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Biological ScienceVII. Genetics

A. Definition of GeneticsGenetics is the study of heredity and variations. Heredity is the transfer of traits from

parents to offsprings. Variations are the differences existing between individuals.

B. Johann Mendels Life StoryJohann Mendel is known today as the Father of Genetics. He is an Austrian and he was

born in 1822. The place where he was born is part now of the Czech Republic.

Johann Mendel did not come from a rich family. He came from family of farmers. In hischildhood, he experienced a lot of financial hardships and health problems.Upon finishing his high school, he entered a monastery where he was renamed as Gregor

After studying theology, he was assigned by his monastery as a temporary teacher. However, hewas not able to pass the teachers exam. Later, he was sent by his monastery to University oVienna to learn more. There, he met Prof. Doppler and Prof. Unger. These two professorsinfluenced much the way of thinking of Mendel. Mendel learned from Prof. Doppler the use ofMathematics in explaining natural phenomena. On the other hand, Prof. Unger is the one whoaroused the interest of Mendel in studying the variations among plants.

Upon finishing his studies in University of Vienna, he was assigned to Brnn ModernSchool. Aside from teaching, he did researches. There, he was able to develop the two famousprinciples of heredity.C. Mendelian Principles of Heredity

The two principles of heredity that Mendel was able to develop are the Principle oSegregation and Principle of Independent Assortment. In these principles, based on hisobservations, he assumed that:

1. The genes in DNA that control the traits of organisms exist in different forms. Theseforms are now known as alleles. For example, instead of just having one form of gene forseed color of pea plants, there are forms of genes for seed color of pea plants. There is anallele for yellow seed and there is an allele for green seed.2. Both the male parent and female parent contribute to the alleles of their offspring. Anorganism has two alleles for each trait. One is from the male parent and one is from thefemale parent.3. An allele is either dominant or recessive. Whenever dominant allele is present, it will bethe one which will be expressed. (In Genetics, alleles are represented by the first letter ofthe dominant allele. The letter symbolizing the dominant allele is capitalized while theletter symbolizing the recessive allele remains is being written using lower case.)

For example:Seed color of pea plants

Dominant allele: allele for yellow seed (Y)

Recessive allele: allele for green seed (y)

Situations:1. YY the seeds of the pea plant will be yellow2. Yy the seeds of the pea plant will be yellow too3. yy the seed of the pea plant will be green

Based on Principle of Segregation, the alleles of a gene for particular trait are beingseparated from each other during gamete or sex cell formation (meiosis). For example, if a peaplant has the genetic composition Yy, during meiosis, Y will be separated from y. One sex cell mayhave Y or one sex cell may have y but not both Y and y.

Based on the Principle of Independent Assortment, the alleles of different genes fordifferent traits combine with each other by chance or without any pattern during gamete or sexcell formation (meiosis).

For example:Seed color of pea plants (Y-yellow versus y-green)Seed shape of pea plants (R-round versus r-wrinkled)

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Biological ScienceFlower color of pea plants (P-purple versus p-white)

The alleles for seed color, seed shape, and flower color of pea plants may formdifferent combinations by chance during meiosis. Dominant alleles may or may not betogether. Recessive alleles may or may not be together. Dominant and recessivealleles may or may not be combined.

The following combinations in sex cells may be formed: YRP, yRP, YrP, YRp, yrPYrp, yRp, yrp

D. Determination of Probability of Having an Offspring with Particular Trait(s)Considering the concepts from the two Mendelian Principles of Heredity, one can

determine the probability or chance of having a particular organism with particular set of traits.

Examples:

Situation 1Given: Rr x rr (round x wrinkled)Find: Chance of having an offspring pea plant with round seedsSolution: via the use ofPunnett Square (a table being used in Genetics to determine thepossible traits of offsprings of two parents)

R rr Rr(roun

d)

rr(wrinkl

ed)

r Rr(roun

d)

rr(wrinkl

ed)

Situation 2Given: RrYy x RrYy (round/yellow x round/yellow)Find: Chance of having an offspring pea plant with wrinkled and green seedsSolution: via the use ofPunnett Square

RY Ry rY ryRY

RRYY(round/yel

low)

RRYy(round/yel

low)

RrYY(round/yello

w)

RrYy(round/yello

w)

Ry

RRYy(round/yel

low)

RRyy(round/gr

een)

RrYy(round/yello

w)

Rryy(round/gree

n)rY RrYY

(round/yellow)

RrYy(round/yel

low)

rrYY(wrinkled/ye

llow)

rrYy(wrinkled/ye

llow)

ry RrYy(round/yel

low)

Rryy(round/gr

een)

rrYy(wrinkled/ye

llow)

rryy(wrinkled/gr

een)

Situation 2 (Additional Technique Only)Given: RrYy x RrYy (round/yellow x round/yellow)Find: Chance of having an offspring pea plant with wrinkled and green seeds

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Ratio of probable genetic compositions is 2Rr:2rr or 1Rr:1rr Ratio of probable traits is 1round:1wrinkled

Chance of having an offspring with round seeds = or 50%

Ratio of probable genetic compositions is1RRYY: 2RRYy: 2RrYY: 1RRyy: 4RrYy: 1rrYY: 2Rryy: 2rrYy: 1rryy

Ratio of probable traits is9round/yellow: 3round/green: 3wrinkled/yellow: 1wrinkled/green

Chance of having an offspring with wrinkled and green seeds =

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Biological ScienceSolution: via the use ofBranching Method

1. Consider each gene first:

Rr x Rr ------> 1RR:2Rr:1rr

Yy x Yy -----> 1YY:2Yy:1yy2. Do the branching:

3. Computation:

Chance of having an offspring with wrinkled and green seeds = 1/16 or (1/16 x 100% =6.25%)

E. Additional Notes1. Non-Mendelian Traits

The two Mendelian Principles of Heredity are, indeed, helpful in determining theprobability of having an offspring with particular traits. However, these two principles donot work for all traits. Some traits are following these principles and are being called asMendelian Traits. Some traits are not following these principles and are being called asNon-Mendelian Traits. Through experiments, one can determine which traits areMendelian and which traits are Non-Mendelian.

2. Other Modes of Inheritance

Non-Mendelian traits exist because they do not follow the assumptions GregorMendel used in his principles.

There are traits that are not only being controlled by two alleles. Some arecontrolled by more than two alleles. In fact, in more complicated situations, thealleles of a gene may be influenced by the alleles of other genes.

Also, some traits do not allow the complete expression of the dominant allele.

Lastly, among plants and animals, some alleles are only coming from the mother ofemale parent. During fertilization, only the nucleus is being contributed by the sexcell of the father/male parent. On the other hand, the whole cell is beingcontributed by the sex cell of the mother/female parent. Chloroplasts and/omitochondria that are located in cytoplasm are said to have their own DNA. Thus, ionly the mother/female parent contributes its cytoplasm, then, the DNA of the

father/male parent in its chloroplasts and/or mitochondria are not being passed tothe offspring.3. Other Factors that Cause Variations

Genes in DNA are, indeed, the units of heredity. However, the traits that they arecoding will only be observable once they are expressed. Possession of a gene is differentfrom expression of a gene. Even if one has a gene for particular trait if it will not beexpressed it will not be observed.

One reason why a gene is not being expressed is that that gene might be arecessive allele or an allele being covered by other alleles. Another possible reason is tha

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1RR

2Rr

1rr

1YY2Yy

1yy

1YY2Yy1yy

1YY2Yy1yy

1RRYY (round/yellow)2RRYy (round/yellow)

1RRyy (round/green)

2RrYY (round/yellow)4RrYy (round/yellow)2Rryy (round/green)

1rrYY (wrinkled/yellow)2rrYy (wrinkled/yellow)1rryy (wrinkled/green)

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Biological Sciencethat gene is being expressed with high influence of the environment. An environmentafactor may be internal or external. Internal factors are those that are dependent to theorganism such as age and sex. External factors are those that are independent from theorganism such as its temperature, its exposure to light, its nutrition, and its extent ofetus-mother relation. Even if two individuals have the same genetic composition, if theywill be exposed to different environment, there will still be the chance that they will havecontrasting characteristics being expressed.

In addition, in other situations, related organisms become different from each other

because of mutation. Mutation is the change in genetic composition because of radiationor chemical. Mutation can be beneficial or harmful. If it will add advantageous trait to anindividual or if it will remove a disadvantageous trait from an individual, it will bebeneficial. However, if it will remove an advantageous trait from an individual or if it wiladd disadvantageous trait to an individual, it will be harmful. Some mutations may nocause detectable changes. Some mutations may result to death.

4. Trivia

Criminality and having high blood pressure are indeed traits and, thus, heritablebut their expression is mainly triggered by the environment.

Hair color, eye color, feeblemindedness, and having schizophrenia are traits thaare being expressed with less influence of the environment.

Below are some of traits with measured heritability.

Traits Heritability (%)

Math Apt 12Science Apt 34

BeingHardworker/Achiever

50

Being Imaginative 50Tendency to HaveDivorce

52

Spelling Skill 53

Verbal Apt 68

Weight 78IntelligenceQuotient (IQ)

80

Stature/Height 81

Note: Heritability is the measurement of resemblance between relatives. Iheritability of a trait is less than 50%, it means that this trait can be influencedmore by the environment. Otherwise, the trait is being influenced more by thegene composition of the organism. For example, if you are having a problem inlearning Math, it is not because of the gene(s) dictating the Math Apt that youhave inherited from your parents. Rather, it is because of you or of your teacher oof both of you. Hence, if you are good in Math, big part of it is because of youreffort and/or the teaching skill of your Math teacher or of both. On the other hand

if you became fat, there is the possibility that it is not only because of your eatinghabits but also because of the genes from your father or mother or from both othem. Similarly, if you cannot gain additional weight anymore despite of eatingmore, having small body built is already part of your nature.

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