Lecture 2.2: Mendel's Principle of Segregation · 2 ratios always near 3:1? Mendel’s rst principle: Principle of Segregation For these 7 traits, instead of uid, the genetic material

Phase 1 Phase 2 Phase 3 Phase 4

Unit 2: Transmission Genetics

Lecture 2.2: Mendel’s Principle of Segregation

John D. Nagy

BIO 181: General Biology for Majors, Scottsdale Community College

2019 Revision

John Nagy Lec 2.2: Segregation 1/25


Outline

1 Phase 1: Initial observations

2 Phase 2: Explain the resultsParticulate genesRandomness

3 Phase 3: Demonstrate hypothesis explains dataF1

F2

Practice

4 Phase 4: Test the hypothesisMain conceptTest cross



Summary of phase 1: Mendel’s observations

Trait 1 Trait 2 F1 F2* Ratio

Spherical seed Wrinkled seed All Spherical 5474 : 1850 2.96:1Yellow Cotyl. Green Cotyl. All Yellow 6022 : 2001 3.01:1Purple Flower White Flower All Purple 705 : 224 3.15:1Inflated pod Shriveled pod All Inflated 882 : 299 2.95:1Green pod Yellow pod All Green 428 : 152 2.82:1Axial Flower Terminal Flower All Axial 651 : 201 3.14:1Tall Stem Dwarf Stem All Tall 787 : 277 2.84:1

* Larger number always same as F1.


Phase 1 Phase 2 Phase 3 Phase 4 Particulate genes Randomness

Phase 2: Explain the patterns in single trait crosses

Mendel’s experiments in the context of his time:

Nothing Mendel observed was new—all of theseexperiments had been performed before.

Mendel was the first person to suggest the correctexplanations of the cause (hypotheses, now calledprinciples).

There are two aspects to his first hypothesis:

Why are the F2 ratios always close to 3:1?

Why are they not exactly 3:1?



Why were F2 ratios always near 3:1?

Mendel’s first principle: Principle of Segregation

For these 7 traits,

instead of fluid, the genetic material is a collection ofindivisible particles;

each trait is determined by precisely 2 of these particles inany given individual plant;

one particle is inherited from the male parent and one fromthe female parent;

one particle dominates the other; that is, when foundtogether in a single individual, only one of 2 different typesof particles is expressed, and it’s always the same kind (thedominant).



Modern terms help organize these ideas

Definition: Gene

A gene is the basic physicaland functional unit ofinheritance; it is an orderedsequence of nucleotideslocated at a particular locusthat encodes a specificfunctional product (typicallyprotein or RNA molecule).

Definition: Genetic Locus

A genetic locus, or just locus, is a particular location on aparticular chromosome.

Genes are Mendel’s “particles.”



Modern restatement of Mendel’s principle

For the traits Mendel studied ,

each trait is determined fundamentally by genes;

each trait is determined by two copies of the same gene;

each parent contributes precisely one of these copies to theoffspring;

the genes have different forms called alleles;

one allele is dominant, and the other is recessive.

Definition: Allele

Alleles are any of the alternative forms of a gene that mayoccur at a specific locus.

NOTE: Mendel did not argue that this pattern exists for all traits; he knewfrom his research that such a pattern is rare.



More definitions

Definition: Homozygous

Homozygous refers to an individual or locus with identicalcopies of a given gene on homologous chromosomes.

Definition: Heterozygous

Heterozygous refers to an individual or locus with differentalleles for a given gene on homologous chromosomes.

Definition: Phenotype

Phenotype is the observable properties of a trait or set oftraits.

Definition: Genotype

A genotype is an exact description of the alleles an individualpossesses at homologous loci of a gene or set of genes.



More definitions and examples

Definition: Dominant allele and trait

Any allele expressed in a heterozygote is dominant. Theexpression of the dominant allele is a dominant trait.

Definition: Recessive allele and trait

Any allele that, when present in a heterozygote, has no effect onthe phenotype is recessive. The expression of the recessivealleles in the homozygous recessive condition is a recessivetrait.

Example: Garden peas express round and wrinkledphenotypes. Two alternate alleles exist, R and r. Therefore,three genotypes are possible: RR, Rr and rr. The first twoproduce spherical phenotypes, and the last produces wrinkledbecause R is dominant and r is recessive.



Practice

A plant has wrinkled seeds. What is its phenotype?

A plant has wrinkled seeds. What is its genotype?

A heterozygote has what phenotype and genotype?

A plant has yellow cotyledons. What is its genotype?



Practice

A plant has wrinkled seeds. What is its phenotype?Wrinkled

A plant has wrinkled seeds. What is its genotype?





Practice


A plant has wrinkled seeds. What is its genotype?rr





Practice



A heterozygote has what phenotype and genotype?Spherical phenotype with genotype Rr.NOTE: Not RW; that implies 2 genes.




Practice



A heterozygote has what phenotype and genotype?Spherical phenotype with genotype Rr

A plant has yellow cotyledons. What is its genotype?Unknown; it could be Y Y or Y y since yellow is dominant



Why the deviation from 3:1?

?



Why the deviation from 3:1?

Answer: Randomness or Stochasticity

F2 ratios are not precisely 3:1 because which allele aheterozygous parent bequeaths to its offspring (either A or a) isdetermined by chance.

An event whose outcome cannot be predicted withcertainty is called random.

A sequence of random events is called stochastic.

Example: The stochastic process of flipping a fair coin 4 timeshas the expected outcome of 2 heads and 2 tails. But otheroutcomes are obviously possible, with the following non-obviousprobabilities:

Outcome 0H/4T 1H/3T 2H/2T 3H/1T 4H/0TProbability 0.0625 0.25 0.375 0.25 0.0625


Phase 1 Phase 2 Phase 3 Phase 4 F1 F2 Practice

Demonstrate that the hypothesis explains the data

For the P × P cross:



Demonstrate that the hypothesis explains the data

For the F1 self cross:

Why not exactly 3:1?Where does therandomness enter thepicture?



Human example: Corneal astigmatism

One of the most commonforms of inheritedastigmatism.

Misshapen cornea causesmultiple focal points;image is blurred on someportions of retina.

Genotype ♠♠ causesnormal vision, andgenotypes ♠♣ and ♣♣cause astigmatism.

Which allele is dominant?




One of the most commonforms of inheritedastigmatism.

Misshapen cornea causesmultiple focal points;image is blurred on someportions of retina.

Genotype ♠♠ causesnormal vision, andgenotypes ♠♣ and ♣♣cause astigmatism.

Which allele is dominant?♣




What is the recessive trait?

Rex has astigmatism buthis mother did not. Whatis Rex’s genotype?

Rex has a baby brotherwho does not haveastigmatism. What doesthat tell us, if anything,about Rex’s parents’genotypes?

What is the probabilitythat Rex’s baby sister alsohas astigmatism?


Phase 1 Phase 2 Phase 3 Phase 4 Main concept Test cross

Mendel must now test his hypothesis

Gregor Mendel

What does it mean to Mendel“to test his hypothesis?” Didn’the already do that by showingthat his hypothesis fits thedata?



Mendel must now test his hypothesis

Gregor Mendel

It means Mendel must nowpredict the outcome of a novelexperiment using hishypothesis.

Data used to generate ahypothesis cannot then be usedto test the hypothesis.

Why not?



Single trait test cross

If Mendel’s hypothesis is correct, predict the outcome of thefollowing cross:

F1 × recessive PFirst question: do you know the genotypes of the parents?



Single trait test cross

If Mendel’s hypothesis is correct, predict the outcome of thefollowing cross:

F1 × recessive PFill in the Punnett square:

Mendel’s actual results

106 Round102 Wrinkled

Do these results support,contradict or say nothingabout Mendel’s hypothesis?


Documents

Lecture 2.2: Mendel's Principle of Segregation · 2 ratios always near 3:1? Mendel’s rst principle: Principle of Segregation For these 7 traits, instead of uid, the genetic material