).

Since the Y (yellow) allele is dominant over the

G (green) allele for pea plants, 100% of the YG

offspring will have a yellow phenotype, as

Mendel observed in his breeding experiments.


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In another example, if the parent plants both

have heterozygous (YG) genotypes, there will be

25% YY, 50% YG, and 25% GG offspring onaverage.

These percentages are determined based on the

fact that each of the 4 offspring boxes in aPunnett square is 25% (1 out of 4).

As to phenotypes, 75% will be Y and only 25% will

be G.

These will be the odds every time a new offspring

is conceived by parents with YG genotypes.


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25% YY, 50% YG, and 25% GG


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An offspring's genotype is the result of the

combination of genes in the sex cells or

gametes (sperm & ova) that came together in

its conception.

One sex cell came from each parent.

Sex cells normally only have one copy of the

gene for each trait (e.g., one copy of the Y or

G form of the gene in the example above).


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Each of the two Punnett square boxes in

which the parent genes for a trait are placed

(across the top or on the left side) actually

represents 1 of the 2 possible genotypes for a

parent sex cell.


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Which of the two parental copies of a gene is

inherited depends on which sex cell is

inherited--it is a matter of chance.

By placing each of the 2 copies in its own box

has the effect of giving it a 50% chance of

being inherited.


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Are Punnett Squares Just Academic

Games?

Why is it important for you to know aboutPunnett squares?

The answer is that they can be used as

predictive tools when considering havingchildren.

Let us assume, for instance, that both you &your mate are carriers for a particularlyunpleasant genetically inherited disease suchas cystic fibrosis .


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Of course, you are worried about whether

your children will be healthy & normal.

For this example, let us define "A" as being

the dominant normal allele & "a" as

the recessive abnormal one that is responsible

for cystic fibrosis.

As carriers, you & your mate are both

heterozygous (Aa).

This disease only afflicts those who arehomozygous recessive (aa).


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The Punnett square below makes it clear that at each

birth, there will be a 25% chance of you having a

normal homozygous (AA) child, a 50% chance of ahealthy heterozygous (Aa) carrier child like you &

your mate & a 25% chance of a homozygous

recessive (aa) child who probably will eventually die

from this condition.


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If both parents are carriers of the recessive

allele for a disorder, all of their children will

face the following odds of inheriting it:25% chance of having the recessive disorder

50% chance of being a healthy carrier

25% chance of being healthy and not have

the recessive allele at all Aa Aa

Aa AaAA aa


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If a carrier (Aa) for such a recessive disease mates

with someone who has it (aa), the likelihood of their

children also inheriting the condition is far greater

(as shown below).

On average, half of the children will be heterozygous

(Aa) & therefore, carriers.

The remaining half will inherit 2 recessive alleles (aa)& develop the disease.


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If one parent is a carrier and the other has a

recessive disorder, their children will have the

following odds of inheriting it:

50% chance of being a healthy carrier

50% chance having the recessive disorder

Aa aa

aaaaAaAa


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It is likely that every one of us is a carrier for alarge number of recessive alleles.

Some of these alleles can cause life-

threatening defects if they are inherited fromboth parents.

Other examples: albinism, beta-thalassemia &

Tay-Sachs disease are recessive disorders.


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Some disorders are caused by dominant

alleles for genes.

Inheriting just one copy of such a dominant

allele will cause the disorder.

This is the case with Huntington disease,

achondroplastic dwarfism & polydactyly.

People who are heterozygous (Aa) are not

healthy carriers.

They have the disorder just like homozygous

dominant (AA) individuals.


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If only one parent has a single copy of a

dominant allele for a dominant disorder,

their children will have a 50% chance ofinheriting the disorder and 50% chance

of being entirely normal.

aaAa

Aa Aa aa aa


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Punnett squares are standard tools used by

genetic counselors.

Theoretically, the likelihood of inheriting many

traits, including useful ones, can be predicted

using them.

It is also possible to construct squares for

more than one trait at a time.

However, some traits are not inherited with

the simple mathematical probability

suggested here.


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The End

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mandel genetics