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Mendelian GeneticsMendelian Genetics Gregor MendelGregor Mendel – –

Austrian monk – Austrian monk – performed extensive performed extensive genetics experiments genetics experiments with garden peaswith garden peas

http://www.biology.arizona.edu/mendelian_genetics/mendelian_genetics.html

http://www.quia.com/jfc/65851.html

Gregor MendelGregor Mendel

Important Mendelian Important Mendelian Genetics terms:Genetics terms: trait: trait: physical attribute or characteristic physical attribute or characteristic

displayed by an individual (i.e. seed shape, displayed by an individual (i.e. seed shape, seed colour, etc.)seed colour, etc.)

alleleallele: forms of a trait (i.e. round or wrinkled : forms of a trait (i.e. round or wrinkled allele for the trait seed shape) allele for the trait seed shape)

dominant alleledominant allele: allele that is displayed when : allele that is displayed when both dominant and recessive alleles are both dominant and recessive alleles are present (dominant allele masks the recessive present (dominant allele masks the recessive allele)allele)

recessive allelerecessive allele: allele that is hidden when : allele that is hidden when both dominant and recessive alleles are both dominant and recessive alleles are present (dominant allele masks the recessive present (dominant allele masks the recessive allele), and only expressed when both allele), and only expressed when both recessive alleles are present)recessive alleles are present)

genotypegenotype: actual gene (allele) combination : actual gene (allele) combination present (RR or Rr)present (RR or Rr)

phenotypephenotype: physical outward appearance : physical outward appearance shown (round seed or wrinkled seed)shown (round seed or wrinkled seed)

homozygotehomozygote: an individual which : an individual which contains only one allele at the allelic contains only one allele at the allelic pair; for example DD is homozygous pair; for example DD is homozygous dominant and dd is homozygous dominant and dd is homozygous recessive; pure lines are recessive; pure lines are homozygous for the gene of interest homozygous for the gene of interest

heterozygoteheterozygote: an individual which : an individual which contains one of each member of the contains one of each member of the gene pair; for example the Dd gene pair; for example the Dd heterozygoteheterozygote

Human Eye colorHuman Eye color

Multiple AllelesMultiple Alleles Multiple AllelesMultiple Alleles – the existence of several (more – the existence of several (more

than two) alleles for a gene (e.g. ABO blood group)than two) alleles for a gene (e.g. ABO blood group)

GenotypeGenotype Blood TypeBlood Type

(Phenotype)(Phenotype)

IIAAIIAA or I or IAAii AA

IIBBIIBB or I or IBBii BB

IIAAIIBB ABAB

iiii OO

Incomplete DominanceIncomplete Dominance

shown when a shown when a heterozygote has a heterozygote has a different phenotype different phenotype (blending of both (blending of both homozygotes) than homozygotes) than either homozygote (e.g. either homozygote (e.g. Snapdragons: Snapdragons:

2 alleles - Red – R, White 2 alleles - Red – R, White – W– W

RR (red) x WW (white) RR (red) x WW (white) produce RW (pink)produce RW (pink)

RED

WHITE

PINK

PINK

PINK

PINK

CodominanceCodominance Similar to incomplete dominanceSimilar to incomplete dominance ““co” – togetherco” – together recessive & dominant traits appear recessive & dominant traits appear

togethertogether in the phenotype of hybrid in the phenotype of hybrid organismsorganisms

Ex: roan coat color in cattle (red and Ex: roan coat color in cattle (red and white hair on same animal)white hair on same animal)

2 allelles - R – red, W – white2 allelles - R – red, W – white 3 different phenotypes – red (RR), 3 different phenotypes – red (RR),

white (WW) and roan (RW)white (WW) and roan (RW)

X-linked InheritanceX-linked Inheritance Sometimes called sex-link inheritanceSometimes called sex-link inheritance Males – XY, Females – XXMales – XY, Females – XX Some traits are present on the X chromosomeSome traits are present on the X chromosome E.g. – hemophilia – X-linked disease (“bleeder E.g. – hemophilia – X-linked disease (“bleeder

disease”)disease”) Normal female – XX, carrier female – XXNormal female – XX, carrier female – XXhh

Female with hemophilia – XFemale with hemophilia – XhhXXh h (only if they (only if they have both Xhave both Xhh chromosomes) chromosomes)

Normal male – XY, male with hemophilia – XNormal male – XY, male with hemophilia – XhhYY Color blindness – X-linkedColor blindness – X-linked

Dihybrid CrossDihybrid Cross Monohybrid crossMonohybrid cross – –

dealing with only dealing with only oneone trait trait

Dihybrid crossDihybrid cross – – dealing with dealing with twotwo traits (seed shape traits (seed shape and seed colour)and seed colour)

(P(P11)Yellow, round)Yellow, round pea plant crossed pea plant crossed with with wrinkled, wrinkled, greengreen plants plants produced 100% produced 100% Yellow, round plants Yellow, round plants ((FF11 - YyRr - YyRr))

FF11 generation crossed – generation crossed – YyRr x YyRrYyRr x YyRr Four possible gametes – YR, yR, Yr, yrFour possible gametes – YR, yR, Yr, yr

FF22 Generation Generation

9 yellow, round9 yellow, round

3 green, round3 green, round

3 yellow, wrinkled3 yellow, wrinkled

1 green, wrinkled1 green, wrinkled

F2 Generation – F2 Generation – phenotypic ratiophenotypic ratio

Mendel’s LawsMendel’s Laws 3 laws:3 laws:a.a. Law of dominanceLaw of dominance – In a cross of parents – In a cross of parents

that are pure for contrasting traits, only one that are pure for contrasting traits, only one form of the trait will appear in the next form of the trait will appear in the next generation.  Offspring that are hybrid for a generation.  Offspring that are hybrid for a trait will have only the dominant trait in the trait will have only the dominant trait in the phenotype. phenotype.

b.b. Law of segregationLaw of segregation - During the formation - During the formation of gametes (eggs or sperm), the two alleles of gametes (eggs or sperm), the two alleles responsible for a trait separate from each responsible for a trait separate from each other.  Alleles for a trait are then other.  Alleles for a trait are then "recombined" at fertilization, producing the "recombined" at fertilization, producing the genotype for the traits of the offspring. genotype for the traits of the offspring.

c.c. Law of independent assortmentLaw of independent assortment - Alleles - Alleles for for differentdifferent traits are distributed to sex cells traits are distributed to sex cells (& offspring) independently of one another. (& offspring) independently of one another.

Linked GenesLinked Genes Linked genes - Linked genes - genes that are on the genes that are on the

same chromosome and subsequently same chromosome and subsequently are inherited together as a package are inherited together as a package unless crossing-over separates them. unless crossing-over separates them.

Crossing overCrossing over - process of sections - process of sections of homologous chromosomes of homologous chromosomes breaking and reconnecting onto the breaking and reconnecting onto the other homologous chromosome.other homologous chromosome.

Recombination - creation of Recombination - creation of combinations of alleles in combinations of alleles in chromosomes not present in either chromosomes not present in either parent.parent.

Linked Genes??Linked Genes??

Crossing Over & Crossing Over & RecombinationRecombination

Hardy Weinberg Hardy Weinberg EquationEquation

1.1. p + q = 1.0p + q = 1.0

2.2. pp22 + 2pq + q + 2pq + q22 = 1.0= 1.0

Hardy Weinberg EquationHardy Weinberg Equation

p + q = 1p + q = 1 p2 + 2pq + q2 = 1p2 + 2pq + q2 = 1 ““Unibrow” is a recessive genetic Unibrow” is a recessive genetic

trait in humans in which it occurs 1 trait in humans in which it occurs 1 in 30 people. in 30 people.

N – separate eyebrow N – separate eyebrow n - “unibrow”n - “unibrow”

1.1. qq22 = 1 / 30 = 0.033 = 1 / 30 = 0.033

2.2. q = √qq = √q22 = √0.033 = 0.183 = 18.3% of = √0.033 = 0.183 = 18.3% of the genes in the population are the genes in the population are “unibrow” (nn)“unibrow” (nn)

3.3. p = 1 – q = 1- 0.183 = 0.817 = 81.7% of p = 1 – q = 1- 0.183 = 0.817 = 81.7% of the genes in the population are separate the genes in the population are separate eyebrow.eyebrow.

4.4. pp22 = 0.8172 = 0.668 = 66.8% of the = 0.8172 = 0.668 = 66.8% of the population is NN.population is NN.

5.5. 2pq = 2 (0.817)( 0.183) = 0.298 = 2pq = 2 (0.817)( 0.183) = 0.298 = 29.8% of the population is Nn (carriers 29.8% of the population is Nn (carriers for the “unibrow” gene)for the “unibrow” gene)

ProbabilityProbability

the study of outcomes of events or the study of outcomes of events or occurrencesoccurrences

Probability = # of chances for an event / # of Probability = # of chances for an event / # of possible outcomespossible outcomes

Probability of a red card = ½ (26/52)Probability of a red card = ½ (26/52) Probability of a red 6 = 1/26 (2/52)Probability of a red 6 = 1/26 (2/52) How does this apply to genetics?How does this apply to genetics? RrRr - probability of - probability of RR = ½, probability of = ½, probability of

rr = ½ = ½


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