Patterns of Inheritance

One of the most exciting fields of biology is the study of patterns of inheritance or GENETICS

What is genetics?• It is the study of the mechanism of heredity and

variations by which traits or characteristics are passed from one generation to another

• These mechanisms are dependent on the behavior of chromosomes during the formation

of sex cells (meiosis) and the way in which this• cells are brought together in fertilization

What is heredity?Refers to the similarities between parents and offspring

It is the transfer of genetically controlled characteristics from one generation to the next

Examples: hair color and color of the

eyes, height, intelligence(human)

flower color (plants)

What is variation?

Variation is difference, whether in the expression of somatic characters or in the elements of the germinal substance, exhibited among groups of organisms related by descent (ancestors).

Example of variation:At the time the chromosome theory of inheritance was proposed, scientists began studying the inheritance of traits in the fruit fly (Drosophila melanogaster)

In a bottle containing normal (wild type) red-eyed flies, a white- eyed fly was discovered in a bottle. This variation was produced by mutation- an inherited change in the gene controlling the eye color

Chromosomal mutation affect the number and structure of chromosomes.

Mutation whether genetic or chromosomal are defined as any heritable change and are the sources of all genetic variation

Mutation therefore is a random change in a gene or chromosome resulting in a new trait or characteristic that can be inherited.

Mutation can be a source of beneficial genetic variation, or it can be neutral or harmful in effect.

History ofHistory ofGeneticsGenetics

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Gregor Gregor MendelMendel(1822-1884)(1822-1884)

Responsible Responsible for the Laws for the Laws governing governing Inheritance Inheritance of Traitsof Traits

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Gregor Johann MendelGregor Johann MendelAustrian monkAustrian monkStudied the Studied the inheritanceinheritance of traits of traits in in pea plantspea plantsDeveloped the Developed the laws laws of inheritanceof inheritanceMendel's work was Mendel's work was not recognized until not recognized until the turn of thethe turn of the 20th 20th centurycentury

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Gregor Johann MendelGregor Johann MendelBetween Between 1856 and 1856 and 1863,1863, Mendel Mendel cultivated and tested cultivated and tested some some 28,000 pea 28,000 pea plantsplantsHe found that the He found that the plants' offspring plants' offspring retained retained traits of the traits of the parentsparentsCalled theCalled the “Father “Father of Genetics"of Genetics"

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He bred garden He bred garden peas in the peas in the

monastery garden monastery garden and analyzed the and analyzed the offspring of these offspring of these

crossescrosses

Site of Gregor Site of Gregor Mendel’s Mendel’s

experimental experimental garden in the garden in the

Czech RepublicCzech Republic

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Reasons why Mendel chose Reasons why Mendel chose garden peas (garden peas (Pisum sativum)Pisum sativum)1.1. Can be grown in a Can be grown in a

small area small area

2. Plants Reproduce 2. Plants Reproduce at a fast rateat a fast rate

3. Thus, a number of 3. Thus, a number of generations can generations can

be be produced within a produced within a short timeshort time

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Reasons why Mendel chose Reasons why Mendel chose garden peas (garden peas (Pisum sativum)Pisum sativum)

4.4. They are hardy They are hardy plants which do plants which do not need much not need much caring and caring and cultivatingcultivating

5. Plants Produce 5. Plants Produce lots of offspring lots of offspring

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Reasons why Mendel chose Reasons why Mendel chose garden peas (garden peas (Pisum sativum)Pisum sativum)66. Produce pure plants. Produce pure plants when allowed to self-when allowed to self- pollinate severalpollinate several generations generations

7. Can be artificially 7. Can be artificially cross-pollinatedcross-pollinated 8. Garden pea has8. Garden pea has several contrasting several contrasting characterscharacters

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Reproduction in Flowering PlantsReproduction in Flowering PlantsPollen contains spermPollen contains sperm

Produced by the stamenProduced by the stamenOvary contains eggsOvary contains eggs

Found inside the flowerFound inside the flower

Pollen carries sperm to Pollen carries sperm to the eggs for fertilizationthe eggs for fertilization

Self-fertilizationSelf-fertilization can can occur in the same occur in the same flowerflower

Cross-fertilizationCross-fertilization can can occur between occur between flowersflowers

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Eight Pea Plant TraitsEight Pea Plant Traits

Seed shapeSeed shape --- --- Round (R) or Wrinkled (r)Round (R) or Wrinkled (r)

Seed ColorSeed Color ---- ---- Yellow (Y) or  Green (Yellow (Y) or  Green (yy))

Pod ShapePod Shape --- --- Smooth (S) or wrinkled (Smooth (S) or wrinkled (ss))Pod ColorPod Color ---  ---  Green (G) or Yellow (g)Green (G) or Yellow (g)Seed Coat ColorSeed Coat Color --- ---Gray (G) or White (g)Gray (G) or White (g)Flower positionFlower position------Axial (A) or Terminal (a)Axial (A) or Terminal (a)Plant HeightPlant Height --- --- Tall (T) or Short (t)Tall (T) or Short (t)

Flower color --- Flower color --- Purple (P) or white (Purple (P) or white (pp))

TRAITSCORRESPONDING

CHARACTERS

Mendel’s Peas

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How Mendel Began his How Mendel Began his expt.expt.Mendel produced pure strains by allowing the Mendel produced pure strains by allowing the plants to self-pollinate for several plants to self-pollinate for several generationsgenerations

This way he was sure that the character (ex. round seeds) was “pure”.

All the pea plants which he cultivated for this purpose produced only “round seeds”

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How Mendel Began his How Mendel Began his expt.expt.

•He describe such plants which produce the same character from generation to generation as pure-breeding or “breed true”.

Mendel did the same with the plants bearing wrinkled seeds

Second step: he cross pollinate the pure breed of round seeded plants with the wrinkled seeded plants

Round seed x Wrinkled seed Round seed x Wrinkled seed

F1: All round seed F1: All round seed

TRAIT P1 GENERATION(Cross Pollinated)

F1 GENERATION

Shape of Seed Round X Wrinkled All round

Color of Seed Yellow X Green All yellow

Color of Seed Coats Colored X White All colored

Shape of Pod Inflated X Constricted All inflated

Color of Pod Green X Yellow All green

Position of Flowers Axial X Terminal All axial

Length of Stem Long X Short All long

Color of the Flowers Purple X White All purple

FIND OUT FROM THE TABLE BELOW WHICH OF THE CHARACTERS ARE

DOMINANT AND WHICH ARE RECESSIVE

Mendel’s P1 Crosses (Cross-Pollination)

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MENDELLIAN LAWS OF GENETICS

From the results of his experiments, he formulated the three laws:

1.Law of Dominance2.Law Segregation3.Law of Independent Assortment

Law of Complete DominanceLaw of Complete DominanceStates that a cross between homozygous dominant genes and homozygous recessive genes will result in a progeny of heterozygous genes determining all dominant traits (all the offspring will show only one of the characters)

The trait that is observed in the offspring is the dominant trait The trait that disappears in the offspring is the recessive trait

Parental Phenotypes:Parental Phenotypes: Curly Hair x Straight HairCurly Hair x Straight Hair

Genotypes: Genotypes: CCCC cc cc

Curly HairsCurly Hairs CcCc

EXAMPLE: FOUND IN YOUR BOOK

Phenotype of offspring:

Genotype of offspring:

100% curly100% Cc

Third Step Of His Third Step Of His ExperimentExperiment

F2: 5474 round: 1850 wrinkledF2: 5474 round: 1850 wrinkled (3/4 round to 1/4 wrinkled) (3/4 round to 1/4 wrinkled)

Round seed x Wrinkled seed Round seed x Wrinkled seed

F1: All round seed F1: All round seed

F1 round plants F1 round plants x x F1 round plantsF1 round plants

He allowed the F1 plants to self pollinate

Law of DominanceLaw of Dominance

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Generation “Gap”Generation “Gap”Parental PParental P11 Generation Generation = = the parental the parental

generation in a breeding experimentgeneration in a breeding experiment..

FF11 generation generation = = the first-generation offspring in the first-generation offspring in a breeding experiment. (1st filial generation)a breeding experiment. (1st filial generation)

From breeding individuals from the PFrom breeding individuals from the P11 generationgeneration

FF22 generation generation = = the second-generation the second-generation offspring in a breeding experiment. offspring in a breeding experiment. (2nd filial generation)(2nd filial generation)

From breeding individuals from the FFrom breeding individuals from the F11 generationgeneration

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Following the GenerationsFollowing the Generations

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Cross 2 Cross 2 Pure Pure PlantsPlantsTT x ttTT x tt

Results Results in all in all HybridsHybridsTtTt

Cross 2 HybridsCross 2 Hybridsgetget3 Tall & 1 Short3 Tall & 1 ShortTT, Tt, ttTT, Tt, tt

TRAIT P1 GENERATION(Cross Pollinated

F1 GENERATION Actual Ratio

Shape of Seed Round X Round 5, 474 round1,850 wrinkled7,324 total

2.93:1

Color of Seed Yellow X Yellow 6,022 yellow2001 white8023 total

3.01:1

Color of Seed Coats

Colored X Colored 705 colored224 white929 total

3.15:1

Shape of Pod Inflated X Inflated 882 imnflated229 constricted1181 total

2.95:1

Color of Pod Green X Green 428 green152 yellow580 total

2.82:1

Position of Flowers Axial X Axial 651 axial207 terminal858 total

3.4:1

Length of Stem Long X Long 787 long288 short1064 total

2.84:1

Mendel’s P2 Crosses (Self pollination)

Mendel analyzed the data closely. To his surprise there was a pattern in the results. He also noticed that the greater number of offspring produced, the clearer was the pattern

It shows in the 4th column that in the F2 generation, the ratio of the plants with the dominant character to the plants with the recessive character was almost 3:1

From the results of his P2 crosses, he formulated his second law –The law of Segregation- states that individuals carry two hereditary alleles affecting any given character

According to this law also that, the determiner of a recessive character, which does not appear in the F1 (Rr) generation, separates from the chance for the determiners of the recessive character to combine during reproduction, and appear in the F2

Law of SegregationLaw of Segregation

Mendel reasoned that each of the F1 plants must have contained two determiners, one for round seed and one for wrinkled seed. However , the determiner for round seed masked the determiner fro wrinkled seed . Mendel used the term “unit determiner” for what we called as “GENES” now

The same gene can have many versions

• A gene is a piece of DNA that directs a cell to make a certain protein.

• Each gene has a locus, a specific position on a pair of homologous chromosomes.

• An allele is any alternative form of a gene occurring at a specific locus on a chromosome.

– Each parent donates one allele for every gene.

– Homozygous describes two alleles that are the same at a specific locus.

– Heterozygous describes two alleles that are different at a specific locus.

– The 2 alleles are known to be found on copies of chromosomes – one from each parent–

Alleles can be represented using letters

– A dominant allele is expressed as a phenotype when at least one allele is dominant.

– A recessive allele is expressed as a phenotype only when two copies are present.

– Dominant alleles are represented by uppercase letters; recessive alleles by lowercase letters.

To make easier for him to analyze the results of his experiments, he

designated symbols for the characters

PLANT (GARDEN PEA)

SYMBOLS

Pure breeding P1 plant bearing round seeds

RR

Pure breeding P1 plant bearing wrinkled seeds

rr

F1 Plants bearing round seeds

Rr

Mendel said that during formation of gametes, the determiners separate. Hence only one member of a

pair of determiners goes to a gamete

HomozygousHomozygousDominantDominant

HomozygousHomozygousRecessiveRecessive

HeterozygousHeterozygous

Round seed x Wrinkled seed Round seed x Wrinkled seed RR rrRR rr

F1: All round seed coatsF1: All round seed coats RrRr

R R R R

Homozygous parents can only pass one Homozygous parents can only pass one form of an allele to their offspring. form of an allele to their offspring.

R r R r

Heterozygous parents can pass either Heterozygous parents can pass either of two forms of an allele to their offspring. of two forms of an allele to their offspring.

TERMINOLOGIES

• PHENOTYPE – the way an organism looks and behaves- its physical characteristics (Ex. Tall, short, green, white, brown hair, blue eyes, etc. )

• GENOTYPE – the gene combination (allelic combination) of an organism – Ex. TT, Tt, tt, etc.)

PhenotypePhenotype: : Round seed Round seed Genotype :Genotype : RR (homozygous dominant)RR (homozygous dominant)

Rr (heterozygous dominantRr (heterozygous dominant))

Phenotype: Phenotype: Wrinkled seed Wrinkled seed Genotype: Genotype: rr (homozygous recessive)rr (homozygous recessive)

Principles of Heredity

– HOMOZYGOUS – 2 ALLELES ARE THE SAME

when an organism possesses two identical alleles.

ex. YY or yy– HETEROZYGOUS – 2 ALLELES DIFFERENT

when an organism possesses different alleles.

ex. Yy

Law of Segregation

In the formation of gametes, the members of a pair of alleles separate (or segregate) cleanly from each other so that only one member is included in each gamete.

Each gamete has an equal probability of containing either member of the allele pair.

When gametes (sperm, egg) are When gametes (sperm, egg) are formed, each gamete will receive one formed, each gamete will receive one

allele or the other.allele or the other.

Law of Segregation• 1. A pea plant contains two discrete hereditary factors, one from each parent• 2. The two factors may be identical or different• 3. When the two factors of a single trait are• different

– One is dominant and its effect can be seen– The other is recessive and is masked

• 4. During gametogenesis (meiosis), the paired factors segregate randomly so that half of the gametes received one factor and half of the gametes received the other– This is Mendel’s Law of Segregation 2-25

Monohybrid cross (cross with only 1 trait)

• Problem:• Using this is a several step process,

look at the following example– Tallness (T) is dominant over shortness

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

Punnett Squares

• Genetic problems can be easily solved using a tool called a Punnett Square.– Tool for calculating genetic probabilities

A punnett squareA punnett square

Punnet Square Process

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

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

Punnett process continued

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

Punnett process continued

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

F1 GENERATIONF1 GENERATIONFATHERFATHER MOTHERMOTHER

T tT t T tT t

TT TT TT tt tt tt

F2 GENERATIONF2 GENERATION

- the - the law of dominance law of dominance explained the heredity ofexplained the heredity of

the offspring of the offspring of the f1 generationthe f1 generation

- the - the law of segregation law of segregation explained the heredity ofexplained the heredity of

the the f2 generationf2 generation

Another Example of Monohybrid Cross

Smooth and wrinkled parental seed strains crossed.

F1 genotypes F1 phenotypes 4/4 Ss 4/4 smooth

With the previous example , apply the law of segregation and find the phenotypic ratio and genotypic ratio of the F2.

Seat work:

ANSWERParentals: Parentals: SS x ssSS x ss

S S s s S S s s

SS

SS

s ss s

F1 x F1: F1 x F1: Ss x SsSs x Ss

R r R r R r R r

½ S½ S

½ s½ s

½ S ½ s ½ S ½ s

¼ SS¼ SS ¼ Ss¼ Ss

¼ Ss¼ Ss ¼ ss¼ ss

SsSs SsSs

SsSs SsSs

Genotypic RatioGenotypic Ratio: ¼ RR + ½ Rr + ¼ rr or 1:2:1: ¼ RR + ½ Rr + ¼ rr or 1:2:1

Phenotypic RatioPhenotypic Ratio: ¾ Round + ¼ Wrinkled: ¾ Round + ¼ Wrinkled

Law of Independent Assortment

-states that alleles of different genes are distributed randomly to the gametes and fertilization occurs at random

- To explain this law, a dihybrid cross will be used. This type of cross uses two traits.

Dihybrid crosses

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

• Process is very similar to monohybrid crosses.

• Example:– 2 traits are being analyzed

– Plant height (Tt) with tall being dominant to short,

– Flower color (Ww) with Purple flowers being dominant to white.

Dihybrid cross example

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

F1 generationF1 generation

Dihybrid cross example continued• Take the offspring and cross them since they are donating

alleles for 2 traits, each parent in the F1 generation can give 4 possible combination of alleles. TW, Tw, tW, or tw. The cross should look like this. (The mathematical “foil” method can often be used here)

F2 GenerationF2 Generation

F1 generationF1 generation

Dihybrid cross example continued• Note that there is a

9:3:3:1 phenotypic ratio. 9/16 showing both dominant traits, 3/16 & 3/16 showing one of the recessive traits, and 1/16 showing both recessive traits.

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

Mendel’s Principle of Independent Assortment

When gametes are formed, the alleles of one gene segregate independently of the alleles of another gene producing equal proportions of all possible gamete types.

Genetic Segregation + Independent Assortment

Parentals: Parentals: RRYY x rryyRRYY x rryy

RY RY RY RY ry ry ry ryRY RY RY RY ry ry ry ry

ryry

RYRY RrYyRrYy

F1: 100% RrYy, round, yellowF1: 100% RrYy, round, yellow

F1 x F1: RrYy x RrYy

RY Ry rY ry RY Ry rY ry

¼ RY ¼ Ry ¼ rY ¼ ry

¼ RY

¼ Ry

¼ rY

¼ ry

1/16 RRYY 1/16 RRYy 1/16 RrYY 1/16 RrYy

1/16 RRYy 1/16 RRyy 1/16 RrYy 1/16 Rryy

1/16 RrYY 1/16 RrYy 1/16 rrYY 1/16 rrYy

1/16 RrYy 1/16 Rryy 1/16 rrYy 1/16 rryy

1 432

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8765

16151413

10 11 13

F2 Genotypes and PhenotypesF2 Genotypes and Phenotypes

PhenotypesPhenotypes GenotypesGenotypes

RoundRound

YellowYellow

1/16 RRYY + 2/16 RRYy + 1/16 RRYY + 2/16 RRYy +

2/16 RrYY + 4/16 RrYy 2/16 RrYY + 4/16 RrYy

Total = 9/16 R_Y_Total = 9/16 R_Y_

Round Round

GreenGreen

1/16 RRyy+ 2/16 Rryy1/16 RRyy+ 2/16 Rryy

Total = 3/16 R_yyTotal = 3/16 R_yy

Wrinkled YellowWrinkled Yellow 1/16 rrYY+ 2/16 rrYy1/16 rrYY+ 2/16 rrYy

Total = 3/16 rrY_Total = 3/16 rrY_

Wrinkled GreenWrinkled Green 1/16 rryy1/16 rryy

Refer to figure above:Start with the shape of the seedList down your answers on a piece of paper

Note: Mendel explained these observations as follows:-The determiners for seed shape are inherited separately from the determiners for seed colors-That is Mendel’s Law of Independent Assortment of determiners. The inheritance of seed coats has no effect on the inheritance of seed color, each character is inherited as a unit.

Suppose you describe the appearance of the offspring in the table above , what are their phenotypes? What are their Genotypes?

Meiotic Segregation explains Meiotic Segregation explains Independent AssortmentIndependent Assortment

THE NON-MENDELIAN LAWS OF INHERITANCE

•Refer to hereditary patterns that are not in accordance with Mendel’s principles or those that are not attributed to single autosomal genes.

A. Law of Incomplete Dominance

- this principle states that a cross between homozygous dominant and recessive genes will result in a progeny of heterozygous dominant and recessive trait. Both alleles exert an effect and jointly produce an intermediate phenotype.

See book: A cross using colors of the petals of flower (red and white)

Parentals: Parentals: RR x rrRR x rr

R R r r R R r r

RR

RR

r rr r

RrRr RrRr

RrRr RrRr

Genotype of offspringGenotype of offspring: 100% Rr: 100% Rr

Phenotype of offspringPhenotype of offspring: 100% pink: 100% pink

Genotypic offspring: Rr; Rr; Rr; Rr

Phenotype of offspring: pink; pink; pink; pink

The resulting genotypes of the offspring are all heterozygous (Rr) just like in the first law, however the phenotype that is expressed is not the dominant trait red, rather, an intermediate color between red and white which is PINK

B.LAW OF CO-DOMINANCE

o This principle states that a cross between homozygous dominant genes will result in a progeny of heterozygous genes determining a phenotype where a mix of the dominant recessive traits is expressed.

Look at the diagram in your book to show this kind of cross. This time the trait used is the color of coat of cattle

Parentals: Parentals: CR = red color coatred color coat; ; Cr =white color coatwhite color coat

CRCR x x CrCr

CR CR Cr Cr

CR

CR

Cr Cr

CRCrGenotype of offspringGenotype of offspring: 100% : 100% CRCr

Phenotype of offspringPhenotype of offspring: 100% roan: 100% roan

Genotype of offspring:

CRCr - CRCr – CRCr- CRCr

Phenotype of offspring: roan; roan; roan; roan

CRCr

CRCr CRCr

The roan phenotype appears pink in color, but with respect to the hair composition, some are red and some are white. The blending of red and white hair will appear pink. The difference of this cross is that the dominant trait (red hair) is expressed equally with the recessive trait (white hair). There is the absence of the intermediate trait that was present in then previous cross of incomplete dominance

SEAT WORK: Open Notes

• DO AT LEAST THREE OTHER TRAITS OF THE PEA PLANT FOLLOWING THE PROCEDURE DONE BY MENDEL:

• FIND F1 AND F2• FIND GENOTYPES PHENOTYPES RATIO

THANK YOU