Transcript Fundamentals of Genetics Chapter 9

Fundamentals of Genetics
Chapter 9
Topics:
Mendel’s Legacy (9-1)
Genetic Crosses (9-2)
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Gregor Mendel
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Austrian monk/scientist
Studied statistics (science of the
collection, organization, analysis, &
interpretation of data)
Studied garden pea (Pisum sativum)
Grew many generations of the plant
& applied statistical analyses to
results to research the transmission
of characteristics from parent to
offspring (heredity)
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Understanding Pisum sativum
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Pea characteristics observed: (pg 167)
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Plant height: long stem or short stem
Flower position on stem: axial or terminal
Pod appearance: inflated or constricted
Pod color: green or yellow
Seed (cotyledon)color: yellow or green
Seed texture: smooth or wrinkled
Flower Color: purple or white
Mendel noted that for each of these
characteristics, the plants exhibited one of 2
possible traits
Mendel studied pea generations taking meticulous
data to set the stage for genetics
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Pisum sativum
Characteristics
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Important vocabulary terms
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Genotype – genetic make-up of an
organism
Phenotype – the external appearance of
an organism based on the genetic makeup
Trait – characteristics of an organism,
what is “seen”
P1 generation – parental cross (mating)
F1 generation – offspring from parental
cross
F2 generation – offspring from F1 cross
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Mendel’s Experimental Set-up
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Anther – male reproductive part of a flower
Pistil – female reproductive part of a flower
Pollen – the male gamete, must move to fuse
ovum (female gamete)
Pollination – union of pollen & ovum
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Peas normally self pollinate, but cross pollination
is possible
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Self pollination – flower that self fertilizes
Cross pollination – pollen from one flower fertilizes
ova of another flower
Insects, weather, animals, etc all help in cross
pollination of plants
Mendel “controlled” all these possibilities &
controlled which pollen pollinated which ova
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Pea Experiments
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Mendel began with peas pure for the 7 traits he
chose to study; purebred means that the
offspring from that parent will always yield that
characteristic (trait)
Each strain of pure pea he obtained for each trait
he named it Parental 1 (P1) generation
He then proceeded to manually cross pollinate his
P1 plants to obtain offspring, named Filial 1 (F1)
generation (each trait studied separately)
The F1 plants were allowed to self pollinate
creating the F2 generation, and the seeds
collected
F2 seeds grown to see traits
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Cross Results
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One cross example: pod color
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Crossed (P1) pure green w/ pure yellow
All offspring (F1) had green pods
(100% green), no yellow
F1 plants allowed to self pollinate for F2
generation
F2 plant pods: ¾ green, ¼ yellow
All other crosses for other traits
yielded similar results…..interesting
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Cross conclusions
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When Mendel saw P1 traits fail to be seen in F1
plants, he found that in F2 the traits were always
in a 3:1 ratio
He concluded:
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“something” was responsible for the trait which he
called “factors”
“factors” came in a pair of alternative forms
controlling the trait
these alternative forms were either dominate or
recessive
For a recessive trait to be expressed, it must have
both recessive “factors”
For a dominate trait to be expressed, it may have
either 2 dominate “factors” or 1 dominate & 1
recessive “factor”
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Law of Segregation
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Mendel concluded that each gamete
receives only 1 “factor” from each
parent, so the resulting cell has 2
So, he proposed the Law of
Segregation which states:
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A pair of factors is segregated or
separated during gamete formation
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Law of Independent Assortment
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Mendel further crossed plants that differed in 2
traits
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He found that this was not always the case
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P1 - flower color (purple) x seed color (yellow)
Might hypothesize that F1 will all have purple
flowers with yellow seeds
The traits of the offspring do not necessarily appear
together; green seeds could be from white flowered
plants, etc.
He suggested the Law of Independent Assortment
which states:
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factors for different traits are distributed to gametes
independently
Wait a minute! What does this sound like?
Didn’t we just learn about independent assortment of something????
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Chromosomes & genes
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DNA strand composed of genes
which code for a trait
Alleles – alternative forms of a
gene
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Mendel’s “factors” are alleles
Homologous pairs of chromosomes
contain the genes from maternal
parent & paternal parent; each
gene is an allele
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Genotype v. Phenotype
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Genotype – the alleles the organism
inherited from both parents
Phenotype – the appearance of the
organism; how the 2 alleles manifest
themselves in the outward appearance of
the organism
Alleles may be dominate or recessive
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(careful, all alleles are not just dom or rec)
Use letter to denote a trait; capitol letter
for dominant trait & lower case letter for
recessive trait
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Alleles
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Homozygous alleles – alleles are
alike
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Homozygous dominate – 2 dom alleles
Homozygous recessive – 2 rec alleles
Heterozygous alleles – alleles are
different
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How to use letters to represent alleles
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Flower color in peas
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Flowers can be purple or white
Purple flowers are dominant trait
White flowers are recessive trait
Pick the letter “P” to represent
flower color
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P is purple
p is white
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Punnett Squares
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Punnett square – term for the
diagram representing a parental
cross to predict the possible
offspring
Monohybrid cross is a cross
between individuals involving a pair
of contrasting traits
Look at handout of Punnett squares
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Carry out the crosses
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Testcross
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Test crosses are used when one is trying to determine
the genotype of an individual for a specific trait.
Problem: I have a black haired (phenotype) guinea pig,
what is his genotype?
I know black hair is a dominate allele, brown hair is
recessive allele
So a black haired guinea pig can have a genotype of BB
or Bb. How do I find out? USE A TESTCROSS
In a testcross, the unknown individual is mated with
homozygous recessive individual and the offspring are
counted. The only 2 possibilities of genotype are BB or
Bb. So, you work out what type of offspring each cross
would be and match that data to the actual offspring.
The data set that matches, will “tell” you what the
parental genotype is.
Try the testcross example on the handout.
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Incomplete Dominance
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Occurs when 2 or more alleles influence
phenotype resulting in an intermediate
phenotype between dominate and
recessive
Four o’clock flowers exhibit this
incomplete dominance in flower color
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White flowers are homozygous recessive, rr
Red flowers are homozygous dominate, RR
Pink flowers are hererozygous, Rr
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Four o’clock flower crosses
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P1 cross RR (red) x rr (white)
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P1 cross RR (red) x Rr (pink)
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F1 is 100% Rr (pink)
F2 (Rr x Rr cross) is 25% RR, 25%rr,
50% Rr for genotype & 1:2:1 ratio of
red: pink: white for phenotype
F1 is 50% RR (red) & 50% Rr (pink)
P1 cross rr (white) x Rr (pink)
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F1 is 50% rr (white) & 50% Rr (pink)
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Codominance
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Occurs when both alleles are expressed
(unblended) in phenotype of
heterozygous individual
Neither allele is dominate or recessive,
nor do they blend
Roan coat color in horses another
example: white hair (R) & red hair (R’) so
RR’ is roan (shows both white and red
hairs)
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Multiple Alleles
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Some traits are
controlled by 3 or
more genes
This is called
multiple allele
inheritance
Human ABO blood
type is controlled by
3 alleles
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Polygenic Traits
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Polygenetic trait – trait which is controlled
by many genes
Genes may be located on same or different
chromosomes
Due to independent assortment and
crossing over during meiosis, many
phenotypes are possible from the
genotypes
Human polygenic traits include:
 Eye color
 Height
 Weight
 Hair color
 Skin color
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Dihybrid Crosses
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Cross between individuals involving
2 pairs of contrasting traits
Use a punnett square to predict
possible offspring, but it is a bit
more complicated
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Set up punnett squares just like for
monohybrid crosses, but there will be
more possibilities for gametes
Do sample crosses on handout
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Dihybrid Crosses
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Some things to be careful about when
doing these crosses
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Make sure the gamete possibilities are correct
Write the offspring possibilities keeping the
traits in order with capitol letter first; SsRr not
SRsr
Write the phenotype in the box with the
genotype
Count carefully, making it a fraction of x/16 for
both genotype and phenotype (you may
reduce fractions)
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Quiz #1
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1. Alternative forms of a gene are ____.
2. The outward characteristics of an
organism are known as the _____.
3. If the genotype is RR, then it is
heterozygous. T/F
4. Using the first 2 letters of the English
alphabet, state the genotype for a
homozygous recessive for both traits.
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Quiz #1 (continued)
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5. If one parent has the genotype
Ttrr, what are the possible gametes
that this parent can provide?
6 – 8. Draw and fill in the Punnett
square for the following cross for
the trait Curly (dominant) leaves &
Straight (recessive) leaves.
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Cc x cc
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Quiz #1 (continued)
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9. What are the possible genotypes
of the offspring from that cross?
10. What are the possible
phenotypes of the offspring from
that cross?
Bonus: What are the ratios of the
genotypes & phenotypes from that
cross?
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Quiz #1 Answers
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6-8. Cc x cc
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Alleles
Phenotype
False
aabb
Tr, tr
C
c
c
Cc
cc
c
Cc
cc
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Quiz #1 Answers
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9. possible genotypes: Cc, cc
10. possible phenotypes: curly
leaves (Cc) & flat leaves (cc)
Bonus: 50% Cc, 50% cc: genotypes
& for phenotypes 50% curly, 50%
straight leaves
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