Chapter 11

Observing Patterns in Inherited Traits

People did not know that sperm and eggs transmitted information about traits

Blending Theory – fluids at fertilization mixed to give rise to offspring- later disapproved

Problem:Would expect variation to disappearVariation in traits persists

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Darwin’s Theory of Natural Selection seemed to support this

Mendel studies on pea plants showed observable evidence of how parents transmit genes to offspring

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Monk from Czech RepublicRaised on farmDeveloped improved varieties of

vegetables & fruitsStrong background in plant breeding

and mathematicsUsing pea plants (Pisum sativum), found

indirect but observable evidence of how parents transmit genes to offspring

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For thousands of years farmers and herders have been selectively breeding their plants and animals to produce more useful hybrids

It was somewhat of a hit or miss process since the actual mechanisms governing inheritance were unknown

Mendel discovered patterns of dominance and recessiveness in certain traits, which were later connected with pairs of alleles on pairs of homologous chromosomes.

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Mendel's research was with plants, the basic underlying principles of heredity that he discovered also apply to people and other animals because the mechanisms of heredity are essentially the same for all complex life forms.

Mendel discovered that certain traits show up in offspring without any blending of parent characteristics. 

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Mendel used pea plants partly because they are self-fertilizing and can “breed true” for certain traits.

The plants can be cross-fertilized artificially by a breeder.

Genes- unit of hereditary trait info parents transmit to offspring have specific locus on chromosomal DNA

Diploid- 2n pr genes on homologous chromosomes

Mutation – alteration of gene’s molecular structure Can change trait Molecular forms of same gene is called an allele

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Hybrids- cross between two individuals that breed true for different forms of a trait

Homologous – pr identical alleles Heterozygous – pr of non-identical alleles Dominant – mask traits in control Recessive – only in control if dominant absent Heterozygous – Aa

Pr non identical alleles Homologous dominant – AA

pr dominant alleles Homologous recessive – aa

pr recessive alleles

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Genotype – alleles carried represented by letters

Phenotype – observable traits words to describe

P -stands for true breeding parentsF1 – first generation

F2 – crossed F1 organisms (second generation)

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Parental generation P

mates to produce

First-generation offspring F1

mate to produce

Second-generation offspring F2

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Two homologous parents with different traits crossedAA (P)x aa(P) Aa (F1)

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 method –possible outcomes of offspring

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Mendel used monohybrid experiments to test a hypothesis: Pea plants inherit two units of information (genes) for a

trait, one from each parent.

Mendel observed seven traits that are easily recognized and apparently only occur in one of two forms:

1.  flower color is purple or white 2.  seed color is yellow or green 3 . flower position is axil or terminal  4. pod shape is inflated or constricted 5. stem length is long or short 6. pod color is yellow or green 7. seed shape is round or wrinkled

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http://learning.mgccc.cc.ms.us/jd/science/carter/chapter11/animations/cross_pollination.html

787 tall 277 dwarf

651 long stem

207 at tip

705 purple 224 white

152 yellow428 green

299 wrinkled882 inflated

6,022 yellow 2,001 green

5,474 round 1,850 wrinkled

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

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

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

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http://learning.mgccc.cc.ms.us/jd/science/carter/chapter11/animations/test_cross.html

Punnett Squares of Test Crosses

Homozygous recessive

a a

A

a aa

Aa Aa

aa

Homozygous recessive

a a

A

A Aa

Aa Aa

Aa

Two phenotypes All dominant phenotype16

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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According to the principle of segregation, for any particular trait, the pair of alleles of each parent separate and only one allele passes from each parent on to an offspring. 

Which allele in a parent's pair of alleles is inherited is a matter of chance.  We now know that this segregation of alleles occurs during the process of sex cell formation (i.e., meiosis

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fertilization produces heterozygous offspring

meiosis II

meiosis I

(chromosomes duplicated

before meiosis)

Homozygous dominant

parent

Homozygous recessive

parent

(gametes) (gametes)

Mendel’s Mendel’s Theory of Theory of

SegregationSegregation

Mendel’s Theory of Segregation- Diploid cells have pairs of genes, on pairs of homologous chromosomes.

The two genes of each pair are separated from each other during meiosis, so they end up in different gametes.

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

AABB x aabb -- Pure breed parents

AB AB ab ab– Gametes

AaBb ---------- F1 generation

FIRST, OUTER, INNER, LAST (FOIL)

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AABB aabbx

AaBb

AB AB ab ab

TRUE-BREEDING PARENTS:

GAMETES:

F1 HYBRID OFFSPRING:

purple flowers, tall

white flowers,dwarf

All purple-flowered, tall

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1/16aaBB

1/16aaBb

1/16aaBb

1/16Aabb

1/16Aabb

1/16AAbb

1/16AABB

1/16AABb

1/16AaBB

1/16AaBb

1/16AABb

1/16AaBb

1/16AaBB

1/16AaBb

1/16AaBb

1/4 AB 1/4 Ab 1/4 aB 1/4 ab

1/16aabb

1/4 AB

1/4 Ab

1/4 aB

1/4 ab

AaBb AaBbX

1/16 white-flowered, dwarf

3/16 white-flowered, tall

3/16 purple-flowered, dwarf

9/16 purple-flowered, tall

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http://learning.mgccc.cc.ms.us/jd/science/carter/chapter11/animations/dihybrid_v2.html

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Principle of Independent assortment -different pairs of alleles are passed to offspring independently of each other.  The result is that new combinations of genes present

in neither parent are possible.  Ex. a pea plant's inheritance of the ability to

produce purple flowers instead of white ones does not make it more likely that it will also inherit the ability to produce yellow pea seeds in contrast to green ones. 

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The principle of independent assortment explains why the human inheritance of a particular eye color does not increase or decrease the likelihood of having 6 fingers on each hand. 

Today, we know this is due to the fact that the genes for independently assorted traits are located on different chromosomes

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

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Key Concepts:Not all traits clearly have dominant or

recessive forms.One allele of a pair may be fully or partially

dominant over its partner or codominant with its partner.

Two or more gene pairs often influence the same trait, and some single genes influence many genes.

Mendel happened to focus on traits that have clearly dominant or recessive forms.

However, expression of genes for some traits is not as straight forward.

Complete dominance

Incomplete dominance

Codominance

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A pair of nonidentical alleles affecting two phenotypes are expressed at the same time

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

Three different alleles in humansTwo alleles (IA and IB) are codominant when

pairedThird allele (i) is recessive to others

Range of genotypes:

Blood types:

IA IA

IA i IA IB IB i

IB IB

ii

A AB B O

or or

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Recipient’s immune system will attack blood cells that

have an unfamiliar glycolipid (antigen) on surface

Type O is universal donor because it has neither type

A nor type B glycolipid

Type A can receive from – A & O

Type can receive from – B & O

Type AB can receive from -A, B, AB & O

Type O can receive from - O

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One allele of a pair is not fully dominant over the other allele in the pair

resulting in the phenotype a mix of the two

http://learning.mgccc.cc.ms.us/jd/science/carter/chapter11/animations/abo_genetics.html

XHomozygous parent

Homozygous parent

All F1 are heterozygous

X

F1 shows all offspring are of same phenotypes F2 shows three phenotypes in 1:2:1 ratio (one each like parents and then 2-mixed)

Incomplete Dominance

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9/16 walnut 3/16 rose 3/16 pea 1/16 singlerrpp

RRpp(rose comb)

rrPP(pea comb)

RrPp (all walnut comb)

P:

F2:

F1:

X

RRPPRRPpRrPPRrPp

RRppRrpp

rrPPrrPp

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rose comb

pea comb walnut comb

single comb

all walnutRrPp

F1

RrPp RrPp

F2

roseRRpp

pearrPP

X

9/16 walnutRRPP, RRPp,RrPP, or RrPp

3/16 roseRRpp or Rrpp

3/16 pearrPP or rrPp

1/16 singlerrpp

Comb Shape in PoultryComb Shape in Poultryhttp://learning.mgccc.cc.ms.us/jd/science/carter/chapter11/animations/chicken_combs.html

Co-dominance Rhododendron

Interaction between the products of two or more gene pairs

Common among genes for hair color in mammals

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bbeeBBEE X

F1 puppies are all BbEe

BBEE

BBEe

BbEE

BbEe Bbee

BbEe

Bbee

BBEe BbEE BbEe

BBee BbEe

bbeebbEe

bbEE bbEe

BE Be bE be

BE

Be

bE

be

black

brown

yellow

F2 puppies

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In horses brown coat color (B) is dominant over tan (b).

Gene expression is dependent on a second gene that controls the deposition of pigment in hair.

The dominant gene (C) codes for the presence of pigment in hair

The recessive gene (c) codes for the absence of pigment.

Horse is homozygous recessive for the second gene (cc), it will have a white coat regardless of the genetically programmed coat color (B gene) because pigment is not deposited in the hair.

Alleles at a single locus may have effects on two or more traits (good & bad). This is referred to as pleitropy.

Disorders: Cystic Fibrosis, Sickle cell Anemia & Marfan syndrome

Marfan syndrome – Autosomal dominant mutation; Mutation in gene for fibrillin affects skeleton, cardiovascular system, lungs, eyes, and skin

An allele at a given gene locus may be fully dominant, incompletely dominant, or codominant with its partner on a homologous chromosome.

Some gene products may interact with each other and influence the same trait through epistasis.

A single gene’s product may have pleiotropic effects, or positive or negative impact on two or more traits.

Himalayan Rabbit is 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 (above 91F color is not produced)

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Hydrangea macrophylla

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

Flower color ranges from pink to blue

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Color is due to pH of soil

blue acid soilpink basic soil

Serotonin transport in humans Serotonin is a chemical in our bodies that

helps relieve emotional stress. One of our genes codes for a “serotonin

transporter” (a protein transporter) that transports the serotonin across the membrane of brain cells.

A mutation in this gene disrupts our ability to handle stress and recover when bad things happen.

Variation in traits arise not only from gene mutations and interactions, but also in response to variation in environmental conditions that each individual faces.

Comptodactyly- rare abnormality –Effect

shape and movement of fingers:

Bent fingers on both hands & immoveable

Bent fingers on one hand & moveable

No effect

Many factors affect gene expression

A more or less continuous range of small differences in a given trait among individuals referred to as continuous variation

Polygenic inheritance – inheritance of multiple genes that affect the same trait

The greater the number of genes and

environmental factors that affect a trait, the

more continuous the variation in versions of

that trait

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Some human traits occur as a few discrete typesAttached or detached earlobes Many genetic disorders

Other traits show continuous variationHeightWeightEye color

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