Heredity
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Heredity. Biology 30. Early Theories of Inheritance. Aristotle (384-322 B.C.E.) proposed the first widely accepted theory of inheritance called pangenesis egg and sperm consist of particles called pangenes that come from all parts of the body.

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Heredity

Heredity

Biology 30


Early theories of inheritance

Early Theories of Inheritance

Aristotle (384-322 B.C.E.)

  • proposed the first widely accepted theory of inheritance

    • called pangenesis

      • egg and sperm consist of particles called pangenes that come from all parts of the body.

      • upon fertilization the pangenes develop into the parts of the body from which they are derived.


Heredity

  • egg and sperm consist of particles called pangenes that come from all parts of the body.

  • upon fertilization the pangenes develop into the parts of the body from which they are derived.

    Antony van Leeuwenhoek (1632-1723)

  • discovered sperm in semen.

    • he believed he thought he saw a complete miniature person called a homunculus inside the head of the sperm.

    • other people of Antony’s time thought that the egg contained the entire person.


  • Mendelian genetics

    Mendelian Genetics

    Gregor Mendel (1822-1884)

    • a Augustinian monk in Brunn (Czech Republic), Austria

    • his research laid the foundation for modern genetics and the science of inheritance.

    • for seven years he bred pea plants (Pisum sativum) and analyzed the results.


    Mendelian genetics1

    Mendelian Genetics

    • Mendel focuses on seven different traits of pea plants..


    Mendelian genetics2

    Mendelian Genetics

    • Mendel let plants self-pollinate to ensure they were true breeding.

      • true breeding plants exhibit the same characteristics generation after generation.

      • Mendel called

        • true breeding plants the parental or P generation

        • the first offspring first filial or F1 generation

      • If the F1 generation were to pollinate the offspring would be called the second filial or F2 generation


    Mendelian genetics3

    Mendelian Genetics

    • Mendel called the first offspring first filial or F1 generation

    • If the F1 generation were to pollinate the offspring would be called the second filial or F2 generation

    • because all of Mendel’s initial crosses only involved one trait we call them monohybrid crosses.

    • Mendel observed that:

      • for every trait crossed the F1 generation only showed one of the two parental traits.

        ie. if plants with round seeds were crossed with plants of wrinkled seeds the F1 generation would only have plants of round seeds.


    Mendelian genetics4

    Mendelian Genetics

    • Mendel observed that:

      • for every trait crossed the F1 generation only showed one of the two parental traits.

        ie. if plants with round seeds were crossed with plants of wrinkled seeds the F1 generation would only have plants of round seeds.

      • even though the F1 generation had a copy of both genes only one was expressed.

        • Mendel called this characteristic dominant.

          allele: one of alternative forms of a gene.

          the gene for wrinkled and the gene for round peas are alleles.


    Mendelian genetics5

    Mendelian Genetics

    • even though the F1 generation had a copy of both genes only one was expressed.

      • Mendel called this characteristic dominant.

        allele: one of alternative forms of a gene.

        the gene for wrinkled and the gene for round peas are alleles.

        dominant trait: a characteristic that is expressed when one or both alleles in an individual are the dominant form

        ~ dominant alleles are indicated by an uppercase letter (R)


    Mendelian genetics6

    Mendelian Genetics

    dominant trait: a characteristic that is expressed when one or both alleles in an individual are the dominant form

    ~ dominant alleles are indicated by an uppercase letter (R)

    • Mendel called the characteristic that was not expressed recessive

      recessive trait: a characteristic that is expressed only when both alleles in an individual are the recessive form.

    • Mendel concluded that one form showed complete dominance.

      • an individual with one dominant and one recessive (Rr) had the same characteristics as one with two dominant forms (RR)


    Mendelian genetics7

    Mendelian Genetics

    • Mendel concluded that one form showed complete dominance.

      • an individual with one dominant and one recessive (Rr) had the same characteristics as one with two dominant forms (RR)


    Mendelian genetics8

    Mendelian Genetics

    Important Definitions

    Homozygous: having identical alleles for the same gene

    Heterozygous: having different alleles for the same gene.

    Genotype: the genetic complement of an organism

    Phenotype: the observable characteristics of an organism

    Segregation: the separation of alleles during meiosis.


    Mendelian genetics9

    Mendelian Genetics

    Genotype: the genetic complement of an organism

    Phenotype: the observable characteristics of an organism

    Segregation: the separation of alleles during meiosis.

    Law of Segregation

    • Mendel’s First Law

      • All individuals have two copies of each factor (gene). These copies segregate (separate) randomly during gamete formation, and each gamete receives one copy of every gene.

    • in 1909 Danish Botanist Wilhem Ludwig Johannsen called Mendel’s “factors” genes


    Mendelian genetics10

    Mendelian Genetics

    Analyzing Genetic Crosses

    Reginald Punnett (1875-1967)

    • devised a visual way to analyze the results of crosses, called a Punnett’s square.


    Mendelian genetics11

    Mendelian Genetics

    Punnett Squares

    • are used to predict the genotype and phenotype of potential off-spring

    • very useful when producing economically important cattle and plants.

    Phenotypic Ratio

    Genotypic Ratio


    Mendelian genetics12

    Mendelian Genetics

    In order to see recessive phenotypes the genotype must be homozygous


    Heredity

    Test Cross

    • a test cross of an individual of unknown genotype to an individual that is fully recessive

    • the phenotypes of the F1 generation of the test cross reveals whether the unknown genotype is homozygous or heterozygous

    • example:

      • you have a white ram (white is dominant “W” and black is recessive “w”) and want to know if it is heterozygous or homozygous for breeding purposes.


    Heredity

    • example:

      • you have a white ram (white is dominant “W” and black is recessive “w”) and want to know if it is heterozygous or homozygous for breeding purposes.

    • do a test cross by crossing your unknown ram with one showing a recessive phenotype.

      • it must have a recessive genotype (ww)


    Heredity

    If the ram is Heterozygous it will produce:

    Phenotypic ratio: 50% white 50% black, or 2:2 or 1:1

    Genotypic ratio:

    2:2 or 1:1 hetero:homo recessive

    If the ram is Homozygous it will produce:

    Phenotypic ratio: 100 % White

    Genotypic ratio:

    100% heterozygous


    Heredity

    Analyze:

    Heterozygous Seed shape crossed with a recessive seed shape

    Analyze:

    What are the predicted phenotypes and genotypes?

    Phenotypic Ratio

    • 50% round 50% wrinkled or 2:2 or 1:1, ½, 2/4,

      Genotypic Ratio

    • 50% hetero: 50% homo recessive, 2:2 or 1:1, hetero 2/4 or ½ homo recessive 2/4 or ½


    Heredity

    Example Problem

    • A horticulturist has seeds from a cross but does not know the genotype of the phenotype of the parents. Use the following information to figure out the parental phenotype and genotype

    Solution

    because there is two different phenotypes one the parents must not be homozygous dominant


    Heredity

    Solution

    because there are two different phenotypes one the parents must not be homozygous dominant

    Solution

    5472/1850 = 2.96

    2.96/1 or 2.96 : 1

    ~ 3 : 1

    to get a 3:1 ratio both parents must be heterozygous


    Mendelian genetics13

    Mendelian Genetics

    3:1 Phenotypic ratio

    1:1 (50%) Phenotypic ratio


    Heredity

    Mendel’s Second Law

    • The Law of Independent Assortment

  • Mendel also crossed plants of two traits.

    • because two traits are involved in these crosses they are called a dihybrid cross.

  • Mendel crossed true breeding tall plants that had green pods (TTGG) with true breeding short plants that had yellow pods (ttgg) to produce the F1 generation


  • Heredity

    • in this case the true breeding plants will produce only one type of gametes

      TTGG → will produce gametes with the TG genes

      ttgg→ will produce gametes with the tg genes

    • the phenotypic ratio of the F1 generation:

      100% tall and green pods

    • the genotypic ratio of the F1 generation

      100% heterozygous


    Heredity

    • Mendel then crossed the F1 generation to produce an F2 generation

    • in this case the plants of the F1 generation produce four different types of gametes

      TtGg → will produce gametes with the:

      TG genes (tall, green)

      Tg genes (tall, yellow)

      tG genes (short, green)

      tg genes (short, yellow)


    Heredity

    TtGg → will produce gametes with the:

    TG genesTg genes

    tG genes

    tg genes


    Heredity

    TT = tallGG = green

    Tt = tallGg = green

    tt = shortgg = yellow


    Heredity

    • for every dihybrid cross that Mendel carried he got the 9:3:3:1 ratio (when he crossed the F1 generation).

      • this ratio is what is expected if the segregation of alleles for one gene had no influence on the segregation of alleles of another gene.

        Law of Independent Assortment

      • The two alleles of one gene segregate (assort) independently of the alleles for other genes during gamete formation


    Heredity

    Law of Independent Assortment

    • The two alleles of one gene segregate (assort) independently of the alleles for other genes during gamete formation

      Pleiotropic Genes

    • a gene that affects more than one characteristic

    • example: Sickle-cell anemia

      • the normal hemoglobin is produced by the allele HbA

      • in sicke-cell anemia the individual has two copies of the mutated allele Hbs


    Heredity

    Pleiotropic Genes

    • a gene that affects more than one characteristic

    • example: Sickle-cell anemia

      • the normal hemoglobin is produced by the allele HbA

      • in sicke-cell anemia the individual has two copies of the mutated allele Hbs

    • the mutation cause abnormally shaped hemoglobin that cannot deliver oxygen to the cells.

      • causes fatigue, enlarged spleen, pneumonia and major organ damage.

    • a heterozygous individual has resistance to malaria but an increased chance of having homozygous recessive offspring.


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