7/28/2019 Principles of Inheritance and Variations
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Biology Project Work
TOPIC Principles Of Inheritance And Variations
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Topics Mendel and the process of science
Mendels principles of inheritance Crosses: Parental, F1, F2, and test
Chromosome mapping
Variations on Mendelian inheritance
In-breeding and out-breeding
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Breeders knew of hybrids at the time that Mendel started his
experiments (in 1856), but only knew that progeny resembledeach parent, and that some resembled grandparents. No oneknew why.
Mendel developed a systematic way of understandingheredity, using very good scientific technique.
He applied the scientific method combined with mathematical
analysis to demonstrate:1. Unit characteristics2. Segregation of those characteristics
3. Dominance of some characteristics
4. Independent assortment of characteristics
What Did Mendel Show?
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Mendel Carefully Chose His Organism Pisum sativum: the garden pea
1. Very productive: produces many peas (large N: good statistics)
2. Short life cycle: produce many generations in a short time3. Typically self-pollinating: good for inbreeding4. Easily cross-pollinated due to flower structure
Has 7 distinct phenotypic characteristics:1. Yellow versus green seeds
2. Round versus wrinkled seeds3. Green versus yellow pods4. Tall versus short plants5. Fat versus tight pods
6. White versus grey seed coats7.
Flowers: end o stem versus alon the len th o stem
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The Pea Flower The flower is well-suited to this study, which required
controlled pollination
Protects the reproductive apparatus Male (anther: makes pollen)
Female (ovary: makes ova)
Stigma arises from the ovary
Good for inbreeding and artificial breeding
Can bag flowers, easily cut off the anthers toprevent further pollination
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One of Mendels Crosses Mendel knew nothing of the chemical basis
for inheritance
He had to work very carefully andexamine ONLY the outward appearance ofthe plants
He compared what we call today thephenotype,which is the outwardexpression of the genes
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A Parental Cross Homozygous Tall (TT) X
Homozygous Short (tt)
All of the F1generation aretall.
Demonstrates the concept ofdominance
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Cross of F1 Cross the F1 generationat randomto
itself.
1/4 of F2 (the progeny of theF1 cross)
are short Mendel showed that the short plant bred
true, so it was aparental type.
That a parental phenotype disappeared
and then reappeared destroys the conceptof blending.
It was masked in the F1 Instead, it supports the concept of unit
characteristicsgenes, as we nowunderstand them
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Segregation of Characteristics Mendel recognized that the only way to achieve the masking of
the characteristics was to segregate (separate) them somehow inthe gametes.
The characters must have been separated during formation of thepollen (male) and the ova (female).
This is the concept of segregation.
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Chromosomal Basis of Segregation Segregation occurs in meiosis
at Metaphase I and then at
Metaphase IIAs a result, the ova or sperm
contribute different homologouschromosomes to the progeny
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Mendels Concepts & Modern Terminology Mendel worked in a strictly conceptual framework he had NO IDEA of
the nature of the genetic material
Today we know his characters are the products of genes. The collection ofgenes is the genotype, or genome.
A gene is positioned at a given locus(loci, plural).
Position is very important, it can define the effect of a gene
Regulation of genes is position-dependent.
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Alleles Different varieties of genes are calledalleles.
Since diploid (2n) organisms have 2 sets of chromosomes, thereare two possible alleles for each gene locus in a 2n organism.
If those alleles are the same, the organism is homozygousat thatlocation
If different, it is heterozygous
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Homozygotes & Heterozygotes We indicate the genotype by the letters:
Dominant alleles are represented by
capital letters Recessive alleles are represented by lowercase letters
Examples:
TT is homozygous dominant
Phenotype is dominant
tt is homozygous recessive
Phenotype is recessive
Tt is heterozygous
Phenotype is dominant
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Homologs & Alleles 2n organisms have a chromosome received from
each parent.
Called homologous chromosomes These have alleles of A, B, C & D.
A & a, B & b, C & c, D & d
C is NOT an allele to D; nor is A
or B A locusis a physical location of DNA that
encodes for a protein product; i.e. agene
A, B, C & D are at specific loci
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Diploid cells
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Homologous and Non homologous Chromosomes: Definitions
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A Test Cross What if you only know that
you have a black and a
brown guinea pig (i.e. youdont know the genotype)?
Do a test cross
Cross with a known true-
breeding recessive brownguinea pig
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Test Crosses: Mechanism On inspection, heterozygotes are
indistinguishable from homozygotedominants.
Phenotypes can be tested for genotype withtest crosses
Test crosses are easily checked w/ Punnettsquares.
Always cross the unknown into homozygous
recessive to reveal the dominant genotype. The distribution of alleles follows the
product law and the sum law.
A
A
a a
Aa
Aa
Aa
Aa
A
a
a
Aa
aa
Aa
aa
a
All A phenotype: Homozygous A
A phenotype; a phenotype
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Mapping Chromosomes If there is 8% crossover
between A & C, 5%crossover between A & B,and 3% between B and C,we can arrange them inthis manner (a):
If the crossover between Aand C was 2% instead of8%, we could rearrange as
in (b).
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Incomplete Dominance
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Inbreeding Inbreeding was useful to Mendel for producing true-breeders
But it can concentrate undesirable attributes, which are often recessive.
Human inbreeding tends to increase the frequency of rare geneticdisorders, although the Tamil of India extensively intermarry with littleor no ill effect.
Australian sheepdogs must be euthanized if they are white puppies: they
become blind, deaf, and have many skeletal muscle problems The English bulldog has significant bone structure problems many
members of this breed can barely mate and bear puppies, and many havedifficulty breathing
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Outbreeding Outbreedingmeans breeding of different strains or stocks of the same organism
Breeding to different strains tends to produce more robust individuals, referred to as hybridvigor
Produces individuals that are multiply heterozygous at many loci Masks recessives in many different traits
In many cases, multiple heterozygosis seems to confer additional advantage beyond the maskingof recessive traits. This is referred to as over dominanceor the Heterozygote Advantage.
Most modern grain crops are multiple hybrids: they are more resistant to disease, tolerate
changes in weather, etc. A simple example: Sickle Cell Anemia
People heterozygous for sickle-cell anemia have a distinct advantage in Africa, where thetrait improves resistance to malaria by interfering with the parasite life cycle.
The moderate illness due to the heterozygote sickle cell condition is not severely disabling.
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Thank YouForWatching
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