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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

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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

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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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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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STEP 3:
  • 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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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

Genotype:

  • The genetic makeup of an organism

Phenotype:

  • 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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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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2. SEX-LINKED TRAITS

-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

Hemophilia

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3. SEX-INFLUENCED TRAITS

-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.

Examples:

  • 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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A. SICKLE-CELL ANEMIA
  • 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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B. HUNTINGTONS DISEASE
  • 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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C. TAY-SACHS DISEASE
  • 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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D. CYSTIC FIBROSIS
  • 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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D. CYSTIC FIBROSIS
  • 1 in 2500 people are born with it
  • More than 10 million people
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NONDISJUNCTIONS
  • Failure of chromatids to separate during Meiosis
  • Results in an egg or sperm with extra or missing chromosomes
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A. MONOSOMY

Condition where the zygote has only 45 chromosomes

Examples:

  • Turner Syndrome (XO)
  • CRI-DU-CHAT SYNDROME
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B. TRISOMY-
  • A condition where the zygote has 47 chromosomes
  • Examples:
  • Supermale (XYY)
  • Klinefelters Syndrome (XXY)
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DETECTING GENETIC DISORDERS
  • 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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Fetoscopy
  • Tiny camera into the uterus
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Karyotyping-
  • Picture of a persons chromosomes
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Amniocentesis-
  • Removal of amniotic fluid
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Ultrasound
  • High frequency sound waves = picture
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