Unit 6 Outline

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Unit 6 Outline- Patterns of Inheritance
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I. Genetics developed from curiosity about
inheritance A. The Blending Hypothesis of
Inheritance 1. Trait- a variation of a
particular character 2. Blending
hypothesis a) Example Cross red flowers
with yellow flowers and they produce orange
flowers
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B. Mendels Plant Breeding Experiments 1.
Genetics- study of heredity 2. Gregor Mendel-
Father of Genetics
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a) particulate hypothesis of inheritance- states
that parents pass on to their offspring separate
and distinct factors that are responsible for
inherited traits b) Mendels first step was to
identify pea plants that were true-breeding 1)
When self-fertilized, a true- breeding plant
produces offspring identical in appearance to
itself generation after generation.
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3. Cross-fertilization- sperm from the pollen of
one flower fertilizes the eggs in the flower of a
different plant
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II. Mendel discovered that inheritance follows
rules of chance A. Mendels Principle of
Segregation 1. Hybrids- the offspring of 2
different true-breeding varieties a) The
parental plants are called the P generation
and the hybrid offspring are the F1
generation. When the F1 plants self-fertilize
or fertilize each other, their offspring
are the F2 generation.
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2. Monohybrid cross- the parents only differ in
one character a) Example
Figure 10-3For each monohybrid cross, Mendel
cross-fertilized true-breeding plants that were
different in just one characterin this case,
flower color. He then allowed the hybrids (the F1
generation) to self-fertilize.
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1) Mendel concluded that the F1 plants must have
carried two factors for the flower color
characteristic (one for purple and one for
white). 2) Today, Mendels factors are called
genes.
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3. Mendel used monohybrid crosses to investigate
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Figure 10-4Mendel examined the inheritance
patterns of seven different pea-plant characters.
For each character, one of the two parent traits
disappeared in the F1 hybrids, but reappeared in
approximately one quarter of the F2 generation
(not shown here).
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a) Mendel developed 4 hypotheses 1) There are
alternative forms of genes called alleles.
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2) For each inherited character, an organism has
two alleles for each gene controlling that
character (one from each parent). a)
Homozygous- when the two alleles are the
same b) Heterozygous- when the two alleles
are different
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3) When only one of the two alleles in a
heterozygous individual appears to affect the
trait, that allele is said to be dominant. The
other allele that does not appear to affect the
trait is said to be recessive.
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4) The 2 alleles for a character segregate or
separate during the formation of gametes or sex
cells, so that each gamete carries only one
allele for each character. This is called the
principle of segregation.
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B. Probability and Punnett Squares 1. Punnett
square- a diagram that shows the probabilities
of the possible outcomes of a genetic cross
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Figure 10-5Punnett squares are used to calculate
the probabilities of outcomes resulting from a
genetic cross. The grid on the left predicts the
probabilities of certain combinations of the two
sides (alleles) of two pennies. The Punnett
square on the right predicts that the two alleles
for flower color will randomly pair in the same
proportions as the two sides of the pennies.
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C. Genotype and Phenotype 1. Genotype- the
genetic makeup or combination of alleles (like
PP) 2. Phenotype- an observable trait (like
purple flowers)
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D. The Testcross 1. A testcross breeds an
individual of an unknown genotype, but
dominant phenotype with a homozygous
recessive individual.
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E. Mendels Principle of Independent
Assortment 1. Dihybrid cross- crossing
organisms differing in 2 characters a) Mendel
crossed a true- breeding plant from a round
yellow seed (genotype RRYY) with a
true-breeding plant grown from a wrinkled
green seed (genotype rryy).
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1) This produced hybrid peas that were
heterozygous for both traits (RrYy). Their
phenotype was round and yellow.
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b) Mendel allowed the F1 generation to
self-fertilize. This produced four
phenotypes 1) A phenotypic ratio of 9331 was
produced. 2) Principle of Independent
assortment- during gamete formation in and F2
cross, a particular allele for one character
can be paired with either allele of another
character
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Figure 10-8A Punnett square predicts a 9 3 3
1 phenotypic ratio for the F2 offspring
resulting from the dihybrid cross for seed shape
and color.
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III. There are many variations of Inheritance
patterns A. Intermediate Inheritance 1.
Inheritance in which heterozygotes have a
phenotype intermediate between the
phenotypes of the 2 homozygous. a)
example Andalusian chickens cross a black
parent with a white parent and you get
blue offspring
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B. Multiple Alleles 1. Codominance
heterozygotes express both traits. a)
example Blood type
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Figure 10-10There are three alleles for blood
type in the human population. (But note that any
one person has only two alleles for blood type.)
The combinations of these alleles result in six
genotypes and four phenotypes. Alleles IA and IB
are codominant. Allele i is recessive.
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C. Polygenic Inheritance 1. The combined effect
of 2 or more genes on a single character a)
example height and skin color
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IV. Meiosis explains Mendels principles A.
Chromosome Theory of Inheritance 1. States
that genes are located on chromosomes, and the
behavior of chromosomes during meiosis and
fertilization accounts for inheritance
patterns a) gene locus specific
location of a gene on a chromosome
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B. Genetic Linkage and Crossing over 1. genetic
linkage the tendency for alleles on one
chromosome to be inherited together
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Figure 10-14Crossing over can recombine gene
loci on homologous chromosomes. This is unlikely
when the genes are very close together (Scenario
1). A crossover is more likely to recombine the
alleles when the genes are far apart (Scenario 2).
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V. Sex-linked traits have unique inheritance
patterns A. Sex-linked genes 1. Any gene that
is located on the sex chromosome is called a
sex- linked gene. 2. In humans, most
sex-linked genes are found on the X
chromosome. 3. Sex-linked genes were
discovered by Thomas Hunt Morgan.
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