Gregor Mendel Mendelian Genetics

Beliefs about Heredity Fig. 1. De la propagation du genre humain, ou manuel indispensable pour ceux qui veulent avoir de beaux enfants de l’un ou l’autre sexe (Paris, Year VII). Image courtesy of the Bibliothèque Interuniversitaire de Médecine, Paris.

Homunculus How is “heredity passed on: Spermist vs Ovists Spermist conception of a human sperm

Homunculus Leeuwenhoek’s black male and white female rabbit experiments: spermist “proof”

Darwin What he got right.What he got wrong. Acquired characteristics Ex. Blind cave animals Sex. Repro.- gemmules from all over body are packed in sperm and egg Blended inheritance Likes produce likes Change can be permanent There is no limit to cumulative change

Mendel’s Three Principles Dominance Segregation Independent Assortment The foundation of “classical” science ( )

Dominance Traits of both parents inherited, but one shows over the other Traits are not blended

Dominance Mechanism Two alleles are carried for each trait In true-breeding individuals, both alleles are the same. Hybrids, on the other hand, have one of each kind of allele. One trait is dominant, the other trait is recessive

Segregation Half the gametes (egg or sperm) will carry the traits of one parent and half the traits for the other parent Pairs of alleles are separated (=segregated) during meiosis

Two different parental characteristics will be inherited independently of one another during gamete formation. Independent Assortment Example: flower color and leaf shape

Mendel’s Genetics Experiments with Peas

Pea Characters Investigated

Genetic Loci & Alleles allele for purple flowers allele for white flowers gene locus for flower color

Genetic Recombination at Fertilization

Mendel’s Test for Evaluating the Inheritance of Seed Shape in Pea Plants l Recognized two different traits for the seed shape character in pea plants: round versus wrinkled seeds. l Established true-breeding varieties for each of these traits.

Mendel’s Test for Evaluating the Inheritance of Seed Shape in Pea Plants l Crossed true-breeding round seed variety with the true breeding wrinkled seed variety (this represents the Parental, or P, cross). l Offspring (F 1 generation) are called “hybrids”.

l 100% of these hybrids produced round seeds. l Crossed these F 1 generation hybrids among each other (individual crosses being selected at random). Mendel’s Test for Evaluating the Inheritance of Seed Shape in Pea Plants

l Results for their offspring (F 2 generation): ó 5474 (74.7%) were plants that produced round seeds. Mendel’s Test for Evaluating the Inheritance of Seed Shape in Pea Plants

l Results for their offspring (F 2 generation): ó 1850 (25.3%) were plants that produced wrinkled seeds. Mendel’s Test for Evaluating the Inheritance of Seed Shape in Pea Plants

l Results for their offspring (F 2 generation): ó Ratio of round:wrinkled = approx. 3:1 Mendel’s Test for Evaluating the Inheritance of Seed Shape in Pea Plants

Mendel’s Explanation for his Results l For every character (e.g., seed shape) an individual possess two instruction sets (alleles). l One of these alleles was originally derived from the individual’s mother, the other allele being originally derived from the individual’s father.

Mendel’s Explanation for his Results l In true-breeding individuals, both alleles are the same. l Hybrids, on the other hand, have one of each kind of allele.

Mendel’s Explanation for his Results l Mendel believed that only two alleles were possible for a given genetic character, and that one of the alleles (the dominant one) masked the expression of the other (the recessive one) in the hybrid.

Mendel’s Explanation for his Results l When the hybrid formed gametes, only one of the two possible alleles would end up in a gamete. l However, both alleles possessed an equal chance of appearing in a gamete.

Mendel’s Explanation for his Results l Defining alleles: R = the round allele (dominant). r = the wrinkled allele (recessive).

Mendel’s Explanation for his Results l Defining genotypes & their phenotypes: RR genotype (homozygous dominant) = round phenotype. rr genotype (homozygous recessive) = wrinkled phenotype. Rr genotype (heterozygous) = round phenotype

Pea Shapes RRRr rr

Mendel’s Explanation for his Results Parental (P) Cross Round X Wrinkled Round X Wrinkled RR rr Gametes: R r

Mendel’s Explanation for his Results F 1 Generation Hybrids 100% Round 100% Rr

Mendel’s Explanation for his Results F 1 Generation Cross Round X Round Rr Rr Gametes: R r R r

Mendel’s Explanation for his Results F 1 Gametes: R r R r F 2 Generation Outcome Round Wrinkled Round Wrinkled RR Rr Rr rr RR Rr Rr rr Note: 3:1 ratio of round:wrinkled

Punnett Square Diagram Rr rR r RRRRr rr gametes

Mendel’s Law of Segregation In the formation of gametes, two members of a gene pair (alleles) segregate into different haploid gametes with equal probability.

A Testcross F 1 Hybrids (Rr) x F 2 Recessives (rr)

Punnett Square Diagram for Testcross Rr r R rr r gametes Rr x rr

Mendel’s Law of Independent Assortment Whenever two or more pairs of contrasting characters are brought together in a hybrid, the alleles of the different pairs segregate independently of one another during gamete formation.

Mendel’s Test Using the Seed Shape Character with the Seed Color Character Define Alleles and Associated Traits: Seed Shape Character R = round seed trait (dominant) r = wrinkle seed trait (recessive)

Mendel’s Test Using the Seed Shape Character with the Seed Color Character Define Alleles and Associated Traits: Seed Color Character Y = yellow seed trait (dominant) y = green seed trait (recessive)

Mendel’s Test Using the Seed Shape Character with the Seed Color Character l Parental (P) cross: crossed true- breeding plants that produce round- yellow (RRYY) seeds with true- breeding plants that produce wrinkled-green seeds (rryy).

Mendel’s Test Using the Seed Shape Character with the Seed Color Character l F 1 Generation: hybrids for both characters (Rr & Yy; or RrYy) were100% round-yellow. l F 1 Generation Individuals (RrYy) crossed among each other.

Mendel’s Test Using the Seed Shape Character with the Seed Color Character l F 2 Generation Results: (56.7%) round-yellow (19.4%) round-green (18.2%) wrinkled-yellow 3 32 (5.8%) wrinkled-green

No Independent Assortment No Independent Assortment

With Independent Assortment

Calculating Expected Frequencies l Expect 9 / 16 of the F 2 generation offspring to be round-yellow. l Therefore, of a total of 556 offspring the expected number (frequency) of round-yellow offspring may be calculated as follows: Frequency = ( 9 / 16 )*556 = Round-Yellow

Calculating Expected Frequencies l Expect 3 / 16 of the F 2 generation offspring to be round-green. l Therefore, of a total of 556 offspring the expected number (frequency) of round-yellow offspring may be calculated as follows: Frequency = ( 3 / 16 )*556 = Round-Green

Calculating Expected Frequencies l Expect 3 / 16 of the F 2 generation offspring to be wrinkled-yellow. l Therefore, of a total of 556 offspring the expected number (frequency) of round-yellow offspring may be calculated as follows: Frequency = ( 3 / 16 )*556 = Wrinkled- Yellow

Calculating Expected Frequencies l Expect 1 / 16 of the F 2 generation offspring to be wrinkled-yellow. l Therefore, of a total of 556 offspring the expected number (frequency) of round-yellow offspring may be calculated as follows: Frequency = ( 1 / 16 )*556 = Wrinkled- Green

Comparing Observed to Expected Results ObservedRound-Yellow315Round-Green108Wrinkled-Yellow101Wrinkled-Green32 Expected Round-Yellow Round-Green Wrinkled-Yellow Wrinkled-Green 34.75