Lecture 2: Mendelian Genetics                                                                                        pdf _download pdf _download 

 

Gregor Mendel's hypotheses:

1. Hereditary determinants are of a particulate nature. Each genetic trait is governed by unit factors , which "hang around" in pairs within individual organisms.

2. When two different unit factors governing the same phenotypical trait occur in the same organism, one of the factors is dominant over the other one, which is called the recessive trait.

3. During the formation of gametes the "paired" unit factors separate or segregate randomly so that each gamete receives either one or the other of the two traits, but only one .

4. The union of one gamete from each parent to form a resultant zygote is random with respect to that particular characteristic.

 

Mendel's First Law: Two members of a gene pair segregate from each other into the gametes, whereby one half of the gametes carries one of the traits, the other half carries the other.

 

    

Mendel's Second Law: During gamete formation the segregation of one gene pair is independent of all other gene pairs


This second law describes the outcome of a dihybrid (two character) cross, or hybrid cross involving additional characters/traits. 

 

A dihybrid is an individual that is a double heterozygote (e.g., with the genotype SsYy).
Mendel’s second law states that the Ss alleles assort into gametes completely independent of the Yy alleles.

Thus, the dihybrid, Ss Yy, produces gametes that have one allele of each gene.

  

In this cross, Four different gametes are possible and will be produced in equal proportions: SY, Sy, sY, and sy.

Random fertilization of gametes yields the outcome visible in the more complex Punnett square.

Note that it is now 4 x 4 table construction to accommodate 16 possible genotypes.

Filling in the table and adding the like cells reveals a 9:3:3:1 ratio of the four possible phenotypes (smooth yellow, smooth green, wrinkled yellow, and wrinkled green).

Monohybrid crosses, dihybrid crosses -

 

-and even more complex by a trihybrid cross.. but with the same principles.

 

Mendel's Laws and the uses of Probability:

The product rule; two independent events occurring simultaneously is the product of each of their respective probabilities.

Roll of the di(c)e ---> 1/6 x 1/6 = 1/36. 

The sum rule; the probability of either one of two events happening is the sum of their individual probabilities.

Roll of the di(c)e ---> 1/36 + 1/36 = 1/18.

This idea gives rise to an understanding of the probability of passing on one of the two traits...."Branch diagrams", where "R-" equals the presence of any of the "R" (or dominant) genotypes. 3/4 x 3/4 = 9/16

 

   

Know how to test for Mendel's laws.

 

While Mendel could not comprehend any of the genetic intricaies that his laws might suggest, his determination of his abstract "unitary" traits had far reaching implications in understanding inheritance of these genetic traits and how they could be inherited.

As we no know the genetic sequence that underpins each allele can be randomly mutatagenized to become a different allele, depending on the DNA sequence changes and how they are maintained or lost -again in a random manner- unless acted upon by a selective pressure -details in a later lecture..

Given the variety of gene expressions in any organism, we describe the "typical phenotype of each organism as being the "Wild Type". This term used to demonstrate the most common presentation of alleles/traits of a given population. Again, being the "most common" allele in a population does not always mean that the wild-type allele will be the dominant or "superior" allele.

However, some definition of "normalcy" in the genetic expression of traits does provide an significant advantage in the understanding of how genes work in the whole organism...

In the early twentieth century, G. H. Shull crossed two varieties of corn, and the yield went from 20 to 80 bushels per acre, thus defining a, now common, agricultural practice to increase production in plants.

This is called either hybrid vigor or heterosis, whereby crossbred strains show superior qualities, and it is a result of the fact that continued genetic "in-breeding" often leads to deleterious traits becoming "fixed" in the gene pool (an aspect we will address later). The mixing of two strains, therefore, reintroduces a number of selectible traits which can enhance the gene pool of the newly defined strains... Use of such hybridization techniques has been a common agricultural practice ever since -to increase production in crops and broadened to cattle etc...

An hypothesis called overdominance even proposes that the heterozygous condition -in certain important genes- is often considered to be selectively "superior" and potentiall has a higher fitness than the presence of either homozygote. This occurs far more frequently in cases where a single trait controls more than one phenotype. Such heterosis, however, would be slightly at odds with Mendel's idea of "independent" traits.

Such variations in genetic thought would effectively clash with Mendelian geenetics, and would, in effect, threaten the very crux of Mendel's ideas which are premised on the "particulate" nature of genes, and an appreciation that one of the alleles in any given gene pair is simply dominant over the other

Perhaps, Mendel wasn't perfect...

 

EXTENSIONS or "Modifications" to Mendelian Genetics -tend to vindicate Mendelian Genetics at the genotypic level.

Same basic scheme, but different phenotypical expression......

Heterozygotes may also show an "intermediate" phenotypewehen compared to the their homozygotic counterparts.
For example, red-flowered snap dragons when crossed with white snap dragons will generate pink-flowered plants.\

   gametogenesis gametogenesis

While this phenotype (on the surface) might tend to support the blending theory that Mendel was so avidly trying to overcome, the F2 progeny, of a "selfing" of the pink flowers admirably demonstrates, Mendelian "particulate" genetics, the reason for the change in colour results from a penomenon called "incomplete dominance".

Another example, where Mendelian Genetics apparantly falls down, can be seen in blood types. In this instance, however, Mendel’s laws are also not compromised -at the genetic level.

These blood-types are said to exhibit codominance, where both alleles are expressed. Note that in codominance the phenotype of the heterozygote is completely different (not just a "blend" of the two homozygotic phenotypes).

Mendel made a series of artful choices in choosing traits in peas that "just happened" to be examples of complete dominance

So, let's look a little closer at the behaviour of genes and their alleles.

Alleles and their Interactions
Differences in alleles of genes are often slight differences in the DNA sequence at the same locus, which result in slightly different products, and can give rise to quite different phenotypes.

Many "genes" can have "multiple alleles".

A population might have more than two alleles for a given gene, even though only a maximum of TWO can be present in any given "diploid individual".

In rabbits, coat color is determined by one gene with four different alleles. Five different colors result from the combinations of these alleles.

      

Please note, even if more than two alleles exist in a population, any given individual can have no more than two of them: one from the mother and one from the father.

Again, as can be observed, Dominance need not be complete.

Multiple Alleles:The "ABO" blood-type system

The ABO system is now known to be a polymorphism of complex carbohydrate structures of glycoproteins and glycolipids expressed at the surface of red blood cells.

  n      

The alleles for blood type are IA, IBand IO. They all occupy one locus.
These alleles determine which antigens (proteins) are present on the surface of red blood cells.

These "antigens" react with proteins called antibodies in the serum of certain individuals and cause the blood to clot.

 

n    abo1  abo2   abo3

 

The "ABO" blood-type system, however, is not just a series of antigens on the surface of each of red blood cell, but also feature their corresponding "antibodies" that exist within the blood serum.

If A antigen (blood cell) and A antibody (serum) are present in the blood... the blood will clot!!.

Therefore, the result of mixing blood from each of the different ABO blood types can result in red blood cell agglutination, or clumping, which may prove to be fatal.

 

Alternative chart Of A-B-O Blood Donor & Recipient Compatibility

 

RECIPIENT
D
O
N
O
R
Alleles &
Antibodies
  O
anti-A
anti-B
  A
anti-B
  B
anti-A
  AB
None
O
None
None
None
None
A
Clump
None
Clump
None
B
Clump
Clump
None
None
AB
Clump
Clump
Clump
None

 

 

Note that blood-types A and B can be considered to be "codominant", as the presence of both alleles AB gives rise to a completely different phenotype, when compared to a homozgous AA or a BB equivalent.

 

Mendelian Ratios: 3:1 and 1:1 (test cross) for monihybrid crosses and 9:3:3:1 for dihybrid crosses.

Extension of Mendelian Genetics:-

Incomplete dominance: eg. Four o' clocks, carnations. BLENDING 1:2:1
Codominance: eg. M and N blood groups on chromosome 4 having specific antigens M and N

1:2:1 but where the heterozygotes (MN) gives rise to a distinct phenotype

Multiple alleles ABO blood types, A and B are dominant to O , but A and B codominant to each other
Lethal alleles eg. Yellow colouration in mice fur. 2 : 1 . Pleiotropy(?)
Several genes/same character Coat colour in mammals,

eg. mice

A (agouti), B (black/brown), C, (colour) D (intensity), S. (distribution) genes

Epistasis essentially "eliminates" or masks phenotypic expression of other genes,

eg. Labrador dogs fur colour, albinoism in mice

Complementary gene activity eg. Pea plants, purple colouration. 9 : 7
Duplicate gene activity eg. Shepherd's purse, Round over narrow fruits, where

both A 1 - and A 2 - can cause heart shape 15 : 1

Recessive suppression eg. Case of purple eye colour in the fruit fly. 13 : 3

 

Gene Interactions
Some genes give rise to gene products that alter the effects (phenotypes) of other genes.

 

An example of gene interactions would be coat colour in mice -the example shown in the picture:

The B allele determines a banded pattern, called agouti.
The recessive b allele results in unbanded hairs.
The genotypes BB or Bb are, therefore, agouti.

The genotype bb is coloured solidly black .

 

Another gene, at an entirely different locus, determines if any coloration occurs at all.

The genotypes AA and Aa have color, whereas the double recessive aa are albino, which do not allow any colour to show through as the aa genotype blocks all pigment production.

Mice that are heterozygous for both genes are agouti.


An F2 phenotypic ratio of an initial parental cross between a BB, AA and a bb, aa would be: 9 agouti: 3 black: 4 white.

The corresponding genotypes are 9 agouti (1 BBAA + 2 BbAA + 4 BbAa):3 black (1 bbAA + 2 bbAa):4 albino (1 BBaa + 2 Bbaa + 1 bbaa).

 

Epistasis

Given that genes of any organism do not operate in total isolation from one another, but function in a common cellular environment (remember concept of "penetrance" and "expressivity" from last lecture) the phenomenon of epistasis occurs when the alleles of one gene cover up alters or masks the expression of alleles of another gene.

Other, more overt examples of gene/allelic expression variation, which differ from expected patterns of Medelian inheritance, one of these is readily found in Labrador fur colouration:

 

epistasis   epistasis

Labrador coat colour is an example of Epistasis in that there are two genes which affect coat colour -the "E" gene, which affects the presence of a dark pigment in the coat and the "B" gene, which governs the degree to which that pigment is present in the coat, where B_ gives rise to a "black lab", but only if the genotypes at the E locus are E_ or EE.

While blood groups would be an more than adequate example of this phenomenon, a more overt example of this, albeit more indirect, would be life and death consequences! Some might even argue that it is not technically "true" epistasis

 

In this instance the same gene that affects coat colour in the mouse also has some influence upon development of the embryo. Thus, one gene, does not always mean one function.   If, as in this case, It has more than one function and is therefore considerred to be pleiotropic.

Complementary gene activity, occurs when two distinct genes affect the same phenotype in a complementary way, whereby the presence of BOTH dominant alleles are required to give a particular phenotype that is distinct from either being present by itself.

Recessive Suppression

b  b  b b

 

Duplicate genes, occur when two distinct genes affect the same phentype in the same way.

One of the best examples of this would be the shape of the shepherd's purse pollen sac.

  

Polygenes also mediate quantitative inheritance.
Individual heritable characters are often found to be controlled by groups of several genes, called polygenes.

Each allele intensifies or diminishes the phenotype.
Variation is continuous or quantitative (“summation" or "adding up” of all the traits).
Examples of continuous characters are height, skin color, and (possibly) intelligence.

Nature vs. Nurture?
When thinking about the importance of either genotypes or phenotypes, one must always consider the immense importance of.....the environment , and how it can affect the action(s) of any given gene.

Variables such as light, temperature, and nutrition can dramatically affect the translation of a genotype into a phenotype.

For example, the darkness of the fur on extremities of a Siamese cat is affected by the temperature of that region. Darkened extremities normally have a lower temperature than the rest of the body.

Such colouration can be manipulated experimentally.

Bunnies

Chromosomes and Mendel:

Sutton and Boveri (at the turn of the century) independently linked Mendel's particulate theory (1866) to the behaviour of chromosomes, coining the term " allele" as being a variation of a gene. At this point the ideals of Genetics and Cytology began to merge.

 red ball

Because of the density of the DNA as the cell transitions into Prophase 1 red ball all of these features can be readily observed under the microscope.

Indeed, at the turn of the century the whole process of the "dancing chromosomes" was correllated to the behavior of Mendel's particles by Sutton and Boveri (1902) red ball.

Consequently, Mendelian genetics can, could be easily explained through a knowledge of Meiosis, and the predictable nature of chromosomal moviement -cellular activities and an understanding of Meiosis came togther beautifully.

      red ball

This was (and is) entirely True ....to a point.  That point being what?....

That any given organism has only a fixed number of chromosomes, far less than the number of potential traits governed by Mendel's particulate theory of hereditary should be a concern for Mendelian Genetics. Even so, random assortment of these numerous genetic "particles" still applies -for the most part......???

So, what about genes that are located on the same chromosome?   According to a cellular appreciation of meiosis, wouldn't that suggest that these genes could not randomly asssort relative to each other, and would be considered to be linked on the same chromosome?

At this stage you may want to revisit the fundamentals of Meiosis and carefully observe the process, and the formation of chiasmata, which were observed at diplotene and diakinesis phases in Meiosis I, where chromosomes appear to be interacting at "mobile nodes". These nodes apparently move toward the ends of the chromosome giving a distinctively unzipping-like aspect as the chromosome appears to "unzip" as the chromosomes move to the poles of the dividing cells....Potential for these interactions to give rise to crossover events?

  chiasmata

It turns out that the probability of these chiasmata (exchange of genetic information from one paired chromosome to another) are critical ingredients for the Mendelian school of thought. They appear to happen randomly -at least once per meiotic event.

Now, if that were true, then genes at loci that are far enough apart on a chromosome are highly likely to experience a chiasmatic event. which would have a dramatic effect upon their degree of "random assortment"?

 

The role of the sex chromosome:

Mendel did all his analyses with plants, which -like corn- exhibit both male and female reproductive structures in the same adult plant (termed monoecious, or "all in one house"). 

What about animals and plants, which have individuals that are either one or the other sex (termed dioecious, or having “two houses”).

In most dioecious organisms, the sex of the organism is determined by differences in the presence/absence of a set of chromosomes, or the presence/absence of discrete chromosomes.

In honeybees, for example, eggs are either fertilized -and become diploid females, or they are not fertilized -and become haploid males, drones.

Female grasshoppers have two X chromosomes, whereas the males have just one. Consequently, it is the sperm that determines the sex of the zygote. If a sperm (without an X) fertilizes an egg, the zygote becomes a male grasshopper.

As for humans...

Discovery of Linkage : This phenomenon was first hypothesized to exist by Bateson and Punnett, who observed "asssociation" of heritable traits.  However, it was really verified by Thomas Hunt Morgan (1909) working on non-autosomal or sex chromosome -linked genes of Drosophila melanogaster.

This distinction was critical for the first analysis as a female that is heterozygous for two distinctively X-chromosomal traits will give male offspring that are hemizygous for these genes so that the gametic genotype -derived from the mother- will directly produce the male offspring's phenotype.

Morgan worked with a mutation in Drosophila melanogaster, which caused a white eye instead of the red eye normally found in his stocks of Drosophila

He crossed a white-eyed male fly with a normal, red-eyed female. ALL the F1's were red-eyed, as might be expected.... if the red-eye allele were dominant.

   

 

Subsequently, when he crossed a male and a female of this F1 generation (giving rise to  the F2) he got the expected 3:1 ratio. He then carried out, what he thought would be a series of test crosses with the equivalent to Mendel's "test cross" using an F1 red-eyed female with a white eyed male and he got the expected expected 1:1 ratio. He then did the same thing with an F1 red eyed male with a white eyed female and he again got the expected 1:1 ratio. The only problem with all this was that when he looked at the original F1 generation -there was a distinctive sex bias toward the white F1 generation, they were ALL MALE.

Eventually, Morgan figured out that these results could be explained if he assumed that the particular gene for eye color was located on the X chromosome.

An understanding of this phenomenon allowed for a special type of genetic inheritance......sex-linked inheritance.   In essence, while a female can be heterozygous for a particular gene that is present on her X chromosome, her male offspring will be hemizygous

Thus, if the allele on the X chromosome is a recessive of the trait it will ALWAYS show through in her male offspring. 

it is relatively easy to trace an X-linked trait that has an overt phenotype, as long as you have enough progeny or a well defined family tree.  One such trait, which has become somewhat infamous among genticists (especially in England) is the passage of Haemophilia within the English royal family.

  

On a less morbid note about another type of X-linked trait -Red Green Colour blindness.... -true story.

  b  b   b  b   b   b

While the X and Y  chromosomes normally behave according to the laws of meiotic cell division (and thus according to Mendelian genetics) the ability to "visualize" through genetic crosses the presence of each of the alleles that is located on either of the X-chromosomes of a mother directly challenges the ubiquity of Mendel's 2nd law).

Remember, that ... to equate Mendel with meiosis we had to invoke the role of chiasmata occuring between two gene loci on adjacent chromatids in paired chromosomes during prophase I of meiosis.   This CANNOT occur for X and Y chromosomes,

 

But what about ALL the genes on the other autosomal chromosomes.

 

 
 

 


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