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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