Exam II Lectures and Text Pages

1
Exam II Lectures and Text Pages

• I. Cell Cycles
• Mitosis (218 228)
• Meiosis (238 249)
• II. Mendelian Genetics (251 270)
• III. Chromosomal Genetics
• IV. Molecular Genetics
• Replication
• Transcription and Translation
• V. Microbial Models
• VI. DNA Technology

2
Monohybrid Crosses

• When Mendel crossed contrasting, true-breeding
  white-flowered and purple-flowered pea plants
• All of the F1 offspring were purple-flowered
• When Mendel crossed the F1 plants
• Many of the F2 plants had purple flowers, but
  some had white flowers
• The traits did NOT blend

3
Large Samples and Accurate Quantitative Records

• Mendel hypothesized that if the inherited factor
  for white flowers had been lost, then a cross
  between F1 plants should produce only
  purple-flowered plants in the F2.

Figure 14.3
4
Genes

• Mendel reasoned that since the inheritable factor
  for white flowers was not lost in the F1
  generation, it must be masked by the presence of
  the purple-flower factor.
• Mendel's factors are now called genes and in
  Mendel's terms, purple flowers is the dominant
  trait and white flowers is the recessive trait.

5
Repeated Experiments

• Mendel observed the same pattern in many other
  pea plant characters

6
Mendels Model

• Mendel developed a hypothesis
• To explain the 31 inheritance pattern that he
  observed among the F2 offspring
• Four related concepts make up this model

7
Alleles

• First, alternative versions of genes
• Account for variations in inherited characters,
  which are now called alleles

8
Alleles Occur in Pairs in Diploid Organisms

• Second, for each character
• An organism inherits two alleles, one from each
  parent
• A genetic locus is actually represented twice
• Homologous loci may have identical alleles as in
  Mendel's true-breeding organisms, or the two
  alleles may differ, as in F1 hybrids.

9
Dominance vs. Recessiveness

• Third, if the two alleles at a locus differ
• Then one, the dominant allele, is completely
  expressed (designated by a capital letter)
• The other allele, the recessive allele, is
  completely masked (designated by a lowercase
  letter)

10
Law of Segregation

• Fourth, the law of segregation
• The two alleles for a heritable character
  separate (segregate) during gamete formation and
  end up in different gametes
• Without any knowledge of meiosis, Mendel deduced
  that a gamete carries only one allele for each
  inherited characteristic, because the alleles of
  a pair separate (segregate) from each other
  during gamete production.
• Gametes of true-breeding plants will all carry
  the same allele.
• If different alleles are present in the parent,
  there is a 50 chance that a gamete will receive
  the dominant allele, and a 50 chance that it
  will receive the recessive allele.

11
Law of Segregation, Probability and the Punnett
Square

• Does Mendels segregation model account for the
  31 ratio he observed in the F2 generation of his
  numerous crosses?

12
Genetic Vocabulary

• An organism that is homozygous for a gene
• Has a pair of identical alleles (PP or pp)
• All gametes carry that one type of allele
• Exhibits true-breeding
• An organism that is heterozygous for a gene
• Has a pair of alleles that are different (Pp)
• Half the gametes carry one allele and half carry
  the other
• Is not true-breeding

13
Phenotype versus genotype
The phenotype is expressed traits - In the
flower color experiment, the F2 generation had a
31 phenotypic ratio of purple-flowered to
white-flowered plants. The genotype is genetic
makeup - The genotypic ratio of the F2
generation was 121
14
The Testcross

• In pea plants with purple flowers
• The genotype is not immediately obvious
• It may be homozygous dominant (PP) or
  heterozygous (Pp).
• To determine whether such an organism is
  homozygous dominant or heterozygous, we use a
  testcross.

15
The Testcross

• Crossing an individual of unknown genotype with a
  homozygous recessive

Example If a cross between a purple-flowered
plant of unknown genotype (P_) produced only
purple-flowered plants, the parent was probably
homozygous dominant since a PP x pp cross
produces all purple-flowered progeny that are
heterozygous (Pp). If the progeny of the
testcross contains both purple and white
phenotypes, then the purple-flowered parent was
heterozygous since a Pp X pp cross produces Pp
and pp progeny in a 11 ratio.
?
Dominant phenotype, unknown genotype PP or Pp?
Recessive phenotype, known genotype pp
If PP, then all offspring purple
If Pp, then 1/2 offspring purple and 1/2
offspring white
p
p
p
p
P
P
Pp
Pp
Pp
Pp
P
p
Pp
Pp
pp
pp
Figure 14.7
16
The Law of Independent Assortment

• The law of segregation was derived
• From monohybrid crosses using F1 monohybrids
  heterozygous for one character
• The Law of Independent Assortment requires
• Using dihybrid crosses between F1 dihybrids
• Crossing two, true-breeding parents differing in
  two characters
• Produces F1 dihybrids, heterozygous for both
  characters

17
The Dihybrid Cross

• Illustrates the inheritance of two characters
• Produces four phenotypes in the F2 generation
• When the F1 dihybrid progeny self-pollinate.
• If the two characters segregate together, the F1
  hybrids can only produce the same two classes of
  gametes (RY and ry) that they received from the
  parents, and the F2 progeny will show a 31
  phenotypic ratio.
• If the two characters segregate independently,
  the F1 hybrids will produce four classes of
  gametes (RY, Ry, rY, ry), and the F2 progeny will
  show a 9331 phenotypic ratio.

Figure 14.8
18
The Law of Independent Assortment

• Using the information from a dihybrid cross,
  Mendel developed the law of independent
  assortment
• Each pair of alleles segregates independently
  from every other pair during gamete formation

19
Probability

• Segregation, independent assortment and
  fertilization are random events and
• Reflect the rules of probability
• From the genotypes of parents, we can predict the
  most likely genotypes of their offspring using
  simple laws of probability.

20
Probability Scale

• The probability scale ranges from 0 to 1 an
  event that is certain to occur has a probability
  of 1, and an event that is certain not to occur
  has a probability of 0.
• The probabilities of all possible outcomes for an
  event must add up to 1.
• Random events are independent of one another.
• The outcome of a random event is unaffected by
  the outcome of previous such events.
• Example it is possible that five successive
  tosses of a normal coin will produce five heads
  however, the probability of heads on the sixth
  toss is still 1/2.

21
Two basic rules of probability
1. Rule of multiplication states that the
probability that independent events will occur
simultaneously is the product of their individual
probabilities.

• Question In a monohybrid cross between pea
  plants (Rr), what is the probability that the
  offspring will be homozygous recessive?
• Answer
• Probability that an egg from the F1 (Rr) will
  receive an r allele 1/2.
• Probability that a sperm from the F1 will receive
  an r allele 1/2.
• The overall probability that two recessive
  alleles will unite at fertilization 1/2 x 1/2
  1/4.

22
Multiplication also applies to dihybrid crosses

• Question For a dihybrid cross, YyRr x YyRr, what
  is the probability of an F2 plant having the
  genotype YYRR?
• Answer
• Probability that an egg from a YyRr parent will
  receive the Y and R alleles 1/2 x 1/2 1/4.
• Probability that a sperm from a YyRr parent will
  receive the Y and R alleles 1/2 x 1/2 1/4.
• The overall probability of an F2 plant with the
  genotype YYRR 1/4 x 1/4 1/16.

23
Two Rules of Probability

• 2. Rule of addition states that the probability
  of an event that can occur in two or more
  independent ways sum of the separate
  probabilities of the different ways.
• Question In this cross between pea plants, Pp x
  Pp, what is the probability of the offspring
  being heterozygous?
• Answer There are two ways a heterozygote may be
  produced the dominant allele (P) may be in the
  egg and the recessive allele (p) in the sperm, or
  vice versa.
• So, the probability that the offspring will be
  heterozygous is the sum of the probabilities of
  those two possible ways
• Probability that the dominant allele will be in
  the egg with the recessive in the sperm is 1/2 x
  1/2 1/4.
• Probability that the dominant allele will be in
  the sperm and the recessive in the egg is 1/2 x
  1/2 1/4.
• So, the probability that a heterozygous offspring
  will be produced is 1/4 1/4 1/2.

24
Complex Genetics Problems

• A dihybrid or other multicharacter cross
• Is equivalent to two or more independent
  monohybrid crosses occurring simultaneously
• In calculating the chances for various genotypes
  from such crosses
• Each character first is considered separately and
  then the individual probabilities are multiplied
  together

25
Multiple Locus Problem

• Question What is the probability that a
  trihybrid cross between organisms with genotypes
  AaBbCc and AaBbCc will produce an offspring with
  genotype aabbcc?
• Answer Segregation of each allele pair is an
  independent event, we can treat this as three
  separate monohybrid crosses
• Aa x Aa probability for aa offspring 1/4
• Bb x Bb probability for bb offspring 1/4
• Cc x Cc probability for cc offspring 1/4
• The probability that these independent events
  will occur simultaneously is the product of their
  independent probabilities (rule of
  multiplication).
• The probability that the offspring will be aabbcc
  is 1/4 aa x 1/4 bb x 1/4 cc 1/64

26
Problem 2

• Question Using garden peas, where and assuming
  the cross is PpYyRr x Ppyyrr what is the
  probability of obtaining offspring with
  homozygous recessive genotypes for at least two
  of the three traits?
• Answer Write the genotypes that are homozygous
  recessive for at least two characters, (note that
  this includes the homozygous recessive for all
  three). Use the rule of multiplication to
  calculate the probability that offspring would be
  one of these genotypes. Then use the rule of
  addition to calculate the probability of
  offspring in which at least two of the three
  traits would be homozygous recessive.
• Genotypes with at least two homozygous recessives
  
• ppyyRr - 1/4 x 1/2 x 1/2 1/16
• ppYyrr - 1/4 x 1/2 x 1/2 1/16
• Ppyyrr - 1/2 x 1/2 x 1/2 2/16
• PPyyrr - 1/4 x 1/2 x 1/2 1/16
• ppyyrr - 1/4 x 1/2 x 1/2 1/16
• 6/16 or 3/8 chance of two recessive traits

27
Particulate Behavior of Genes

• Reviewing Mendels discoveries
• If a seed is planted from the F2 generation of a
  monohybrid cross, we cannot predict with absolute
  certainty that the plant will grow to produce
  white flowers (pp). We can say that there is a
  1/4 chance that the plant will have white
  flowers. Alternatively, we can say that if
  there are several offspring, it is likely that
  1/4 of them will have white flowers.
• Alleles are discrete units that segregate into
  separate gametes at meiosis. Each gene pair
  separates independently of all the other pairs.