CHAPTER 9:
Fundamentals of Genetics
Genetics: The science of heredity
& the mechanisms by which traits
are passed from parent to offspring
Introduction:
• People knew- connection
between sex & reproduction.
• Knew idea of “like begets like”.
• Breeding in plants and animals
– Example: corn was developed from a
wild grass in Mexico and Central America, about 5000 years
ago.
– Example: dogs were domesticated by 12,000 years ago;
selective crossing has created many breeds we have today.
– Example-artificial pollination done to increase yields in fruit
• Sperm were discovered by van Leeuwenhoek ~1680
• Eggs were not observed in mammals until 1827
• Chromosomes - seen in the 1800s, but their role in
heredity was not understood until the early 1900s
I. Gregor Mendel
(1822-1884)
A. History
• Austrian Monk
• Taught high school,worked
in the garden
• 1851-Entered University of
Vienna to study science, math
& statistics.
• He started breading pea plants (Pisum sativum)
about 1855, studied heredity.
• Mendel formulated 2 fundamental laws of
heredity in the early 1860's.
The “Father of Modern Genetics”
• Mendel’s
experiments
established the basic
ideas & laws that
predict the pattern
of inheritance from
generation to
generation.
Photo from:
http://www.savoirs.essonne.fr/dossiers/le-patrimoine/histoire-des-sciences/johann-gregor-mendel-la-foi-en-la-science
/
Mendel
• Did his work from 1856 to
1865, with out a microscope,
without ever having heard of
a “chromosome”.
• His results were published
in one paper in 1866.
• With the lack of communication at those
times, his work went unnoticed until 1900
scientists repeated the experiments and then
found that Mendel had already developed the
ideas some 40 years previous!
http://www.exploringnature.org/db/detail.php?dbID=22&detID=54
http://kentsimmons.uwinnipeg.ca/cm1504/mendel.htm
Vocabulary:
Heredity
• the transmission of characteristics from
parents to offspring.
Characteristic
• An inheritable feature (flower color,
height)
Trait (or strain)
• A genetically determined variant of a
characteristic. (white or red flower)
B. Work with Peas: Mendel used
statistics to study 7 Characteristics,
each occurred in 2 Contrasting Traits:
1. Flower Color – purple or white
2. Flower Position along Stem – axial or terminal
3. Seed or Pea Color - yellow or green
4. Seed Texture or Shape – round (smooth) or
wrinkled
5. Pod Color – green or yellow
6. Pod Appearance or Shape – inflated or
constricted
7. Plant Height - long or short stems
Seven pea plant characteristics studied:
http www.exploringnature.org/db/detail.php?dbID=22&detID=54
://
• Pollination: when pollen grains (from anther- male)
are transferred to stigma(female).
• Self-pollination- occurs when pollen is transferred
from flower anthers to stigma of either same flower or
flowers on the same plant.
• Cross-pollination- occurs between flowers of 2
plants.
Pea plants usually
Reproduce through
self-pollination.
2. Pollination Methods
• Mendel first collected seeds that were true
breeding (always produce offspring with that
trait) for many generations, (collected 14
traits)
• Then he interrupted self-pollination &
performed cross-pollination
• Did this by removing all the Anthers from
flowers on a plant,
• Then manually pollinating the Stigma of this
plant with pollen from another plant.
Cross section of a flower
3. Three Steps of Mendel’s Experiments
1. Produce a true-breeding or PURE P generation
-
By self-pollination for several generations
“P” is for parent saw: 2 pure traits- 1 for each parent
2. Produce an F1 generation
-
By cross-pollination of parents pure for different traits.
Example- yellow pods w/ green
“F1” is for first filial generation
saw: only 1 trait (dominant - the recessive 1 disappeared)
3. Produce an F2 generation.
-
By allowing F1 generation to self-pollinate
“F2” is for second filial generation
saw reappearance of recessive trait in 3: 1 ratio
http://www.anselm.edu/homepage/jpitocch/genbio/geneticsnot.html
4. Mendel’s results & conclusions
Experiments were repeated carefully for
hundreds of crosses
• See table 9-1 page 176 in textbook
• Results of F1 generation- showed all
offspring of crossed true-breeding plants
were of one trait.
– Example: purple flowers vs white parents
– Offspring all purple
• Results of F2 generation- showed a 3:1
ratio
– Example: 705 purple and 224 white flowers
Results of Mendel’s Experiments:
P generation
– 2 different pure traits
F1 generation- all
offspring show
same trait.
F2 Generation
-3 out of 4 offspring
(3:1 ratio)
Show the trait
hidden in F1
a. Dominant & Recessive
• When strains of plants pure for a trait were crossed,
one trait always failed to appear in the F1 generation.
The absent trait reappeared in ¼ of the F2 generation.
• Mendel concluded that each trait was inherited by
means of a separate factor, so a pair of factors
controlled each trait.
• Dominant- Term for a factor (we now call these
alleles) that masks the presence of another factor .
• Recessive - Term for a factor (allele) that is
masked by the presence of another allele.
Law of Segregation
Basic Principle of genetic inheritance stating that:
• for any particular trait, the pair of
factors of each parent separate (during
the formation of sex cells) and only one
gene from each parent passes on to an
offspring.
Law of Independent Assortment
Basic principle of genetic inheritance stating that:
• different pairs of factors are passed to offspring
independently
• so that new combinations of genes, present in
neither parent, are possible.
– Example: flower color & plant height not connected
• In other words, the distribution of one pair of alleles
does not influence the distribution of another pair-the genes controlling different traits are inherited
independently of one another.
Karyotype
• A picture (photomicrograph) of chromosomes.
• Notice that humans have 46 chromosomes
– 22 homologous pairs of autosomes
– Plus 2 sex chromosomes. (Is this person male or female?)
Karyotype Copied from: http://homepages.uel.ac.uk/V.K.Sieber/human.htm
Mendel succeeded due to a
combination of good work & luck
• He used excellent experimental technique
• He kept detailed records of crosses for several years
• Collected numerical data- it was the ratios of the
crosses that clinched the arguments for his theories
• The chosen characters all showed
dominant/recessive traits- this made his analysis
much easier
• He was lucky- some of his characters were on the
same chromosome, but were so far apart that
crossing-over made them sort nearly independently
C. Modern Support of Mendel
• Much of his work was not understood until
well into the 1900s.
• Modern genetics verified what he discovered.
• Biologist Walter Sutton read Mendel’s papers
and proposed the chromosome theory, linked
meiosis with Mendel’s work.
• Molecular Genetics- the study of the structure
& function of chromosomes & genes.
Factors now called “alleles” &
now represented by letters.
• Dominant are CAPITAL letters
• Recessive are small letters.
• Example 1: Brown eyes dominant: Blue recessive
– “B” for brown & “b” for blue
• Example 2: Red flower dominant-White recessive
– “R” for red and “r” for white
– REMEMBER- USE ONLY 1 LETTER PER PAIR:
Bb or Rr for example - don’t mix!!! Never: Rw!!!
II. Genetic Crosses
• How do we predict the likely outcome of a genetic cross?
• Genotype WHAT IS ON THE GENES
– the genetic make- up of an organism for a trait
– the alleles that the organism inherits from its parents
– Not always observable.
• Phenotype
WHAT YOU SEE
– An organism’s appearance or other observable characteristic
– the detectable expression that results from the organism’s
genotype
Genotype & Phenotype:
http://kentsimmons.uwinnipeg.ca/cm1504/mendel.htm
Homozygous & Heterozygous
• Homozygous
(2 OF THE SAME)
-When both alleles of a pair are alike.
– Example: RR (Both alleles red color)
rr (both white color)
• Heterozygous
(2 DIFFERENT)
– When the alleles in the pair are different.
– Examples: Rr (one Red & one white)
– Or Tt (one tall & 1 short)
( REMEMBER not to use Ts or Rw !!!!!!!!)
Note: there are
2 different
genotypes that
produce the
dominant
Phenotype;
but only 1
genotype
produces the
recessive
phenotype
http://cikgurozaini.blogspot.com/2010_06_01_archive.html
Probability:
• The likelihood that something will happen.
• Predicted results more likely for more trials.
• Example: flipping a coin.
Q1- What is the chance that it will be head or tails?
Q2- Discuss what happens if you flip it 10 times? 100?
Q3- If you flip 2 coins- Does that result have anything to
do with the other- why or why not?
• REVIEW: Relate this back to the law of independent
assortment.
Practice Problem:
• Tongue Rolling is Dominant
(R) You can roll your tongue but your
spouse cannot. Your first child also
cannot roll his tongue.
Q1 What is your genotype for this trait?
Q2 What is the probability of having a
tongue roller child next time?
Predicting Monohybrid Cross Results
• Monohybrid Cross-A cross in which only 1
trait is tracked.
• Punnett square – A graphic or diagram
used to predict the results of a genetic cross.
Draw a Punnett square - 4 small squares
in the shape of a window. Write the
possible gene(s) of one parent across the
top and the gene(s) of the other parent
along the side of the Punnett square.
Example
• Fill in each box of the Punnett square by transferring
the letter above and in front of each box into each
appropriate box.
• As a general rule, the capital letter goes first and a
lowercase letter follows.
• The letters inside the
boxes indicate probable
genotypes.
Predict the phenotype
of these offspring:
Homozygous X Homozygous
• Black- BB
homozygous
dominant
• White- bb
homozygous recessive
• 100%probability
off-spring are:
Heterozygous (Bb)
genotype
& black phenotype.
Homozygous recessive bb
X
Heterozygous Bb
Bb (Heterozygous genotype,
Black phenotype)
Bb (homozygous recessive genotype
white phenotype)
Offspring:
Genotype50% Bb, 50% bb
Phenotype50% Black
50% White
Heterozygous X Heterozygous
• Can cross 2 heterozygous parents (Bb) x (Bb)
such as 2 black rabbits.
B
b
• Offspring:
BB
Bb
• Genotypic ratio:
B
– 1 BB: 2Bb: 1bb
• Phenotypic ratio– 3 black: 1 white
b
Bb
bb
Remember that:
• AN ORGANISM WITH A RECESSIVE
TRAIT ALWAYS HAS A
HOMOZYGOUS RECESSIVE
GENOTYPE (two lowercase letters).
• BUT -a DOMINANT TRAIT MAY
HAVE EITHER BB OR Bb
GENOTYPE!
Testcross
• Since you don’t know if a black rabbit might
have a genotype of BB or Bb, you perform a
test cross.
• A test cross is an individual of
unknown genotype crossed with a
homozygous recessive (bb).
• Try this on your white boards to see what
happens with BB, then Bb crosses.
You always know that bb is recessive…
Is the black phenotype BB or Bb?
Use a test cross to find out.
"Key Points" to remember about
a TEST CROSS.
1. the organism with the dominant trait is always crossed with
an organism with the recessive trait
2. if ANY offspring show the recessive trait, the unknown
genotype is heterozygous
3. if ALL the offspring have the dominant trait, the unknown
genotype is homozygous dominant
4. large numbers of offspring are needed for reliable results
List reasons why would you Never use a
test cross on humans.
http://www.hobart.k12.in.us/jkousen/Biology/testcrss.htm
Incomplete Dominance
• Mendel was lucky to pick traits with complete
dominance. (one trait masks the other, shows dominate
OR recessive)
• Incomplete Dominance occurs when a heterozygous trait
is intermediate between phenotypes of the parents.
• The dominant allele is unable to express itself fully.
• Example: Japanese four o’clock flowers
RR is Red
Rr is pink
Rr is white
Codominance
• When both alleles for a gene
are expressed in
heterozygous offspring.
• Neither allele is dominant
• Alleles do not blend.
• If a red & a white allele are
present in a flower- BOTH
will be expressed!
Human blood types M, N & MN also show codominance.
MN, M,N is different than the better known ABO blood
type which actually a type of trait expressed by Multiple
Alleles .
http://blog.savcds.org/swanson/2010/01/08/codominance-and-incomplete-dominance-upcoming-assessments/
Dihybrid Crosses
• A cross in which 2 characters are tracked.
• More complicated
ry
ry
ry
RY
ry
RrYy
RrYy
RrYy
RrYy
RrYy
RrYy
RrYy
RrYy
RrYy
RrYy
RrYy
RrYy
RrYy
RrYy
RrYy
RY
RY
• On the white boards,
practice rryy x RRYY RY RrYy
Homozygous vs Homozygous
Heterozygous vs Heterozygous
RrYy x RrYy