1.what are homologous chromosomes? 2.give an example of a heterozygous dominant genotype. 3.give an...
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1.What are homologous chromosomes?
2.Give an example of a heterozygous dominant genotype.
3.Give an example of a homozygous recessive genotype.
4.What does “mono-hybrid” mean?
** Look at your notes from last time, if there is anything you didn’t understand, let me know and we will go over it.
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Heredity• Heredity is the passing on of characteristics
from parents to offspring.
These characteristics are called
traits
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• It was not until the mid-nineteenth century that Gregor Mendel, an Austrian monk, carried out important studies of heredity.
Gregor Mendel
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• Mendel argued that parents pass on to their offspring factors (now called genes) that are responsible for inherited traits.
• Mendel was the first person to succeed in predicting how traits are transferred from one generation to the next.
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• A complete explanation requires the careful study of genetics—the branch of biology that studies heredity.
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• Mendel chose to use the garden pea in his experiments for several reasons.
• Garden pea plants reproduce sexually, which means that they produce male and female sex cells, called gametes.
Mendel chose his subject carefully
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Stigma
Style
Ovary
Pistil
Filament
AntherStamen
Mendel chose his subject carefully
• The male gamete forms in the pollen grain, which is produced in the male reproductive organ.
• The female gamete forms in the female reproductive organ or ovary.
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• In a process called fertilization, the male gamete unites with the female gamete.
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• The resulting fertilized cell, called a zygote, then develops into a seed.
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Stigma
Style
Ovary
Pistil
Filament
AntherStamen
• The transfer of pollen grains from a male reproductive organ to a female reproductive organ
in a plant is called pollination.
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• When Mendel wanted to breed, or cross, one plant with another, he opened the petals of a flower and removed the male organs.
Remove male parts
Mendel wanted to pick his own parent plants
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• He then dusted the female organ with pollen from the plant he wished to cross it with.
Female part
Transfer pollen
Pollen grains
Maleparts
• The process of combining male cells from one plant with female cells from another is called cross-pollination.
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• He only wanted to study one trait at a time at first.
• Mendel started by cross pollinating short pea plants with tall pea plants.
• The only differing trait was height.
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• The tall pea plants he worked with were from populations of plants that had been tall for many
generations and had always produced tall offspring.
• They were said to be pure-bred tall
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• Likewise, the short plants he worked with were
pure-bred for shortness.
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• A hybrid is the offspring of parents that have different forms of a trait, such as tall and short height.
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Monohybrid crosses• Mendel’s first experiments are called monohybrid crosses
• Mono means “one”
• The two parent plants differed from each other by a one trait—
which in this case is height.
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• He cross-pollinated the tall pea plant with pollen from a short pea plant.
• All of the offspring grew to be as tall as the taller parent.
The First Generation
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• Mendel allowed the tall plants in this first generation to self-pollinate.
• Three-fourths of the plants were as tall as the tall plants in the parent and first generations.
The Second Generation
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• One-fourth of the offspring were as short as the short plants in the parent generation.
• In the second generation, tall and short plants occurred in a ratio of about three tall plants to one short plant.(3:1)
Short pea plant Tall pea plant
All tall pea plants
3 tall: 1 short
P1
F1
F2
The Second Generation
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Short pea plant Tall pea plant
All tall pea plants
3 tall: 1 short
P1
F1
F2
• In every case, he found that one trait of a pair seemed to disappear in the F1 generation.
• only to reappear unchanged in one-fourth of the F2 plants.
The Second Generation
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• Mendel concluded that each organism has two genes that control each of its traits.
• We now know that these genes are located on chromosomes.
• Genes exist in alternative forms. We call these different gene forms alleles.
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• An organism’s two alleles are located on their homologous chromosomes—one inherited from the female parent and one from the male parent.
B b
Bb
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• Mendel called the observed trait “dominant” and the trait that disappeared “recessive”.
• Mendel concluded that the allele for tall plants is dominant to the allele for short plants.
Rule of Dominance
Bb
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• It is customary to use the same letter for different alleles of the same gene.
T T
T
T
t t
t
t
Tall plant Short plant
All tall plants
F1
Tfor Tall
tfor Short
The letter “T” equals height.
Big T is Tall
Little T is Short
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• An uppercase letter is used for the dominant allele and a lowercase letter for the
recessive allele.
• The dominant allele is always written first.
T T
T
T
t t
t
tAll tall plants
F1
Tall plant Short plant
Tt
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• The law of segregation states:
every individual has two alleles of each gene and when gametes (or sex cells) are produced, each gamete receives one of these alleles.
The Law of Segregation
Bb
B b
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• Two organisms can look alike but have different underlying allele combinations.
Law of segregation Tt Tt cross
F1
Tall plant Tall plant
TTT
T t T t
t T t tt
3
Tall Tall Short
1
Tall
F2
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Phenotypes and GenotypesPhenotypes and Genotypes
• The way an organism looks and behaves is called its phenotype.
• The allele combination an organism contains is known as its genotype.
• An organism’s genotype can’t always be known by its phenotype.
Bb BB
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• An organism is homozygous for a trait if its two alleles for the trait are the same.
• The pure-bred tall plant that had two alleles for tallness (TT) would be homozygous for the trait of height.
TT
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• An organism is heterozygous for a trait if its two alleles for the trait differ from each other.
• Therefore, the tall plant that had one allele for tallness and one allele for shortness (Tt) is heterozygous for the trait of height.
Tt
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What is the phenotype of the pea plant below?
What is the genotype of the pea plant ?
ttSo therefore, what would you call this trait?
Homozygous short
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• In 1905, Reginald Punnett, an English biologist, devised a shorthand way of finding the expected proportions of possible genotypes in the offspring of a cross.
• This method is called a Punnett square.
Punnett Squares
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• A Punnett square for this cross is two boxes tall and two boxes wide because each parent can produce two kinds of gametes for this trait.
Heterozygous tall parent
T t
T t
T t
T
t
Heterozygous tall parent
Monohybrid cross
T t
T
t
TT Tt
Tt tt
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Punnet Square Activity
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• Mendel performed another set of crosses in which he used peas that differed from each other in two traits rather than only one.
• Such a cross involving two different traits is called a dihybrid cross.
Dihybrid crosses
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• Mendel took pure-bred pea plants that had round yellow seeds (RRYY) and crossed them with pure-bred pea plants that had wrinkled green seeds (rryy).
• He already knew the round-seeded trait was dominant to the wrinkled-seeded trait.
• He also knew that yellow was dominant to green.
The first generation
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Dihybrid Cross round yellow x wrinkled green
Round yellow Wrinkled green
All round yellow
Round yellow Round green Wrinkled yellow Wrinkled green
9 3 3 1
P1
F1
F2
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• The Law of Independent Assortment states:
genes for different traits—for example, seed shape and seed color—are inherited independently of each other.
The Law of Independent Assortment
Bb bb
rr RRBb
Rr
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• A Punnett square for a dihybrid cross will need to be four boxes on each side for a total of 16 boxes.
Dihybrid crossesDihybrid crosses
Punnett Square of Dihybrid Cross
Gametes from RrYy parentRY Ry rY ry
Ga
me
tes
from
RrY
y p
are
nt
RY
Ry
rY
ry
RRYY RRYy RrYY RrYy
RRYy RRYy RrYy Rryy
RrYY RrYy rrYY rrYy
RrYy Rryy rrYy rryy
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• In reality you don’t get the exact ratio of results shown in the square.
• That’s because, in some ways, genetics is like flipping a coin—it follows the rules of chance.
Probability
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Question 1The passing on of characteristics from parents to offspring is __________.
D. allelic frequency
C. pollination
B. heredity
A. genetics
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The answer is B. Genetics is the branch of biology that studies heredity.
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Question 2What are traits?
AnswerTraits are characteristics that are inherited. Height, hair color and eye color are examples of traits in humans.
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Question 3Gametes are __________.
D. fertilized cells that develop into adult organisms
C. both male and female sex cells
B. female sex cells
A. male sex cells
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The answer is C. Organisms that reproduce sexually produce male and female sex cells, called gametes.
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Question 4Which of the following genotypes represents a plant that is homozygous for height?
D. tt C. tT
B. Hh
A. Tt
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The answer is D. An organism is homozygous for a trait if its two alleles for the trait are the same. It can be either homozygous dominant or homozygous recessive.
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Head, C-curly, c-straight
Eyes, S-slanted, s-straight
Father, CcSs (Curly hair, Slanted eyes)M
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CcS
s (C
urly
hai
r, S
lant
ed e
yes)
CS Cs cS cs
CS
Cs
cS
cs
CCSS CCSs CcSS CcSs
CCSs CCss CcSs Ccss
CcSS CcSs ccSS ccSs
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