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Transcript of 2.1.2. About blending theory of heredity - 2011-12-02¢  2.1.2. About blending theory of...

  • INTRODUCTION TO GENETICS

    From Proteins to Mendel Supplementary Materials

    Teacher’s Reference

    Rosa Macaya IES Pobla de Segur 1

    2.1.2. About blending theory of heredity

    Blending theory was the commonly held belief that characteristics were mixed in

    each generation. For example, breeding two horses, one with a light-coloured

    coat, the other dark, would result in offspring that were all intermediate in coat

    colour. If this held true, then eventually all organisms would become more alike

    in each generation. Although this theory persisted for many years, it was

    eventually supplanted by the work of Mendel and the modern geneticists.1

    � Pre-Mendelian theory of heredity proposing that hereditary material from

    each parent mixes in the offspring; once blended like two liquids in

    solution, the hereditary material is inseparable. And the offspring’s traits

    are the result of this blend.

    � Individuals of a population should reach a uniform appearance after many

    generations

    � Once traits blended, they cannot be separated out to appear again in later

    generations

    QUESTIONS TO THINK ABOUT AND DISCUSS AT THE END

    � Why do you think Mendel is known as the Father or genetics?” (Because

    he discovered the basic underlying principles of heredity)

    � “Do you think that using mathematics to work out the data was usual in

    Mendel’s time? (No, he was the first one to made use ot this scientific

    language)

    1 Blending inheritance is similar to the modern legitimate idea of incomplete dominance and the terms are rarely, but incorrectly, used interchangeably by some. However, incomplete dominance results in blending only of the phenotype, keeping the alleles within the heterozygote distinct (and, thus still inheritable in successive generations), whereas the theory of blending inheritance referred to an actual blending of the genetic material (i.e. in modern terms, alleles would blend together to form a completely new allele).**** To be used when explaining incomplete dominance..

  • INTRODUCTION TO GENETICS

    From Proteins to Mendel Supplementary Materials

    Teacher’s Reference

    Rosa Macaya IES Pobla de Segur 2

    � What do you think Mendel carried out his breeding experiments with pea

    plants? (Because he could observe inheritance patterns in up two

    generations a year)

    � Think and compare how scientific findings are communicate and published

    today and in Mendel’s time. (Here can be told that today the language of

    science is English and that should help the communication. Remember

    Mendel published his work in German. Relate this fact with other cases,

    like Wegener…)

  • INTRODUCTION TO GENETICS

    From Proteins to Mendel Supplementary Materials

    Teacher’s Reference

    Rosa Macaya IES Pobla de Segur 3

    2.1.3.Words Cards for Definition Team Game

    01 Hybrid

    02 Genetics

    03 Gregor Mendel

    04 Blending theory

    05 Purebred

    06 Genes

    07 Alleles

    08 Genotype

    09 Homozygous genotype

    10 Heterozygous genotype

    11 Phenotype

    12 Recessive allele

    13 Dominant allele

    14 Principle of segregation

    15 Principle of independent assortment

  • INTRODUCTION TO GENETICS

    From Proteins to Mendel Supplementary Materials

    Teacher’s Reference

    Rosa Macaya IES Pobla de Segur 4

    2.1.4. Glossary of Mendelian Genetic Terms 1. Alleles

    alternate forms or varieties of a gene. The alleles for a trait occupy the

    same locus or position on homologous chromosomes and thus govern the

    same trait. However, because they are different, their action may result in

    different expressions of that trait.

    2. Blending theory

    an incorrect 19th century theory about the inheritance of

    characteristics. It proposed that inherited traits blend from generation to

    generation. Through his plant cross-breeding experiments, Gregor Mendel

    proved that this was wrong

    3. Chromosomes

    tread-like, gene-carrying bodies in the nucleus of a cell. Chromosomes are

    composed primarily of DNA and protein. They are visible only under

    magnification during certain stages of cell division. Humans have 46

    chromosomes in each somatic cell and 23 in each sex cell.

    4. Codominance

    the situation in which two different alleles for a trait are expressed

    unblended in the phenotype of heterozygous individuals. Neither allele is

    dominant or recessive, so that both appear in the phenotype or influence

    it. Type AB blood is an example. Such traits are said to be codominant.

    5. Cross-pollination

    the mating of two genetically different plants of the same

    species. Usually, the term is used in reference to the crossing of two pure

    breeding (homozygous) plants.

    6. Dominant allele

    an allele that masks the presence of a recessive allele in the

    phenotype. Dominant alleles for a trait are usually expressed if an

    individual is homozygous dominant or heterozygous.

    7. f1 generation

  • INTRODUCTION TO GENETICS

    From Proteins to Mendel Supplementary Materials

    Teacher’s Reference

    Rosa Macaya IES Pobla de Segur 5

    the first offspring (or filial) generation. The next and subsequent

    generations are referred to as f2, f3, etc.

    8. Genes

    units of inheritance usually occurring at specific locations, or loci, on a

    chromosome. Physically, a gene is a sequence of DNA bases that specify

    the order of amino acids in an entire protein or, in some cases, a portion of

    a protein. A gene may be made up of hundreds of thousands of DNA

    bases. Genes are responsible for the hereditary traits in plants and

    animals.

    9. Genetics

    the study of gene structure and action and the patterns of inheritance of

    traits from parent to offspring. Genetic mechanisms are the underlying

    foundation for evolutionary change. Genetics is the branch of science that

    deals with the inheritance of biological characteristics.

    10. Genotype

    the genetic makeup of an individual. Genotype can refer to an organism's

    entire genetic makeup or the alleles at a particular locus. .

    11. Heterozygous

    a genotype consisting of two different alleles of a gene for a particular

    trait (Aa). Individuals who are heterozygous for a trait are referred to as

    heterozygote.

    12. Homologous chromosomes

    chromosomes that are paired during the production of sex cells in

    meiosis. Such chromosomes are alike with regard to size and also position

    of the centromere. They also have the same genes, but not necessarily the

    same alleles, at the same locus or location.

    13. Homozygous

    having the same allele at the same locus on both members of a pair of

    homologous chromosomes. Homozygous also refers to a genotype consisting

    of two identical alleles of a gene for a particular trait. An individual may be

  • INTRODUCTION TO GENETICS

    From Proteins to Mendel Supplementary Materials

    Teacher’s Reference

    Rosa Macaya IES Pobla de Segur 6

    homozygous dominant (AA) or homozygous recessive (aa). Individuals who

    are homozygous for a trait are referred to as homozygote.

    14. Hybrids

    offspring that are the result of mating between two genetically different

    kinds of parents--the opposite of purebred.

    15. Intermediate expression

    the situation in which a heterozygous genotype results in a phenotype that

    is intermediate between those resulting from the homozygous

    genotypes. The mid-range baritone male voice is an example.

    16. Meiosis

    cell division in specialized tissues of ovaries and testes which results in the

    production of sperm or ova. Meiosis involves two divisions and results in

    four daughter cells, each containing only half the original number of

    chromosomes--23 in the case of humans.

    17. Mendelian genetics

    inheritance patterns which can be explained by simple rules of dominance

    and recessiviness of genes.

    18. Phenotype

    the observable or detectable characteristics of an individual organism--the

    detectable expression of a genotype.

    19. Principle of independent assortment

    Gregor Mendel's second principle of genetic inheritance. It states that

    different pairs of genes are passed to offspring inde