Mendels Genetic Patterns

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Mendels Genetic Patterns
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History of Genetics

• 1. DNA is the molecule that stores genetic
  information
• Griffith- discovered bacterial transformation
• Avery- showed that DNA transforms bacteria
• Hershey and Chase-proved that DNA carries
  genetics, not protein

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• 2. DNA stores genetic information in long strands
  of nucleotides arranged in an inherited, specific
  order.
• Genes are segments of DNA
• Genes are passed to offspring in egg and sperm
  cells
• -Egg an sperm each carry one complete set of
  genes (N N 2N)

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New Question Are there observable patterns to
genetic inheritance?

• Gregor Mendel
• 19th century Austrian monk
• Studied pea plants in monastery garden
• Applied statistics to 20 years of data and
  noticed patterns of inheritance in peas
• Known as the father of genetics

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Mendels Observations

• 1. Some genetic traits are dominant, while others
  are recessive
• 2. The two copies of a trait in each parent
  segregate, and are NEVER passed on together to
  the offspring.
• 3.Traits from the male and female parents mix and
  recombine in the offspring independent of each
  other

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Mendels Law of Segregation

• 1. Each parent has two genes for each trait.
• This makes them diploid
• Symbol 2N
• 2. Male parent produces sperm cells
• Sperm cells are haploid
• Symbol N
• 3. Female parent produces egg cells
• Egg cells are haploid
• Symbol N

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• 4. Fertilization combines egg and sperm
• N N 2N
• 2N zygote produced

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Mendels Law of Dominance

• 1. Purebred always produces offspring that look
  like the parents. Both parents must be purebred
  to produce 100 purebred offspring.
• Purebred tall plant x purebred tall plant 100
  purebred tall offspring.
• 2. Hybrid result from crossing opposite
  purebred parents.
• Purebred tall x Purebred short 100 hybrid tall
  offspring.

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• 3. Mendel noticed that DOMINANT traits always
  show in hybrids.
• Tall is dominant to short
• Short is recessive
• Smooth seeds dominant to wrinkled seeds
• Wrinkled seeds are recessive
• Yellow seeds dominant to green seeds
• Green seeds are recessive

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• This pattern is called Complete Dominance.

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Punnett Squares

• A way to predict probability of future offspring
  traits.
• Capital letters dominant trait
• Lowercase letters recessive traits
• Column to left possible male gametes
• Row across top possible female gametes
• Boxes inside possible offspring

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Practice Problem 1

• In pea plants, tall is dominant to short. What is
  the probability of short offspring if a purebred
  tall and a hybrid tall pea plant are crossed?

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Practice Problem 2

• In pea plants, tall is dominant to short. What is
  the probability of short offspring if two hybrid
  tall pea plants are crossed?

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Labeling generations

• The parental (first generation) is called the P1
  generation.
• The offspring from the P1 are called the F1
  generation.
• The offspring from the F1 are called the F2
  generation.

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Practice Problem 3

• A farmer crosses a purebred smooth-seeded pea
  plant with a purebred wrinkled-seed pea plant and
  gets offspring. He then inbreeds two of these
  offspring to produce the next generation. What is
  the probability that the last generation will
  have wrinkled seeds?

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Alleles

• The letters used in Punnett squares are called
  alleles.
• Alleles are actually genes
• One gene can have more than one allele
• T dominant allele gene that causes tallness
• T recessive allele gene that causes shortness
• Homozygous means alleles are the same
• Heterozygous means alleles are different

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Phenotype vs Genotype

• The phenotype of an organism is how it looks
• The genotype of an organism shows which alleles
  are actually present.
• If you know the genotype, then you know the
  phenotype

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Practice Problem 4

• What would be the genotype of a hybrid tall,
  hybrid yellow pea plant? (Use T for height and Y
  for seed color)

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Practice Problem 5

• A. What is the genotype of a hybrid tall, hybrid
  yellow pea plant? (Use T for height and Y for
  seed color)
• B. What is the genotype of a pea plant homozygous
  for the tall allele and heterozygous for the seed
  color allele?
• C. What is the genotype of a homozygous short and
  green pea plant?

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Human Phenotypes

• PTC Tasting Phenotype
• You are either a taster or a nontaster
• The allele for tasting PTC is dominant to the
  allele for nontasting.
• Nationally, about 55 of Americans are tasters.

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Practice Problem 6

• A. How many people are in your class?
• B. Of these how many are
• The taster phenotype
• Nontaster phenotype
• C. What is the genotype of a nontaster?
• D. What percentage of the class are
• Tasters
• nontasters
• E. What are the two possible genotypes of a
  taster?
• F. How might a taster figure out there real
  genotype?

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Test Crosses

• Test crosses reveal hidden genotypes in genetics
  problems
• To make a test cross, always cross the unknown
  with a purebred recessive.

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Practice Problem 7

• A farmer has a male black rabbit he bought on
  eBay cheap. He was told the rabbit is purebred,
  but wants to know for sure. (In rabbits the
  allele for black fur is dominant to the allele
  for brown fur)
• What are the two possible genotypes of this male
  rabbit?
• How can he prove his rabbit is purebred?

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Dihybrid Crosses

• Punnett squares can be used to predict offspring
  for more than one trait at a time.
• If two parents are crossed and they are hybrid
  for two traits, then a dihybrid cross is produced
• AaBb x AaBb
• Dihybrid crosses always produce a 9331 ratio
  of offspring phenotypes
• 9 A_B_ shows both dominant traits
• 3 A_bb shows one dominant trait and one
  recessive
• 3 aaB_ shows one dominant trait and one
  recessive
• 1 aabb shows two recessive traits

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Practice Problem 8

• A. What is the genotype of a hybrid tall, hybrid
  yellow-seeded pea plant?
• B. What offspring are possible if two hybrid
  tall, hybrid yellow-seeded pea plants are crossed?

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Practice Problem 9

• A. What is the genotype of a hybrid tall,
  purebred yellow-seeded pea plant?
• B. What offspring are possible if a hybrid tall,
  hybrid yellow-seeded pea plant is crossed with a
  hybrid tall, purebred yellow-seeded pea plant?

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Practice Problem 10

• Ttall, t short
• Yyellow seeds, y green seeds
• Aaxial flowers (along stem), a terminal
  flowers (at tip only)
• What are the possible offspring phenotypes for
  this cross?
• Tt YY AA x TT YyAa

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Practice Problem 11

• In mice eye color is determined by a single gene.
  Black eye color is dominant to red eye color.
• A. Complete a Punnett square showing a cross
  between a heterozygous male and a heterozygous
  female.
• B. What is the predicted ratio of red-eyed
  offspring to black eyed offspring?
• C. What is the predicted ratio of purebred
  black-eyed to hybrid black-eyed to red-eyed
  offspring?

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Practice Problem 12

• In guinea pigs, black fur is dominant to brown
  fur. A rough coat is dominant to a smooth coat.
• A. What are four possible genotypes for guinea
  pigs with black,rough coats?
• B. What percentage of the offspring will be
  smooth and brown if the parents are both hybrid
  black and rough?