single gene inheritance chapter 2. - the two members of an homologous pair carry alleles for the...
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Single Gene InheritanceChapter 2
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- the two members of an homologous pair carry alleles for the same genes and, therefore, affect the same traits.
Gene are located on chromosomes
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Chromatin – 25% protein + 75% DNAChromatin condenses
into chromosomes at cell division
The stuff that makes up chromosomes
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Packaging of DNA
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Helix Nucleosome Fibers Loops
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Loops Coils Chromosome
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A chromosome, A closer view
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DefinitionsPhenotype - the visible expression of
information contained in the genetic make-up of an individual
Genotype - the genetic make-up, latent or expressed. The sum of all genes present in an individual.
P1 - the parental lines
F1 - the first filial generation, the offspring resulting from a cross. F2/ F3 the second and third generations.
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Genetic polymorphismsThe stuff of genetic variation
1 locus (monogenic), 2 different alleles 3 combos possible
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Mendel’s Principles of Inheritance
1) Genetic characteristics are controlled by unit factors (elemente) that exist in pairs in individual organisms.
These unit factors represent units of inheritance today called genes.
Alternative forms of a single gene are called alleles.
Individuals can have identical alleles (homozygous) or different alleles (heterozygous) for a single gene.
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An example: seed color
+o oX P1 (parental)
F1
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Self-pollination of the F1
+o oX
3 1
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Dominance/Recessiveness
When two unlike alleles responsible for a single character are present in a single individual, one is dominant (expressed) to the other which is said to be recessive (silent).
+o oXGG gg
P1
Gg
F1
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Segregation
During formation of the gametes, the paired alleles separate or segregate randomly so that each gamete receives either with equal likelihood.
G
G
g
g
Possible egg alleles
Gg
GgGG
gg
Punnett square
Possible sperm alleles
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Sample Problem
• Flower position in pea plants is another single gene trait. Axial flower position is dominant to terminal. If we cross a true breeding plant with axial flowers to one with terminal flowers, what phenotypes can we expect in the F1 progeny? in the F2 of a selfing of the F1?
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Sample Problem
• Having dimples is a dominant trait in humans. I have no dimples but my father and mother do. All my siblings have dimples as well. Am I adopted??
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More on Single Gene Inheritance
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P1 red X white
F1 pink
Query
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Self F1 pink X pink
red pink white 1 2 1
F2
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Self F1 pink X pink (R1RR22) ) (R1RR22))
Incomplete (partial) dominance
R1 RR22
Possible sperm cells
Possible egg cells
R1
RR22
R1RR22
pinkpink
R1RR11 R1RR22
pinkpink
R2RR22
Intermediate expression of the phenotype.
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But what about the yellow ?
This could be the effect of multiple alleles involved in color production.
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Multiple alleles
An example: the ABO blood types in humans
Genotype Phenotype
IAIA
IAIO
IBIB
IBIO
IAIB
IOIO
A
B
AB
O
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Sample problem
• Red-green color blindness is a sex linked, recessive allele of a single gene trait. My brother-in-law is red-green color blind. My sister is not, but my father might have been. My sister and her husband have 4 sons. My mother (their grandmother) was worried that they would be color blind. Should she have been concerned?
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Genes can be Pleiotropic
-single gene, multiple effects
ex. dwarf
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Chromosomal Basis of Inheritance
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Meiosis (the prelude to
sexual reproduction)
For sexual reproduction to occur, chromosomes must be duplicated and divided between the gametes.
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Meiosis I
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Meiosis II
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Meiosis
Each gamete contains only one member of each homologous pair.
There are 2 steps of meiosis - 2 cell divisions, but only
1 replication of chromosomes.
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Summary
Mendel’s “rules of the game”
3) Segregation- during formation of the gametes, the paired unit factors separate or segregate randomly.
1) Genes occur in pairs - Genetic characteristics are controlled by genes that exist in pairs.
2) Dominance/Recessiveness- When two unlike alleles responsible for a single character are present in a single individual, one is dominant (expressed) to the other which is said to be recessive (silent).
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Discovering genes via Mutant analysis
• Generating mutants
– Chemical mutagenesis (EMS)
– base transition, point mutation
– Radiation
– deletions
– Transposons/ T-DNA tags
– insertion/deletions (indels)
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And observing segregation ratios
• Planned crosses and Punnett squares, Pedigree analysis
– Mutant or polymorphism analysis• Autosomal dominant/recessive
• Sex-linked genes