FUNDAMENTALS OF GENETICS Modern Biology Chapter 9 Pages 164 -
182
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Objectives: Describe how Mendels results can be explained by
the scientific knowledge of genes and chromosomes. Differentiate
between a monohybrid cross and a dihybrid cross. Predict &
perform results of monohybrid and dihybrid crosses Fundamentals of
Genetics
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Do Now What is the genetic code? What molecule carries the
genetic code? What is genetics?
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K W - L What do you know about inheritance? What do you want to
know about inheritance? What have you learned about
inheritance?
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Fundamentals of Genetics All of your characteristics or traits
are unique to you. Parents may pass many of their own traits to
their children, or offspring. For example, the color of your hair,
the size of your feet and the shape of your nose are some of your
traits.
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heredity. The passing of these traits from parents to offspring
is called heredity. genetics. The study of heredity is called
genetics. Biologists who study heredity are called Geneticists.
Fundamentals of Genetics
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Look at the photographs to the right. What traits have these
babies inherited from their parent?
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Gregor Mendel The Father of Genetics Gregor Johann
Mendel.Genetics was founded with the works of an Austrian Monk,
scientists and mathematician Gregor Johann Mendel. He experimented
with garden pea plants. rpee Seeds and Plants Homerpee Seeds and
Plants Home> Vegetables > Peas > Pea, Easy
PeasyVegetablesPeas
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Gregor Mendel His task of tending the garden gave him time to
observe the passing of traits from parent pea plants pea plants to
their offspring. He became interested in why certain patterns of
traits showed up in living things.
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Mendel began his experiments by collecting seeds from his pea
plants, carefully recording the traits of each plant. Seeds from
tall plants usually produced tall plants but sometimes produced
short plants. Seeds from short plants only produced short plants.
but, WHY?
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He studied 7 different characteristics in his pea plants, each
with 2 contrasting traits. CHARACTERISTIC CHARACTERISTIC-a
distinguishing quality that an organism exhibits. Ex: height, hair
color, eye color, skin color. TRAIT TRAIT- specific hereditary
options available for each characteristic. Ex: tall height/short
height, smooth/ blonde hair, brown/blue eyes, Dark/light skin.
Mendels Experiments
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CHARACTERISTICSTRAIT 1. Plant Height Tall vs. Short 2. Seed
Color Yellow vs. Green 3. Seed Shape Round vs. Wrinkled 4. Pod
Color Green vs. Yellow 5. Pod/Flower Location Axial vs. Terminal 6.
Pod Shape Inflated vs. Constricted 7. Flower ColorPurple vs.
White
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purebredHe decided to grow plants that were purebred - having a
trait that will always be passed to the next generation The term
strain denotes all plants that are pure for a trait. Mendels
Methods
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He produced 14 strains (one for each of the 14 traits he
observed) by allowing the plants to self-pollinate for several
generations This became his Parent generation or his P 1 Generation
P 1 Generation Mendel Controlled Pollination
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Pollination-transfer of pollen from anther (male flower part)
to stigma (female flower part) SelfPollination occurs on same plant
Cross Pollination occurs between different plants Mendel Controlled
Pollination
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cross pollinatedThen, Mendel cross pollinated plants that had
contrasting traits to see what the offspring would look like. (P 1
X P 1 - i.e. pure tall x pure short) F 1 GenerationWould the
offspring (F 1 Generation offspring of P 1 ) be tall, short, or
medium ? Mendels Methods
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In his first crosses, Mendel found that only one of the two
traits appeared in the offspring plants (F 1 generation). (F 1 )For
example, when he crossbred tall pea plants with short pea plants,
the offspring (F 1 ) were always tall. Mendels Results
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F1After his first crosses, Mendel took those offspring plants
(F1) and crossed them. F2 generationIn these second crosses, both
traits showed up again in the F2 generation. (F2 GENERATION (F2
GENERATION-offspring of crosses between the F1 generation). He
observed that of the plants had the same trait as the F1
generation.
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The same results happened in every experiment. One trait, like
being tall, was always there in the first generation (F 1 ). The
other trait, like being short, seemed to go away; only to reappear
again in the second generation (F 2 ). This happened with every set
of traits that Mendel studied.
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EXAMPLES: Plant Height Cross Seed Color Cross Seed Shape Cross
Parent P 1 x P 1 Tall x Short All Tall Plants Tall x Tall Tall
Short Yellow x Green All Yellow Plants Yellow x Yellow Yellow Green
Round vs. Wrinkled All Round Plants Round X Round Round Wrinkled
First Generation F 1 x F 1 Second Generation F 2
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Mendel hypothesized that something in the pea plants was
controlling the characteristics that came through factorsHe called
these controls factors Genes) (We now know that these factors are
really traits controlled by Genes)
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Because each characteristic had two forms, he said there must
be a pair of factors controlling each trait. allelesEach pair
consists of alternate forms (we now call alleles) of the same
trait; one from mother and one from father.
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MENDELS 3 CONCLUSIONS MENDELS 3 CONCLUSIONS : Based on his
findings, Mendel formulated three laws or principles of heredity:
1. Principle of Dominant and Recessiveness 2. Principle of
Segregation 3. Principle of Independent Assortment
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(alleles Through crossing thousands of pea plants, he was able
to conclude that that both of these factors (alleles) together
controlled the expression of a trait. Dominant traits dominant
alleles recessive traits Dominant traits were controlled by
dominant alleles and recessive traits were controlled by recessive
alleles. Principle of Dominant & Recessiveness
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DOMINANT DOMINANT-can mask or dominate the other factor and is
displayed most often. RECESSIVE RECESSIVE-the factor that can be
covered up; is displayed less often. Ex: the factor (allele) for
tall is dominant over the factor (allele) for short, so the short
allele would be the recessive allele. Principle of Dominant &
Recessiveness
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Letters are used to represent the alleles that carry the trait
found on genes If the gene that controls the trait is dominant, the
letter is written in uppercase. If the gene is recessive, the
letter is written in lowercase. i.e. T- represents a dominant trait
for tallness; t represents a recessive trait for lack of tallness,
or shortness Principle of Dominant & Recessiveness
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T dominant allele for tallness t recessive allele for lack of
tallness or shortness. W dominant allele for round or smooth seeds
w recessive allele for wrinkled seeds P dominant for flower color
(purple) p recessive allele for white flower
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GENE GENE- a segment of DNA that codes for a specific
characteristic. Ex: height ALLELE ALLELE-the different forms of a
gene (Mendels factor) Ex: allele for brown eyes is B/ allele for
blue eyes is b SOif BB is a brown eyed person and bb is a blue eyed
person, what color eyes does someone with Bb have? Vocabulary
Review
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Principle of Segregation Each parent has two factors (copies of
each trait) and they segregate, or separate into different sex
cells (gametes) Each gamete gets only 1 factor (allele) of each
trait
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Principle of Independent Assortment Mendel also crossed plants
that differed in 2 characteristics, such as flower height and
flower color. The data from these crosses showed that dominant
traits do not always appear together ttP?
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Principle of Independent Assortment The alleles for different
genes on different chromosomes are not connected. The alleles for
different traits are distributed into gametes independently
(randomly) from each other.
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Principle of Independent Assortment
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Gregor Mendel and his pea plants experiments (1857-1865)
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Do Now Who is the father of genetics? What type of organism did
he work with? What are dominant and recessive traits?
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Vocabulary Review ChromosomesChromosomes made of DNA GeneGene
segment of DNA that controls a specific hereditary trait. Because
chromosomes occur in pairs, genes occur in pairs Allele -Allele -
(Mendels factor) contrasting form of a gene Dominant allele capital
letter Recessive allele lowercase letter
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AFTER MENDEL Today, Geneticists rely on Mendels work to predict
the likely outcome of genetic crosses. Why would geneticists want
to predict the probable genetic make up and appearance of offspring
resulting from specified crosses?
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GENOTYPE & PHENOTYPE GENOTYPEGENOTYPE-the genetic makeup of
an organism (the combination of alleles an organism inherits). Use
2 letters together to represent genotype. PHENOTYPEPHENOTYPE-the
trait displayed based on the genotype. Ex: BB Brown eyes bb Blue
eyes Bb Brown eyes
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GENETIC CROSSES
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bb Blue alleles Genotype Phenotype
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Organisms with different genotypes may have the same phenotype.
For example, a brown-eyed organism (BB) and a brown eyed organism
(Bb) have different genotypes. However, they have the same
phenotype, which is brown eyes GENOTYPE & PHENOTYPE
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b Brown Alleles One pair of chromosomes for eye color One pair
of chromosomes for eye color B BB
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AFTER MENDEL
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HOMOZYGOUSHOMOZYGOUS- organism has 2 of the same alleles for a
trait. Homozygous Dominant-has 2 dominant alleles; dominant trait
is displayed Ex: BB = Brown-eyed organism Homozygous Recessive-has
2 recessive alleles; recessive trait is displayed Ex: bb =
blue-eyed HETEROZYGOUSHETEROZYGOUS-organism has 1 dominant and 1
recessive allele; the dominant trait is displayed. Ex: Bb = brown
eyes GENOTYPE & PHENOTYPE
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bb bB Blue Allele Brown AlleleBlue alleles Homozygous alleles
are the same Heterozygous alleles are different
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Do Now What are Mendels Laws of Inheritance? What is an allele?
What is homozygous vs. heterozygous? What is genotype vs.
phenotype?
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Probability In order to understand genetics you need to have
some basic concepts concerning probability. ProbabilityProbability
the likelihood that a specific event will occur Can be expressed as
a decimal, percentage, ratio or fraction. P= number of times an
event is expected to happen number of opportunities for an event to
happen
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Probability If you flip a coin once, what is the probability
that it will land on heads? P(Event)= 1 (Heads) 2 (Heads or Tails)
2 (Heads or Tails) P= 1 2 ;.5; or 50%; 1:2 If you flip a coin
twice, what is the probability that it will land on heads twice? P1
(Heads) P = 1 (Heads) 4 (Heads, Tails; Tails, Heads; Tails, Tails;
Heads, Heads) P = ??
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Remember, probability is the likelihood that a chance event
will occur. The value of studying genetics is in understanding how
we can predict the likelihood of inheriting particular trait.
Predicting the Results of Genetic Crosses
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MONOHYBRID CROSSMONOHYBRID CROSS a genetic cross between 2
individuals involving 1 pair of contrasting traits. Predicting the
Results of Genetic Crosses
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One of the easiest ways to calculate the mathematical
probability of inheriting a specific trait was invented by an early
20 th century English geneticist, Reginald Punnett. Predicting the
Results of Genetic Crosses
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Punnett square. His technique employs what we now call a
Punnett square. Punnett square A Punnett square is a chart that
shows possible gene combinations of offspring of two parents whose
genotypes are known.
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HOW TO DRAW A PUNNETT SQUARE 1. Write what each allele means.
2. Write the genotypes of the parents. 3. Draw a grid. 4. Put the
alleles for one parent along the top; put the alleles for the other
parent along the side. 5. Fill in the grid.
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EXAMPLE 1:HOMOZYGOUS X HOMOZYGOUS T= tall plantTALL X SHORT t =
short plant (TT x tt) Genotype Genotype = 4 Tt Phenotype Phenotype
= 4 tall plants Probability = number of times an event(tall) is
expected to happen number of opportunities (total) for an event to
happen Probability Ratio : 4/4Probability percent: 100% T Tt T t
t
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EXAMPLE 2:HOMOZYGOUS X HETEROZYGOUS T= tall plantTALL X SHORT t
= short plant (Tt x tt) Genotype Genotype = 2 Tt, 2 tt Phenotype
Phenotype = 2 tall, 2 short Probability = number of times an
event(tall/short) is expected to happen number of opportunities
(total) for an event to happen Probability: 2/4 tall plants; 50%
tall plants; 2:4 2/4 short plants; 50% short plants; 2:4 T Tt t t t
tt Tttt
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EXAMPLE 2:HOMOZYGOUS X HETEROZYGOUS T= tall plantTALL X TALL t
= short plant (TT x Tt) Genotype Genotype = 2 TT, 2 Tt Phenotype
Phenotype = 4 Tall Probability = number of times an
event(tall/short) is expected to happen number of opportunities
(total) for an event to happen Probability: 4/4 tall plants; 100%
tall plants; 4:4 0% short T Tt T T t TT Tt TT Tt
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EXAMPLE 2: HETEROZYGOUS X HETEROZYGOUS T= tall plantTALL X TALL
t = short plant (Tt x Tt) Genotype Genotype = 1 TT, 2 Tt, 1 tt
Phenotype Phenotype = 3 Tall, 1 short Probability = number of times
an event(tall/short) is expected to happen number of opportunities
(total) for an event to happen Probability: tall plants; 75%; 3:4
short plants; 25%; 1:4 T Tt t T t TT Tt tt
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TESTCROSS TESTCROSSTESTCROSS - Cross to determine genotype of
parent with dominant phenotype. Use to determine if the unknown is
heterozygous or homozygous dominant genotype. Ex: A plant with
green seed pods could have a genotype of GG or Gg. Cross the
unknown parent with a homozygous recessive.
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EXAMPLE 2: ? ? X HOMOZYGOUS T= tall plantTALL X SHORT t = short
plant (T? x tt) If Phenotype If Phenotype = 4 Tall Genotype of
Unknown Genotype of Unknown = TT T Tt ? t t ? t T T t
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EXAMPLE 2: ? ? X HOMOZYGOUS T= tall plantTALL X SHORT t = short
plant (T? x tt) If Phenotype If Phenotype = 3 Tall, 1 short
Genotype of Unknown Genotype of Unknown = Tt T Tt ? t t ? t t t t
t
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The way genes control traits can be complex and interact in
different ways. More Complex Patterns of Heredity
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COMPLETE DOMINANCEAll of these crosses we just did were
examples of COMPLETE DOMINANCE. COMPLETE DOMINANCECOMPLETE
DOMINANCE-one allele is totally dominant over the other allele.
EXAMPLE: PP and Pp = purple flower plants
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When one gene for a certain trait is not completely dominant
over the other gene, a blending effect occurs. INCOMPLETE DOMINANCE
INCOMPLETE DOMINANCE is a type of inheritance in which one allele
(dominant) for a specific trait is not completely dominant over the
other (recessive) allele. This results in a combined phenotype
(expressed physical trait). Incomplete Dominance
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R R r r R r EXAMPLE: Four oclocks (flowers) RR = red rr = white
Rr = pink RED (RR) X WHITE (rr) Genotype = 4Rr Phenotype = 4
pink
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Incomplete Dominance R r r R R r R r EXAMPLE: Four oclocks
(flowers) RR = red rr = white Rr = pink PINK (Rr) X PINK (Rr)
Genotype = 1 RR;2Rr;1rr Phenotype = 1 red, 2 pink, 1 white
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Codominance Another pattern of heredity can occur when two
dominant genes are present for a certain trait. co-dominance This
pattern of heredity is called co-dominance (both variations of the
gene appearing at the same time). Neither allele is dominant or
recessive, nor do they blend.
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Codominance R R R R R EXAMPLE: roan horse: RR red coat color RR
white coat color RR roan coat both red and white hairs Genotype = 4
RR Phenotype = 4 Roan
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Many traits are controlled by one gene that has more than two
possible variations. multiple allelesThese traits are controlled by
multiple alleles. Human blood groups are controlled by multiple
alleles. Codominance & Multiple Alleles
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There are 3 alleles for the gene that determines blood type. A,
B, O (Remember: You have just 2 of the 3 in your genotype - 1 from
mom & 1 from dad). With three alleles, we have a higher number
of possible combinations in creating a genotype. There are 6
different genotypes and four different phenotypes blood type.
Codominance & Multiple Alleles
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Blood Type A (I A I A ) X Blood Type B (I B I B ) Genotype = 4
I A I B Phenotype = 4 Blood Type AB IAIA IBIB IBIB IAIA IAIBIAIB
IAIBIAIB IAIBIAIB IAIBIAIB
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Possible Blood Type Combinations
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The A and B alleles are equally dominant. A child who inherits
and A allele from one parent and a B allele from the other parent
will have type AB blood. What type of dominance is
this?co-dominance The O allele is recessive to both A and B
alleles. A child who inherits an A allele from one parent and an O
allele from the other parent will have a genotype of AO and a
phenotype of Type A blood. A child who inherits on O allele from
one parent and an O allele from the other will have: Genotype?
Phenotype? FYI
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Predicting the Probability of a Dihybrid Crosses Cross between
individuals studying one trait is Monohybrid Cross Dihybrid
CrossCross between individuals studying two traits is Dihybrid
Cross
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Predicting the Probability of a Dihybrid Crosses DIHYBRID
CROSSA DIHYBRID CROSS is more complicated than monohybrid because
there are more possible combinations. MONOHYBRID CROSSMONOHYBRID
CROSS = 2 traits/4 possible offspring DIHYBRID CROSSDIHYBRID CROSS
= 4 Traits/ 16 possible offspring
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Predicting the Probability of a Dihybrid Crosses Example: AA or
Aa = purple; aa = white BB or Bb = tall; bb = short AaBb x AaBb
(Purple Flower, Short Plant x Purple Flower, Short Plant)
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FYI One really important thing that Mendel noticed from this
type of cross was that traits (like flower color, height) are
inherited independently - not together as a unit. This is type of
cross helped Mendel develop the Law of Independent Assortment. Law
of Independent AssortmentREMEMBER - Law of Independent Assortment -
Genes for various traits assort into gametes independently (due to
homolouges lining up randomly at the metaphase plate.)
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Dihybrid Cross Example: Homozygous x Homozygous Tall, Round
Plant (TT RR) X Tall, Round Plant (TT RR) First, we need to
determine what alleles each parent could possibly give - all
possible combinations of the alleles from each trait. TTRR TR, TR,
TR, TR
Dihybrid Cross Example: Heterozygous x Homozygous LETS TRY IT
!! Cross Tall, Round Plant (TtRr) X Short, Wrinkled Plant (ttrr)
Determine what alleles each parent could possibly give - all
possible combinations of the alleles from each trait. TtRr ttrr TR,
Tr, tR, tr tr tr tr tr
Dihybrid Cross Example: Heterozygous x Heterozygous Cross Tall,
Round Plant (TtRr) X Tall, Round Plant (TtRr) Determine what
alleles each parent could possibly give - all possible combinations
of the alleles from each trait. TtRr TR, Tr, tR, tr TR Tr tR
tr
