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Mendelian Genetics
Preeyanan Sanpote,University of Phayao
Heredity & Genetics
• Heredity is …..the passing on of characteristics (traits) from parents to offspring
• Genetics is….. the study of heredity
• GeneticsGenetics is the study of genes. is the study of genes.
• InheritanceInheritance is how traits (characteristics) are passed is how traits (characteristics) are passed on from generation to generation.on from generation to generation.
• ChromosomesChromosomes are made up of are made up of genesgenes, which are made , which are made up of up of DNADNA..
• Genetic material Genetic material (genes,chromosomes, DNA) is found (genes,chromosomes, DNA) is found inside the nucleus of a cell.inside the nucleus of a cell.
• Gregor Mendel Gregor Mendel is considered is considered “The Father of Genetics"“The Father of Genetics"
HistoryHistory
Father of Genetics
Mendel……Austrian monkAustrian monk • used the scientific approach to identify two laws of inheritance
• discovered the basic principles of heredity by breeding garden peas (Pisum sativum) in carefully planned experiments
• reproduce sexually, reproduced quicklyreproduced quickly• have two distinct, male and female, sex
cells called gametes• their traits are easy to isolate• there are many varieties with distinct
heritable characters • showed obvious differences in the traits showed obvious differences in the traits • Mating,cross-pollination can be controlled
Mendel used peas..(Advantages of pea plants for genetic study)
Understood that there was something that Understood that there was something that carried traits from one generation to carried traits from one generation to
the next- the next- “FACTOR“FACTOR”.”.
Mendel stated that physical traits are Mendel stated that physical traits are inherited as inherited as “particles”“particles”
Mendel did not know that the Mendel did not know that the “particles” “particles” were actually were actually Chromosomes & DNAChromosomes & DNA
Mendel chose only characters that occurred in two distinct alternative forms
He also used varieties that were true- breeding (plants that produce offspring of the same variety when they self-pollinate)
Particulate InheritanceParticulate Inheritance
cross-fertilized true-breeding that were cross-fertilized true-breeding that were different in just different in just one characterone character
““Law of Law of Segregation”Segregation”
Monohybrid Monohybrid CrossesCrosses
• Mendel mated two contrasting,true-breeding varieties, a process called hybridization
• The true-breeding parents are the P generation
• The hybrid offspring of the P generation are called the F1 generation
• When F1 individuals self-pollinate or cross- pollinate with other F1 hybrids, the F2 generation is produced
Experiment
StamensCarpel
TECHNIQUE
First filialgenerationoffspring
(F1)
Parentalgeneration
(P)
RESULTS
3
2
1
4
5
EXPERIMENT
P Generation(true-breeding
parents) Purpleflowers
Whiteflowers
F1 Generation(hybrids) All plants had purple flowers
Self- or cross-pollination
F2 Generation
705 purple-flowered
plants
224 whiteflowered
plants
• Mendel reasoned that only the purple flower factor was affecting flower color in the F1 hybrids
• Mendel called the purple flower color a dominant trait and the white flower color a recessive trait
• The factor for white flowers was not diluted or destroyed because it reappeared in the F2
generation
• Mendel discovered a ratio of about three to one, purple to white flowers, in the F2 generation
• Mendel observed the same pattern of inheritance in six other pea plant characters, each represented by two traits
• What Mendel called a “heritable factor” is what we now call a gene
What Do What Do the Peas Look Like?the Peas Look Like?
Mendel studies seven characteristics
in the garden pea
The results of Mendel’s F1 crosses for 7
characters in Pea plants
• Mendel was the first biologist to use mathematics to explain his results quantitatively.
• Mendel predicted the concept of genes that
genes occur in pairs that one gene of each pair
is present in the gametes
F2 ratio 705 : 224
or 3.15 : 1
The theoretical or The theoretical or expected ratio expected ratio of plants of plants producing round or wrinkled seeds is producing round or wrinkled seeds is 3 round : 3 round : 1 wrinkled1 wrinkled
Mendel’s Mendel’s observed ratio observed ratio was was 2.96 : 12.96 : 1
The discrepancy is due to The discrepancy is due to statistical errorstatistical error
The The larger the samplelarger the sample the more nearly the the more nearly the results approximate to the theoretical ratioresults approximate to the theoretical ratio
Did the observed ratio Did the observed ratio match the theoretical match the theoretical
ratio?ratio?
• Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2 offspring
• During the During the formation of gametes formation of gametes (eggs or (eggs or sperm), thesperm), the two alleles two alleles responsible responsible for a trait for a trait separateseparate from each other.from each other.
• Alleles for a trait are then Alleles for a trait are then "recombined" at "recombined" at fertilizationfertilization, , producing the genotype for the producing the genotype for the traits of the offspringtraits of the offspring.
““Law of Segregation”Law of Segregation”
• An organism with two identical alleles for a character is said to be homozygous for the gene controlling that character
• An organism that has two different alleles for a gene is said to be heterozygous for the gene controlling that character
• Unlike homozygotes, heterozygotes are not true-breeding
Alleles
allele for purple flowers
Locus for flower-color gene
allele for white flowers
Pair ofhomologouschromosomes
Alleles
Gene – a unit of heredity; a section of DNA sequence encoding
a single proteinGenome – the entire set of genes in an organism
Alleles – the different forms of a characteristicthe different forms of a characteristic..
Locus – a fixed location on a strand of DNA where a gene or
one of its alleles is located.
ProbabilityProbability-- the chances/percentages that something will occur. the chances/percentages that something will occur.
Punnett Squares- Punnett Squares- show how crosses are made.show how crosses are made.
DominantDominant traitstraits- traits that are expressed.- traits that are expressed.
RecessiveRecessive traitstraits- traits that are covered up.- traits that are covered up.
Genetic TerminologyGenetic Terminology
Monohybrid cross- Monohybrid cross- a genetic cross involving a single pair of genes
(one trait);looking at only one traitlooking at only one trait
Dihybrid cross Dihybrid cross –– a genetic cross involving two pairs of genes
(two traits);looking at only two traitslooking at only two traits P - Parental generation F1 - First filial generation;offspring from a genetic cross.
F2 - Second filial generation of a genetic cross
GenotypeGenotype-- the types of genes (alleles) present. the types of genes (alleles) present.
PhenotypePhenotype--the way organism looks and behaves/what it looks the way organism looks and behaves/what it looks
like.like. HomozygousHomozygous-- two of the same alleles. two of the same alleles.
HeterozygousHeterozygous-- two different alleles. two different alleles.
Trait or CharacterTrait or Character--any characteristic that can be passed from any characteristic that can be passed from
parent to offspring parent to offspring
Trait:Trait: Seed Shape Seed Shape Alleles:Alleles: RR – – RoundRound rr – – WrinkledWrinkled
Cross:Cross: Round seeds x Wrinkled seeds Round seeds x Wrinkled seeds
RRRR rrrr
GenotypeGenotype : : Rr Rr Phenotype:Phenotype: RoundRound
Genotypic Ratio:Genotypic Ratio: all alikeall alike
Phenotypic Ratio: Phenotypic Ratio: all alikeall alike
Monohybrid CrossMonohybrid Cross
gametes
rr rr
RR RrRr RrRr
RR RrRr RrRr Homozygous dominant x Homozygous recessiveHomozygous dominant x Homozygous recessive Offspring all Heterozygous (hybrids)called Offspring all Heterozygous (hybrids)called
FF11generationgeneration Genotypic & Phenotypic ratio is Genotypic & Phenotypic ratio is all alikeall alike
gametes
RR rr
RR RRRR RrRr
rr RrRr rrrr
Genotype :Genotype : RR,Rr,rrRr,rr
Phenotype :Phenotype : Round & Round & wrinkledwrinkled
Genotypic Ratio:Genotypic Ratio: 1:2:11:2:1
Phenotypic Ratio: Phenotypic Ratio: 3:13:1
Trait:Trait: Seed Shape Seed Shape Alleles:Alleles: RR – Round– Round rr – – WrinkledWrinkled
Cross:Cross: Round seeds x Round seeds Round seeds x Round seeds RrRr x x Rr Rr
Heterozygous Heterozygous x x HeterozygousHeterozygous Offspring have 3 genotypes ,called Offspring have 3 genotypes ,called FF2 2
generationgeneration Genotypic & Phenotypic ratio Genotypic & Phenotypic ratio is not is not the samethe same
P Generation
F1 Generation
F2 Generation
phenotype:
Genotype:
Gametes:
Sperm from F1 (Pp) plant
Purple flowersPp
Purple flowers White flowersPP pp
P
P
P
P
p
p
p
p
Eggs from F1 (Pp) plant
PP
ppPp
Pp
1/21/2
3 : 1
phenotype:
Genotype:Gametes:
Phenotype
Purple
Purple
Purple
White
3
1
1
1
2
Ratio 3:1 Ratio 1:2:1
Genotype
PP(homozygous)
Pp(heterozygous)
Pp(heterozygous)
pp(homozygous)
Applying the Law of Applying the Law of SegregationSegregation
Test cross
• How can we tell the genotype of an individual with the dominant phenotype ?
• Such an individual could be either homozygous dominant or heterozygous
• The answer is to carry out a testcross : breeding the mystery individual with a homozygous recessive individual
Test cross : Cross with a homozygous recessive individual.
• When you have an individual with an unknown genotype, you do a test cross.
• If any offspring display the recessive phenotype, the mystery parent must be heterozygous
Example, a plant with purple flowers can either be PP or Pp
therefore, you cross the plant with a pp (white
flowers, homozygous recessive)
P- pp
• If you get all 100% purple flowers,then the
unknown parent was PP
P p P p
P p
P p
P P
p
p
P p p p
P p
p p
P p
p
p
•If you get 50% white, 50% purple flowers,then the unknown parent was Pp
Dominant phenotype,unknown genotype:
PP or Pp?
Recessive phenotype,known genotype:
pp
PredictionsIf purple-floweredparent is PP
If purple-floweredparent is Pp
or
Sperm Sperm
Eggs Eggs
or
All offspring purple 1/2 offspring purple and 1/2 offspring white
Pp Pp
Pp Pp
Pp Pp
pp pp
p p p p
P
P
P
p
TECHNIQUE
RESULTS
cross-fertilized true-breeding that were cross-fertilized true-breeding that were different in just different in just two characterstwo characters
Dihybrid Dihybrid CrossesCrosses
““The Law of Independent The Law of Independent Assortment”Assortment”
• Using a dihybrid cross, Mendel developed the law of independent assortment
• The law of independent assortment states that each pair of alleles segregates independently of each other pair of alleles during gamete formation
• Strictly speaking, this law applies only to genes on different, nonhomologous chromosomes or those far apart on the same chromosome
• Genes located near each other on the same chromosome tend to be inherited together
P Generation
F1 Generation
Predictions
Gametes
EXPERIMENT
RESULTS
YYRR yyrr
yrYR
YyRr
Hypothesis ofdependent assortment
Hypothesis ofindependent assortment
Predictedoffspring ofF2 generation Sperm
Spermor
EggsEggs
Phenotypic ratio 3:1
Phenotypic ratio 9:3:3:1
Phenotypic ratio approximately 9:3:3:1315 108 101 32
1/21/2
1/2
1/2
1/41/4
1/41/4
1/4
1/4
1/4
1/4
9/163/16
3/161/16
YR
YR
YR
YRyr
yr
yr
yr
1/43/4
Yr
Yr
yR
yR
YYRR YyRr
YyRr yyrr
YYRR YYRr YyRR YyRr
YYRr YYrr YyRr Yyrr
YyRR YyRr yyRR yyRr
YyRr Yyrr yyRr yyrr
RYRY RyRy rYrY ryry
RYRY
RyRy
rYrY
ryry
Dihybrid CrossDihybrid Cross
RRYY
RRYy
RrYY
RrYy
RRYy
RRyy
RrYy
Rryy
RrYY
RrYy
rrYY
rrYy
RrYy
Rryy
rrYy
rryy
Round/Yellow : 9Round/green : 3wrinkled/Yellow : 3wrinkled/green : 1
9:3:3:1 phenotypic ratio
RYRY RyRy rYrY ryry
RYRY
RyRy
rYrY
ryry
Dihybrid CrossDihybrid Cross
Dihybrid test cross??
If you had a tall, purple plant,how would you
know what genotype it is?
tt pp?? ??
1. TTPP 2. TTPp3. TtPP 4. TtPp
The laws of probability govern The laws of probability govern Mendelian inheritanceMendelian inheritance
• Mendel’s laws of segregation and independent assortment reflect the rules of probability
• When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss
• In the same way, the alleles of one gene segregate into gametes independently of another gene’s alleles
Segregation ofalleles into eggs
Segregation ofalleles into sperm
Sperm
Eggs
1/2
1/2
1/21/2
1/41/4
1/41/4
Rr Rr
R
R
RR
R
R
r
r
r
r r
r
Inheritance patterns are often more complex
than predicted by simple Mendelian genetics
• The relationship between genotype and phenotype is rarely as simple as in the pea plant characters Mendel studied
• Many heritable characters are not determined by only one gene with two alleles
• However, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance
Extending Mendelian Genetics for a Single Gene
• Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations :
– When alleles are not completely dominant
– When a gene has more than two alleles
– When a gene produces multiple phenotypes
Incomplete DominanceIncomplete Dominance
F1 hybrids F1 hybrids have an appearance somewhat in in betweenbetween the phenotypes phenotypes of the two parental varieties.
Example :Example : snapdragons (flower)snapdragons (flower)red (RR) x white (rr)
RR = red flowerRR = red flower
rr = white flower
R
R
r r
P Generation
F1 Generation
1/21/2
Red White
Gametes
Pink
Gametes
CWCWCRCR
CR CW
CRCW
CR CW
Incomplete Dominance
P Generation
F1 Generation
F2 Generation
1/21/2
1/21/2
1/2
1/2
Red White
Gametes
Pink
Gametes
Sperm
Eggs
CWCWCRCR
CR CW
CRCW
CR CW
CWCR
CR
CW
CRCR CRCW
CRCW CWCW
Summary of Mendel’s lawsSummary of Mendel’s laws
LAWLAW PARENT PARENT CROSS CROSS OFFSPRINGOFFSPRING
DOMINANCEDOMINANCE TT x tt TT x tt tall x shorttall x short
100 %100 % Tt Tt tall tall
Tt x Tt Tt x Tt tall x talltall x tall
75% or (3/4)75% or (3/4) tall tall 25% or (1/4)25% or (1/4) shortshort
INDEPENDENT INDEPENDENT ASSORTMENTASSORTMENT
RrGg x RrGgRrGg x RrGground & round & greengreen x x round & round & greengreen
9/169/16 round seeds & green pods round seeds & green pods
3/163/16 round seeds & yellow pods round seeds & yellow pods
3/163/16 wrinkled seeds & green wrinkled seeds & green
pods pods
1/161/16 wrinkled seeds & yellow wrinkled seeds & yellow
podspods
QuestionsQuestions
• What did Mendel cross?
• What are traits?• What are gametes?• What is fertilization?• What is heredity?• What is genetics?• What Did Mendel Find?What Did Mendel Find?
• The two alleles for a trait must separate when gametes are formed
• A parent randomly passes only one allele for each trait to each offspring
• The genes for different traits are inherited independently of each other.
Law of Independent Law of Independent AssortmentAssortment
Law of SegregationLaw of Segregation
• How many alleles are there for each trait?• What is an allele?• How many alleles does a parent pass on to
each offspring for each trait
Questions..???Questions..???
Dominantly Inherited Disorders
Achondroplasia, a form of dwarfism with an incidence of one case among every 10,000 people. Heterozygous individuals have the dwarf phenotype.
Huntington’s disease, a degenerative disease of the nervous system, is caused by a lethal dominant allele that has no obvious phenotypic effect until the individual is about 35 to 45 years old.
Key
Male Female Affectedmale
Affected female
1stgeneration
2ndgeneration
3rdgeneration
Is a widow’s peak a dominant or recessive trait?
Widow’speak
No widow’speak
WWor
Ww
Ww ww ww Ww
Ww Ww Wwww ww ww
ww
1stgeneration
2ndgeneration
3rdgeneration
Is an attached earlobe
a dominant or recessive?
Attachedearlobe
Freeearlobe
FForFf
Ff Ff Ff
Ff Ff
ff
ffffffFF or Ff
ff
Widow’speak
No widow’speak
Attachedearlobe
Parents
NormalAa
Sperm
Eggs
NormalAa
AANormal
AaNormal(carrier)
AaNormal(carrier)
aaAlbino
A
A
a
a
Albinism
Parents
DwarfDd
Sperm
EggsDd
Dwarfdd
Normal
DdDwarf
ddNormal
D
d
d
d
Normaldd
Dwarfism
• Chromosomes carry hereditary (genes)• Chromosomes (and genes) occur in pairs• New combinations of genes occur in sexual
reproduction• Monohybrid vs. Dihybrid crosses• Mendel’s Principles:
Dominance: one allele masks anotherSegregation: genes become separated in gamete formationIndependent Assortment: Members of one gene pair segregate independently from other gene pairs during gamete formation
Summary of Genetics
Preeyanan Preeyanan Sanpote Sanpote
School of Science
University of Phayao
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