Classical Genetics — Lecture I

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Classical Genetics — Lecture I

• Dr. Steven J. Pittler

• VH375B• Office 4-6744• Cell 612-9720

Suggested Reading: Lewis 2nd Edition

Chapter on Mendelian Inheritance


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Inheritance• Parents and offspring

often share observable traits.

• Grandparents and grandchildren may share traits not seen in parents.

• Why do traits disappear in one generation and reappear in another?


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Gregor Mendel: The father of modern genetics

Combined • plant breeding• Statistics• Careful recordkeeping

Described hypothesis of transmission of traits now considered the laws of inheritance


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Mendel studied pea traits with two distinct forms


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True breeding plants

Plants which consistently have offspring with same trait as parent are true breeding plants.


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Monohybrid cross• What happens

when true breeding plants with two distinct forms of a trait are crossed?

Progeny show only one form of the trait.

The observed trait is called dominant.The masked trait is called recessive.


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Test crossIs a plant showing the dominant trait true-breeding or not?

Test by crossing with a plant showing the recessive trait.

All tall offspring indicateparent is true-breeding

Mixed offspring indicateparent is hybrid


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Crossing hybrids to each otherHybrid parents show the dominant trait (tall).

Offspring:

Mendel concluded that among the hybrid parents the short trait (recessive) was hidden but not absent

• Dominant trait (tall) and true breeding (1/4 total)

• Recessive trait (short) and always true breeding (1/4 total)

• Dominant trait (tall) and NOT true breeding (1/2 total)


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Mendel’s data1. Crossed true-breeding plants differing at one of seven traits. 2. Crossed hybrid offspring to each other (all show dominant trait). 3. Counted offspring of hybrid crosses.

Trait Offspring

# with dominant

trait# with

recessive trait

Ratiodominant : recessive

Seed form 7,324 5,474 1,850 2.96 : 1Seed color 8,023 6,022 2,001 3.01 : 1Seed coat color 929 705 224 3.15 : 1Pod form 1,181 882 299 2.95 : 1Pod color 580 428 152 2.82 : 1

Flower position 858 651 207 3.14 : 1Stem length 1,064 787 277 2.84 : 1


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• If you know the genotype of the parents, it is possible to determine the gametes and use a Punnett square to determine the phenotypic ratio among the offspring.

• When a monohybrid reproduces with a monohybrid, the results are 3 : 1.

• This ratio is used to state the chances of a particular phenotype.

• A 3 : 1 ratio means that there is a 75% chance of the dominant phenotype and a 25% chance of the recessive phenotype.


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Punnett Squares

t Tt

Tt T

The genes from one parent go here.

The genes from the other parent go here.


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Punnett Squares

T T

t Tt Tt

t Tt TtF1 generation


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Gamete Formation (sperm and eggs)• Because homologous pairs separate

during meiosis, a gamete has only one allele from each pair of alleles.

• If the allelic pair is Tt, a gamete would contain either a T or a t, but not both.

• Tt represents the genotype of an individual.

• Gametes are represented by T or t.


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One-Trait Crosses and Probability

• Laws of probability alone can be used to determine results of a cross.

• The laws are: • (1) the probability that two or more

independent events will occur together is the product of their chances occurring separately, and

• (2) the chance that an event that can occur in two or more independent ways is the sum of the individual chances.


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• In the cross of Tt x Tt, what is the chance of obtaining either a T or a t from a parent?

• Chance of T = ½, or chance of t = ½• The probability of these genotypes is:

• The chance of TT = ½ x ½ = ¼• The chance of Tt = ½ x ½ = ¼• The chance of tT = ½ x ½ = ¼• The chance of tt = ½ x ½ = ¼

• The chance of tall plants (TT, Tt, tT) is ¼ + ¼ + ¼ = ¾ or 75%.


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Law of segregation

Why do traits “disappear” in one generation only to reappear in a subsequent generation?

• Each plant has two distinct separable units (alleles) for each trait inherited from each parent. (sister chromatids that together make a chromosome)

• Only one version is observed in an individual.• Gametes contain ONE allele for each trait.

The unit (allele) does not disappear. It may be present but hidden.


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Gene locus (locus = location)


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Law of segregation


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Alleles

Mendel’s units (or “elementen”) are called alleles.

• versions of the same gene or DNA sequence.

• differ in DNA sequence at one or more sites.


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Genotype indicates the combination of alleles present• Homozygous alleles are the same• Heterozygous alleles differ

Phenotype indicates the trait observed.

Terms distinguish the observed form and the underlying alleles present.


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Genotype and phenotype

Phenotype

Tall plant

Short plant

Genotype

Homozygous dominant “tall-associated” alleles

Heterozygous

Homozygous recessive “short-associated” alleles

Abbreviationof genotype

TT

Tt

tt


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Modern terms for Mendel’s crosses

• Mendel’s true-breeding plants were homozygous for the alleles of a trait.

• Mendel’s hybrid plants were heterozygous for the alleles of a trait.


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Wildtypemost common version in the general population

• wildtype phenotype • mutant phenotype • wildtype allele • mutant allele

• most common phenotype

• phenotype different from the wildtype• most frequent allele associated with the common phenotype homozygous.• allele associated with the mutant phenotype.


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Law of segregation: the monohybrid cross

Two heterozygous parents produce gametes with T or t allele equally frequently.

Offspring genotypes 1/4 TT : 1/2 Tt : 1/4 tt

Offspring phenotypes 3/4 tall : 1/4 short


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Mode of inheritanceindicates the patterns with which the mutant phenotype is associated.

Most common Autosomal recessive Autosomal dominantX-linked recessiveX-linked dominantY-linked (holandric)mitochondrial


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Autosomal dominant inheritance

• Heterozygotes exhibit the affected phenotype.

• Males and females are equally affected and may transmit the trait.

• Affected phenotype does not skip generations.


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Autosomal recessive inheritance

• Heterozygotes carry the recessive allele but exhibit the wildtype phenotype.

• Males and females are equally affected and may transmit the trait.

• May skip generations.


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Comparison of autosomal dominant and autosomal recessive inheritance

NoYesAt least one parent of affected child must be affected?

YesNoTrait skips generations?

YesYesMales and females transmit the trait?

YesYesMales and females affected?

Autosomal recessive

Autosomal dominant


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Law of independent assortment

• Two genes on different chromosomes segregate their alleles independently.

• The inheritance of an allele of one gene does not influence which allele is inherited at a second gene.


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Law of independent assortment


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Independent assortment of two traits• In a dihybrid cross, parents with two

differing traits are crossed. • Which allele is dominant?

Heterozygous peas are round and yellow.

Therefore round is dominant to wrinkled yellow is dominant to green


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Two traits segregating independently


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Pedigreessymbolic representations of family relationships and inheritance of a trait


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Autosomal dominant inheritance of brachydactyly

Heterozygotes exhibit the phenotype.


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Autosomal recessive inheritance of albinism

Heterozygotes carry the recessive allele but exhibit the wildtype phenotype


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Genetic predictions

What is the chance that Ellen’s child has a sickle cell anemia allele (a)?

Ellen Michael

?

Ellen and Michael’s parents must be carriers.

A a

A

a

AA Aa

Aa aa

Ellen is not affected andcannot carry aa genotype

chance child inherits sickle cell allele = 1/2

Overall chance child carries sickle cell allele from Ellen = 2/3 x 1/2 = 1/3

chance Ellen is a carrier = 2/3

Ellen’s brother Michael hassickle cell anemia, an autosomal recessive disease.

Classical Genetics — Lecture I

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