3a3
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3A3. Transmission of Genes from Parent to offspring. Mystery of heredity. Before the 20 th century, 2 concepts were the basis for ideas about heredity Heredity occurs within species Traits are transmitted directly from parent to offspring - PowerPoint PPT PresentationTRANSCRIPT
TRANSMISSION OF GENES FROM
PARENT TO OFFSPRING
3A3
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Mystery of heredity
Before the 20th century, 2 concepts were the basis for ideas about heredity Heredity occurs within species Traits are transmitted directly from parent to offspring
Thought traits were borne through fluid and blended in offspring
Paradox – if blending occurs why don’t all individuals look alike?
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Early work
Josef Kolreuter – 1760 – crossed tobacco strains to produce hybrids that differed from both parents Additional variation observed in 2nd generation
offspring contradicts direct transmissionT.A. Knight – 1823 – crossed 2 varieties of
garden pea, Pisum sativa Crossed 2 true-breeding strains 1st generation resembled only 1 parent strain 2nd generation resembled both
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Gregor Mendel
Chose to study pea plants because:1.Other research showed that pea hybrids
could be produced2.Many pea varieties were available3.Peas are small plants and easy to grow4.Peas can self-fertilize or be cross-fertilized
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Mendel’s experimental method
Usually 3 stages1.Produce true-breeding strains for each trait
he was studying2.Cross-fertilize true-breeding strains having
alternate forms of a trait Also perform reciprocal crosses
3.Allow the hybrid offspring to self-fertilize for several generations and count the number of offspring showing each form of the trait
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Monohybrid crosses
Cross to study only 2 variations of a single trait
Mendel produced true-breeding pea strains for 7 different traits Each trait had 2 variants
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F1 generation
First filial generationOffspring produced by crossing 2 true-
breeding strainsFor every trait Mendel studied, all F1 plants
resembled only 1 parent Referred to this trait as dominant Alternative trait was recessive
No plants with characteristics intermediate between the 2 parents were produced
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F2 generation
Second filial generationOffspring resulting from the self-fertilization
of F1 plantsAlthough hidden in the F1 generation, the
recessive trait had reappeared among some F2 individuals
Counted proportions of traits Always found about 3:1 ratio
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3:1 is 1:2:1
F2 plants ¾ plants with the dominant form ¼ plants with the recessive form The dominant to recessive ratio was 3:1
Mendel discovered the ratio is actually: 1 true-breeding dominant plant 2 not-true-breeding dominant plants 1 true-breeding recessive plant
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Conclusions
His plants did not show intermediate traits Each trait is intact, discrete
For each pair, one trait was dominant, the other recessive
Pairs of alternative traits examined were segregated among the progeny of a particular cross
Alternative traits were expressed in the F2 generation in the ratio of ¾ dominant to ¼ recessive
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5 element model
1. Parents transmit discrete factors (genes)2. Each individual receives one copy of a gene
from each parent3. Not all copies of a gene are identical
Allele – alternative form of a gene Homozygous – 2 of the same allele Heterozygous – different alleles
4. Alleles remain discrete – no blending5. Presence of allele does not guarantee
expression Dominant allele – expressed Recessive allele – hidden by dominant allele
Genotype – total set of alleles an individual contains
Phenotype – physical appearance
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Principle of Segregation
Two alleles for a gene segregate during gamete formation and are rejoined at random, one from each parent, during fertilization
Physical basis for allele segregation is the behavior of chromosomes during meiosis
Mendel had no knowledge of chromosomes or meiosis – had not yet been described
Punnett square
Cross purple-flowered plant with white-flowered plant
P is dominant allele – purple flowersp is recessive allele – white flowersTrue-breeding white-flowered plant is pp
Homozygous recessiveTrue-breeding purple-flowered plant is PP
Homozygous dominantPp is heterozygote purple-flowered plant
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Human traits
Some human traits are controlled by a single gene Some of these exhibit dominant and recessive
inheritancePedigree analysis is used to track inheritance
patterns in familiesDominant pedigree – juvenile glaucoma
Disease causes degeneration of optic nerve leading to blindness
Dominant trait appears in every generation
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Recessive pedigree – albinism Condition in which the pigment melanin is not
produced Pedigree for form of albinism due to a nonfunctional
allele of the enzyme tyrosinase Males and females affected equally Most affected individuals have unaffected parents
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Dihybrid crosses
Examination of 2 separate traits in a single cross
Produced true-breeding lines for 2 traitsRR YY x rryyThe F1 generation of a dihybrid cross (RrYy)
shows only the dominant phenotypes for each trait
Allow F1 to self-fertilize to produce F2
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F1 self-fertilizesRrYy x RrYyThe F2 generation shows all four possible
phenotypes in a set ratio 9:3:3:1 R_Y_:R_yy:rrY_:rryy Round yellow:round green:wrinkled yellow:wrinkled
green
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Principle of independent assortment
In a dihybrid cross, the alleles of each gene assort independently
The segregation of different allele pairs is independent
Independent alignment of different homologous chromosome pairs during metaphase I leads to the independent segregation of the different allele pairs
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Probability
Rule of addition Probability of 2 mutually exclusive events occurring
simultaneously is the sum of their individual probabilities
When crossing Pp x Pp, the probability of producing Pp offspring is probability of obtaining Pp (1/4), PLUS probability of
obtaining pP (1/4) ¼ + ¼ = ½
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Rule of multiplication Probability of 2 independent events occurring
simultaneously is the product of their individual probabilities
When crossing Pp x Pp, the probability of obtaining pp offspring is Probability of obtaining p from father = ½ Probability of obtaining p from mother = ½ Probability of pp= ½ x ½ = ¼
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Testcross
Cross used to determine the genotype of an individual with dominant phenotype
Cross the individual with unknown genotype (e.g. P_) with a homozygous recessive (pp)
Phenotypic ratios among offspring are different, depending on the genotype of the unknown parent
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Extensions to Mendel
Mendel’s model of inheritance assumes that Each trait is controlled by a single gene Each gene has only 2 alleles There is a clear dominant-recessive relationship
between the alleles
Most genes do not meet these criteria
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Polygenic inheritance
Occurs when multiple genes are involved in controlling the phenotype of a trait
The phenotype is an accumulation of contributions by multiple genes
These traits show continuous variation and are referred to as quantitative traits For example – human height Histogram shows normal distribution
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Pleiotropy
Refers to an allele which has more than one effect on the phenotype
Pleiotropic effects are difficult to predict, because a gene that affects one trait often performs other, unknown functions
This can be seen in human diseases such as cystic fibrosis or sickle cell anemia Multiple symptoms can be traced back to one
defective allele
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Multiple alleles
May be more than 2 alleles for a gene in a population
ABO blood types in humans 3 alleles
Each individual can only have 2 allelesNumber of alleles possible for any gene is
constrained, but usually more than two alleles exist for any gene in an outbreeding population
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Incomplete dominance Heterozygote is intermediate in phenotype between
the 2 homozygotes Red flowers x white flowers = pink flowers
Codominance Heterozygote shows some aspect of the phenotypes of
both homozygotes Type AB blood
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Human ABO blood group
The system demonstrates both Multiple alleles
3 alleles of the I gene (IA, IB, and i) Codominance
IA and IB are dominant to i but codominant to each other
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Environmental influence
Coat color in Himalayan rabbits and Siamese cats Allele
produces an enzyme that allows pigment production only at temperatures below 30oC
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Epistasis
Behavior of gene products can change the ratio expected by independent assortment, even if the genes are on different chromosomes that do exhibit independent assortment
R.A. Emerson crossed 2 white varieties of corn F1 was all purple F2 was 9 purple:7 white – not expected
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Sickle Cell Anemia
Tay-Sachs Disease
Huntington’s Disease
X-linked Color Blindness
Trisomy 21/Down’s Syndrome
Klinefelter’s Syndrome
What is Right? Ethical
What is Wrong? Not Ethical