Mendelism: The Basic Principles of Heredity Md. Habibur Rahaman (HbR) Lecturer Dept. of Biology & Chemistry North South University

Heredity What genetic principles account for the transmission of traits from parents to offspring? One possible explanation of heredity is a “blending” hypothesis - The idea that genetic material contributed by two parents mixes in a manner analogous to the way blue and yellow paints blend to make green An alternative to the blending model is the “particulate” hypothesis of inheritance: the gene idea - Parents pass on discrete heritable units (or genes)

Gregor Mendel Gregor Mendel (1822 – 1884) – Austrian monk who first discovered the basic rules of inheritance – his work was rediscovered in 1900 and he came to be known as the Father of Genetics Documented a “particulate” mechanism of inheritance through his experiments with garden peas

Genetic Terminology Locus – specific location of a gene on a chromosome Homologous chromosomes carry the same type of gene, located at the same place (same type, but may not be identical) Alleles – alternative forms of the same gene. e.g., – human blood types produced by three different alleles A, B, and O Homozygous – an organism that has the same allele on both homologous chromosomes at a given gene locus Heterozygous – when the two homologous chromosomes have different alleles at a given gene locus (e.g., hybrid)

Genetic Terminology Pure-breeding (true-breeding) – It involves creating offspring by mating two parents that are genetically similar, homozygous (opposite of a hybrid) Phenotype – the physical appearance of an organism Genotype – the actual combination of alleles carried by an organism Genome – the whole set of the genetic information of an organism Dominant traits- traits that are expressed Recessive traits- traits that are covered up Monohybrid crosses—Crosses between parents that differed in a single characteristic

Mendel’s Success Mendel worked with the pea plant, Pisum sativum Easy to cultivate, rapid growth, produce many offspring He focused on traits that were easy to contrast He selected pure-breeding plants (self-fertilizing) Traits were located on separate chromosomes (he was lucky!) No linkage among the traits

Antagonistic traits Dominant Recessive

True Breeding Plants Mendel also made sure that he started his experiments with varieties that were “true-breeding” X

Monohybrid Crosses When Mendel crossed contrasting, true-breeding white and purple flowered pea plants all of the offspring were purple When Mendel crossed the F1 plants, many of the plants had purple flowers, but some had white flowers A ratio of about three to one, purple to white flowers, in the F2 generation P Generation (true-breeding parents) Purple flowers White  F1 Generation (hybrids) All plants had purple flowers F2 Generation

Mendel’s Rationale In the F1 plants, only the purple trait was affecting flower color in these hybrids Purple flower color was dominant, and white flower color was recessive Mendel developed a hypothesis to explain the 3:1 inheritance pattern that he observed among the F2 offspring There are four related concepts that are integral to this hypothesis

Heredity Concepts Alternative versions of genes account for variations in inherited characters, which are now called alleles For each character an organism inherits two alleles, one from each parent, A genetic locus is actually represented twice If the two alleles at a locus differ, the dominant allele determines the organism’s appearance The law of segregation - the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes Allele for purple flowers Locus for flower-color gene Homologous pair of chromosomes Allele for white flowers

Mendel’s Monohybrid Cross White (pp) Purple (Pp) Gametes Gametes p p P p Purple (PP) Purple (Pp) P P Gametes Pp Pp PP Pp Gametes p P Pp Pp Pp pp F1 generation All purple F2 generation ¾ purple, ¼ white The Punnett Square

Test Cross To determine whether an individual with a dominant phenotype is homozygous for the dominant allele or heterozygous, Mendel crossed the individual in question with an individual that had the recessive phenotype: PP If all offspring are purple; unknown plant is homozygous. P pp p Pp Recessive phenotype Dominant Phenotype Gametes Alternative 1 – Plant with dominant phenotype is homozygous ? Pp If half of offspring are white; unknown plant is heterozygous. P p pp Recessive phenotype ? Alternative 2 – Plant with dominant phenotype is heterozygous Dominant Phenotype Gametes

Dihybrid Cross Monohybrid cross: determined that the 2 alleles at a single gene locus segregate, when the gametes are formed Dihybrid cross: determined that alleles at 2 different gene loci segregate independently of one another Also known as law of independent assortment Basically, an extension of the law of segregation

Dihybrid Cross Crossed pea varieties that differed in two traits Mendel crossed 2 pure-breeding plants: round yellow seeds X green wrinkled seeds Seed shape is controlled by one gene locus where round (R) is dominant to wrinkled (r) Seed color is controlled by a different gene locus where yellow (Y) is dominant to green (y)

Dihybrid Cross RrYy What are the expected F1 gametes? True-breed parents Parental Gametes RY ry RrYy F1 Offspring What are the expected F1 gametes? What should be the phenotypic ratio for the F2?

F2 with independent assortment Phenotypic ratio is 9 : 3 : 3 : 1 RY Ry rY ry RR YY Yy Rr yy rr RY Ry rY ry Phenotypic ratio is 9 : 3 : 3 : 1

Summary of Mendel’s Principles Mendel’s Principle of Uniformity in F1: F1 offspring of a monohybrid cross of true-breeding strains resemble only one of the parents Why? Some traits are completely dominant over other traits Mendel’s Law of Segregation: Recessive characters masked in the F1 progeny of two true-breeding strains, re-appear in a specific proportion of the F2 progeny Two members of a gene pair segregate (separate) from each other during the formation of gametes Mendel’s Law of Independent Assortment: Alleles for different traits assort independently of one another Genes on different chromosomes behave independently in gamete production

Exceptions To Mendel’s Original Principles Incomplete dominance Co-dominance Polygenic traits Epistasis Pleiotropy

Incomplete dominance Neither allele is dominant and heterozygous individuals have an intermediate phenotype For example, in Japanese “Four o’clock”, plants with one red allele and one white allele have pink flowers: P Generation F1 Generation F2 Generation Red CRCR Gametes CR CW  White CWCW Pink CRCW Sperm Cw 1⁄2 Eggs CR CR CR CW CW CW

Does it support blending inheritance? Incomplete Dominance Gametes CR CW CRCR CR CRCR CRCW Gametes CW F1 generation All CRCW CRCW CWCW F2 generation CWCW 1 : 2 : 1 Does it support blending inheritance?

Co-dominance Neither allele is dominant and both alleles are expressed in heterozygous individuals Example ABO blood types

Misconception of Dominant Phenotype A genetic factor/trait may be dominant in one species, but can be recessive or incompletely dominant in other species Even though the two genetic factors appear to produce identical phenotypes (red flower trait in both the pea and the Four O’clock) A factor for black fur may be dominant in one species (guinea pigs) but recessive in another (sheep)

Polygenic Traits Most traits are not controlled by a single gene locus, but by the combined interaction of many gene loci. These are called polygenic traits. Polygenic traits often show continuous variation, rather then a few discrete forms:

Epistasis When two or more genes influence a trait, an allele of one of them may have an overriding effect on the phenotype Labrador retrievers: a dominant allele (B) produces a black coat while the recessive allele (b) produces a brown coat However, a second gene locus controls the color. Dogs that are homozygous recessive at this locus (ee) will have golden fur no matter which alleles are at the first locus Gene 1: Represented by B : Controls color Gene 2: Represented by E : Controls expression of B

Epistasis BBEE or BbEe --> Black retrievers bbEE or bbEe --> Brown retrievers BBee, Bbee, or bbee --> Golden retrievers Golden Brown Black

Pleiotropy This is when a single gene locus affects more than one trait For example, in Labrador retrievers the gene locus that controls the pigment in the hair also affects the color of the nose, lips, and eye rims

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