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Introduction to Genetics


Introduction to Genetics Chapter 11 – PowerPoint PPT presentation

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Title: Introduction to Genetics

Introduction to Genetics
  • Chapter 11

DNA Basics
  • DNA contains the code for making proteins.
  • Proteins control the chemistry of life.
    Everything that occurs within a living thing is
    done through chemistry, thus making DNA/proteins
  • DNA is made of base pairs
  • A-T
  • C-G
  • 4. The sequence of A, T, C, and G determines the
    construction of the particular protein.
  • 5. DNA is inheritable. It is passed on from
    generation to generation.

Genes Chromosomes
  • What is a gene?
  • A segment of DNA that codes for a particular
    protein (trait).
  • Where are genes found?
  • On a particular location on a chromosome called
    a locus.

Genes Chromosomes
  • What does diploid mean?
  • 1. Chromosomes come in pairs.
  • 2. The pairs are called homologous chromosomes.
  • - one chromosome came from the mother
    (maternal chromosome)
  • - the other chromosome came from the father
    (paternal chromosome)
  • 3. The symbol for diploid is 2N or 2n.
  • 4. Sexual reproduction creates diploidy.

Genes Chromosomes
  • What does haploid mean?
  • The chromosomes do not come in pairs.
  • In humans, only the gametes (ovum sperm cell)
    are haploid.
  • Meiosis creates haploid cells in a diploid

  • Meiosis is also known as reduction division.
  • The daughter cells will have ½ the number of
    mother cell chromosomes.
  • How is this done?
  • - Replicate the DNA once
  • - Divide twice
  • The 1st division is known as Meiosis I
  • - crossing-over occurs here
  • The 2nd division is known as Meiosis II
  • - this is when the reduction occurs

Meiosis I
  • What is the coolest thing that occurs during
    Meiosis I?
  • Homologous chromosomes pair up in a process
    called synapsis and form a tetrad.
  • The homologous chromosomes exchange portions of
    their chromatids in a process called
  • This creates a new combination of genes and
    greatly increases genetic variety.

Meiosis II
  • What occurs during Meiosis II?
  • No DNA replication occurs during Interphase II!
  • When Meiosis II is complete, 4 haploid (n)
    daughter cells are produced.
  • Meiosis Animation

  • What is gametogenesis?
  • It is meiosis that creates gametes.
  • There are two kinds of gametogenesis
  • Spermatogenesis
  • - creates 4 viable sperm cells
  • Oogenesis
  • - creates 1 viable ovum and 3 unviable polar
  • - this process conserves cytoplasm because the
    sperm cell contributes no cytoplasm or
    organelles to the fertilized eggs.

Comparing Mitosis Meiosis
Comparison of Mitosis Meiosis
Mitosis Meiosis
Start End of chromosomes
Cells genetically same/not
Cells produced
Produce more body cells
Produce sex cells/gametes
Genetically different crossing over
Genetically identical
1 makes 4
1 makes 2
Gregor Mendel
  • Who was Gregor Mendel?
  • He is the father of genetics.
  • He was an Austrian monk who lived during the
  • He was trained in math science.
  • While working in his peas garden, he noticed that
    the plants displayed different traits.

Gregor Mendel
  • What pea traits did Mendel study?

Mendelian Vocabulary
  • What is a trait?
  • A characteristic of an organism.
  • What is a gene?
  • The genetic code for the trait.
  • What is an allele?
  • An alternative form of a gene
  • Example Trait height Alleles tall (T)
    short (t)
  • Alleles come in pairs one from dad, one from

Mendelian Vocabulary
  • What does homozygous mean?
  • Both parents contributed the same allele. (TT or
  • What does heterozygous mean?
  • Parents contributed different alleles. (Tt)
  • What does dominant allele mean?
  • This allele will always be expressed if present.
    (TT Tt are tall). a.k.a. principle of
  • What does recessive allele mean?
  • This allele will only be expressed is homozygous.
    (tt short)

Mendelian Vocabulary
  • What does genotype mean?
  • The alleles an organism has. Expressed in terms
    of recessive/dominant homozygous/heterozygous.
    (TT, Tt, tt)
  • What does phenotype mean?
  • Physical expression of the genotype. What an
    organism actually looks like.

Mendelian Vocabulary
  • What does true-breeding (purebred) mean?
  • The organisms genotype is homozygous. (TT or
    tt) All of the organisms gametes will have the
    same allele.
  • What does hybrid mean?
  • The organisms genotype is heterozygous. (Tt)
    The parents were two different purebreds. The
    organisms gametes could get either allele.

Mendelian Vocabulary
  • Putting it all together
  • Trait height Alleles tall (T), short (t)
  • Genotypes Phenotypes
  • TT (homozygous dominant) Tall
  • Tt (heterozygous) Tall
  • tt (homozygous recessive) Short
  • TT tt are true-breeding/purebred Tt is a

Mendelian Laws
  • The Law of Segregation
  • The alleles separate during meiosis.

Probability Punnett Squares
  • Probability - The likelihood that an event will
  • In a coin toss, there is a 1 in 2 chance of
    getting heads with each toss.
  • Punnett Square - diagram to show possible gene
    combinations from a genetic cross.

Mendels Laws
  • The one-factor (monohybrid) cross
  • Only one trait is studied
  • 1st parental generation (P1) are different
    true-breeding/purebred genotypes phenotypes.
  • The first generation of offspring (F1) are all
    hybrids expressing the dominant phenotype. F
    filial (Latin for son.

Mendels Laws
  • The one-factor (monohybrid) cross
  • Use the F1 generation as the second set of
    parents (P2).
  • This cross will create a phenotype ratio of 31
    (3 dominant phenotype 1 recessive phenotype).
  • This cross will create a genotype ratio of 121
    (1 homozygous dominant 2 heterozygous 1
    homozygous recessive).

Mendels Monohybrid Crosses
F1 Generation
P Generation
F2 Generation
Mendels Laws
  • The Law of Independent Assortment
  • This law applies to two-factor (dihybrid)
  • Alleles for different traits segregate
    independently of each other
  • FOIL (First, Outside, Inside, Last)
  • Each gamete gets one allele for each trait.

Mendels Laws
  • The dihybrid cross
  • The P2 are heterozygous for both traits
  • This will create a phenotype ratio of 9331
  • 9 dominant phenotype for both traits
  • 3 dom. pheno. for 1st trait rec. for 2nd
  • 3 rec. pheno. for 1st trait dom. for 2nd
  • 1 recessive phenotype for both traits

Incomplete Dominance
  • Four Oclock Plants
  • R red flower allele
  • W white flower allele
  • Neither allele is dominant or recessive.
  • Genotypes Phenotypes
  • RR red flowers
  • WW white flowers
  • RW pink flowers

Codominance both alleles contribute to the
phenotype 2 phenotypes present at same time
  • Human ABO Blood Types display this
  • IA type A codominant
  • IB type B codominant
  • i type O recessive
  • Genotypes/Phenotypes
  • IAIA IAO type A
  • IBIB IBO type B
  • IAIB type AB (universal recipient)
  • ii type O (universal donor)

Multiple Alleles
  • More than two alleles exists for the trait.
  • Rabbit fur is a good example

Polygenic Traits
  • These traits require the interaction of several
  • Human skin color is a good example.

Genes the Environment
  • The phenotype of an organism is only partially
    determined by the genotype, the environment can
    play a key role.
  • The western white butterfly is an example
  • Springtime hatchlings are darker (shorter days)
  • Summertime hatchlings are lighter (longer days)

Spring Hatchling
Summer Hatchling
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