Classical Genetics

Classical (Mendelian) Classical (Mendelian) GeneticsGenetics

Gregor MendelGregor Mendel

VocabularyVocabulary

• GeneticsGenetics: The scientific study of heredity: The scientific study of heredity• AlleleAllele: Alternate forms of a gene/factor.: Alternate forms of a gene/factor.• GenotypeGenotype: combination of alleles an organism has.: combination of alleles an organism has.• PhenotypePhenotype: How an organism appears.: How an organism appears.• DominantDominant: An allele which is expressed (masks the : An allele which is expressed (masks the

other).other).• RecessiveRecessive: An allele which is present but remains : An allele which is present but remains

unexpressed (masked)unexpressed (masked)• HomozygousHomozygous: Both alleles for a trait are the same.: Both alleles for a trait are the same.• HeterozygousHeterozygous: The organism's alleles for a trait are : The organism's alleles for a trait are

different.different.

HistoryHistory

• Principles of genetics were developed in the Principles of genetics were developed in the mid 19th century by mid 19th century by Gregor MendelGregor Mendel an an Austrian MonkAustrian Monk

• Developed these principles without ANY Developed these principles without ANY scientific equipment - only his mind.scientific equipment - only his mind.

• Experimented with pea plants, by crossing Experimented with pea plants, by crossing various strains and observing the various strains and observing the characteristics of their offspring.characteristics of their offspring.

• Studied the following characteristics:Studied the following characteristics:– Pea color (Green, yellow)Pea color (Green, yellow)– Pea shape (round, wrinkled)Pea shape (round, wrinkled)– Flower color (purple, white)Flower color (purple, white)– Plant height (tall, short)Plant height (tall, short)

• Made the following observations (example Made the following observations (example given is pea shape)given is pea shape)

• When he crossed a round pea and wrinkled When he crossed a round pea and wrinkled pea, the offspring (F1 gen.) always had pea, the offspring (F1 gen.) always had round peas.round peas.

• When he crossed these F1 plants, however, When he crossed these F1 plants, however, he would get offspring which produced he would get offspring which produced round and wrinkled peas in a 3:1 ratio.round and wrinkled peas in a 3:1 ratio.

Laws of InheretanceLaws of Inheretance

• Law of SegregationLaw of Segregation: When gametes : When gametes (sperm egg etc…) are formed each (sperm egg etc…) are formed each gamete will receive one allele or the other.gamete will receive one allele or the other.

• Law of independent assortmentLaw of independent assortment:: Two or Two or more alleles will separate independently of more alleles will separate independently of each other when gametes are formed each other when gametes are formed

Punnett SquaresPunnett Squares

• Genetic problems can be easily solved Genetic problems can be easily solved using a tool called a using a tool called a punnett squarepunnett square..– Tool for calculating genetic probabilitiesTool for calculating genetic probabilities

A punnett squareA punnett square

Monohybrid cross Monohybrid cross (cross with only 1 trait)(cross with only 1 trait)

• Problem:Problem:

• Using this is a several step process, look Using this is a several step process, look at the following exampleat the following example– Tallness (T) is dominant over shortness (t) in Tallness (T) is dominant over shortness (t) in

pea plants. A Homozygous tall plant (TT) is pea plants. A Homozygous tall plant (TT) is crossed with a short plant (tt). What is the crossed with a short plant (tt). What is the genotypic makeup of the offspring? The genotypic makeup of the offspring? The phenotypic makeup ?phenotypic makeup ?

Punnet processPunnet process

1.1. Determine alleles of Determine alleles of each parent, these each parent, these are given as TT, and are given as TT, and tt respectively.tt respectively.

2.2. Take each possible Take each possible allele of each parent, allele of each parent, separate them, and separate them, and place each allele place each allele either along the top, either along the top, or along the side of or along the side of the punnett square.the punnett square.

Punnett process continuedPunnett process continued

• Lastly, write the letter Lastly, write the letter for each allele across for each allele across each column or down each column or down each row. The each row. The resultant mix is the resultant mix is the genotype for the genotype for the offspring. In this case, offspring. In this case, each offspring has a Tt each offspring has a Tt (heterozygous tall) (heterozygous tall) genotype, and simply a genotype, and simply a "Tall" phenotype."Tall" phenotype.


• Lets take this a step Lets take this a step further and cross these further and cross these F1 offspring (Tt) to see F1 offspring (Tt) to see what genotypes and what genotypes and phenotypes we get.phenotypes we get.

• Since each parent can Since each parent can contribute a T and a t to contribute a T and a t to the offspring, the the offspring, the punnett square should punnett square should look like this….look like this….


• Here we have some more Here we have some more interesting results: First interesting results: First we now have 3 we now have 3 genotypes (TT, Tt, & tt) in genotypes (TT, Tt, & tt) in a 1:2:1 a 1:2:1 genotypic ratiogenotypic ratio. . We now have 2 different We now have 2 different phenotypes (Tall & short) phenotypes (Tall & short) in a 3:1 in a 3:1 Phenotypic Phenotypic ratioratio. This is the . This is the common outcome from common outcome from such crosses. such crosses.

Dihybrid crossesDihybrid crosses

• Dihybrid crossesDihybrid crosses are made when phenotypes are made when phenotypes and genotypes composed of 2 independent and genotypes composed of 2 independent alleles are analyzed.alleles are analyzed.

• Process is very similar to Process is very similar to monohybridmonohybrid crosses. crosses.• Example:Example:

– 2 traits are being analyzed2 traits are being analyzed– Plant height (Tt) with tall being dominant to short,Plant height (Tt) with tall being dominant to short,– Flower color (Ww) with Purple flowers being dominant Flower color (Ww) with Purple flowers being dominant

to white.to white.

Dihybrid cross exampleDihybrid cross example

• The cross with a pure-breeding (homozygous) The cross with a pure-breeding (homozygous) Tall,Purple plant with a pure-breeding Short, Tall,Purple plant with a pure-breeding Short, white plant should look like this.white plant should look like this.

F1 generationF1 generation

Dihybrid cross example continuedDihybrid cross example continued

• Take the offspring and cross them since they are Take the offspring and cross them since they are donating alleles for 2 traits, each parent in the f1 donating alleles for 2 traits, each parent in the f1 generation can give 4 possible combination of alleles. generation can give 4 possible combination of alleles. TW, Tw, tW, or tw. The cross should look like this. (The TW, Tw, tW, or tw. The cross should look like this. (The mathematical “foil” method can often be used here)mathematical “foil” method can often be used here)

F2 GenerationF2 Generation

Dihybrid cross example continuedDihybrid cross example continued

• Note that there is a Note that there is a 9:3:3:1 9:3:3:1 phenotypic ratiophenotypic ratio. 9/16 . 9/16 showing both dominant traits, showing both dominant traits, 3/16 & 3/16 showing one of the 3/16 & 3/16 showing one of the recessive traits, and 1/16 recessive traits, and 1/16 showing both recessive traits. showing both recessive traits.

• Also note that this also Also note that this also indicates that these alleles are indicates that these alleles are separating independently of separating independently of each other. This is evidence each other. This is evidence of Mendel's Law of of Mendel's Law of independent assortmentindependent assortment

Chromosomes and Classical GeneticsChromosomes and Classical Genetics

• Walter Sutton in 1902 proposed that Walter Sutton in 1902 proposed that chromosomes were the physical carriers of chromosomes were the physical carriers of Mendel's allelesMendel's alleles

• Problems arose however regarding the following Problems arose however regarding the following question:question:

• Why are the number of alleles which undergo Why are the number of alleles which undergo independent assortment greater than the independent assortment greater than the number of chromosomes of an organism?number of chromosomes of an organism?

• This was explained understanding of 2 additional This was explained understanding of 2 additional factors; Sex Linkage and crossing over factors; Sex Linkage and crossing over

Sex LinkageSex Linkage

• All chromosomes are homologous All chromosomes are homologous except on except on sex chromosomessex chromosomes..

• Sex chromosomes are either Sex chromosomes are either XX or or YY..• If an organism is XX, it is a female, if XY it is If an organism is XX, it is a female, if XY it is

male.male.• If a recessive allele exists on the X If a recessive allele exists on the X

chromosome. It will chromosome. It will notnot have a corresponding have a corresponding allele on the Y chromosome, and will therefore allele on the Y chromosome, and will therefore alwaysalways be expressed be expressed

Sex linkage exampleSex linkage example• Recessive gene for white eye Recessive gene for white eye

color located on the Xcolor located on the Xww chromosome of Drosophila.chromosome of Drosophila.

• All Males which receive this All Males which receive this gene during fertilization (50%) gene during fertilization (50%) will express this.will express this.

• If a female receives the XIf a female receives the Xww chromosome. It will chromosome. It will usuallyusually not be expressed since she not be expressed since she carries an X chromosome with carries an X chromosome with the normal gene the normal gene

Human Sex LinkageHuman Sex Linkage

• Hemophilia:Hemophilia:– Disorder of the blood Disorder of the blood

where clotting does not where clotting does not occur properly due to a occur properly due to a faulty protein.faulty protein.

– Occurs on the X Occurs on the X chromosome, and is chromosome, and is recessive.recessive.

• Thus a vast majority of Thus a vast majority of those affected are males.those affected are males.

– First known person known First known person known to carry the disorder was to carry the disorder was Queen Victoria of England. Queen Victoria of England. Thus all those affected are Thus all those affected are related to European related to European royalty.royalty.

Hemophilia and RoyaltyHemophilia and Royalty

Other Factors: Multiple AllelesOther Factors: Multiple Alleles• Phenotypes are controlled by more than 1 allele. Eg. Blood types

are regulated by 3 separate genes.

• ABO Blood typing– Humans have multiple types of surface antigens on RBC's – The nature of these surface proteins determines a person's

Blood Type.– There are 3 alleles which determine blood type IA, IB, or IO. This

is referred to as having multiple alleles– Human blood types are designated as A, B or O.

• Type A denotes having the A surface antigen, and is denoted by IA

• Type B denotes having the B surface antigen, and is denoted by IB

• Type O denotes having neither A or B surface antigen, and is denoted by IO

– There are several blood type combinations possible• A• B• AB (Universal recipient)• O (Universal donor)

Blood & ImmunityBlood & Immunity• A person can receive blood only when the donor's blood type does A person can receive blood only when the donor's blood type does

not contain any surface antigen the recipient does not. This is not contain any surface antigen the recipient does not. This is because the recipient has antibodies which will attack any foreign because the recipient has antibodies which will attack any foreign surface protein.surface protein.

• Thus, Type AB can accept any blood types because it will not attack Thus, Type AB can accept any blood types because it will not attack A or B surface antigens. However, a type AB person could only A or B surface antigens. However, a type AB person could only donatedonate blood to another AB person. They are known as blood to another AB person. They are known as Universal Universal RecipientsRecipients..

• Also, Type O persons are Also, Type O persons are Universal donorsUniversal donors because they have NO because they have NO surface antigens that recipients' immune systems can attack. Type surface antigens that recipients' immune systems can attack. Type O persons can ONLY receive blood from other type O persons.O persons can ONLY receive blood from other type O persons.

• There is another blood type factor known as Rh.There is another blood type factor known as Rh.• People are either Rh+ or Rh- based on a basic dominant/recessive People are either Rh+ or Rh- based on a basic dominant/recessive

mechanism.mechanism.• Not usually a problem except with pregnancy.Not usually a problem except with pregnancy.• It is possible that an Rh- mother can carry an Rh+ fetus and develop It is possible that an Rh- mother can carry an Rh+ fetus and develop

antibodies which will attack & destroy the fetal bloodantibodies which will attack & destroy the fetal blood• This usually occurs with 2nd or 3rd pregnancies, and is detectable This usually occurs with 2nd or 3rd pregnancies, and is detectable

and treatable.and treatable.

Other Factors: Incomplete DominanceOther Factors: Incomplete Dominance

• Some alleles for Some alleles for a gene are not a gene are not completely completely dominant over dominant over the others. This the others. This results in results in partially masked partially masked phenotypes phenotypes which are which are intermediate to intermediate to the two the two extremes.extremes.

Other Factors: Continuous VariationOther Factors: Continuous Variation

• Many traits may have a wide range of continuous values. Eg. Human height can vary considerably. There are not just "tall" or "short" humans

Other FactorsOther Factors

• Gene interaction: Gene interaction: – Many biological pathways are governed by multiple Many biological pathways are governed by multiple

enzymes, involving multiple steps. If any one of these enzymes, involving multiple steps. If any one of these steps are altered. The end product of the pathway steps are altered. The end product of the pathway may be disrupted.may be disrupted.

• Environmental effects:Environmental effects:– Sometimes genes will not be fully expressed owing to Sometimes genes will not be fully expressed owing to

external factors. Example: Human height may not be external factors. Example: Human height may not be fully expressed if individuals experience poor nutrition.fully expressed if individuals experience poor nutrition.

The The Average Average

American American PhenotypePhenotype

Documents

Classical Genetics