Download Genetics Lecture III

Mendelian
Genetics
Genetics Lecture III
Biology Standards Covered
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2c ~ students know how random chromosome
segregation explains the probability that a particular
allele will be in a gamete
2e ~ students know why approximately half of an
individual’s DNA sequence comes from each
parent
2g ~ students know how to predict possible
combinations of alleles in a zygote from the genetic
makeup of the parents
Biology Standards Covered
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3a ~ Students know how to predict the probable
outcome of phenotypes in a genetic cross from the
genotypes of the parents and mode of inheritance
(autosomal or x-linked, dominant or recessive)
3b ~ Students know the genetic basis for Mendel’s
laws of segregation and independent assortment
Gregor Mendel
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Thought of as the “founder” of modern
genetic research
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He worked on an Austrian monastery in
the mid 1800’s
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His work with the “garden peas”
explained many unanswered questions
about genetics
Important Vocabulary
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Heredity – the passing on of
characteristics from parents to
offspring
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Traits – characteristics that are
inherited (for example your eye color)
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Genes – chemical factors that
determine traits
Important Vocabulary
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Gametes – individual male and female
sex cells
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Pollination – the transfer of pollen
(male gametes) to the pistil (female
part of a flower)
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Fertilization – the uniting of male and
female gametes
Chromosome Number
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Each organism on
this planet has a
specific number of
chromosomes

Humans (for example)
have 23 pairs of
chromosomes with a
total of 46
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Apes have 24 pairs for
a total of 48
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We share 99% of our
genome with chimps
and bonobos
Garden Pea plants
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The garden pea plants were “self
pollinating” when Mendel first
observed them
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This means that the offspring of each
pea plant was an exact copy of the
parent plant
Garden Pea plants
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Mendel wanted to change the “exact
copy” offspring
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He did so by “cross – pollinating”
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He basically pollinated one pea plant
with a completely separate one
Genes and Dominance
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Mendel found that when you make
certain “crosses” some of the traits
were hidden or masked
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When he made further crosses some of
these traits came back
Genes and Dominance
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This led Mendel to believe that there
were Dominant traits and less
dominant or recessive traits
The dominant trait is expressed by a
capital letter (for example D)
 The recessive trait is expressed by a
lower case letter (for example d)
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Mendel’s Experiment
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The original parent pea plants were called
the P generation
P Generation
Tall
Short
Mendel’s Experiment
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The first set of offspring in the experiment is
called the F1 – Generation
P Generation
Tall
Short
F1 Generation
Tall
Tall
Mendel’s Experiment
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The second set of offspring from the F1 –
Generation is called the F2 - Generation
P Generation
Tall
Short
F2 Generation
F1 Generation
Tall
Tall
Tall
Tall
Tall
Short
Alleles
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Alleles are different forms of a gene
 By
“form” we mean the amount or variety of traits
 If
there are three different possible colors for flowers
on a pea plant, then there are three possible Alleles
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For example:
a
TALL allele may be written as
a
short allele may be written as
T
t
Genotype vs. Phenotype
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The Genotype is the “genetic makeup”
 Homozygous TALL plants would be: T T
 Heterozygous TALL plants:
Tt
 Homozygous small:
tt
* The recessive trait is “short” and in order for
the plant to be short both alleles must be
lowercase (recessive)
Genotype vs. Phenotype
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The Phenotype is the “physical expression” of
the gene
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Dominance Rule:
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If a dominant allele is present in the Genotype, the
Phenotype will show the dominant trait
T is a dominant allele for TALL
 T T will yield a TALL plant
 T t will also yield a TALL plant
* The ONLY way a short plant will occur
is if BOTH alleles are recessive!! (tt)
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F1 Generation Cross
Tt x Tt Cross
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This diagram shows a
cross between the F1 –
generation
Both plants being
crossed are
Heterozygous
That is; they have both
alleles in their genotype
What would be the
outcome?
Probability in Genotypes
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In a Heterozygous cross
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1 out of 4 will be
homozygous dominant
(25%)
2 out of 4 (1/2) will be
heterozygous dominant
(50%)
1 out of 4 will be
homozygous recessive
(25%)
Mendel’s Principles
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Biological characteristics are inherited as genes
from parents to offspring
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Some forms (alleles) of genes are dominant and
others are recessive
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In most sexually reproducing organisms, each adult
has two copies of a gene – one from each parent
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The alleles for different genes usually segregate
independently from each other
Exceptions to Mendel’s Principles
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Some alleles are
neither dominant nor
recessive
Many traits are
controlled by multiple
alleles or genes
Incomplete Dominance –
when there isn’t a
dominant allele at all
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This appears as a third
allele (red, white, & pink)
Exceptions to Mendel’s Principles
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Codominance – a case in which both alleles
contribute to the phenotype of the organism
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If a red and white flower were crossed, codominance
would result in a red flower with white stripes or white
with red stripes
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Both alleles are expressed at the same time in the offspring
Multiple Alleles – if a gene has more than two
alleles
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This means that more than two possible alleles exist in
a population
Multiple Alleles
Exceptions to Mendel’s Principles
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Polygenic traits – when a trait is controlled by two
or more genes that can be found on similar or
completely different chromosomes
Human skin color is controlled by more than four individual
genes
 Human eye color is also considered polygenic
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