Introduction to Genetics

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Introduction to Genetics

• Biology
• Chapter 11

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11-1 The Work of Gregor Mendel
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Into. To Genetics

• Genetics - the scientific study of heredity
• Gregor Mendel
• Austrian monk
• Considered the father of genetics
• The first person to succeed in predicting how
  traits would be transferred from one generation
  to another
• using the garden pea plant

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Parts of a Flower
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Gregor Mendels Peas

• Mendel used true-breeding pea plants
• Offspring is identical to the parents
• Fertilization
• When the male gamete unites with the female
  gamete
• Self-pollination produce true-breeding pea
  plant
• Cross-pollination dusting pollen from one plant
  onto the pistil or another plant

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Types of Pollination
Cross-pollination
self-pollination
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Genes and Dominance

• Trait a specific characteristic, such as seed
  color or plant height, that varies from one
  individual to another
• Hybrid - the offspring of parents that have
  different forms of a trait, such as tall and
  short
• Monohybrid cross (mono one)
• The two parent plants differed by a single trait
  height
• P1 parent generation
• F1 first generation
• F2 second generation

P parent F filial son or
daughter
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Genes and Dominance
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Genes and Dominance

• The First Generation
• Crossed 2 true breeding plants
• 1 tall and 1 short
• All offspring of the 2 parent plants were tall

P1
X
F1
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Genes and Dominance

• Mendel came to two conclusions
• Factors that are passed from one generation to
  the next determine inheritance.
• Scientist call the factors that determine these
  traits genes.
• The different forms of a single gene are called
  alleles. (ex tall short)
• The Principle of Dominance - states that some
  alleles are dominant and some are recessive.
• An organism with a dominant trait will always be
  dominant
• An organism with a recessive trait will only show
  up when a dominant allele is not present

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Dominate and Recessive
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Segregation

• Where were the recessive alleles?
• Mendel self-pollinated the plants from the first
  generation
• Segregation The separation of alleles during
  the formation of gametes (sex cells).
• ¾ the offspring were tall
• ¼ the offspring were short
• 31 ratio tall to short

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Segregation

• Law of segregation
• During fertilization, male and female gametes
  randomly pair to produce 3 combinations of
  alleles.
• Concluded that each plant in the F1 generation
  carried one dominate allele and one recessive
  allele and the F2 generation either received 2
  dominate 2 recessive or one of each

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Segregation

• Recording the results for crosses
• Dominate allele is always written first
• Uppercase letter is used for dominate
• T tall
• Lowercase letter is used for recessive
• t short

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Segregation
X
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11-2 Probability and Punnett Squares
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Genetics and Probability

• Probability likelihood that a particular event
  will occur
• Flipping a coin 2 possible outcomes (head or
  tails) with each flip there is a 50 chance it
  will be heads or tails
• The principles of probability can be use to
  predict the outcomes of genetic crosses.

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Punnett Squares

• Punnett square can be used to predict and
  compare the genetic variations that will result
  from a cross.
• Homozygous organisms that have two identical
  alleles for a trait (TT or tt) true-breeding
• Heterozygous organisms that have two different
  alleles for a trait (Tt) these are hybrid

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Punnett Squares

• Phenotype
• The way an organism looks or behaves
• What you see (i.e. tall, green, etc.)
• Genotype
• The gene combination an organism contains
• The genetic makeup (i.e. TT, tt and Tt)

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Probability and Segregation

• If you self-pollinated a heterozygous organisms
  it would result in
• Genotypic ratio
• ¼ will be TT (both tall alleles) 2/4 will be Tt
  (one of each allele) ¼ will be tt (both short
  alleles)
• Phenotypic ratio
• 31 (3/4 tall and ¼ short)

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Probability and Segregation

• White and purple garden pea flowering plants
• Purple is dominate (P)
• White is recessive (p)

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Homozygous Dominate Cross
Cross Purple (PP) X Purple (PP)
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Homozygous Recessive Cross
Cross White (pp) X White (pp)
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Heterozygous Cross
Cross Purple (Pp) X Purple (Pp)
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Monohybrid Cross
Heterozygous tall parent Tt Cross Tt X Tt
?
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Monohybrid Punnett Square
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11-3 Exploring Mendelian Genetics
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Independent Assortment

• Independent assortment
• Genes from different traits are inherited
  independently of each other helps account for
  the many genetic variations
• Ex. A pea plant that is RrYy, the alleles will
  separate and the traits will separate

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Independent Assortment

• Dihybrid crosses - cross where the peas differ in
  2 traits
• A cross involving two traits
• Ex. Seed color and Seed shape

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Independent Assortment

• The First Generation
• Two true breeding plants (P1)
• RRYY round yellow seed (homozygous
  dominate)
• rryy wrinkled green seed (homozygous
  recessive)
• When they were crossed all the plants had round
  yellow seeds (F1)

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Independent Assortment
Dihybrid Cross round yellow (RRYY) X wrinkled
green (rryy)
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Independent Assortment

• The Second Generation (F2)
• Self-pollinated plants from the first generation
• Resulted in 9 round yellow, 3 round green, 3
  wrinkled yellow, 1 wrinkled green
• A ratio of 9331

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Independent Assortment
Heterozygous Cross round yellow (RrYy) X round
yellow (RrYy)
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Independent Assortment
Heterozygous round yellow seed parents
RrYy Cross RrYy X RrYy
?
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Independent Assortment
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Independent Assortment
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A Summary of Mendels Principles

• The inheritance of biological characteristics is
  determined by individual units known as genes.
  Genes are passed from parents to their offspring.
• In cases in which two or more forms (alleles) of
  the gene for a single trait exist, some forms of
  the gene may be dominant and others may be
  recessive.
• In most sexually reproducing organisms, each
  adult has two copies of each geneone from each
  parent. These genes are segregated from each
  other when gametes are formed.
• The alleles for different genes usually segregate
  independently of one another.

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Beyond Dominant and Recessive Alleles

• Some alleles are neither dominant nor recessive,
  and many traits are controlled by multiple
  alleles and multiple genes

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Beyond Dominant and Recessive Alleles

• Incomplete dominance
• Cases in which one allele is not completely
  dominant over another
• The phenotype of the heterozygous is intermediate
  between those of the two homozygotes
• Ex. Color of snapdragons

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Beyond Dominant and Recessive Alleles

• Codominance
• both alleles contribute to the phenotype
• Cause the phenotype of both alleles to be
  expressed equally

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Beyond Dominant and Recessive Alleles

• Multiple alleles
• Traits controlled by more than two alleles
• In Labrador retriever, coat color is determined
  by one gene with four different alleles.

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Beyond Dominant and Recessive Alleles

• Polygenic traits
• The inheritance pattern of a trait that is
  controlled by two or more genes
• Genes may be on same or different chromosomes
• Each allele intensifies or diminishes the
  phenotype
• Example Eye color skin color

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