Medelian Genetics

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Chapter 8

• Medelian Genetics

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Important Terms
Free Lobe
Attached Lobe

• A trait is a variation of a characteristic.
• Example attached earlobes vs. free earlobes
• Characteristic is earlobe attachment trait is
  attached or free
• Heredity is the transmission of characteristics
  from parents to offspring.
• Genetics is the branch of Biology that studies
  heredity.

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Gregor Mendel

• Gregor Mendel - father of modern genetics
• Born 1822 - Austria monk
• One of the first to use a quantitative approach
• Mendel performed genetic experiments and
  accurately predicted patterns of heredity in pea
  plants
• Later scientists found that traits were
  determined by genes encoded in DNA

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Mendels Garden Peas

• Why the garden pea?
• It can self-pollinate
• Many varieties that grow quickly and easily
• Many characteristics with readily observable
  contrasting traits

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Self-pollinating and Crosses

• A cross refers to mating of two individuals
• In cross-pollination, one plant fertilizes a
  different plant when pollen from one plants
  flowers is transferred to a flower on the other
  plant
• Self-pollination involves a
• flower fertilizing itself.

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Monohybrid Crosses

• A monohybrid cross refers to a cross that
  involves only one pair of contrasting traits
• Mendel performed his experiments in three steps
• 1. He produced a true-breeding parent (P)
  generation to ensure that a cross with
  individuals with the same trait would produce
  only offspring with that trait

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Monohybrid Crosses

• 2. Mendel crossed two P generation plants with
  contrasting traits
• Ex. Purple flower plant with White flower plant
• Offspring of this cross are called the F1
  Generation (First filial)
• He then recorded the number of F1 individuals
  expressing each trait

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Monohybrid Crosses

• 3. Mendel then allowed each F1 individual to
  self-pollinate
• These offspring of are called the F2 generation
• He then counted the number of F2 individuals
  expressing each trait

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Mendels Results

• P generation expressed both forms of the trait
• Some purple flower plants, some white flower
  plants
• F1 generation expressed only ONE form of the
  trait
• Only purple flower plants
• The missing trait
• reappeared in the F2
• generation!
• White flower plants and purple plants both present

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Mendels Hypotheses

• 1. For each trait, there are 2 different forms of
  the trait (gene)
• These forms are called alleles
• Tall Plant (T) vs. Short Plant (t)
• The physical appearance of the organism is called
  its phenotype
• The genetic make-up (alleles present) of an
  organism is called its genotype

Genotype pp Phenotype White Flowers
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Mendels Hypotheses Continued

• For each trait, an individual receives two copies
  (alleles)
• of the gene for that trait
• One copy from the mother
• One copy from the father
• If the two alleles for a trait are identical,
  individual is homozygous.
• If two alleles are different, individual is
  heterozygous.

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Mendels Hypotheses Continued

• When an individual has two different alleles for
  a trait, only one allele is expressed.
• The expressed form of the trait is dominant
• The form of the trait which is not expressed is
  called recessive

A heterozygous individual produces only purple
flowers
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Mendels Hypotheses Continued

• 4. Law of Segregation
• When gametes are formed, the alleles for each
  gene separate (segregate) from
• one another during Meiosis I
• Each gamete receives one
• allele for each trait (gene)
• During fertilization, the
• zygote receives one allele from the sperm and one
  allele from the egg for each trait

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Laws of Heredity

• Mendels Hypotheses are sometimes referred to as
  the laws of heredity
• Law of Segregation
• Law of Independent Assortment The inheritance
  of one characteristic (height) does not affect
  the inheritance of another characteristic (flower
  color)
• Different genes separate independently of one
  another during gamete formation
• We now know that this is only true of genes which
  lie on different chromosomes

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Punnett Square

• A Punnett square is a diagram that predicts the
  outcome of a genetic cross by considering all
  possible combinations of gametes in the cross.
• The possible gametes of one parent are written
  across the top of the square.
• The possible gametes of the other parent are
  written along the side of the square.

Parent 1
P a r e n t 2
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Predicting Results of Monohybrid Crosses

• Punnett squares are used to calculate the
  probabilities of outcomes resulting from a
  genetic cross.

The Punnett square predicts that the two alleles
for flower color will randomly pair in the same
proportions as the two sides of the pennies.
This grid predicts the probabilities of certain
combinations of the two sides (alleles) of two
pennies.
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Monohybrid CrossesHomozygous x Homozygous

• A monohybrid cross between a homozygous dominant
  and a homozygous recessive individual results in
  all heterozygous (hybrid) individuals.

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Monohybrid CrossesHomozygous x Heterozygous

• A monohybrid cross between a homozygous and
  heterozygous individual yields homozygous
  dominant and heterozygous offspring with a 11
  genotypic ratio and a 10 phenotypic ratio.

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Monohybrid CrossesHeterozygous x Heterozygous

• A monohybrid cross between two heterozygous
  individuals yields homozygous dominant,
  heterozygous and homozygous recessive offspring
  with a 121 genotypic ratio and a 31 phenotypic
  ratio.

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Test Cross Determining Unknown Genotypes

• How can you determine whether an organism that
  displays a dominant phenotype is homozygous (PP)
  or heterozygous (Pp)?
• Perform what geneticists call a testcross (or
  back cross).

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Probability and Punnett Squares

• Probability is the likelihood that a specific
  event will occur.
• They can be expressed in words, in decimals,
  fractions or percentages
• Probability of 1 kind of possible outcome
• total of all possible
  outcomes
• The possibility that a coin tossed will land on
  heads one possible outcome
• The total number of all possible outcomes 2
  heads or tails

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Mendels Principle of Independent Assortment

• Mendel also crossed plants that differed in two
  characteristics, such as height and seed color.
• This is an example of a dihybrid cross.

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Dihybrid Crosses

• To determine the possible gametes in a dihybrid
  cross, use the FOIL rule.
• Then place all possibilities for one parent
  across the top and all possibilities for the
  other parent along the side of the Punnett square.

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Dihybrid Crosses Homozygous x Homozygous

• A dihybrid cross between two homozygous
  individuals with contrasting traits yields
  heterozygous offspring expressing the dominant
  trait.

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Dihybrid CrossesHeterozygous x Heterozygous

• A dihybrid cross (F2)
• between two heterozygotes yields offspring in a
  phenotypic ratio of 9331
• 9 dom both traits (R_Y_)
• 3 dom one trait (R_yy)
• 3 dom other trait (rrY_)
• 1 rec both traits (rryy)