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GENE SEGREGATION & INTEGRATION
A. Law of Segregation
• B. Law of Independent Assortment
• C. Segregation and Assortment in Haploid Organisms
• D. Dominance Relationship
• E. Multiple Alleles
• F. Lethal Genes
• G. Modifier Genes
• H. Gene interactions
• I. Pseudoalleles
• J. Environmental Influence
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A GENE is an operational region of the chromosomal DNA, part of which can be transcribed into a
functional RNA at the correct time and place during development. Thus,
the gene is composed of the transcribed region and adjacent
regulatory regions (Griffiths et al.)
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ALLELE – any 2 or more related genes of a trait
DOMINANT allele: expresses its effect over another allele; masks the recessive allele in the heterozygous organism
RECESSIVE allele: masked in a heterozygous individual by the presence of dominant allele
DOMINANT vs. RECESSIVE
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IMPORTANT!!! Dominance does not determine the relative FREQUENCY of a gene in a population.
GENOTYPE: the genetic constitution
PHENOTYPE: visible
appearance
GENOTYPE GENOTYPE PHENOTYPE
PP Homozygous
purple
Purple
Pp Heterozygous
purple
Purple
pp Homozygous
white
white
GENOTYPE vs. PHENOTYPE
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DEFINE THE FOLLOWING:
P: parents
F1: first filial generation
F2: 2nd filial generation
Self pollination
Cross pollination
True-breeding
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Law of Segregation
MONOHYBRID CROSS – a cross between 2 individuals involving
1 character
Allele pairs separate and each goes into a different gamete.
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Punnett Square
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Law of Independent Assortment The pair of factors for one trait segregate independently of the factors for other traits. All possible combinations of factors can occur in the gametes.
Alleles from one locus segregate into gametes independently of those from another locus
OR
DIHYBRID CROSS - A cross between 2 individuals involving 2 characters
PHENOTYPES: 9 Tall plant, green pod: T_G_ 3 Tall plant, yellow pod: T_gg 3 Short plant, green pod: ttG_ 1 short plant, yellow pod: ttgg
GENOTYPES: TTGG (homozygous tall, homozygous green) = 1 TTGg (homozygous tall, heterozygous green) = 2 TTgg (homozygous tall, homozygous yellow) = 1 TtGG (heterozygous tall, homozygous green) = 2 TtGg (heterozygous tall, heterozygous green) = 4 Ttgg (heterozygous tall, homozygous yellow) = 2 ttGG (homozygous short, homozygous green) = 1 ttGg (homozygous short, heterozygous green) = 2 ttgg (homozygous short, homozygous yellow) = 1
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DOMINANCE RELATIONSHIP
• Complete Dominance
• Incomplete or No Dominance
• Co-dominance
• Overdominance
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the heterozygote displays a phenotype
that is an intermediate between the phenotypes
of both homozygote parents
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occurs when the phenotypes
of both parents are
simultaneously expressed in
the same offspring
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Question: Could a child with blood type O be produced from parents with blood types A and B?
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/AO /BO
OO / AA/ BB / AB
P
F
P F
P F
P F
Question: A mother and her child have blood type O. Which blood group can the father NOT belong to?
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/AO /BO
OO / AA/ BB / AB
• ABO blood groups: Bombay phenotype
The ABO blood types are based on the presence or absence of specific sugar residues on the surface of RBC. Almost all individuals possess molecules called the H substance on their RBC.
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• ABO blood groups: Bombay phenotype
Individuals with the IA allele express an enzyme that attaches the sugar N-acetylglucosamine (AcGalNH) to the H substance. The addition of the terminal AcGalNH on the H substance produces the A antigen and this antigen on the surface of RBC is the basis of the type A phenotype.
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• ABO blood groups: Bombay phenotype
Individuals with the IB allele express an enzyme that attaches the sugar galactose to the H substance. The presence of a terminal Gal results in the B antigen on RBC and the type B phenotype.
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• ABO blood groups: Bombay phenotype
Individuals with the IOIO
allele have a mutation in the I gene that results in a nonfunctional protein. Thus individuals who are homozygous for the IO allele lack an enzyme to modify the H substance. - only have the H substance on their RBC (test negative for either A or B antigens) They are blood type O.
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• ABO blood groups: Bombay phenotype
The formation of the H substance requires the H gene. H gene produces an enzyme that adds a sugar residue (fucose) to the H substance precursor, forming the complete H substance. Most people have the dominant H allele of this gene, which codes for the functional enzyme.
Some people have the recessive h allele which does not code for a functional enzyme. Thus hh people cannot produce the H substance.
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• Predicting the ABO blood type of children based on the phenotypes of their parents should be approached with caution.
• There is a third antigen (H) on the surface of red cells that can prevent the expected ABO blood type from occurring.
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type O blood child can also be born to parents who do not have the O allele if a
recessive form of the allele for the H antigen (or h) also is inherited
from both parents.
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Alleles that cause an organism to die only when present in homozygous condition, where the gene involved is an essential gene.
Fully dominant lethal allele: the carrier is killed in both homozygous and heterozygous conditions.
Completely lethal genes usually cause death
…of the zygote
…later in the embryonic development or
…even after birth or hatching.
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Conditional lethal
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Some lethal alleles exert their effects only under certain environmental conditions.
Some developing Drosophila larvae are killed at 30 C. Larvae will survive if grown at 22 C.
EXAMPLE: ts lethals
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