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Foundations of Genetics Observable Patterns of Inheritance. Chapter 8. Earlobe Variation . Whether a person is born with attached or detached earlobes depends on a single gene Gene has two molecular forms (alleles). Earlobe Variation.

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Foundations of genetics observable patterns of inheritance l.jpg

Foundations of GeneticsObservable Patterns of Inheritance

Chapter 8


Earlobe variation l.jpg
Earlobe Variation

  • Whether a person is born with attached or detached earlobes depends on a single gene

  • Gene has two molecular forms (alleles)


Earlobe variation3 l.jpg
Earlobe Variation

  • You inherited one allele for this gene from each parent

  • Dominant allele specifies detached earlobes

  • Recessive allele specifies attached lobes


Dominant recessive alleles l.jpg
Dominant & Recessive Alleles

  • If you have attached earlobes, you inherited two copies of the recessive allele

  • If you have detached earlobes, you may have either one or two copies of the dominant allele


Early ideas about heredity l.jpg
Early Ideas About Heredity

  • People knew that sperm and eggs transmitted information about traits

  • Blending theory

  • Problem:

    • Would expect variation to disappear

    • Variation in traits persists


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Gregor Mendel

  • Strong background in plant breeding and mathematics

  • Using pea plants, found indirect but observable evidence of how parents transmit genes to offspring


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The Garden Pea Plant

  • Self-pollinating

  • True breeding (different alleles not normally introduced)

  • Can be experimentally cross-pollinated


Genes l.jpg
Genes

  • Units of information about specific traits

  • Passed from parents to offspring

  • Each has a specific location (locus) on a chromosome


Alleles l.jpg
Alleles

  • Different molecular forms of a gene

  • Arise by mutation

  • Dominant allele masks a recessive allele that is paired with it


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Allele Combinations

  • Homozygous

    • having two identical alleles at a locus

    • AA or aa

  • Heterozygous

    • having two different alleles at a locus

    • Aa


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Genetic Terms

A pair of homologous chromosomes

A gene locus

A pair of alleles

Three pairs of genes


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Genotype & Phenotype

  • Genotype refers to particular genes an individual carries

  • Phenotype refers to an individual’s observable traits (that are expressed)

  • Cannot always determine genotype by observing phenotype


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Tracking Generations

  • Parental generation P

    mates to produce

  • First-generation offspring F1

    mate to produce

  • Second-generation offspring F2


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F1 Results of One Monohybrid Cross


F 2 results of monohybrid cross l.jpg
F2 Results of Monohybrid Cross


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Mendel’s Monohybrid Cross Results

5,474 round

1,850 wrinkled

6,022 yellow

2,001 green

882 inflated

299 wrinkled

428 green

152 yellow

F2 plants showed dominant-to-recessive ratio that averaged 3:1

705 purple

224 white

651 long stem

207 at tip

787 tall

277 dwarf


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Mendel’s Theory of Segregation

  • An individual inherits a unit of information (allele) about a trait from each parent

  • During gamete formation, the alleles segregate from each other


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Probability

The chance that each outcome of a given event will occur is proportional to the number of ways that event can be reached


Punnett square of a monohybrid cross l.jpg

Female gametes

A a

A

AA

Aa

Male

gametes

a

Aa

aa

Punnett Square of a Monohybrid Cross

Dominant

phenotype can

arise 3 ways,

recessive only

one


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Test Cross

  • Individual that shows dominant phenotype is crossed with individual with recessive phenotype

  • Examining offspring allows you to determine the genotype of the dominant individual


Slide21 l.jpg

Homozygous

recessive

Homozygous

recessive

a a

a a

A

A

Aa

Aa

Aa

Aa

a

A

aa

Aa

aa

Aa

Punnett Squares of Test Crosses

Two phenotypes

All dominant phenotype


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Dihybrid Cross

Experimental cross between individuals that are homozygous for different versions of two traits


A dihybrid cross f 1 results l.jpg
A Dihybrid Cross - F1 Results

purple

flowers, tall

white

flowers,

dwarf

TRUE-

BREEDING

PARENTS:

AABB

x

aabb

GAMETES:

AB

AB

ab

ab

AaBb

F1 HYBRID

OFFSPRING:

All purple-flowered, tall


F 1 results of mendel s dihybrid crosses l.jpg
F1 Results of Mendel’s Dihybrid Crosses

  • All plants displayed the dominant form of both traits

  • We now know:

    • All plants inherited one allele for each trait from each parent

    • All plants were heterozygous (AaBb)


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Phenotypic Ratios in F2

Four Phenotypes:

  • Tall, purple-flowered (9/16)

  • Tall, white-flowered (3/16)

  • Dwarf, purple-flowered (3/16)

  • Dwarf, white-flowered (1/16)

AaBbX

AaBb


Explanation of mendel s dihybrid results l.jpg

ab

ab

aB

AB

AB

Ab

Ab

aB

Explanation of Mendel’s Dihybrid Results

If the two traits are coded for by genes on separate chromosomes, sixteen gamete combinations are possible

1/4

1/4

1/4

1/4

1/4

1/16

1/16

1/16

1/16

AABB

AABb

AaBB

AaBb

1/4

1/16

1/16

1/16

1/16

AABb

AAbb

AaBb

Aabb

1/4

1/16

1/16

1/16

1/16

AaBB

AaBb

aaBB

aaBb

1/4

1/16

1/16

1/16

1/16

AaBb

Aabb

aaBb

aabb


16 allele combinations in f 2 l.jpg

ab

ab

aB

AB

AB

Ab

Ab

aB

16 Allele Combinations in F2

1/4

1/4

1/4

1/4

1/4

1/16

1/16

1/16

1/16

AABB

AABb

AaBB

AaBb

1/4

1/16

1/16

1/16

1/16

AABb

AAbb

AaBb

Aabb

1/4

1/16

1/16

1/16

1/16

AaBB

AaBb

aaBB

aaBb

1/4

1/16

1/16

1/16

1/16

AaBb

Aabb

aaBb

aabb


Independent assortment l.jpg
Independent Assortment

  • Mendel concluded that the two “units” for the first trait were to be assorted into gametes independently of the two “units” for the other trait

  • Members of each pair of homologous chromosomes are sorted into gametes at random during meiosis


Independent assortment29 l.jpg
Independent Assortment

Metaphase I

OR

A

A

a

a

A

A

a

a

B

B

b

b

b

b

B

B

Metaphase II:

A

A

a

a

A

A

a

a

B

B

b

b

b

b

B

B

Gametes:

B

B

b

b

b

b

B

B

A

A

a

a

A

A

a

a

1/4 AB

1/4 ab

1/4 Ab

1/4 aB


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Tremendous Variation

Number of genotypes possible in offspring as a result of independent assortment and hybrid crossing is

3n

(n is the number of gene loci at which the parents differ)


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Impact of Mendel’s Work

  • Mendel presented his results in 1865

  • Paper received little notice

  • Mendel discontinued his experiments in 1871

  • Paper rediscovered in 1900 and finally appreciated


Dominance relations l.jpg
Dominance Relations

  • Complete dominance

  • Incomplete dominance

    • Heterozygote phenotype is somewhere between that of two homozygotes

  • Codominance

    • Non-identical alleles specify two phenotypes that are both expressed in heterozygotes


Flower color in snapdragons incomplete dominance l.jpg
Flower Color in Snapdragons: Incomplete Dominance

Red-flowered plant X White-flowered plant

Pink-flowered F1 plants

(homozygote)

(homozygote)

(heterozygotes)


Flower color in snapdragons incomplete dominance34 l.jpg
Flower Color in Snapdragons: Incomplete Dominance

Pink-flowered plant X Pink-flowered plant

White-, pink-, and red-flowered plants

in a 1:2:1 ratio

(heterozygote)

(heterozygote)


Flower color in snapdragons incomplete dominance35 l.jpg
Flower Color in Snapdragons: Incomplete Dominance

  • Red flowers - two alleles allow them to make a red pigment

  • White flowers - two mutant alleles; can’t make red pigment

  • Pink flowers have one normal and one mutant allele; make a smaller amount of red pigment


Genetics of abo blood types three alleles l.jpg
Genetics of ABO Blood Types: Three Alleles

  • Gene that controls ABO type codes for enzyme that dictates structure of a glycolipid on blood cells

  • Two alleles (IA and IB) are codominant when paired

  • Third allele (i) is recessive to others


Abo blood type allele combinations l.jpg
ABO Blood Type:Allele Combinations

  • Type A - IAIA or Iai (core glycolipid H + N-acetyl-D-galactosamine)

  • Type B - IBIBor IBi (core glycolipid H + D-galactose)

  • Type AB - IAIB (both glycolipids)

  • Type O - ii (core glycolipid H only)


Abo blood type glycolipids on red cells l.jpg
ABO Blood Type: Glycolipids on Red Cells

  • Type A - Glycolipid A on cell surface

  • Type B - Glycolipid B on cell surface

  • Type AB - Both glyocolipids A & B

  • Type O - Neither glyocolipid A or B,

    but H shared by both Types A and B


Abo and transfusions l.jpg
ABO and Transfusions

  • Recipient’s immune system will attack blood cells that have an unfamiliar glycolipid on surface

  • Type O is universal donor because it has neither type A nor type B, but a core glycolipid H common to both A and B.


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Pleiotropy

  • Alleles at a single locus may have effects on two or more traits

  • Classic example is the effects of the mutant allele at the beta-globin locus that gives rise to sickle-cell anemia


Genetics of sickle cell anemia l.jpg
Genetics of Sickle-Cell Anemia

  • Two alleles

    1) HbA

    Encodes normal beta hemoglobin chain

    2) HbS

    Mutant allele encodes defective chain

  • HbS homozygotes produce only the defective hemoglobin; suffer from sickle-cell anemia


Pleiotrophic effects of hb s hb s l.jpg
Pleiotrophic Effects of HbS/HbS

  • At low oxygen levels, cells with only HbS hemoglobin “sickle” and stick together

  • This impedes oxygen delivery and blood flow

  • Over time, it causes damage throughout the body


Epistasis l.jpg
Epistasis

  • Interaction between the products of gene pairs

  • Common among genes for hair color in mammals


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Genetics of Coat Color in Labrador Retrievers

  • Two genes involved

    - One gene influences melanin production

    • Two alleles - B (black) is dominant over b (brown)

      - Other gene influences melanin deposition

    • Two alleles - E promotes pigment deposition and is dominant over e


Allele combinations and coat color l.jpg
Allele Combinations and Coat Color

  • Black coat - Must have at least one dominant allele at both loci

    • BBEE, BbEe, BBEe, or BbEE

  • Brown coat - bbEE, bbEe

  • Yellow coat - Bbee, BBee, bbee


Albinism l.jpg
Albinism

  • Phenotype results when pathway for melanin production is completely blocked

  • Genotype - Homozygous recessive at the gene locus that codes for tyrosinase, an enzyme in the melanin-synthesizing pathway


Comb shape in poultry l.jpg
Comb Shape in Poultry

Alleles at two loci (R and P) interact

  • Walnut comb - RRPP, RRPp, RrPP, RrPp

  • Rose comb - RRpp, Rrpp

  • Pea comb - rrPP, rrPp

  • Single comb - rrpp


Campodactyly unexpected phenotypes l.jpg
Campodactyly: Unexpected Phenotypes

  • Effect of allele varies:

    • Bent fingers on both hands

    • Bent fingers on one hand

    • No effect

  • Many factors affect gene expression


Continuous variation l.jpg
Continuous Variation

  • A more or less continuous range of small differences in a given trait among individuals

  • The greater the number of genes and environmental factors that affect a trait, the more continuous the variation in versions of that trait


Human variation l.jpg
Human Variation

  • Some human traits occur as a few discrete types

    • Attached or detached earlobes

    • Many genetic disorders

  • Other traits show continuous variation

    • Height

    • Weight

    • Eye color


Describing continuous variation l.jpg

(line of bell-shaped curve indicates continuous variation in population)

Number of individuals with

some value of the trait

Number of individuals with

some value of the trait

Range of values for the trait

Range of values for the trait

Describing Continuous Variation


Temperature effects on phenotype l.jpg
Temperature Effects population)on Phenotype

  • Himalayan rabbits are Homozygous for an allele that specifies a heat-sensitive version of an enzyme in melanin-producing pathway

  • Melanin is produced in cooler areas of body


Environmental effects on plant phenotype l.jpg
Environmental Effects on Plant Phenotype population)

  • Hydrangea macrophylla

  • Action of gene responsible for floral color is influenced by soil acidity

  • Flower color ranges from pink to blue

  • Soil pH of 6.0 to 6.5 will yield pink flowers

  • Soil pH of 5.0 to 5.5 will yield blue flowers.

.