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Mendelian Genetics. SBI3U0. Gregor Johann Mendel. Modern day genetics has its roots in the 1800s An A ugustinian monk named Gregor Johann Mendel independently discovered some of the most important laws governing the inheritance of traits

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gregor johann mendel
Gregor Johann Mendel
  • Modern day genetics has its roots in the


  • 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’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
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 characteristics1
Pea Characteristics

Note: the traits are different versions of a specific characteristic.

Eg: purple is a trait the for flower colour characteristic

  • 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
  • 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
f 2 generation
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
  • 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
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
  • 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
  • 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
  • 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
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
dominant vs recessive
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
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
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 squares1
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


green gametes


green gametes

  • 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
f 2 generation1
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

  • 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
  • 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!
  • 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
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
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
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