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Chapter 12Patterns of Inheritance
• 12.1 What is the physical basis of inheritance?
• 12.2 How did Gregor Mendel lay the foundation for modern genetics?
• 12.3 How are single traits inherited?• 12.4 How are multiple traits on different
chromosomes inherited?• 12.5 How are genes located on the same
chromosome inherited?
Chapter 12Patterns of Inheritance, cont.
• 12.6 How is sex determined, and how are sex-linked genes inherited?
• 12.7 Do the Mendelian rules of inheritance apply to all traits?
• 12.8 How are human genetic disorders investigated?
• 12.9 How are human disorders caused by single genes inherited?
• 12.10 How do errors in chromosome number affect humans?
12.1 What is the physical basis of inheritance?
• Inheritance is the process by which the characteristics of individuals are passed to their offspring
• Genes encode these characteristics
Genes
• A gene is a unit of heredity that encodes information for the form of a particular characteristic
• The location of a gene on a chromosome is called its locus
Alleles
• Homologous chromosomes carry the same kinds of genes for the same characteristics
• Genes for the same characteristic are found at the same loci on both homologous chromosomes
Alleles
• Genes for a characteristic found on homologous chromosomes may not be identical
• Alternate versions or forms of genes found at the same gene locus are called alleles
Alleles
• Each cell carries two alleles per characteristic, one on each of the two homologous chromosomes
• If both homologous chromosomes carry the same allele (gene form) at a given gene locus, the organism is homozygous at that locus
Alleles
• If two homologous chromosomes carry different alleles at a given locus, the organism is heterozygous at that locus (a hybrid)
12.2 How Did Gregor Mendel Lay the Foundation for Modern Genetics?
• Gregor Mendel studied many subjects (including botany and math) in the 2 years at the Univ. of Vienna
• He settled down as a monk in the monastery of St. Thomas (in what is now Brno in the Czech Republic)
• He did groundbreaking research on peas, despite no knowledge of genes or chromosomes
Portrait painted about 1888
12.1 How Did Gregor Mendel Lay the Foundation for Modern Genetics?
• Doing It Right: The Secrets of Mendel’s Success
• 3 steps to doing an experiment right1. Choosing the right organism2. Designing and performing the experiment
correctly3. Analyzing the data properly
12.2 How Did Gregor Mendel Lay the Foundation for Modern Genetics?
• Each flower has pollen grains (male gametes) which fertilize the eggs (female gametes)
• Peas are usually self-fertilized• A plant which is homozygous for a trait is
called true-breeding• Peas can easily be cross-fertilized• The traits Mendel looked at (like color) only
had one gene that coded for that trait.
Section 12.3 Outline
• 12.3 Inheritance of Single Traits– The Language of a Genetic Cross – Mendel’s Flower Color Experiments– Alleles of a Gene Are Dominant or Recessive– How Meiosis Separates Genes: Segregation– Understanding the Results of Mendel’s Flower
Color Experiments– “Genetic Bookkeeping”– Practical Application: The Test Cross
The Language of a Genetic Cross
• The parents used in a cross are part of the parental generation (known as P)
• The offspring of the P generation are members of the first filial generation (F1)
• Offspring of the F1 generation are members of the F2 generation, etc.
true-breeding,purple-flowered
plant
true-breeding,white-flowered
plant
cross-fertilize
pollen
pollen
all purple-flowered plants
Parental generation (P)
First-generationoffspring (F1)
Copyright © 2005 Pearson Prentice Hall, Inc.
3/4 purple 1/4 white
Second- generationoffspring (F2)
First- generationoffspring (F1)
self-fertilize
Copyright © 2005 Pearson Prentice Hall, Inc.
12.3 How are single traits inherited?
• After that, Mendel allowed the F2 plants to self-fertilize a third generation (F3)
• The white plants always produced white plants – no matter how many generations
• The purple-flowered F2 plants were of 2 types– 1/3 true-breeding purple– 2/3 were hybrids and produced purple and white
flowered peas
• F2 generation ratio was ¼ true breeding white : ½ hybrid : ¼ true breeding purple
Inheritance of dominant & recessive• Each trait is determined by a pair of
discrete units now called genes• The pairs separate during gamete
formation (meiosis). This is called Mendel’s Law of Segregation
• Which allele goes in which gamete is random (otherwise the math doesn’t work)
• One type of allele can mask the other allele (dominant and recessive) but the dominant does not alter the recessive allele
homozygous parent
A A AA
gametes
True-breeding organisms have two of the same allele for a given gene. This is also called homozygous.
heterozygous parent
A a aA
gametes
Hybrids organisms have two of the different alleles for a given gene. This is also called heterozygous.
Capital letters for dominant and lower case for recessive
PP P P
purple parent
all P sperm and eggs
pp p
white parent
all p sperm and eggs
p
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F1
offspring
p
p
Pp
Pp
sperm eggs
P
P
or
ppp
Ppp
p
gametes fromF1 plants
eggs
F2 offspring
p
PP
P
P
PP
sperm
Pp
1/4
1/4
1/4
1/4
Dominant and Recessive Alleles
• The particular combination of the two alleles carried by an individual is called the genotype (PP, Pp, or pp)
• The physical expression of the genotype is known as the phenotype (e.g. purple or white flowers)
Genetic Bookkeeping
• Punnett Square Method predicts offspring genotypes from combinations of parental gametes
1. First assign letters to the different alleles of the characteristic under consideration (uppercase for dominant, lowercase for recessive)
2. Determine the gametes and their fractional proportions (out of all the gametes) from both parents…
Genetic Bookkeeping
3. Write the gametes from each parent, together with their fractional proportions, along each side of a 2 x 2 grid (Punnett square)
4. Fill in the genotypes of each pair of combined gametes in the grid, including the product of the fractions of each gamete (e.g. ¼ P with ½ p = 1/8 Pp)…
P p
PP
P
p
pp
Pp
eggs
Ppself-fertilize
pP
12
12
12
12
14
sper
m
14
14
14
P P PP PP
P p Pppurple
P P pP
Pp
P p pp pp white
sperm eggsoffspring
genotypesgenotypic
ratio(1:2:1)
phenotypicratio(3:1)
12
12
12
12
12
12
12
12
14
14
14
14
14
24
14
14
34
A test cross
• Take a dominant phenotype (with unknown second allele)
• Cross it with a known recessive homozygous
• And you will get one of two results...
allsperm
pp
P
pp
Pp
ppall eggs
PP or Ppsperm unknown
if PP if Pp
egg egg
pollen
p
12
12
12
P
p
12
all Pp
sper
m
12.4 How Are Multiple Traits on Different Chromosomes Inherited?• Mendel hypothesized that genes on different
chromosomes are inherited independently
• He looked at many pea traits
• If these traits were linked in some way, then the ratios shouldn’t agree with hypothesis
• Remember that for the math to work out, you have to do lots of replicates – and Mendel also did that correctly.
Seedshape
Seedcolor
Podcolor
Podshape
Flowercolor
Flowerlocation at leaf
junctionsat tips ofbranches
tall(1.8 to 2 meters)
dwarf(0.2 to 0.4meters)
Plantsize
smooth
Dominant formTrait Recessive form
wrinkled
yellow
yellow
inflated
green
green
white
constricted
purple
Traits Are Inherited Independently
• Punnett Square from SSYY x ssyy crossGametes
¼sy ¼sy ¼sy ¼sy
¼SY SsYy SsYy SsYy SsYy F1: All SsYy Smooth
yellow peas
¼SY SsYy SsYy SsYy SsYy
¼SY SsYy SsYy SsYy SsYy
¼SY SsYy SsYy SsYy SsYy
1
16
1
16
1
16
1
16
1
16
1
16
1
16
1
16
1
16
1
16
1
16
1
16
1
16
1
16
1
16
1
16
SY
SY
SSYY SsYY
ssYY
ssyY
SsyY
SSYy SsYy
SsYy
ssyy
SsyySSyy
sSyY sSyy
sSYY sSYy
SSyY
sY
sY
sy
sy
Sy
Sy
eggs
self-fertilize
ssYy
14
14
14
14
14
14
14
14
sper
m
116
116
116
116
116
116
116
116
116
116
116
116
116
116
116
116
seed shape seed color phenotypic ratio(9:3:3:1)
smooth yellow smooth yellow
smooth green
wrinkled yellow
wrinkled green
yellow
green
green
smooth
wrinkled
wrinkled
34
34
34
34
14
14
14
14
316
316
916
116
Copyright © 2005 Pearson Prentice Hall, Inc.
And of course, his experiments came out just like this. This idea is called the Law of Independent Assortment
The End of the Mendel Story
• In 1865, Gregor Mendel presented his work and published the following year
• He was pretty much ignored
• In 1900, 3 biologists were about to publish on the same subject
• They did a literature search and found Mendel’s work, and gave him all the credit
• He had died in 1884
12.5 How are genes located on the same chromosome inherited?
• Genes on the same chromosome tend to be inherited together.
• This is called linkage.
• One of the first pairs of linked genes was found on the sweet pea (a different species from Mendel’s garden pea)
Genes on the Same Chromosome
• Not all genes independently assort
• Mendel’s Law of Independent Assortment only works for genes whose loci are on different chromosomes
SS s ss s
Y
S
Y Y Yy y y
S
y
independent assortment produces four equallylikely allele combinations during meiosis
SY sy Sy sY
meiosis II
meiosis I
S
S
S
S
s
s
s
s
Y
Y
Y
Y
y
y
y
y
chromosomesreplicate
SS
ss
Y
Yy
y
replicated homologuespair during metaphaseof meiosis I,orienting like this
or like this
pairs of alleles on homologouschromosomes in diploid cells
Ss
Yy
flower color gene pollen shape gene
purpleallele, P
redallele, p
longallele, L
roundallele, l
12.5 How are genes located on the same chromosome inherited?
• Despite being on the same gene, crossing over can occur
• Thereby creating new linkages.• Exchanging DNA between homologous
chromosomes is genetic recombination• The farther apart genes are on a
chromosome, the more likely crossing over will occur between them
• Mendel wasn’t just a good scientist, he was really lucky he didn’t find linked traits
flower color gene
purple allele, P long allele, L
red allele, p round allele, l
pollen shape gene
sister chromatids
homologouschromosomes(duplicated)at meiosis I
sister chromatids
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crossing over
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recombinedchromatids
P L
p L
P l
p l
Copyright © 2005 Pearson Prentice Hall, Inc.
P L
p L
P l
p l
12.6 How Is Sex Determined, and How Are Sex-Linked Genes Inherited?
• Females have two identical sex chromosomes XX
• Men have two different sex chromosomes XY
• That means the men control the sex of the offspring
X chromosome
Y chromosome
12.6 How Is Sex Determined, and How Are Sex-Linked Genes Inherited?
• Sex linked genes are only found on the X or the Y gene
• But the Y gene is small, so most sex linked problems are found on the X
• Women have XX, so they can have dominant and recessive X alleles
• Men only have one X, and so whatever is on their X is expressed
X1
X1
X2
X2
Xm
Xm
XmXmX1
X2
Y
Y
Y YX1 X2
eggs
female parent
female offspring
male offspring
male parent
sper
m
all the F2
femaleshave red eyes
half the F2 maleshave red eyes,half have whiteeyes
XRX
rX
r
XR
r
XR
R
R
Xr
r
YY
XR
R
XR
R
eggs
XR
R
XR
R
Xr
r
XR
R
XR
R
Y Xr
r
Y
XRY
female parent
female offspring
male offspring
male parent
sper
m
12.7 Do the Mendelian Rules of Inheritance Apply to All Traits?
• Incomplete dominance: The phenotype of heterozygotes is intermediate between the phenotypes of the homozygotes
• In other words, neither allele is completely dominant or recessive
• This is more like sharing or blending
RR
eggsF2:
RR
P:
F1:
RR
R
R
RR RR
R
RR’
RR R’R
R12
12
12
12
14
14
14
14
sper
m
12.6 Do the Mendelian Rules of Inheritance Apply to All Traits?
• A single gene may have multiple alleles
• Eye color in Drosophilia has more than a 1,000 possible alleles
• There are hundreds of alleles for Marfan syndrome and cystic fibrosis
• Blood type has multiple alleles with dominant and recessive and codominance
Polygenic Inheritance
• Phenotypes produced by polygenic inheritance are governed by the interaction of more than two genes at multiple loci
• Human skin color is controlled by at least 3 genes, each with pairs of incompletely dominant alleles
Polygenic Inheritance
• Phenotypes produced by polygenic inheritance are governed by the interaction of more than two genes at multiple loci
• Human skin color is controlled by at least 3 genes, each with pairs of incompletely dominant alleles
R1R1R2R2
eggs
R1R1R2R2
R1R1R2R2R1R1R2R2
R1R1R2R2R1R1R2R2
R1R1R2R2R1R1R2R2
R1R1R2R2
R1R1R2R2
R1R1R2R2
R1R1R2R2
R1R1R2R2
R1R1R2R2
R1R1R2R2
R1R1R1R2
R1R1R2R2
sper
m
R1R2
R1R2
R1R2
R1R2
R1R2
R1R1R2R2
R1R2R1R2R1R2
Pleiotropy
• Some alleles of a characteristic may create multiple phenotypic effects (pleiotropy)
– Mendel’s rules specify only one phenotype possible for any allele
• Example: The SRY gene in male humans– SRY gene stimulates development of gonads
into testes, which in turn stimulate development of the prostate, seminal vesicles, penis, and scrotum
Environmental Influence
• The environment can module how genes are expressed. Examples:
• Intelligence and height is based on genetics AND environment
• Himalayan rabbit– Himalayan rabbits have the genotype for black
fur all over the body– Black pigment is only produced in colder
areas of the body: the nose, ears, and paws– The Himalayan rabbit has dark skin below 34º
C (93º F)
12.8 How Are Human Genetic Disorders Investigated?
• Many experiments on humans are not allowed (not ethical)
• Human geneticists study medical, historical and family records
• Family pedigrees are records extending across several generations
• These help figure out which diseases are genetic, and how they are passed
12.9 How Are Human Disorders Caused by Single Genes Inherited?
• Some human genetic disorders are caused by recessive alleles
• Heterozygous individuals are called carriers and often don’t show any symptoms (although sometimes they show some)
• Related couples are more likely to express a recessive genetic disorder
Human Rattlesnake Wallaby
Albinism is a single, recessive allele
12.9 How Are Human Disorders Caused by Single Genes Inherited?
• Sickle-cell anemia is caused by a defective allele for hemoglobin synthesis
• The heterozygous have some abnormal hemoglobin, but not enough to cause much problems
• The heterozygous also help protect against malaria
12.8 How Are Human Disorders Caused by Single Genes Inherited?
• Some human genetic disorders are caused by dominant alleles– Wikipedia Huntington's Disease
• Some human genetic disorders are sex-linked– Color blindness and hemophilia
• Queen Victoria and Hemophilia
III
IV? ?
or = colorblind
= heterozygous carrier female, normal color vision
or = normal color vision (not carrier)
maternalgrandfather
mother father
sister G. Audesirk T. Audesirk
daughter
aunts
II
I
unaffected male
unaffected female
EdwardDuke of Kent
Victoria Princess of
Saxe-Coburg
AlbertPrince
of Saxe-Coburg-Gotha
VictoriaQueen
of England
Louis IVGrand Duke of
Hesse-Darmstadt
AlicePrincessof Hesse
Victoria Mary
Elizabeth Alexandra Tsarina
Frederick Ernest Mary Victoria
Irene
Olga Tatiana Maria Anastasia Alexis Tsarevitch
Edward VIIKing of
England
Alexandra of Denmark
Leopold Duke
of Albany
HelenPrincess of
Waldeck-Pyrmont
Henry Prince of Battenburg
Beatrice
present Britishroyal family(unaffected)
AlexanderAlbert
AlfonsoXII
Victoria Queen of Spain
Leopold Maurice
AlfonsoCrown Prince
Juan Beatrice died in
infancy
Marie Jaime Gonzalo
carrierdaughter
and hemophiliac
grandson
several unaffected
chidren
carrier female
hemophiliac male
?
Nicholas IIof Russia
?? ? ? ? ? ?
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12.9 How Do Errors in Chromosome Number Affect Humans?
• Some genetic disorders are caused by abnormal numbers of sex chromosomes
• Nondisjunction means errors in meiosis which cause a different number of chromosomes
• Most embryos of this type would abort
Klinefelter Syndrome Gives a Genetic Twist to Tales about George Washington, Napoleon, Lincoln
Rosy periwinkle – Leukemia drug
Calophyllum lanigerum – AIDS drug