In this lesson, you will learn how to predict the probable genetic makeup and appearance of offspring resulting from specific crosses.

1
Genetic Crosses

• In this lesson, you will learn how to predict the
  probable genetic makeup and appearance of
  offspring resulting from specific crosses.

2
Standards

• Demonstrate an understanding of the key features
  of DNA, genes, and chromosomes, and the
  relationship that exists among them.
• Make predictions concerning inheritance based on
  Gregor Mendels laws of heredity.

3
Essential Question

• How can we predict the possible traits of an
  offspring, considering the traits of parental
  generations. ?

4
Objectives This is what you are expected to know

• Explain how probability is used to predict the
  results of genetic crosses.
• Use a Punnett square to predict the results of
  monohybrid and dihybrid genetic crosses.
• Explain how a testcross is used to show the
  genotype of an individual whose phenotype is
  dominant.
• Differentiate a monohybrid cross from a dihybrid
  cross.

5
Before we get started

• We need to understand the meaning of some
  important genetic terms. The slides that follow
  will explain this.
• If you want to practice, click on the link below
  to go to a web site that has flash cards of
  genetic words.
• Flash cards

6
Relationship between Gene and Allele

• Gene - a segment of DNA that controls a specific
  trait.
• Allele - the alternate (or contrasting) form of a
  gene.

7
Difference between Dominant and Recessive

• Dominant - refers to an allele that masks the
  expression of another allele for the same trait.
• Recessive - the allele that is masked by the
  presence of another allele for the same trait.
• Unattached (right) earlobes is dominant to
  attached earlobes (left).
• A widows peak (left) is a dominant trait over a
  rounded face.

8
More examples

• To see more dominant and recessive traits, click
  here.

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Difference betweenHomozygous and Heterozygous

• Heterozygous - alleles that are mixed, (dominant
  and recessive) showing the dominant trait.
• Homozygous - alleles that are the same, dominant
  for the trait or recessive for the trait.

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Difference betweenGenotype and Phenotype

• Genotype - genetic makeup of an organism refers
  to the alleles for a trait.
• Phenotype - physical or outward expression of the
  alleles for that trait. (What it looks like)
• Genotype codes ( or determines) for phenotype

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More About Genotype

• The genetic makeup of an organism is called its
  genotype.
• It consists of the alleles that the organism
  inherits from its parents.
• Alleles are designated with letters of the
  alphabet.
• Dominant alleles are capital letters (P for
  purple flower color).
• Recessive alleles are lower case letters (p for
  white flower color).

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More about Phenotype

• The outward (or physical) appearance of an
  organism is called its phenotype.
• It is an expression of the genotype of an
  organism.
• For example purple flower color or white flower
  color.
• There are two ways that a dominant phenotype can
  be expressed.
• heterozygous containing both the dominant and
  recessive alleles (Purple flower color Pp)
• homozygous dominant containing two alleles for
  the dominant trait. (Purple flower color PP).
• There is only one way that a recessive trait can
  be expressed.
• homozygous recessive - containing only the
  recessive alleles (white flower color pp).

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Probability

• Predicts the likelihood that a specific event
  will occur.
• May be expressed a a decimal, a percentage, or a
  fraction.
• Determined by the following formula
• Probability number of times an event
  is expected to happen
• number of opportunities for an event to
  happen

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Why probability is important to genetics

• Mendel used probability to determine how likely
  the dominant trait would appear over the
  recessive trait.
• The yellow pea appeared 6,022 times in the F2
  generation. The green pea appeared 2,001 times.
• The total number of individual was 8023
  (60222001)
• Using the formula
• 6022 8023 0.75
• 2001 8023 0.25
• Percentage 75 green peas 25 yellow peas
• Ratio 31 ratio of yellow to green peas
• Fraction 1/4 chance of green peas and 3/4 chance
  of yellow peas

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Results of the F1 generation

• PP Pp x 2 pp
• ratio 121 or 31 purple to white
• 75 purple to 25 white

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F1 generation yielded 100 purple flowers,
heterozygous for the purple trait.

• white (pp) x purple(PP)
• yields
• 100 purple flowers that are heterozygous for
  the purple flower trait (Pp).

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Punnett Square

• A diagram used to predict the probability of
  certain traits by offspring.
• The following examples will illustrate the
  outcome of different types of crosses.

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How to set up and work a Punnett square

• Draw a four-square box.
• Place one set of alleles on the side of the box
  as shown at right.

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How to set up and work a Punnett square

• One set of alleles for a trait go on top of the
  box (usually male) and the other set of alleles
  go on the side of the box.
• Each letter from the set of alleles is placed on
  top of the square.

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Filling in the boxes

• Fill in the top left box with the alleles from
  top left and upper left.
• The dominant letter is placed first.

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Filling in the boxes

• The second box gets the top left and bottom left
  allele

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Filling in the boxes

• The third box gets the top right and the top left
  letters
• Remember that the capital letter goes first.

23
Filling in the boxes

• The fourth box gets the top right and the lower
  left letter.

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Ok. So what does this mean?

• Each box represents a possible zygote.
• The alleles are for a single trait, in this case
  T is tall and t is short.
• Tt is the genotype for a heterozygous tall.
• tt is the genotype for homozygous recessive
  short.
• From this cross, 50 of the offspring will be
  tall and 50 will be short. This is the
  phenotype.

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Lets apply this to Mendels experiment.

• Two homogeneous parental generations were crossed
  to yield the F1 generation.
• The results were 100 purple flowers,
  heterozygous for the trait (Pp).

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Lets apply this to Mendels experiment.

• Two heterozygous F1 generations were
  self-pollinated.
• The results were 25 heterozygous purple flowers
  50 homozygous purple flowers, and 25 white
  flowers (homozygous recessive)
• This 31 ratio hold true for all heterozygous
  monohybrid crosses!

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Genetic Crosses

• Genetic crosses are used to predict the
  probability of offspring resulting from the union
  of sperm and egg.
• Types of crosses
• Monohybrid cross - cross between one pair of
  contrasting traits.
• Dihybrid cross - cross between two pairs of
  contrasting traits.
• Test cross - an unknown genotype is crossed with
  a homozygous recessive individual.

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Examples of Monohybrid Genetic Crosses

• Homozygous x Homozygous
• Heterozygous x Heterozygous
• Homozygous x Heterozygous
• Testcross
• Incomplete Dominance
• Codominance

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Homozygous x Homozygous pp x
PP

• This Punnett represents Mendels P1 generation
• The recessive alleles for white flowers (pp) are
  crossed with the homozygous dominant purple
  flower (PP)
• All of the offspring are heterozygous (Pp) and
  show the dominant trait of purple.
• Genotype 100 Pp
• Phenotype 100purple flower color

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Heterozygous x Heterozygous Bb
x Bb

• This is an example of Mendels F2 generation that
  shows 75 dominant and 25 recessive trait (31
  ratio).
• This cross represents a cross between two
  heterozygous black haired rabbits (brown hair is
  the recessive trait).
• Genotype 25 BB 50 Bb 25 bb or 121 ratio.
• Phenotype 75 black hair and 25 brown hair (31
  ratio).

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Homozygous x Heterozygous BB
x Bb

• This cross represents a homozygous dominant
  allele for black coat (BB) crossed with a
  heterozygous allele for black coat (Bb)
• Genotype 50 of the offspring are homozygous
  dominant (BB) and 50 are heterozygous (Bb)
• Phenotype 100 black coat.

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Test Cross

• Useful when you want to determine whether a trait
  is homozygous or heterozygous for the trait.
• An unknown genotype is crossed with a homozygous
  recessive individual.
• Left If no recessive traits appear, then the
  unknown genotype if most likely homozygous for
  the trait.
• Right If any of the offspring show the recessive
  trait, then the unknown genotype is likely
  heterozygous for the trait.

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Incomplete Dominance

• Occurs when two or more of the alleles influence
  phenotype, resulting in a phenotype intermediate
  between the dominant and recessive trait.
• The heterozygous individual (Rr) shows the
  intermediate trait of pink.
• RR (homozygous dominant) is red flower
• rr (homozygous recessive) is white flower color.

34
Codominance

• Occurs when both alleles for a gene are expressed
  in heterozygous offspring.
• Neither the dominant or recessive allele is
  dominant, nor do they blend in phenotype.

35
Dihybrid Crosses

• Cross between individuals that involves two pairs
  of contrasting traits
• Four alleles allows for 16 possible combinations
  of alleles. (16 box Punnett square)
• Four combinations of alleles can be determined by
  using the foil method of distribution. YyTt
• First pair of alleles YT (dominant )
• Outer pair of alleles Yt (heterozygous)
• Inner pair of alleles yT (heterozygous)
• Last pair of alleles yt (recessive)

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Dihybrid Crosseshomozygous x homozygous

• The example at right crosses two homozygous
  monohybrid traits
• This is representative of a dihybrid cross of
  Mendels P generation
• Notice that all of the offspring are heterozygous
  (RrYy) for the dominant trait-- yellow (R) and
  smooth (Y)

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Dihybrid Crossheterozygous x heterozygous

• RrYy is a heterozygous trait for yellow, smooth
  peas
• This represents Mendels cross of the F1
  generation, with two traits.
• Using the foil method to determine possible
  gametes, the choices are RY, Ry, rY, and ry
• After placing the allele combinations along the
  top and side, you follow the basic rule for
  combining alleles, remembering to place capital
  letters first, and like combinations of alleles
  together.

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Dihybrid Crossheterozygous x heterozygous

• Yellow color ( R ) is dominant to green ( r )
  yellow green
• Round ( Y ) is dominant to wrinkled ( y )
• The possible combinations are
• 9/16 - round and yellow seeds (genotype RRYY,
  RRYy, RrYY, RrYy)
• 3/16 - round, green seeds (genotype Rryy, Rryy)
• 3/16 wrinkled,yellow seeds (genotype rrYY, rrYy)
• 1/16 wrinkled, green seeds (genotype rryy)
• The ratio of 9331 holds true for every
  dihybrid heterozygous cross!
• Nine different genotypes and four different
  phenotypes.

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Study Questions

• Explain the difference between the following
  terms
• self-pollination, cross-pollination
• F1,F2 generation
• pure, hybrid
• dominant, recessive
• law of segregation, law of independent assortment
• gene and allele
• genotype and phenotype
• homozygous and heterozygous
• monohybrid, dihybrid cross
• Complete dominance, incomplete dominance,
  codominance

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Genetic Problem

• Assume that black hair is dominant to brown hair.
  Cross a heterozygous black haired trait with a
  homozygous recessive brown hair trait.
• Draw a Punnett square and predict the offspring.
• Give the percentages and ratios of the phenotype
  and genotypes of the offspring.

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Questions

1. What is the ratio of a dihybrid cross between two
   heterozygous traits?
2. In an dihybrid cross between two heterozygous
   parents, what is the probability of obtaining an
   offspring that is homozygous for both traits?
3. What is the most likely explanation for two
   parents with dominant phenotypes producing
   offspring with a recessive phenotype?
4. You cross a red-flowering plant with a
   yellow-flowering plant and notice that some of
   the offspring have orange flowers. What is the
   most likely explanation for this occurrence?
5. Explain the difference between the P generation,
   F1 generation, and F2 generation.

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Questions

• When the dominant and recessive traits are known,
  why is it not necessary to use the term
  homozygous when referring to the genotype of an
  individual with a recessive phenotype?
• In pea plants, smooth texture is dominant over
  wrinkled texture. A gardener has a pea plant that
  produces smooth seeds. How can the gardener
  determine whether the plant is homozygous or
  heterozygous for allele that determines seed
  texture?
• In rabbits, the allele for black coat color (B)
  is dominant over the allele for brown coat color
  (b). Predict the results of a cross between a
  rabbit homozygous for black coat color (BB) and a
  rabbit homozygous for brown coat color (BB).

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The End!