Mendelian Genetics

Mendelian Genetics SBI3U0

Gregor Johann Mendel • Modern day genetics has its roots in the 1800s • An Augustinian monk named Gregor Johann Mendel independently discovered some of the most important laws governing the inheritance of traits • His work was not accepted at the time, and was dismissed, until it was rediscovered in the early 1900s • His work was independently developed by Hugo de Vries and Carl Correns just before it was rediscovered

Mendel’s Experiments • Mendel ran experiments on pea plants based on the idea that “traits are inherited directly from parents; some are visible in the offspring and some are not” • The prevailing scientific thought at the time was that an offspring’s traits are a blending of the traits of its parents • To illustrate how traits can be inherited Mendel crossbred thousands of pea plants and meticulously recorded all of the results

Pea Characteristics • Mendel chose seven specific characteristics of the pea plants to study • Flower colour, flower position, stem length, seed shape, seed colour, pod shape, and pod colour • Mendel created true breeding plants for each trait • Meaning that the plants always produce offspring genetically identical to itself for one or more traits when self-pollinated

Pea Characteristics Note: the traits are different versions of a specific characteristic. Eg: purple is a trait the for flower colour characteristic

Experiments • In his experiments Mendel would cross (breed) two true-breeding plants that had only a one trait difference (eg: purple vs. white flowers) • He did this for all of the characteristics several times • The initial parents are referred to as the parental generation or P generation • The offspring of these crosses are referred to as hybrids • Because they have the genetic material for BOTH traits rather than just one as their parents did

Experiments • These crosses are called monohybrid crosses • Mendel noticed something significant from these monohybrid crosses • All of the offspring (called the first filial generation or F1 generation) showed the same traits • Eg: if a purple flowering and white flowering plant were bred, the F1 generation had all purple flowers • The white flower trait was being masked by the purple flower trait

F2 Generation • When the F1 generation plants were allowed to self pollinate, an interesting thing happened; • Both traits were visible in their offspring, the second filial generation (F2) • Eg: there were both white and purple flowered offspring • Therefore, the genetic material for white flowers had not been lost, it was still present in the F1 generation even though they all appeared purple

Ratios • Even more interesting was that regardless of what trait he investigated, the F2 generation consistently showed the same ratio of traits • The ratio was always 3:1 • Eg: out of every four plants (on average) 3 had purple flowers and 1 had white flowers • From these observations Mendel developed “Mendel’s laws of inheritance” • The Law of segregation • The Law of independent assortment

Law of Segregation • For every characteristic, an organism carries two factors (genes): one from each parent • Parent organisms donate only one of their factors to their children • This is something we now know more about due to microbiology and the study of meiosis • Gamete cells contain only one set of chromosomes • They contain only one copy of each type of gene

Terms • The law of segregation can now be used to predict the characteristics of a filial generation • To do this we must define a few terms • Alleles • Different versions of a particular gene • Homozygous • An individual who carries two copies of the same allele • Heterozygous • An individual who carries two different alleles for the same characteristic

Alleles • We have specific symbols used to describe genes and alleles • We use one letter to describe the characteristic • Eg: flower colour might be represented by the letter C for colour • Specific alleles are shown as superscripts • Eg: the purple flower allele might be Cp • The white flower allele might be Cw

Terms • Genotype • The genetic makeup of an individual • The specific combination of alleles that an individual carries • Eg: CwCw, or CpCp, or CwCp • Phenotype • An individuals outward appearance for a specific characteristic • Eg: purple or white flowers

Dominant and Recessive • Mendel noticed that in the F1 generation, even though each plant was a hybrid (they were heterozygous for a certain characteristic), only one allele was expressed • Meaning that only one phenotype was shown (eg: all purple flowers) • We say that one allele is dominant, while the other is recessive • Meaning that if an individual is heterozygous, only the dominant allele will be expressed

Pea Characteristics

Dominant Vs. Recessive • Dominant alleles are often written as capital letters • Purple flower colour is dominant CP • Axial flower position is dominant PA • Recessive alleles are often written as lower case letters • White flower colour is recessive Cw • Terminal flower position is recessive Ct

Predicting Inheritance • Now we are ready to use the genotypes of two parents to predict the genotypes and phenotypes of the offspring • We use a Punnett square to do this • Punnett squares are diagrams summarizing every possible combination of gametes between two parents

Punnett Squares • Let’s breed two homozygous parents, one homozygous for green seeds (dominant), while the other is homozygous for yellow seeds (recessive) • We say “homozygous green” and “homozygous yellow” to describe the parent’s genotypes • Gametes • Each gamete gets one allele for a given characteristic • Since both parents are homozygous, their gametes are identical • One parent has only the green allele, while the other has only the yellow allele

Punnett Squares • The square below shows a monohybrid cross for seed colour • The gametes of each parent are drawn along the edges and the possible combinations are shown in the squares Homozygous green gametes Homozygous green gametes

Results • Genotypes • All offspring in the F1 generation are heterozygous • Phenotypes • All offspring in the F1 generation have green seeds since the green allele is dominant

F2 Generation • Let’s try a cross of the heterozygous F1 plants with one another • Both parents are heterozygous, therefore there are two possible gamete types: SG, Sy Genotypes: SGSG, SGSY, SYSY Phenotypes: Green seeds, Yellow seeds

Probability • Notice that there are a different amount of possible ways to make each genotype • ¼ ways to make homozygous dominant SGSG • ¼ ways to make homozygous recessive SySy • ½ (or 2/4) ways to make heterozygous SGSy • These fractions can be used as the probability that a certain offspring will have a specific genotype • Remember that it is entirely random which gametes combine with one another

Probability • If the probability of forming a homozygous dominant genotype is ¼ or 25%, does this mean if the plant has 4 offspring 1 will for sure be heterozygous dominant? • No, it doesn’t! These are only probabilities • Due to the randomness of fertilization, it is possible that more or less than 1 offspring will be homozygous dominant • Test it!

Phenotypes • We can calculate phenotype probabilities as well • ¾ of the offspring have green seeds, therefore the probability of making an offspring with green seeds is 75% • ¼ of the offspring have yellow seeds, therefore the probability of making an offspring with yellow seeds is 25%

Test Crosses • A test cross is used to determine the genotype of an unknown individual • A test cross is performed by mating the unknown individual with a homozygous recessive individual • This way the dominant and recessive alleles of the unknown individual will be expressed • If all the offspring show the dominant phenotype, the unknown individual is homozygous dominant • If the offspring show both phenotypes, the unknown individual is heterozygous

Test Cross • If we look at seed shape in pea plants, round is dominant and wrinkled is recessive. • Therefore the possible genotypes of a plant with round seeds is SRSR or SRSw • The possible crosses are;

Test Cross Results • If the unknown is homozygous dominant • All of the offspring have the heterozygous genotype • Thus all of the offspring have the dominant phenotype • If the unknown is heterozygous • ½ of the offspring are heterozygous • ½ of the offspring are homozygous recessive • Thus both the dominant and recessive phenotype may be shown in the offspring

Mendelian Genetics