TRANSMISSION OF GENES FROM

PARENT TO OFFSPRING

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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