Chapter 14: Mendelian Genetics PDF

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Chapter 14, page 1

Chapter 14: Mendel and the Gene Idea, Part I  What are the two basic explanations for heredity?  Blending-   genetic material from parents “blends” in the offspring  like two colors of paint in a bucket (yellow + blue = green)  A problem with this idea- over a long period of time in a population all of the traits would be blended and organisms would all look alike  Particulate- the gene idea   parents pass discrete heritable units to offspring in the form of genes  Like marbles or cards in a deck genes can be shuffled and sorted and passed on to offspring in undiluted form  Gregor Mendel, an Austrian monk,  discovered the basic principles of genetics by breeding garden peas in carefully planned experiments.  Mendel’s educational background:  High school education: basic education plus agriculture  Theological studies: Olmutz Philosophical Institute  Augustinian monastery at age 21- failed teacher exam  University of Vienna- Mendel was influenced by two professors:  Doppler- physicist and mathematician (Doppler effect)  Unger- botanist interested in variation in plants  Where and when did Mendel conduct his experiments? The garden at the monastery at around 1857 was the site of Mendel’s plant breeding experiments.

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Class Notes

Chapter 14, page 2

 Why did Mendel choose to work with garden peas for his studies?  They were available in many varieties.  Definitions:  What is a character? A heritable feature  Example: flower color  What is a trait? Each variant for a character  Example: purple or white  By using garden peas Mendel had strict control over which plants mated with which.  How did Mendel control this? In the diagram below, label the steps in one of Mendel’s genetic crosses of pea plants. 1. Reproductive parts of a flower: a. Male = stamen b. Female = carpel 2. Removed male flower part (stamen) 3. Pollen was dusted on female flower part (carpel) 4. Male and female parents could be planned and controlled  Mendel chose to track only those characters that varied in an either or rather than a more or less manner.

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Class Notes

Chapter 14, page 3

 Example: flowers were either purple or white (not in between)  Mendel also made sure he started his experiments with plants that were true breeding.  How did he ensure this? He allowed plants to selfpollinate to assure that the offspring were exactly like the parents for the trait he was studying.  Example: When self-pollinated plants with purple flowers always had offspring with purple flowers.  In a typical breeding experiment, Mendel would follow three generations of plants. Describe these three generations:  Generation 1 is called the P generation (parents).  This consists of pure-breeding plants being crossed.  Example: purple flowers and white flowers  Define hybridization- the crossing of pure-breeding parents  Generation 2 is called the F1 generation.  This consists of offspring of P generation.  These offspring are called hybrids.  Generation 3 is called the F2 generation.  This consists of offspring of F1 generation.  What would have happened if Mendel had stopped with two generations of crosses? He would not have discovered the principles of inheritance!  How many pea plants were involved in a single one of Mendel’s crosses? MANY (1,000 for a single cross)  Why is this important? He was able to see the patterns of different characters of plants in his experiments.

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Class Notes

Chapter 14, page 4

In the diagram below label the P, F1, and F2 generations of one of Mendel’s breeding experiments.

 P generation (pure-breeding parents) o pure purple X pure white  F1 generation (hybrids) o all purple

 F2 generation o 3 purple to 1 white

 By the law of segregation, the two alleles for a character are packaged into separate gametes.

In the diagram of the breeding experiment of 3 generations of flowers with purple and white color, several interesting things can be noted:  In the F1 generation, all flowers are all purple; none are white.  In the F2 generation, the flowers are both purple and white in a ratio of 3 purple flowers to 1 white flower.  Why did the white color seem to “disappear” in the F1 generation? The purple flower color was “dominant” over the white flower color.

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Class Notes

Chapter 14, page 5

 Define dominant: a trait that is fully expressed when both dominant and recessive traits are present (hybrid)  Usually represented as a capital letter  Example: P = purple flower color  Define recessive: a trait that is NOT expressed when both dominant and recessive traits are present (hybrid)  Usually represented as a lower case letter  Example: p = white flower color  Why did the white color reappear in the F2 generation? The trait for white color had been hidden in the F1 generation, but reappeared in the F2 generation when it was in plants without the dominant trait. Based on his pea crosses, Mendel developed a hypothesis for inheritance of traits in pea plants. What 4 important ideas did Mendel develop?  1. Alternative versions of genes account for variations in inherited characters.  Define allele- alternative versions of a gene  Example: flower color may be purple (P) or white (p) In the drawing below label the alleles on the chromosomes.

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Class Notes

   

Chapter 14, page 6

Homologous pair of chromosomes Allele for purple flower color (P) Allele for white flower color (p) Locus is the “location”

 2. For each character, an organism inherits two alleles, one from each parent.  Did Mendel know about chromosomes? NO!  How does this idea fit with our current understanding of chromosomes?  Diploid cells have two chromosomes  The alleles are each located on one of the chromosomes.  3. If the two alleles differ, then:  The dominant allele determines the appearance.  The recessive allele has no effect on appearance.  Example: F1 generation (hybrid = Pp) is all purple in appearance  See page 4 class notes  4. The two alleles for each character separate during gamete production. This is called Mendel’s Law of Segregation.  Thus, an ovum or a sperm gets only one of the two alleles that are present in a somatic cells of an organism.  How does this relate to meiosis? (For review see chapter 13.)

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Class Notes

Chapter 14, page 7

Recall that homologous chromosomes separate in meiosis I, so that an egg or sperm get only one chromosome of each type. If an organism has identical alleles for a particular character, what is the chance its egg or sperm cells will contain that allele? 100% Why? It is the only allele present.  Example: true-breeding white flowers always make white flowers. If an organism has different alleles for a particular character, what is the chance its egg or sperm cells will contain that allele? 50% Why? If two alleles are present, there is a 50/50 chance that an offspring will get either allele.  Example: If a plant has both purple and white color alleles (a hybrid), 50% of the eggs and sperm will get each allele.

 Some useful vocabulary:  genotype- genetic makeup of organism  phenotype- appearance of organism  homozygous- an organism with two identical alleles for a trait  Example: PP or pp  heterozygous- an organism with two different alleles for a trait  Example: Pp

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Class Notes

Chapter 14, page 8

 The inheritance of a single gene can be illustrated using a type of diagram called a Punnett square.  In the diagram below label the three generations: P, F1, and F2.  What would be the genetic makeup (genotype) of each plant?  What would be the appearance (phenotype) of each plant?

P generation: Genotype: PP and pp Phenotype: purple flowers X white flowers

F1 generation: Genotypes: all Pp Phenotype: all purple

F2 generation: Genotypes: PP, Pp, and pp Phenotypes: 3 purple and 1 white

 If we have a pea plant that has purple flowers, what are the two possible genotypes? PP or Pp  If we want to determine which of the genotypes the plant has, we can do a test cross.

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Class Notes

Chapter 14, page 9

 Cross the purple plant of unknown genotype with a recessive homozygote (pp).  In the diagram below of a test cross, predict the two possible outcomes.

 If the original purple plant were homozygous for purple color, what would the offspring be?  Genotypes: all Pp  Phenotypes: all purple  If the original purple plant were heterozygous for purple color, what would the offspring be?  Genotypes: Pp and pp  Phenotypes: half purple and half white   By the law of independent assortment, each pair of alleles  segregates into gametes independently.  Mendel derived the law of segregation by studying pea plants in which he followed only a single character.

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Class Notes

Chapter 14, page 10

 The F1 generation of a cross considering a single character is called a monohybrid (Pp).  Recall, when pure-breeding purple and pure-breeding white flowers were crossed, the (F1 generation) offspring all had the genotype Pp and the phenotype purple. (see page 8 class notes)  In all Mendel studied 7 different individual characters. List them below:  Flower color  Flower position  Seed color  Seed shape  Pod shape  Pod color  Stem length

 Mendel wondered what would happen when he followed two different characters at the same time.  Would the genes for the two traits “stay together” or would they “be inherited separately”?

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Class Notes

Chapter 14, page 11

 For this experiment, Mendel used pea plants that contained characters for:  Seed color- yellow (Y) or green (y)  Seed shape- round (R) or wrinkled (r)  Mendel developed two hypotheses:  Hypothesis #1: The traits stayed together when eggs and sperm were formed. He called this dependent assortment.  This hypothesis predicted that the genes that started together in a parent plant would stay together in the offspring.  Hypothesis #2: The traits would separate when eggs and sperm were formed.  This hypothesis predicted that the genes that started together would not necessarily stay together in the offspring.  They would be independently assorted based on the laws of probability.

In the diagram below, complete the Punnett squares. Remember that we are crossing two traits,  seed color: yellow Y or green y  seed shape: round R or wrinkled r Which is the correct hypothesis?

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Class Notes

Dependent Assortment 

Chapter 14, page 12

Independent Assortment 

Phenotypic ratio in offspring? ___Yellow round ___Yellow wrinkled ___Green round ___Green wrinkled

 Which of the hypotheses proved to be correct, dependent or independent assortment? Independent assortment  This is now called Mendel’s Law of Independent Assortment.  Mendelian inheritance reflects the rules of probability.  The rules of chance involved in tossing a coin or rolling dice also apply to Mendel’s laws of segregation and independent assortment.  Probability is the basis for genetic predictions.

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Class Notes

Chapter 14, page 13

 What is the probability that a (fair) tossed coin will land heads? 50% (or ½) tails? 50% (or ½)   Will this definitely occur or just probably? Probably  How can you increase the likelihood that the ratio of heads to tails will be 50% to 50%? Increase the number of tosses  The fact that Mendel used many plants in his breeding experiments shows that he understood the laws of probability.  Mendel used the law of probability to predict that (for example) if a heterozygous purple plant with the genotype Pp is bred, it will produce:  What percent P eggs or sperm? 50%  This is a probability of ½  What percent p eggs or sperm? 50%  This is a probability of ½

 Two laws of probability:  The rule of multiplication:  The probability that two independent events will happen at the same time is the product of the two probabilities.

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Class Notes

Chapter 14, page 14

 Example: The probability of getting two heads on two different coins at the same time is ½ X ½ = ¼ or 25%.  Example: The probability of getting a P egg and a P sperm if both parents are Pp is ¼ or 25%.  The rule of addition:  The probability of something that can happen in more than one way is the sum of the independent probabilities.  Example: (Refer back to the Punnett square on page 8 of class notes.) In how many ways could two heterozygous parents produce a heterozygous offspring?  Sperm P and Egg p = Pp  Sperm p and Egg P = Pp  If the probability of each of these events is ¼, what is the probability of Pp occurring? ¼ + ¼ = ½ (or 50%)

Complete the diagram below, which compares tossing a coin to the segregation and random fertilization of genes in a monohybrid cross. Assume the following:

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Class Notes

Chapter 14, page 15

 The parents in this cross are each Pp, where P is the dominant trait and p is the recessive trait.  What genes would the eggs and sperm have? P or p  What is the probability of a P egg? ½ Of a p egg? ½  What is the probability of a P sperm? ½ Of a p sperm? ½

Genotype total probabilities: PP = ¼ Pp = ¼ + ¼ = ½ pp = ¼

In the figure above,  How did you use the rule of multiplication?  How did you use the rule of addition?  Practice question: Phenylketonuria (PKU) is an inherited disease caused by a recessive allele. If a woman and her husband are both carriers (that is, each has both the dominant and recessive gene), calculate the probability that

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Class Notes

Chapter 14, page 16

their baby will have the disease. Let P = normal and p = PKU gene and use a Punnett square to solve the problem. Woman = Pp Man = Pp P p P p

PP Pp

Pp pp

 What is the probability that the baby will have PKU? 25%  What is the probability that the baby will be a carrier? 50%  What is the probability that the baby will be normal (and not a carrier or have the disease)? 25%  For more genetics practice, see your textbook pages 272-3, questions number 1, 3, 5, 15, 16.  For each question do the following:  Decide what abbreviations you will use for the dominant and recessive traits.  Determine what the genotypes are given for the parents and/or the offspring.  Make a Punnett square to fill in the missing egg and sperm genes and the offspring genes....