INTRO TO GENETICS LECT 1

AGRC 311 - Lecture 2 AGRC 311 - Lecture 2 Patterns of InheritancePatterns of Inheritance

Eric Yirenkyi Danquah

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Objectives of the LectureObjectives of the Lecture

• To provide an in-depth background in classical Mendelian genetics

– Develop analytical and problem solving skills through learning how geneticists solve problems and make new discoveries

– understand basic vocabulary

– thoroughly master the basic laws of Mendel

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Some Key QuestionsSome Key Questions

• How is it possible to tell if a phenotypic variant has a genetic basis?

• What principle of gene inheritance did Mendel discover?

• How is Mendel’s principle applied to human inheritance?

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OutlineOutline

• Mendel’s Experimental System

• Human Pedigree Analysis

• Sex Chromosomes and Sex-linked Inheritance

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The Nature of InheritanceThe Nature of Inheritance

Genesis Chapter 30: vs. 25-43, earliest reference of selective breeding

Greek philosophers and theories of inheritance

- Pythagoras (500 BC)

- Hippocrates (460-377 BC)- Aristotle (384-322 BC)

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Gregor Johann Mendel, Gregor Johann Mendel, (b. 22 July 1822; d. 6 January 1884)(b. 22 July 1822; d. 6 January 1884)Moravia, Austro-Hungarian EmpireMoravia, Austro-Hungarian Empire

Originator of the concept of the gene (autosomal inheritance) 8

The Idea of Blending Inheritance

Spermatozoon and egg contained essences from various parts of the body; at conception, these essences somehow blended to form a pattern for the new individual

Ideas in Science come in fashions called paradigms

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Mendel’Mendel’s Experimental s Experimental SystemSystem

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Birthplace of Modern Genetic AnalysisBirthplace of Modern Genetic AnalysisAugustinian Monastry Garden,Augustinian Monastry Garden,

St. Thomas, Brünn, Austria St. Thomas, Brünn, Austria

Experiments

1856-1870

Brno (Czech Rep.) 11

MendelMendel’’s Studiess StudiesConstitute an outstanding example of good

scientific technique

He chose research material well suited to the problem at hand

Designed his experiments carefully

Collected large amounts of data

Used mathematical analysis to show that the results were consistent with his explanatory hypothesis

He tested the predictions of the hypothesis in a new round of experimentation 12

MendelMendel Not the first to study patterns of

inheritance

First to use true-breeding strains as parents and to quantify his results

– He studied simple well-defined variations in some characters (phenotypes)

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Garden Pea (Garden Pea (Pisum sativumPisum sativum))

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Main Reasons Why Mendel Main Reasons Why Mendel Chose to Study Garden PeaChose to Study Garden Pea

1. Peas were available from seed merchants in a wide array of distinct variant shapes and colours that could be easily identified and analyzed

2. Peas can either self-pollinate (self) or cross-pollinate (cross)

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A Pea Flower with the Keel Cut A Pea Flower with the Keel Cut and Opened to Expose the and Opened to Expose the

Reproductive PartsReproductive Parts

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Artificial Cross PollinationArtificial Cross Pollination

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Other Practical Reasons Other Practical Reasons for Mendelfor Mendel’’s Choices Choice

• Peas are inexpensive and easy to obtain

• Take up little space

• Have short generation time

• Produce many offspring18

What is a Pure Line?What is a Pure Line?

An inbred line of genetic descent e.g. A stock of seeds all of the

same genetic constitution, so that a cross of any two will always give the same phenotype

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The Seven Different Character The Seven Different Character Studied by MendelStudied by Mendel

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Purple-flowered (F) x White-Purple-flowered (F) x White-flowered (M)flowered (M)

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Purple flowered (M) x White-Purple flowered (M) x White-flowered (F)flowered (F)

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HybridHybrid an individual produced from

genetically different lines

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Results of All Mendel’s Results of All Mendel’s Crosses in which Parents Crosses in which Parents Differed in One characterDiffered in One character

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ProgenyProgeny

First fillial generation (F1)

Subsequent generations produced by selfing are symbolised F2, F3 and so on

Reciprocal crosses gave the same result

not good news for Aristotle who assigned a “vitalizing” effect to semen, which he suggested was highly purified blood (a notion that influenced thinking for 2,000 years)

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Mendel’Mendel’s Conclusions Conclusion It makes no difference which way the

cross is made

Purple F1 identical to purpled flowered parental plants

In all cases, one parental phenotype disappeared in the F1 and re-appeared (in its original form) in one fourth of the F2 plants (This is not blending inheritance)26

Mendel’Mendel’s Inferences Inference

F1 plants receive from their parents the ability to produce both purple and white phenotypes and that these abilities are retained and passed on to future generations rather than blended

The terms “Dominant” and “Recessive”

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FF22 individuals individuals Two genetically distinct subclasses

Yellow pure line x Green pure line

F1, yellow seeds F2, yellow and green seeds in 3:1 ratio

Next, 519 yellow F2 seeds selfed individually

Of these, 166 produced yellows seeds; remaining 353 produced either yellow or green seeds in a 3:1 ratio (Selfed F2 green seeds produced green seeds i.e. , one-third of the F2 yellow plants were true breeding and two-thirds were like F1 yellows)

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3:1 ratio3:1 ratio

More fundamental 1:2:1 ratio

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What Kind of Hypothesis What Kind of Hypothesis Can Account for These Can Account for These

Observations?Observations?

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Mendel’s HypothesisMendel’s Hypothesis

The difference between yellow and green is caused by differences in discrete determinants he called “factors” (what we now call genes)

The factors exist in pairs, one pair for each character

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One of the pairs of phenotypes was dominant over the other in each character studied e.g., in seed colour, yellow is dominant over green, which is recessive and in flower colour, purple is dominant to white

At gamete formation the members of a gene pair separate, each into one half of the gametes

Male and female gametes fuse randomly

Mendel’s HypothesisMendel’s Hypothesis

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MendelMendel’s model’s model

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MendelMendel’’s Model in Modern Termss Model in Modern Terms

1. A hereditary factor, gene is necessary for pea colour

2. Each plant has a pair of this type of gene3. The gene comes in two forms called alleles,

Y and y4. A plant can be either Y/Y, y/y or Y/y5. In the Y/y plant, the Y allele dominates so

the phenotype is yellow (Y allele is dominant and y allele, recessive) 34

MendelMendel’’s Model in Modern Termss Model in Modern Terms

6. In meiosis, the members of gene separate equally into the eggs and sperm (Mendel’s First Law or the Law of equal segregation)

7. Hence a gamete contains only one member of a gene pair

8. At fertilization, gametes fuse randomly, regardless of which of the alleles they bear

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Monohybrid TestcrossMonohybrid Testcross

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Mendel had developed an analytical scheme for the identification of major genes defining any biological character or function

Also, after thousands of years he had solved the mystery of Jacob’s goats!

Out of a Series of Crossing Out of a Series of Crossing ExperimentsExperiments

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The concept of equal segregation

Mendel’s first law:The two members of a gene pair segregate from each other into the gametes; so that half the gametes carry one member of the pair and the other half of the gametes carry the other member of the pair.

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Plants Differing in Plants Differing in Two CharactersTwo Characters

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Round (R/R or R/r) and Wrinkled (r/r) Round (R/R or R/r) and Wrinkled (r/r) Peas in a Pod of a Selfed Heterozygous Peas in a Pod of a Selfed Heterozygous

(R/r) Plant(R/r) Plant

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The FThe F22 Generation from a Dihybrid Cross Generation from a Dihybrid Cross

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Punnet Square of the Predicted Genotypic and Phenotypic Punnet Square of the Predicted Genotypic and Phenotypic Constitution of the FConstitution of the F22 Generation from a Dihybrid Cross Generation from a Dihybrid Cross

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ResultResult

Consistent with independent segregation of two pairs of alleles

Mendel’s second law:different gene pairs assort independently during gamete formationbut, genes on the same chromosome do not assort independently. Modern version “gene pairs on separate chromosome pairs assort independently at meiosis” 45

Dihybrid testcrossDihybrid testcross

Each student to write down the parents of a dihybrid testcross

What is the outcome of the cross?

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Test of 1:1:1:1 ratio of RY, Ry, Test of 1:1:1:1 ratio of RY, Ry, rY and ryrY and ry

Cross F1 dihybrid (RrYy) to rryy genotype which produces only gametes with recessive alleles

• Products of the cross should be a direct reflection of the gametic proportions of the dihybrid i.e.:

¼ RrYr, ¼ Rryy, ¼ rrYy and ¼ rryy.• This was the result he obtained; perfectly

consistent with the expectations in all the characters studied. 47

Methods of Calculating Genetic Ratios

Two main methods:

1. Punnet square

2. Branch method

Application of simple statistical rules is another method for calculating the probabilities (expected frequencies) of the specific phenotypes and genotypes from a cross 48

Two Statistical RulesTwo Statistical Rules Product rule

the probability of two independent events occurring together is the product of the individual events (e.g. two 4s when two dice are rolled)

Sum rulethe probability of either of two mutually exclusive events occurring is the sum of their individual probabilities (e.g. two 4s or two 5s when two dice are rolled) 49

5 Minutes Break5 Minutes Break

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Human GeneticsHuman Genetics Controlled crosses?

Pedigree analysis - geneticists resort to scrutinizing records in the hope that informative matings have been made by chance

Propositus - the first person from a family to interest a geneticist e.g. a dwarf 51

Contrasting Human PhenotypesContrasting Human Phenotypes Determined by a pair of alleles e.g.:

autosomal recessive disorders- Phenylketonuria- Cystic fibrosis- Albinism- Majewski osteodysplastic primordial

dwarfism type II autosomal dominant disorders- Achondroplasia- Huntington’s disease- Polydactyly- Brachydacyly- Piebald spotting 52

Symbols Used in Human Pedigree AnalysisSymbols Used in Human Pedigree Analysis

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Centralia, Illinois, USA The dynamic duo suffer from

Majewski osteodysplastic primordial dwarfism type II (This condition is inherited in an autosomal recessive pattern,

which means both copies of the gene, PCNT (encoding pericentrin protein) in each cell have mutations).

a). August 13, 2012. Christie Jordan with Bridgette and Brad; b). Bridgette (23 yrs, 27”, 18 Lbs ) and Brad (21 yrs, 38”, 35Lbs) shopping 54

Pedigree of a Rare Recessive Phenotype Pedigree of a Rare Recessive Phenotype Determined by a Recessive Allele Determined by a Recessive Allele aa

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The Human Pseudo-The Human Pseudo-Achondroplasia PhenotypeAchondroplasia Phenotype

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Pedigree of a Dominant PhenotypePedigree of a Dominant Phenotype Determined by a Dominant Allele ADetermined by a Dominant Allele A

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The Age of Onset of The Age of Onset of Huntington’Huntington’s diseases disease

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Polydactyly and BrachydactylyPolydactyly and Brachydactyly

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Piebald Spotting, a Rare Dominant Phenotype

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MessageMessage In human pedigrees, an autosomal

recessive disorder is revealed by the appearance of the disorder in the male and female progeny of unaffected persons

Pedigrees of Mendelian autosomal dominant disorders show affected males and females in each generation; they also show that affected men and women transmit the condition to equal proportions of their sons and daughters

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Sex Chromosomes and Sex-Sex Chromosomes and Sex-linked Inheritancelinked Inheritance

Sexual dimorphism in animals and plants

Two categories of chromosomes, sex chromosmes and autosomes

Humans, 46 chromosomes: 22 homologous pairs of autosomes plus 2 sex chromosomes

- Females (Homogametic sex, XX)- Males (Heterogametic sex, XY)

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Differential and pairing regions of human sex64

Chromosomal Determination of Chromosomal Determination of Sex in Sex in DrosophilaDrosophila and Humans and Humans

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Dioecious Plant Species, Dioecious Plant Species, Aruncus dioicusAruncus dioicus

Dioecious plants, by definition, bear male or female parts on separate plants. Members of such plant species are said to be either male or female based on the reproductive structures present in the flowers.66

Inheritance of An X-linked Inheritance of An X-linked Recessive DisorderRecessive Disorder

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X-linked Recessive DisordersX-linked Recessive Disorders Many more males than females show the phenotype

under study

None of the offspring of an affected male show the phenotype, but all his daughters are “carriers” bearing the recessive allele masked in the heterozygous condition. In the next generation, half the sons of these carrier daughters show the phenotype

None of the sons of an affected male show the phenotype under study, nor will they pass the condition to their descendants.

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Four Siblings with Testicular Ferminization Syndrome Four Siblings with Testicular Ferminization Syndrome (Congenital Insensitivity to Androgens)(Congenital Insensitivity to Androgens)

Rare X-linked recessive phenotype; individuals are chromosomally males, 44 autosomes + XY but have inherited the recessive X-linked allele conferring insensitivity to androgens (male hormones). One of the sisters (not shown), who was genetically XX, was a carrier and bore a child who also showed TFS 69

Inheritance of An X-linked Inheritance of An X-linked Dominant PhenotypeDominant Phenotype

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X-linked Dominant DisordersX-linked Dominant Disorders

All affected males pass the condition to all their daughters but to none of their sons

Affected heterozygous females married to unaffacted males pass the condition to half their sons and daughters

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Y-linked InheritanceY-linked Inheritance

Hairy ear rims, caused by an allele of a Y-linked gene?

SRY (testis determining factor) - the gene that plays a primary role in maleness. SRY confirmed on the differential region of the Y Chromosome by genomic analysis.

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MessageMessage

• Inheritance patterns with an unequal representation of phenotypes in males and females can locate the genes concerned to one or both of the sex chromosomes

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Significance of Mendel’s Work

The Basis of Modern Plant and Animal Breeding

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Why Was Mendel’Why Was Mendel’s Work s Work Unrecognized and Unrecognized and

Neglected for 35 Years Neglected for 35 Years Following its Publication?Following its Publication?

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Mendel’Mendel’s Prophetic Wordss Prophetic Words

• “Meine Zeit wird schon Kommen” meaning “My time will come”

• “MA DOBA PRIJDE”

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19001900

Rediscovery of Mendel’s work

• Hugo de Vries• Carl Correns• Eric von Tschermak

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Why is Mendel’s Work So Important?

• A method of genetic experimentation and explained how heredity works

• Completely novel idea, and to what is probably the most important concept ever formulated in genetics - that of particulate inheritance

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QuestionQuestion

Assume we have two plants of genotypes “AabbCcDdEe and AaBbCcddEe” and that from the cross, we want to recover a progeny of genotype “aabbccddee”

Estimate how many progeny plants one needs to grow in order to stand a reasonable chance of obtaining the desired genotype.

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AnswerAnswer

• Aa x Aa, ¼ will be aa• bb x Bb, ½ will be bb• Cc x Cc, ¼ will be cc• Dd x dd, ½ will be dd• Ee x Ee, ¼ will be ee

Therefore, the overall probability or expected frequency of genotype

“aabbccddee” will be 1/256.80

Assuming Complete DominanceAssuming Complete Dominance

No of gene pairs: n

Different kinds of gametes produced (F1): 2n

Genotypes in F2: 3n

Phenotypes in F2: 2n

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Take home Take home 1. Define the following terms: monohybrid cross, self fertilisation, cross

fertilisation, segregation, homozygous, heterozygous, allele, dominant, recessive, monohybrid, dihybrid cross, testcross.

2. Consider three yellow, round peas, labeled A, B and C. Each was grown into a plant and crossed to a plant grown from a green, wrinkled pea. Exactly 100 peas issuing from each cross were sorted into phenotypic classes as follows:A: 51 yellow, round; 49 green, roundB: 100 yellow, roundC: 24, yellow, round; 26 yellow, wrinkled; 25 green, round;

25 green, wrinkledWhat were the genotypes of A, B, and C (Choose your own gene symbols)?

3. Deduce the F2 ratios of the trihybrid cross AABBCC x aabbcc.82

THANK YOUTHANK YOU

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