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CHAPTER 8. Mendel and Heredity. I. The Origins of Genetics:. A. Terms Trait : A characteristic determined by a genes INHERITANCE - The passing of traits by heredity HEREDITY - The transmission of traits from parents to offspring. B. The History of Genetics. Gregor Mendel :

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Chapter 8


Mendel and Heredity

Chapter 8

I. The Origins of Genetics:

A. Terms

  • Trait:

    A characteristic determined by a genes


    The passing of traits by heredity


    The transmission of traits from parents to offspring

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B. The History of Genetics

Gregor Mendel:

  • An Austrian monk in the mid-1800’s

  • Studied math and science

  • “Father of Genetics”

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  • Mendel made observations within the monastery's garden.

  • Noticed that in pea plants, seven specific traits could be seen.

  • Mendel was interested in testing the predictability of the traits

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C. Mendel’s Breeding Experiments

  • Crossed pea plants with purple flowers with plants with white flowers

  • Mendel counted all offspring and analyzed the data

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The Genetic Work of Mendel

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Traits Expressed as Simple Ratios

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D. Monohybrid Crosses

  • Cross involving only one pair of contrasting traits.

    STEP 1:

  • Produce TRUE BREEDING strains

  • Ex: Purple plants only produce purple offspring

  • These plants make up the “P Generation” (parental)

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Step 2:

  • Cross-pollinate two P generation plants with contrasting traits

  • F1- First Filial Generation:

  • The offspring of the P generation

  • Record the number of F1 plants with each trait

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  • F1plants self-pollinate

  • The offspring are called F2 Second Filial

  • Each plant was counted

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E. Mendel’s Results

  • F1 showed only one form of the trait

  • Missing trait reappeared in some of the F2 generation

  • 3:1 ratio in the F2 generation

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Mendel’s Work

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II. Mendel’s Theory

A. Mendel’s 4 Hypotheses:

1. For each trait an individual has 2 copies of the gene (one from each parent)

2. There are alternative versions of genes called ALLELES

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Mendel’s Hypotheses Continued

  • Dominance and Recessiveness:

  • One factor isDominant, and hides, the other factorRecessive

  • Segregation:

  • Theeggandspermcarry only one of the possible alleles

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B. Mendel’s Findings in Modern Times

  • Dominant-

    First letter of the trait in capital (P- purple),

  • Recessive-

    First letter of the dominant trait in lower case (p- white)

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Homozygous vs. Heterozygous

  • Homozygous: 2 alleles are the same

    - Homozygous Dominant(PP) ”Pure”

    - Homozygous Recessive(pp)

  • Heterozygous: alleles are different (Pp)

    - Display the characteristics of the dominant trait

  • A genetic trait that appears in every generation of offspring is called dominant.

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Genotype vs. Phenotype


  • The genetic makeup of an organism


  • External “physical” appearance of an organism(trait)

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C. The Laws of Heredity

1. Law of Segregation:

  • The 2 alleles for a trait segregate when gametes are formed

    2. Law of Independent Assortment:

  • Dominant factorsdon’talways go with other dominant factors

  • Recessive factorsdon’talways go with other recessive factors

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III. Studying Heredity

A. Punnet Squares

  • Diagram that predicts the outcome of a monohybrid cross

  • Four Boxes in a square

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Monohybrid Crosses

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Simulating a Dihybrid Cross

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Dihybrid Crosses:

compares 2 contrasting traits at one time

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Test Crosses

  • Used to determine the genotype of an individual

  • A procedure in which an individual of unknown genotype is crossed with a homozygous recessive individual

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B. Probability:

  • Likelihood that a specific event will occur

    Probability = # of one kind of outcome total # of all possible outcomes

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C. Inheritance of Traits

Pedigree :

  • History of how a trait has been inherited over several generations

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1. Autosomal Traits

  • Autosomal will appear in both sexes equally

  • It affects a chromosome other than the 23rd pair

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-Traits with alleles found on the “X” chromosome

-Males are more likely to express recessive sex-linked alleles because the single “X” has nothing to hide/mask the trait

  • Red/Green most common form of

  • color deficiency

    Ex: Colorblindness


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-A trait that is expressed because of the presence of male or female sex hormones

-Alleles that code for these are on the autosomes

-Both males and females have same allele, but the trait is only expressed in one of the sexes

Ex: Pattern Baldness

B = dominant in males

B’ = recessive in females

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IV. Complex Patterns of Inheritance

A.) Polygenic Traits:

-Trait that is controlled by two or more genes.


  • skin color

  • eye color

  • fingerprints

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B. Incomplete Dominance

  • Individual displays a phenotype intermediate “blending of genetic information” between the two parents

    Ex: Red Snapdragon + White Snapdragon = Pink Snapdragon

    Ex: Straight hair + Curly hair = Wavy hair

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C. Multiple Alleles

  • Genes with 3 or more alleles

    EX: Blood … A-B-O

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D. Codominance:

  • Two dominant alleles are expressed at the same time

  • Ex: IA + IB = AB blood type

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  • Genetic Disorders

Mutation: A change in a gene due to damage or being copied incorrectly.

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  • Change in the shape of the RBC:

  • Results in anemia, clogs, pain, decreased blood flow, death….

  • Primarily effects the black population

  • 1 in 500 births in USA

    A = Normal

    A’ = Sickle Cell

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  • Dominant Disorder

  • 30-40’s, forgetfulness, loss of muscle control,mental illness, death

  • genetically programmed degeneration of brain cells

    *Can be detected by identifying short sections of DNA known as GENETIC MARKERS

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  • Recessive Disorder

  • fatty substance accumulate in the nerve cells of the brain

  • A child becomes blind, deaf, and unable to swallow. Muscles begin to atrophy and paralysis sets in

  • Children usually die by age 5

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  • Body produces an abnormally thick, sticky mucus

  • Faulty transport of salt within cells of organs such as the lungs and pancreas

  • very salty-tasting skin;

  • persistent coughing,

  • wheezing or pneumonia;

  • excessive appetite but poor weight gain

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  • 1 in 2500 people are born with it

  • More than 10 million people

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  • Failure of chromatids to separate during Meiosis

  • Results in an egg or sperm with extra or missing chromosomes

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Condition where the zygote has only 45 chromosomes


  • Turner Syndrome (XO)


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  • A condition where the zygote has 47 chromosomes

  • Examples:

  • Supermale (XYY)

  • Klinefelters Syndrome (XXY)

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-DOWN SYNDROME (trisomy) - Extra 21

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PATAU SYNDROME (trisomy 13)

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  • Genetic Screening-

  • The examination of a persons genetic make-up…

  • especially if they have a history of problems in their family

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  • Tiny camera into the uterus

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  • Picture of a persons chromosomes

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  • Removal of amniotic fluid

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  • High frequency sound waves = picture

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