Introduction to Genetics

1
Introduction to Genetics

• Chapter 11

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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
  vital.
• 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.

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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.

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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.

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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
  organism.

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Meiosis

• 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

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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
  crossing-over.
• This creates a new combination of genes and
  greatly increases genetic variety.

chiasma
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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

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Meiosis

• 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
  bodies
• - this process conserves cytoplasm because the
  sperm cell contributes no cytoplasm or
  organelles to the fertilized eggs.

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Comparing Mitosis Meiosis
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Comparison of Mitosis Meiosis
Mitosis Meiosis
Purpose
Start End of chromosomes
Cells genetically same/not
Cells produced
Produce more body cells
Produce sex cells/gametes
2N?2N
2N?N
Genetically different crossing over
Genetically identical
1 makes 4
1 makes 2
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Gregor Mendel

• Who was Gregor Mendel?
• He is the father of genetics.
• He was an Austrian monk who lived during the
  1800s.
• He was trained in math science.
• While working in his peas garden, he noticed that
  the plants displayed different traits.

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Gregor Mendel

• What pea traits did Mendel study?

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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
  mom

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Mendelian Vocabulary

• What does homozygous mean?
• Both parents contributed the same allele. (TT or
  tt)
• 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
  dominance
• What does recessive allele mean?
• This allele will only be expressed is homozygous.
  (tt short)

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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.

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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.

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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
  hybrid.

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Mendelian Laws

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

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Probability Punnett Squares

• Probability - The likelihood that an event will
  occur
• 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.

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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.

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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).

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Mendels Monohybrid Crosses
F1 Generation
P Generation
F2 Generation
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Mendels Laws

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

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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

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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

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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)

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Multiple Alleles

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

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Polygenic Traits

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

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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