Download HEREDITY - GENETICS

Lesson Objective:
HEREDITY - GENETICS
By the end of the lesson (s), I can:
1. Describe Mendelian genetics.
2. Differentiate between dominant and recessive alleles.
3. Define the homozygous, heterozygous, genotype, and phenotype.
4. Discuss Mendel’s laws of inheritance.
5. Diagram and predict the results of mono and dihybrid crosses.
6. Identify the effects of multiple alleles, codominance, and incomplete dominance
on phenotype.
Vocabulary:
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Genetics
Heredity
Trait
Pollination
Self-pollination
Cross-pollination
True-breeding
P generation
F1 generation
F2 generation
Dominant
Recessive
Law of segregation
Law of independent assortment
Molecular genetics
Lesson Questions:
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Allele
Genotype
Phenotype
Homozygous
Heterozygous
Probability
Monohybrid cross
Dihybrid cross
Punnett square
Genotypic ratio
Phenotypic ratio
Testcross
Complete dominance
Incomplete dominance
Codominance
1. What is Medelian inheritance?
2. What is the difference between dominant and recessive alleles?
3. What are the definitions of homozygous, heterozygous, , genotype, and
phenotype?
4. What are Mendel’s laws of inheritance?
5. How are the results of monohybrid and dihybrid crosses diagrammed?
6. What are the effects of multiple alleles, codominance, and incomplete dominance
on phenotype?
Focus Question:
1. How do the structures of organisms enable life’s functions?
2. Why do individuals of the same species vary in how they look, function, and
behave?
Overarching questions:
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How do organisms live, grow, respond to their environment, and reproduce?
How are the characteristics of one generation passed on to the next?
How can individuals of the same species and even siblings have different
characteristics?
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HEREDITY - GENETICS
FUNDAMENTALS OF GENETICS
Genetics is the field of biology that is devoted to the understanding of hoe characteristics are
transmitted from parents to offspring. Genetics was founded with the work of Gregor Johnan
Mendel.
1. Mendels Legacy
 Born 1822, Mendel was a monk who was charged of taking care of the gardens.
 1851 he entered the University of Vienna to study science and mathematics.
 Studies included training in the then-new field of statistics.
 This knowledge proved valuable in his research on heredity
 Heredity being the transmission of characteristics from parent to offspring.
 Mendel studied many plants, but he is remembered for his experiments with
Pisum sativum, a species of garden peas.
2. Mendel’s Garden Peas
 Mendel observed seven characteristics of pea plants. Each characteristic occurred
in 2 contrasting traits.
 Characteristic is a heritable feature, such as flower color.
 A trait is a genetically determined variant of a characteristic, such as
yellow flower color.
 Mendel used his knowledge of statistics to analyze his observations
 The characteristics Mendel observed were
 Plant Height (traits: long and short)
 Flower position along the stem (traits: axial and terminal)
 Pod color (traits: green and yellow)
 Pod appearance (traits: inflated and constricted)
 Seed texture (traits: round and wrinkled)
 Seed color (traits: yellow and green)
 Flower color (traits: purple and white)
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HEREDITY - GENETICS
Mendel collected seeds from his pea plants and carefully recorded each plant’s
traits and seeds.
 Next year he planted the seeds. He observed that purple-flowering plants
grew from most of the seeds obtained from purple-flowering plants but
that white-flowering plants grew from some of the seeds of purpleflowering plants.
 When experimenting with the characteristic of plant height, he observed
that while tall plants grew from most of the seeds obtained from tall
plants, short plants grew from some of the seeds obtained from tall plants.
Mendel’s methods
 He was able to observe how the traits were passed from one generation to
the next by carefully controlling how pea plants were pollinated.
 Pollination occurs when pollen grains produced in the male reproductive
parts of a flower, called anthers, are transferred to the female reproductive
parts of the flower, called stigma.
 Self-pollination occurs when pollen is transferred from the anthers of a
flower to the stigma of either that flower or another flower o the same
plant. Self-pollination can be prevented by removing all of the anthers
from the flowers of a plant.
 Cross-pollination occurs between flowers of two plants. Pea plants
normally reproduce through self-pollination. Cross-pollination can be
performed by manually transferring pollen from the flower of a second
plant to the stigma of the antherless plant.
 By preventing self-pollination and manually cross-pollinating pea
plants, Mendel selected parent plants that had specific traits and
observed the traits that appeared in the offspring.
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Mendel’s experiments
 Began by growing plants that were true-breeding for each trait.
 Plants that are true-breeding, OR PURE, for a trait always produce
offspring with that trait when they self-pollinate.
 He produced true-breeding plants by self-pollinating the pea plants
for several generations. Eventually he obtained 14 true-breeding
plant types, one for each of the 14 traits he observed.
 Mendel cross-pollinated pairs of plants that were true-breeding for
contrasting traits of a single characteristic.
 He called the true-breeding parents the P generation.
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HEREDITY - GENETICS
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When the plants matured, he recorded the number of each type of
offspring produced by each cross. ‘
He called the offspring of the P generation the first filial generation
or F1 generation.
 He allowed the flowers of the F1 generation to selfpollinate and collected the seeds
 Mendel called the plants in this generation the second filial
generation, or F2 generation.
 Following this process Mendel performed hundreds of
crosses and documented the results of each by counting and
recording the observed traits of every cross.
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Mendel’s results and conclusions
 One of Mendel’s experiment illustrated that when one truebreeding green pod plant (P generation) and one true-breeding
yellow pod plant (P generation) was crossed only green podded
plants resulted in the F1 generation.
 He then allowed the F1 generation to self-pollinate and planted the
resulting seeds. The F2 generation grew and he noticed that ¾ of
the plants had green pods and ¼ had yellow pods
 His observations and careful records led him to HYPOTHESIZE
that something within the pea plants controlled the characteristics
observed.
 He called the controls FACTORS or ALLELES.
 He hypothesized that each trait was inherited by means of a
separate allele.
 Because the characteristics had 2 alternative forms, he
reasoned that a PAIR of alleles must control each trait.
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HEREDITY - GENETICS
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 Recessive and Dominant traits
 The above pattern resulted in thousands of crosses. Therefore he
hypothesized that the trait appearing in the F1 generation was
controlled by a dominant allele because it masked, or dominated
the other allele for the other trait in the pair.
 He thought that the trait that did not appear in the F1 generation
but reappeared in the F2 generation was controlled by a recessive
allele.
 Law of segregation
 Mendel concluded that the paired alleles separated during the
formation of reproductive cells (meiosis).
 The law of segregation states that a pair of alleles is segregated, or
separated, during the formation of gametes.
 Law of independent assortment
 Mendel also crossed plants that differed in 2 characteristics, such
as flower color and seed color.
 The data from these more complex crosses indicated that
traits produced by dominant alleles do not necessarily
appear together.
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 A green seed pod produced by a dominant allele
could appear in a white-flowering pea plant.
 He concluded that the alleles for individual
characteristics are not connected.
 The random separation of homologous autosomes is
called independent assortment.
 The law of independent assortment states that alleles
separate independently of one another during the formation
of gametes.
Support for Mendel’s conclusions
 Most of Mendel’s findings agree with what we know about molecular
genetics.
 Molecular genetics is the study of the structure and function of
chromosomes and genes.
 A gene is a segment of DNA on a chromosome that
controls a particular hereditary trait.
 The nomenclature that is used today:
 Dominant alleles use a capital letter and
 Recessive alleles use a lower case letter.
 Mendel’s law of independent assortment is supported by the
independent segregation of chromosomes to gametes during
meiosis.
 Therefore, the law of independent assortment is observed only for
genes located on separate chromosomes or located far apart on the
same chromosome.
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