Download Introduction to Genetics

Chapter 11
Gregor Mendel
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Father of Genetics
Born in 1822 in the Czech Republic
Priest in monastery
Fertilization by testing pea plants
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Cross pollination
Cross Pollination
Genes and Dominance
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Gene: a specific characteristic that varies from
one individual to another.
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Alleles: different forms of a gene.
Principle of Dominance
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Dominant trait: overshadows the recessive
trait
Recessive trait: only can be seen when with
another recessive trait
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Homozygous: organisms that have two
identical alleles for a particular trait.(TT)
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True breeding
Heterozygous: organisms that have two
different alleles for a particular trait. (Tt)
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Hybrid
Terms for Genetic Crosses
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Phenotype: physical characteristics
Genotype: genetic makeup
P group: parental group
F1: First offspring or generation
F2: second generation
Rules for Punnett Squares
1.
2.
3.
4.
Determine the symbols for the traits
Determine the genotype of the parents.
Make the cross
Answer the questions
Practice Problem #1
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Mendel found that the allele for tall (T) pea
plants is dominant to the allele for short (t).
What offspring phenotypes would be expected
from the following parents:
Male: TT
Female: tt
Male: Tt
Female: tt
Male: Tt
Female: Tt
Practice Problem #2
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The allele for axial flowers (A) in peas is
dominant to the allele for flowers borne
terminally (a). What phenotypic ratios would
you expect among the offspring of a cross
between a known heterozygous axialflowered plant and one whose flowers are
terminal?
Practice Problem #3
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Two drosophilas (fruit flies) with normal
wings are crossed. Among 123 progeny, 91
have normal wings and 32 have dumpy wings.
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A. which trait is dominant?
B. what were the genotypes of the parents?
Practice Problem #4
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Black fur in guinea pigs is a dominant trait
and white is the recessive trait. When a
homozygous black (BB) guinea pig is crossed
with a homozygous white one (bb), what is
the phenotype and genotype of the F1
generation?
Law of Segregation
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Alleles segregate from each other during the
formation of gametes
EX: allele for tall separates from allele for
short
Incomplete Dominance
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Cases in which one allele is not completely
dominant over another
Heterozygous phenotype is somewhere
between the two homozygous phenotypes.
Incomplete Problem
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A homozygous red flower is crossed with a
homozygous white flower. This produces
pink flowers in their offspring. What would
the offspring look like if a pink flower crosspollinated with a red flower?
Codominance
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Both alleles contribute to the phenotype.
Both show
Codominance Problem
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In certain varieties of chickens, the allele for
black feathers is codominant with the allele
for white feathers. Offspring are speckled
with white and black. What would happen if
a chicken who is homozygous black feathers
is crossed with a chicken who is speckled.
Multiple Alleles
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Genes that have more than two alleles
EX: coat color in rabbits. Determined by a
single gene that has at least four different
alleles.
Polygenic Traits
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Traits controlled by two or more genes
EX: wide range in skin color in humans
comes about because more than four different
genes control this trait.