Download Chapter 8- Genetics

Chapter 8- Genetics
Genetic Terminology: (in notebook after the test)
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Trait - any characteristic that can be passed from parent to offspring
Heredity - passing of traits from parent to offspring
Genetics - study of heredity
Alleles - two forms of a gene (dominant & recessive)
Dominant - stronger of two genes expressed in the hybrid; represented by a capital letter (R)
Recessive - gene that shows up less often in a cross; represented by a lowercase letter (r)
Genotype - gene combination for a trait (e.g. RR, Rr, rr)
Phenotype - the physical feature resulting from a genotype (e.g. tall, short)
Homozygous genotype - gene combination involving 2 dominant or 2 recessive genes (e.g. RR or Rr);
also called pure
Heterozygous genotype - gene combination of one dominant & one recessive allele (e.g. Rr); also
called hybrid
Monohybrid cross - cross involving a single trait
Dihybrid cross - cross involving two traits
Punnett Square - used to solve genetics problems
Gregor Mendel: (notes)
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Austrian monk
Studied science & math at the University of Vienna
Formulated the laws of heredity in the early 1860's
Did a statistical study of traits in garden peas over an eight year period
Why peas, Pisum sativum? (discussion)
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Can be grown in a small area
Produce lots of offspring
Produce pure plants when allowed to self-pollinate several generations
Can be artificially cross-pollinate
Mendel's Experiments: (notes)
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Mendel studied simple traits from 22 varieties of pea plants (seed color & shape, pod color & shape,
etc.)
Mendel traced the inheritance of individual traits & kept careful records of numbers of offspring
He used his math principles of probability to interpret results
Mendel studied pea traits, each of which had a dominant & a recessive form (alleles)
The dominant (shows up most often) gene or allele is represented with a capital letter, & the recessive
gene with a lower case of that same letter (e.g. B, b)
Mendel's traits included:
a. Seed shape --- Round (R) or Wrinkled (r)
b. Seed Color ---- Yellow (Y) or Green (y)
c. Pod Shape --- Smooth (S) or wrinkled (s)
d. Pod Color --- Green (G) or Yellow (g)
e. Seed Coat Color --- Gray (G) or White (g)
f. Flower position --- Axial (A) or Terminal (a)
g. Plant Height --- Tall (T) or Short (t)
h. Flower color --- Purple (P) or white (p)
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Mendel produced pure strains by allowing the plants to self-pollinate for several generations
These strains were called the Parental generation or P1 strain
Mendel cross-pollinated two strains and tracked each trait through two
generations (e.g. TT x tt )
T
T
Trait - plant height
Alleles - T tall, t short
P1 cross TT x tt
t
t
Tt
Tt
Tt
Tt
genotype -- Tt
phenotype -- Tall
genotypic ratio --all alike
phenotypic ratio- all alike
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The offspring of this cross were all hybrids showing only the dominant trait & were called the
First Filial or F1 generation
Mendel then crossed two of his F1 plants and tracked their traits; known as an F1 cross
Trait - plant height
Alleles - T tall, t short
F1 cross Tt x Tt
T
t
T
TT
Tt
ct
Tt
tt
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genotype -- TT, Tt, tt
phenotype -- Tall & short
genotypic ratio --1:2:1
phenotypic ratio- 3:1
When 2 hybrids were crossed, 75% (3/4) of the offspring showed the dominant trait & 25% (1/4)
showed the recessive trait; always a 3:1 ratio
The offspring of this cross were called the F2 generation
Mendel then crossed a pure & a hybrid from his F2 generation; known as an F2 or test cross
Trait - Plant Height
Alleles - T tall, t short
T
T
F2 cross
T
TT
TT
TT x Tt
t
Tt
Tt
genotype - TT, Tt
phenotype - Tall
genotypic ratio - 1:1
phenotypic ratio - all alike
t
t
F2 cross
T
Tt
Tt
tt x Tt
t
tt
tt
genotype - tt, Tt
phenotype - Tall & short
genotypic ratio - 1:1
phenotypic ratio - 1:1
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50% (1/2) of the offspring in a test cross showed the same genotype of one parent & the other 50%
showed the genotype of the other parent; always a 1:1 ratio
Results of Mendel's Experiments:
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Inheritable factors or genes are responsible for all heritable characteristics
Phenotype is based on Genotype
Each trait is based on two genes, one from the mother and the other from the father
True-breeding individuals are homozygous ( both alleles) are the same
Law of Dominance states that when different alleles for a characteristic are inherited (heterozygous),
the trait of only one (the dominant one) will be expressed. The recessive trait's phenotype only
appears in true-breeding (homozygous) individuals
Law of Segregation states that each genetic trait is produced by a pair of alleles which separate
(segregate) during reproduction
Law of Independent Assortment states that each factor (gene) is distributed (assorted) randomly and
independently of one another in the formation of gametes
Other Patterns of Inheritance:
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Incomplete dominance occurs in the heterozygous or hybrid genotype where the 2 alleles blend to
give a different phenotype
Flower color in snapdragons shows incomplete dominance whenever a red flower is crossed with a white
flower to produce pink flowers
In some populations, multiple alleles (3 or more) may determine a trait such as in ABO Blood type
Alleles A & B are dominant, while O is recessive. This is known as codominance because both traits are
displayed
Genotype Phenotype
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IOIO
Type O
IAIO
Type A
IAIA
Type A
I BI O
Type B
IBIB
Type B
IAIB
Type AB
Polygenic inheritance occurs whenever many variations in the resulting phenotypes such as in hair,
skin, & eye color
The expression of a gene is also influenced by environmental factors (example: seasonal change in fur
color)
Pedigrees:
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Also called a family tree
Squares represent males and circles represent females
Horizontal lines connecting a male and female represent mating
Vertical lines extending downward from a couple represent their children
A shaded symbol means the individual possess the trait
Half-shaded symbols are carriers
Sex Linkage:
If a trait is autosomal, it will appear in both sexes equally.
If a trait is sex linked, it will appear more frequently in males because the trait is located in the x chromosome.
Most sex linked traits are recessive, and since a male only has one x chromosome, a male who carries the trait
will express it. A female who carries the recessive trait will not express the trait if there is a dominant allele on
her other X chromosome.
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In humans, another well-known X-linked traits is hemophilia (free bleeders that lack clotting factors
in their blood)
One of the most famous genetic cases involving hemophilia goes back to Queen Victoria who was a
carrier for the disorder and married Prince Albert who was normal
Their children married other royalty, and spread the gene throughout the royal families of Europe
Linked genes:
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Each chromosomes has 1000's of genes
All genes on a chromosome form a linkage group that stays together except during crossing-over
Some genes located on the same chromosome tend to be inherited together
Linked genes were discovered by Thomas Hunt Morgan while studying fruit flies
Linked alleles do not obey Mendel's laws because they tend to go into the gametes together
Crosses involving linked genes do not give same results as unlinked genes
Environmental effects:
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Sometimes genes will not be fully expressed owing to external factors. Example: Human height
may not be fully expressed if individuals experience poor nutrition.
Gene interaction:
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Many biological pathways are governed by multiple enzymes, involving multiple steps. If any one
of these steps are altered. The end product of the pathway may be disrupted.
Continuous Variation
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Many traits may have a wide range of continuous values. Eg. Human height can vary considerably.
There are not just "tall" or "short" humans