Mendel

1
Mendel the gene idea

• Chapter 14

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Key Vocabulary

• Genetics The scientific study of heredity
• Heredity the passing of traits from parents to
  offspring
• Inheritance You get your genes from your parents
  - in meiosis, half of the chromosomes in a pair
  come from the Dad, half come from the Mom

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Key terms to know

• Allele each form of a gene for a certain trait
  (R or r)
• Gene sequence of DNA that codes for a protein a
  thus determines a trait
• Genotype combination of alleles for a given
  trait (RR or Rr or rr)
• Phenotype Appearance of trait ( round seeds or
  wrinkled seeds
• Homozygous - when you have 2 or the same alleles
  for a given trait (RR or rr)
• Heterozygous when you have 2 different alleles
  for a trait (Rr)

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Characters and Traits

• Character heritable feature that varies among
  individuals
• ex. Flower color
• Trait each variant for a character
• ex. Purple vs. white flowers
• Originally believed that traits of parents
  blended together to give offspring results!!!

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Gregor Mendel's Discoveries

• Gregor Mendel studied pea plants in monastery
  garden COUNTED the plants and compiled data
  (QUANTITATIVE APPROACH to science).
• Mendel discovered the basic principles of
  heredity by breeding garden peas in carefully
  planned experiments.

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Figure 14.1 A genetic cross
For his experiments, Mendel chose to CROSS
POLLINATE (mate different plants to each other)
plants that were TRUE BREEDING (meaning if the
plants were allowed to self-pollinate, all their
offspring would be of the same variety). P
generation parentals true-breeding parents
that were cross-pollinated F1 generation
(first filial) - hybrid offspring of parentals
that were allowed to self-pollinate F2
generation (second filial) - offspring of
F1s
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Figure 14.2 Mendel tracked heritable characters
for three generations
If the blending model of inheritance were
correct, the F1 hybrids from a cross between a
purple-flowered and white-flowered pea plants
would have pale purple flowers (an intermediate
between the two traits of the parentsBUT When
F1 hybrids were allowed to self-pollinate, or
when they were cross-pollinated with other F1
hybrids, a 31 ratio of the two varieties
occurred in the F2 generation. So what happened
to the white flowers in the F1 generation?
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Mendels 4 ideas

• Alternative versions (different alleles) of genes
  account for variations in inherited characters.
• For each character, an organism inherits two
  alleles, one from each parent.
• If the two alleles differ, the dominant allele is
  expressed in the organisms appearance, and the
  other, a recessive allele is masked.
• (Law of Dominance)
• Allele pairs separate during gamete formation.
  This separation correspondes to the distribution
  of homologous chromosomes to different games in
  meiosis.
• (Law of Segregation)

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Figure 14.3 Alleles, alternative versions of a
gene
The gene for a particular inherited character,
such as color, resides at a specific locus
(position) on a certain chromosome. Alleles are
variants of that gene. In the case of peas, the
flower-color gene exists in two versions the
allele for purple flowers and the allele for
white flowers. This homologous pair of
chromosomes represents an F1 hybrid, which
inherited the allele for purple color from one
parent and the allele for white flowers from the
other parent.
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Figure 11-3 Mendels Seven F1 Crosses on Pea
Plants
MENDELS TEST CROSSES ON PEA PLANTS
Seed Shape
Flower Position
Seed Coat Color
Seed Color
Pod Color
Plant Height
Pod Shape
Round
Yellow
Gray
Smooth
Green
Axial
Tall
Wrinkled
Green
White
Constricted
Yellow
Terminal
Short
Round
Yellow
Gray
Smooth
Green
Axial
Tall
Flower color purple (P) vs. white (p)
Seed coat color and flower color are often put in
for one another thus, the EIGHT traits!!!
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Figure 14.4 Mendels law of segregation (Layer 1)
Each true-breeding plant of the parental
generation has matching alleles, PP or
pp. Gametes (circles) each contain only on
allele for the flower-color gene. In this case,
every gamete produced by one parent has the same
allele. Union of the parental gametes produces
F1 hybrids having a Pp combination (because the
purple allele is dominant, all these hybrids have
purple flowers.) When the hybrid plants produce
gametes, the two alleles segregate (separate),
half the gametes receiving the P allele and the
other half the p allele. This Punnett square
shows all possible combinations of alleles in
offspring. Each square represents an equally
probable product of fertilization. Random
combination of the gametes results in the 31
ratio that Mendel observed in the F2
generation. The LAW OF SEGREGATION states that
allele pairs separate during gamete formation,
and then randomly re-form as pairs during the
fusion of gametes at fertilization.
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Figure 14.4 Mendels law of segregation (Layer 2)
The LAW OF SEGREGATION states that during the
formation of gametes, the two traits carried by
each parent separate.
Parent cell with full gene and Tt alleles.
Traits have separated during gamete formation
from meiosis.
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Figure 14.5 Genotype versus phenotype
Grouping F2 offspring from a cross for flower
color according to phenotype results in the
typical 31 ratio. In terms of genotype, there
are actually two categories of purple-flowered
plants (PP and Pp).
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Law of Independent Assortment

• States that each allele pairs of different genes
  segregates independently during gamete formation
  
• applies when genes for two characteristics are
  located on different pairs of homologous
  chromosomes.
• See figure 14.7 (page 253)
• http//www.sumanasinc.com/webcontent/animations/co
  ntent/independentassortment.html

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Punnett Square

• Device for predicting the results of a genetic
  cross between individuals of a known phenotype.
• Developed by R.C. Punnett
• Rules
• must predict possible gametes first
• male gametes are written across top, female
  gametes on left side
• when reading a Punnett, start in upper left
  corner and read as if a book WRITE OUT
  GENOTYPES IN ORDER!

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Board examples

• Character flower color
• Alleles Purple (P) and white (p)
• Genotypic Combos possible
• two dominants PP (homozygous dominant)
• two recessives pp (homozygous recessive)
• One of each Pp (heterozygous)
• Phenotypes possible
• PP looks purple, so phenotype is purple
• pp looks white
• Pp looks purple (white is masked, but still
  part of genotype)

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Testcross

• Designed to reveal the genotype of an organism
  that exhibits a dominant trait
• it is homozygous dominant or heterozygous?
• Involves the breeding of a recessive homozygote
  with an organism of dominant phenotype by unknown
  genotype

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Figure 14.6 A testcross
Is the dominant phenotype homozygous or
heterozygous? A testcross will tell us!
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Monohybrid crosses only one character
considered

• Steps to do
• Write out genotypes of parents
• Write out possible gametes produced
• Draw 4 box Punnett square
• Put male gametes on top, female on left side
• Fill in boxes
• Determine genotypes by reading Punnett starting
  from top left
• Determine phenotypes by reading from genotype
  list
• Ex.
• White flowered plant X Purple flowered plant
• Yellow peas X Green peas
• Tall plant X short plant

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Dihibrid cross

• Developed following TWO characters at the same
  time Dihybrid cross
• Ex.
• Homozygous dominant for seed color, homozygous
  dominant for seed shape
• X
• homozygous recessive for seed color, homozygous
  recessive for seed shape

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Steps to do

• Write out genotypes of parents
• Write out possible gametes produced hopscotch
  method
• Draw 16 box Punnett square
• Put male gametes on top, female on left side
• Fill in boxes
• Determine genotypes by reading Punnett starting
  from top left
• Determine phenotypes by reading from genotype
  list

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Dihybrid practice problems

• 1. heterozygous for shape, heterozygous for
  color X
• heterozygous for shape, heterozygous for color
• 2. heterozygous for shape, homozygous recessive
  for color X
• homozygous dominant for shape, homozygous
  recessive for color

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Beyond Mendel

• Mendels two laws, segregation and independent
  assortment, explain heritable variations in terms
  of alternative forms of genes (hereditary
  particles) that are passed along, generation
  after generation, according to simple rules of
  probability.
• Figure 14.4 in text (be able to explain)
• Figure 14.7 B in text (be able to explain)
• Now lets go beyond basic Mendelian genetics.

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Other Genetic Landmarks

• 1879 Walther Flemming German biologist who
  stained cells with dye and saw tiny, threadlike
  structures in the nucleus ? CHROMOSOMES!
• also observed and described MITOSIS and noted
  that a full set of chromosomes was being passed
  on to each daughter cell.
• Sixteen years after Mendels death, his paper is
  rediscovered and scientists realize that the
  chromosomes are the carriers of heredity
  Mendels FACTORS are ensuring the passing of
  traits from parents to offspring.
• 1902 Walter Sutton American biologist who
  supports idea that factors are located on
  chromosomes

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Other Genetic Landmarks

• 1905 E.B. Wilson and Nettie Stevens Americans
  studying insect chromosomes
• Saw that male insects always showed a chromosome
  that did not seem to have a match (females always
  had a perfect matching set of chromosomes.)
  Thus, they referred to the non-matching
  chromosomes as Sex Chromosomes.
• In females the sex chromosomes do match
• XX
• In males, one of the chromosomes looked as if it
  were missing a part, so called it a Y
• XY

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Other Genetic Landmarks

• 1909 Wilhelm Johannsen Danish biologist who
  coined the term gene to define the physical
  units of heredity.
• GENE segment of DNA molecules that carries the
  instructions for producing a specific trait.

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Other Genetic Landmarks

• 1912 Thomas Hunt Morgan Showed evidence that
  the presence of white eye color in fruit flies
  was associated with a particular gene on a
  particular chromosome.
• Drosophila melanogaster -- scientific name for
  fruit fly .

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Why Study Fruit Flies?

• Produces about 100 offspring per egg lay good
  statistics!
• Matures in only 15-20 days!
• Only have 8 chromosomes (4 pair) so less to look
  at!
• Easy/inexpensive to raise!
• Chromosomes are VERY large and easy to see and
  locate!
• Sexes are easily distinguished
• female is larger
• shapes of abdomen identify sexes at a glance

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Drosophila Crosses

• Normally, fruit flies always have RED eyes, but
  Morgan saw a white eyed one show up, and it was
  MALE!! Thought that this was strange, so he
  conducted an experiment
• P white eyed X red eyed
• F1 all red eyed offspring
• (thus concluded that red is dominant over white
  for color)
• F1 red eyed X red eyed
• F2 ¾ red eyed ¼ white eyed
• (AND ALL OF THE WHITE EYED ONES WERE MALE!!!)
• Determined that this was a sex-linked trait the
  trait for eye color in fruit flies is carried on
  the sex chromosome.
• Examples of other sex-linked traits
  hemophilia color blindness
• C normal vision, c colorblindness
• Xc Y crossed with XCXc.work this problem out!

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Dominance, Multiple Alleles, and Pleiotrophy

• Involve effects of alleles for SINGLE GENES

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DOMINANT Alleles

• See pages 256 and 257
• Definition is NOT clear cut
• Three points
• They range from complete dominance, through
  various degrees of incomplete dominance, to
  codominance.
• They reflect the mechanisms by which specific
  alleles are expressed in phenotype and do not
  involve the ability of one allele to subdue
  another at the level of the DNA.
• Dominant alleles are not necessarily more common.

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Incomplete Dominance

• Incomplete Dominance when BOTH alleles in an
  individual affect the appearance of a trait and
  you get a brand new color that was not found in
  the original parents. Both traits are written in
  capitals and have different letters because BOTH
  control the appearance.
• Example flower color in snapdragons
• Pure red (RR) X Pure white (WW)
• Offspring will be pink (RW)

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Incomplete Dominance
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Codominance

• Codominance when 2 alleles work together and
  BOTH are expressed without one masking the other
  (NO intermediate phenotype)
• TWO ALLELES AFFECT THE PHENOTYPE IN SEPARATE,
  DISTINGUISHABLE WAYS!

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Multiple Alleles

• Multiple Alleles when more than two
  possibilities for a trait are present.
• Example Blood type see pages 257 and 258
• There are 3 alleles for blood type -- A, B, O
• Here, A and B are dominant over O, but if A and B
  are present together, neither dominates!!! This
  is codominance they share the power of
  expression.

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More on Blood Types

• The letters A, B, and O refer to 2 carbohydrates
  found on the surfaced of RED BLOOD CELLS.
• Will often see the A,B designation as
  superscripts with a base of I
• O (since is recessive to A and B) is shown as i.
• Matching compatible blood groups is critical
  proteins called antibodies are produced against
  foreign blood factors.
• Antibodies bind to foreign molecules and cause
  donated blood cells to clump together
  (agglutination).

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Figure 14.10 Multiple alleles for the ABO blood
groups
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Pleiotropy

• Most genes have MULTIPLE phenotypic effects
• Ability of a gene to affect an organism in many
  ways is called PLEIOTROPHY
• This is due to molecular and cellular
  interactions that are responsible for an
  organisms development
• Ex. Sickle-cell disease (page 262)

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Figure 14.15 Pleiotropic effects of the
sickle-cell allele in a homozygote
Sickle cell is a disease caused by the
substitution of a single amino acid in the
hemoglobin protein of red blood cells. When
oxygen concentration of affected individual is
low, the hemoglobin crystallizes into long
rods. Heterozygotes for sickle cell have
increased resistance to malaria because the rod
shape of blood interrupts the parasites life
cycle. So, sickle cell is prevalent among
African Americans.
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Epistasis

• Involves MORE THAN ONE GENE
• Defined as when a gene at one locus alters the
  phenotypic expression of a gene at a second locus
• Mouse coat color page 258
• coat color B black, b brown
• second gene determines whether pigment will be
  deposited in the hair C color, c albino

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Figure 14.11 An example of Epistasis
One gene determines whether the coat will be
black (B) or brown (b). The second gene
controls whether or not pigment of any color will
be deposited in the hair, with the allele for the
presence of color (C) dominant to the allele for
the absence of color (c).
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Polygenic Inheritance

• Additive effect of two or more genes on a single
  phenotypic character
• Ex. Skin color in humans page 259

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Nature vs. Nurture

• Phenotype depends on nature AND genes
• See NORM OF REACTION phenotypic range of
  possibilities due to environmental influences on
  genotypeREAD TEXT PAGE 259!
• Ex. Blood count of RBCs and WBCs depends on
  altitude, physical activity, presence of
  infection
• Ex. Color of hydrangea blooms depends on soil
  acidity

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Figure 14.13 The effect of environment of
phenotype
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Human Genetics

• Humans are difficult to studybut we have
  developed ways to approach these difficulties.
• Pedigree analysis family history for a
  particular trait
• Study of Genetic diseases
• Twin studies Nature vs. nurture
• Population Sampling
• Genetic Technology

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Figure 14.14 Pedigree analysis

• Males are shown as squares, Females are shown as
  circles
• Horizontal lines marriage or mating lines
• Vertical lines offspring lines
• Shaded symbols represent individuals with the
  trait being studied
• CARRIERS of the trait are those individuals that
  are heterozygous (Ww OR Ff) because they may
  transmit the recessive allele to their offspring
  even though they do not express the trait.
• See text page 261 PEDIGREE ANALYSIS

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Errors in Chromosomes

• Mistakes in numbers of chromosomes
• nondisjunction -- members of a pair of
  homologous chromosomes do not move apart
  properlyresult in offspring that have
• Aneuploidy abnormal chromosome number
• Can beTrisomy or Monosomy or Polyploidy

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Chromosomal Mistakes

• 2. Mistakes in shape of chromosomes
• deletion part of chromosome is broken off and
  lost completely
• duplication broken fragment of chromosome
  attaches to sister chromatid so section is
  repeated on that chromatid
• inversion when fragment reattaches to original
  chromosome but in reverse order
• translocation broken fragment attaches to a
  nonhomologous chromosome
• (can exist as reciprocal or nonreciprocal)

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Figure 15.13 Alterations of chromosome structure
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Technology is Providing New Tools for Genetic
Testing and Counseling

• Carrier recognition with genetic screening and
  Fetal testing
• -ultrasound and sonograms
• -amniocentesis
• -chorionic villi sampling
• -fetoscopy
• -blood/urine tests of newborns

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Figure 14.17 Testing a fetus for genetic
disorders
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Probabilities Practice

• What is the probability that the genotype Aa will
  be produced by the parents Aa x Aa?
• ½
• What is the probability that the genotype ccdd
  will be produced by the parents CcDd x CcDd?
• 1/16
• What is the probability that the genotype Rr will
  be produced by the parents Rr x rr?
• ½
• What is the probability that the genotypes TTSs
  will be produced by the parents TTSs x TtSS?
• 1/4

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Genetics Practice Problems

• How many unique gametes could be produced through
  independent assortment by an individual with the
  genotype AaBbCCDdEE?
• 8
• What is the expected genotype ratio for a
  dihybrid heterozygous cross?
• 9331