Patterns of Inheritance
Introduction: Barking Up the Genetic Tree Dogs are one of man’s longest genetics experiments – Dog breeds are the result of artificial selection – Populations of dogs became isolated from each other
Chapter 9
– Humans chose dogs with specific traits for breeding – Each breed has physical and behavioral traits due to a unique genetic makeup
Sequencing of the dog’s genome shows evolutionary relationships between breeds Copyright © 2009 Pearson Education, Inc.
MENDEL S LAWS
Wolf Ancestral canine
Chinese Shar-Pei Akita
Siberian Husky
Basenji
Alaskan Malamute Afghan hound Saluki Rottweiler Sheepdog
Retriever
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9.1 The science of genetics has ancient roots
9.2 Experimental genetics began in an abbey garden
Pangenesis was an early explanation for inheritance
Gregor Mendel discovered principles of genetics in experiments with the garden pea
– It was proposed by Hippocrates – Particles called pangenes came from all parts of the organism to be incorporated into eggs or sperm
– Mendel showed that parents pass heritable factors to offspring (heritable factors are now called genes)
– Characteristics acquired during the parents’ lifetime could be transferred to the offspring
– Advantages of using pea plants
– Aristotle rejected pangenesis and argued that instead of particles, the potential to produce the traits was inherited
Blending was another idea, based on plant breeding – Hereditary material from parents mixes together to form an intermediate trait, like mixing paint Copyright © 2009 Pearson Education, Inc.
– Controlled matings – Self-fertilization or cross-fertilization – Observable characteristics with two distinct forms – True-breeding strains
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White 1
Removed stamens from purple flower
Petal Stamens Carpel Parents (P)
2
Purple
3
Transferred pollen from stamens of white flower to carpel of purple flower
Pollinated carpel matured into pod
4
Stamen Carpel
Offspring (F1)
Planted seeds from pod
Flower color
Purple
White
9.3 Mendel s law of segregation describes the inheritance of a single character Example of a monohybrid cross
Flower position
Axial
Terminal
Seed color
Yellow
Green
Seed shape
Round
Wrinkled
Pod shape
Inflated
Constricted
Pod color
Green
Yellow
Stem length
Tall
Dwarf
– Parental generation: purple flowers × white flowers – F1 generation: all plants with purple flowers – F2 generation: 3/4 of plants with purple flowers 1/4 of plants with white flowers
Mendel needed to explain – Why one trait seemed to disappear in the F1 generation – Why that trait reappeared in one quarter of the F2 offspring Copyright © 2009 Pearson Education, Inc.
P generation (true-breeding parents) Purple flowers
9.3 Mendel s law of segregation describes the inheritance of a single character White flowers
F1 generation
All plants have purple flowers
Fertilization among F1 plants (F1 × F1)
Four Hypotheses 1. Genes are found in alternative versions called alleles; a genotype is the listing of alleles an individual carries for a specific gene 2. For each characteristic, an organism inherits two alleles, one from each parent; the alleles can be the same or different – A homozygous genotype has identical alleles
F2 generation
– A heterozygous genotype has two different alleles 3 – 4
of plants have purple flowers
1 – 4
of plants have white flowers Copyright © 2009 Pearson Education, Inc.
9.3 Mendel s law of segregation describes the inheritance of a single character Four Hypotheses 3. If the alleles differ, the dominant allele determines the organism’s appearance, and the recessive allele has no noticeable effect
Genetic makeup (alleles) pp PP
P plants
Gametes
F1 plants (hybrids)
All Pp 1 – 2
Gametes
– The phenotype is the appearance or expression of a trait – The same phenotype may be determined by more than one genotype
4. Law of segregation: Allele pairs separate (segregate) from each other during the production of gametes so that a sperm or egg carries only one allele for each gene
All p
All P
1 – 2
P
P F2 plants
Phenotypic ratio 3 purple : 1 white
Sperm
p
p
P
PP
Pp
p
Pp
pp
Eggs
Genotypic ratio 1 PP : 2 Pp : 1 pp
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9.4 Homologous chromosomes bear the alleles for each character
Gene loci
For a pair of homologous chromosomes, alleles of a gene reside at the same locus
Dominant allele
P
a
B
P
a
b
– Homozygous individuals have the same allele on both homologues – Heterozygous individuals have a different allele on each homologue Genotype:
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Recessive allele
Bb PP aa Homozygous Heterozygous Homozygous for the for the dominant allele recessive allele
9.5 The law of independent assortment is revealed by tracking two characters at once
9.5 The law of independent assortment is revealed by tracking two characters at once
Example of a dihybrid cross
Law of independent assortment
– Parental generation: round yellow seeds × wrinkled green seeds – F1 generation: all plants with round yellow seeds – F2 generation: 9/16 of 3/16 of 3/16 of 1/16 of
plants plants plants plants
with with with with
round yellow seeds round green seeds wrinkled yellow seeds wrinkled green seeds
– Each pair of alleles segregates independently of the other pairs of alleles during gamete formation – For genotype RrYy, four gamete types are possible: RY, Ry, rY, and ry
Mendel needed to explain – Why nonparental combinations were observed – Why a 9:3:3:1 ratio was observed among the F2 offspring Copyright © 2009 Pearson Education, Inc.
Copyright © 2009 Pearson Education, Inc.
rryy
RRYY
F1 generation
1 – 2
RY
Sperm 1 – 4
ry
RY
Eggs 1 – 2
– Mating between an individual of unknown genotype and a homozygous recessive individual
RrYy
Sperm 1 – 2
Testcross
ry
Gametes RY
RrYy
1 – 2
F2 generation
rryy
RRYY
ry
Gametes RY
9.6 Geneticists use the testcross to determine unknown genotypes
Hypothesis: Independent assortment
Hypothesis: Dependent assortment P generation
ry
1 – 4
RY
1 – 4
rY
Eggs
Hypothesized (not actually seen)
1 – 4
Ry
1 – 4
ry
RY
1 – 4
rY
1 – 4
Ry
1 – 4
– Will show whether the unknown genotype includes a recessive allele
ry
RRYY
RrYY
RRYy
RrYy
RrYY
rrYY
RrYy
rrYy
RRYy
RrYy
RRyy
Rryy
RrYy
rrYy
Rryy
rryy
– Used by Mendel to confirm true-breeding genotypes 9 –– 16
Actual results (support hypothesis)
3 –– 16 3 –– 16 1 –– 16
Yellow round Green round Yellow wrinkled Green wrinkled Copyright © 2009 Pearson Education, Inc.
Bb male F1 genotypes
Mendel s laws reflect the rules of probability
Testcross: B_
Genotypes
bb
Formation of sperm
Bb female Formation of eggs 1 – 2
1 – 2
B
b
Two possibilities for the black dog: BB
Bb
or
1 – 2
B
Gametes b
Bb
b
B
b
Bb
bb
B
B
B
B 1 – 4
1 – 4
1 – 2
b
b
B
b
1 – 4
Offspring
All black
1 black : 1 chocolate
9.8 CONNECTION: Genetic traits in humans can be tracked through family pedigrees
F2 genotypes
Dominant Traits
Recessive Traits
Freckles
No freckles
Widow s peak
Straight hairline
Free earlobe
Attached earlobe
A pedigree – Shows the inheritance of a trait in a family through multiple generations – Demonstrates dominant or recessive inheritance – Can also be used to deduce genotypes of family members
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b
b 1 – 4
First generation (grandparents)
Ff
Ff
Ff
ff
9.9 CONNECTION: Many inherited disorders in humans are controlled by a single gene Inherited human disorders show
Second generation (parents, aunts, and uncles) FF or Ff Third generation (two sisters)
– Recessive inheritance ff
ff
Ff
Ff
ff
– Two recessive alleles are needed to show disease – Heterozygous parents are carriers of the disease-causing allele – Probability of inheritance increases with inbreeding, mating between close relatives
ff
FF or Ff
Female Male Affected Unaffected
– Dominant inheritance – One dominant allele is needed to show disease – Dominant lethal alleles are usually eliminated from the population Copyright © 2009 Pearson Education, Inc.
Parents
Normal Dd
Normal Dd
×" Sperm
D Offspring
D
d
DD Normal
Dd Normal (carrier)
Dd Normal (carrier)
dd Deaf
Eggs d
9.10 CONNECTION: New technologies can provide insight into one s genetic legacy Genetic testing of parents Fetal testing: biochemical and karyotype analyses – Amniocentesis
VARIATIONS ON MENDEL S LAWS
– Chorionic villus sampling
Maternal blood test Fetal imaging – Ultrasound – Fetoscopy
Newborn screening Copyright © 2009 Pearson Education, Inc.
9.11 Incomplete dominance results in intermediate phenotypes
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P generation Red RR
Incomplete dominance – Neither allele is dominant over the other
White rr
r
R
Gametes
F1 generation
– Expression of both alleles is observed as an intermediate phenotype in the heterozygous individual
Pink Rr
Gametes
1 – 2
R
1 – 2
R
1 – 2
r
Sperm
F2 generation
1 – 2
r
1 – 2
R
RR
rR
1 – 2
r
Rr
rr
Eggs
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9.12 Many genes have more than two alleles in the population
9.12 Many genes have more than two alleles in the population
Multiple alleles
Codominance
– More than two alleles are found in the population
– Neither allele is dominant over the other
– A diploid individual can carry any two of these alleles
– Expression of both alleles is observed as a distinct phenotype in the heterozygous individual
– The ABO blood group has three alleles, leading to four phenotypes: type A, type B, type AB, and type O blood
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– Observed for type AB blood
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Blood Group (Phenotype) Genotypes
Red Blood Cells
9.13 A single gene may affect many phenotypic characters Pleiotropy
O
ii!
A
I AI A or IAi!
Carbohydrate A
I BI B or IBi!
Carbohydrate B
B
– One gene influencing many characteristics – The gene for sickle cell disease – Affects the type of hemoglobin produced – Affects the shape of red blood cells – Causes anemia – Causes organ damage – Is related to susceptibility to malaria
AB
I AI B
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9.14 A single character may be influenced by many genes
Individual homozygous for sickle-cell allele Sickle-cell (abnormal) hemoglobin
Polygenic inheritance
Abnormal hemoglobin crystallizes, causing red blood cells to become sickle-shaped
– Many genes influence one trait Sickle cells
– Skin color is affected by at least three genes Clumping of cells and clogging of small blood vessels
Breakdown of red blood cells
Physical weakness
Heart failure
Anemia
Impaired mental function
Pain and fever
Brain damage
Pneumonia and other infections
Paralysis
Accumulation of sickled cells in spleen
Damage to other organs
Rheumatism
Spleen damage
Kidney failure Copyright © 2009 Pearson Education, Inc.
9.15 The environment affects many characters
P generation aabbcc AABBCC (very light) (very dark)
Phenotypic variations are influenced by the environment
F1 generation AaBbCc
AaBbCc
– Skin color is affected by exposure to sunlight –1 8
–1 8
–1 8
Sperm –1 8
–1 8
–1 8
–1 8
–1 8
– Susceptibility to diseases, such as cancer, has hereditary and environmental components
F2 generation
–1 8 –1 8 –1 8
–1 8
Fraction of population
Eggs
20 –– 64
–1 8
–1 8 –1 8 –1 8
15 –– 64
6 –– 64
1 –– 64 1 –– 64
6 –– 64
15 –– 64
20 –– 64
15 –– 64
6 –– 64
1 –– 64
Skin color Copyright © 2009 Pearson Education, Inc.
9.16 Chromosome behavior accounts for Mendel s laws
THE CHROMOSOMAL BASIS OF INHERITANCE
Mendel’s Laws correlate with chromosome separation in meiosis – The law of segregation depends on separation of homologous chromosomes in anaphase I – The law of independent assortment depends on alternative orientations of chromosomes in metaphase I
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Copyright © 2009 Pearson Education, Inc.
F1 generation
R r
9.17 Genes on the same chromosome tend to be inherited together
All round yellow seeds (RrYy) y
Y r
R
y
Y
R
r
Y
y
r
y
Y
Y
Y R
r
R
Y
y
Linked Genes – Are located close together on the same chromosome
Anaphase I of meiosis
R
R
Metaphase I of meiosis (alternative arrangements)
Metaphase II of meiosis
Gametes
y r
1 – RY 4
9
Y
Y r 1 – rY 4
:1
– Tend to be inherited together
Example studied by Bateson and Punnett y
r
:3
y
Y
r
:3
Y
R
Fertilization among the F1 plants F2 generation
R
r
y
1 – ry 4
r
– Parental generation: plants with purple flowers, long pollen crossed to plants with red flowers, round pollen y
y R
R 1 – 4
Ry
– The F2 generation did not show a 9:3:3:1 ratio – Most F2 individuals had purple flowers, long pollen or red flowers, round pollen Copyright © 2009 Pearson Education, Inc.
Explanation: linked genes PL
Parental diploid cell PpLl
Experiment
pl
Purple flower
Meiosis
PpLl
PpLl
Most gametes
Long pollen
pl
PL
Fertilization
Phenotypes Purple long Purple round Red long Red round
Observed offspring
Prediction (9:3:3:1)
284 21 21 55
215 71 71 24
Sperm
Most offspring
PL
pl
PL
PL
PL pl
pl pl
PL
pl
PL Eggs pl
3 purple long : 1 red round Not accounted for: purple round and red long
9.18 Crossing over produces new combinations of alleles Linked alleles can be separated by crossing over – Recombinant chromosomes are formed
AB
A B
A b
a B
a b
– Geneticists measure genetic distance by recombination frequency a b Tetrad
Crossing over Gametes
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9.20 Chromosomes determine sex in many species
SEX CHROMOSOMES AND SEX-LINKED GENES
X-Y system in mammals, fruit flies – XX = female; XY = male
X-O system in grasshoppers and roaches – XX = female; XO = male
Z-W in system in birds, butterflies, and some fishes – ZW = female, ZZ = male
Chromosome number in ants and bees – Diploid = female; haploid = male Copyright © 2009 Pearson Education, Inc.
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(female)
(male) 44 + XY
Parents diploid cells
44 + XX
9.21 Sex-linked genes exhibit a unique pattern of inheritance Sex-linked genes are located on either of the sex chromosomes – Reciprocal crosses show different results
22 + X
22 + Y Sperm 44 + XX
Offspring (diploid)
22 + X Egg 44 + XY
– White-eyed female × red-eyed male and white-eyed males
red-eyed females
– Red-eyed female × white-eyed male and red-eyed males
red-eyed females
– X-linked genes are passed from mother to son and mother to daughter – X-linked genes are passed from father to daughter – Y-linked genes are passed from father to son Copyright © 2009 Pearson Education, Inc.
Female
Male
Female
Male
XR Xr
XR Y
Xr Y
XR XR
Sperm
Eggs XR
Sperm
Xr
Y
XR Xr
XR Y
XR
Y
XR
XR XR
XR Y
Xr
Xr XR
Xr Y
Eggs R = red-eye allele r = white-eye allele
Female
Male
XR Xr
Xr Y
Males express X-linked disorders such as the following when recessive alleles are present in one copy – Hemophilia
Sperm Xr
9.22 CONNECTION: Sex-linked disorders affect mostly males
Y
– Colorblindness – Duchenne muscular dystrophy
XR
XR XR
XR Y
Xr
Xr Xr
Xr Y
Eggs
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9.23 EVOLUTION CONNECTION: The Y chromosome provides clues about human male evolution Fertilization
Homologous Alleles, residing chromosomes at the same locus
Similarities in Y chromosome sequences – Show all men related to the same male ancestor
Meiosis
– Demonstrate a connection between people living in distant locations
Paired alleles, alternate forms of a gene
Gamete from other parent
Diploid zygote (containing paired alleles)
Haploid gametes (allele pairs separate)
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Genes located on
Incomplete dominance
White rr
Red RR
Pleiotropy
Multiple genes
(a)
chromosomes
Pink Rr
Single gene
alternative versions called
at specific locations called
(b)
if both same, if different, genotype called genotype called
(c)
Multiple characters Polygenic inheritance
heterozygous expressed allele called
(d)
Single characters (such as skin color)
unexpressed allele called
(e) inheritance when phenotype In between called
(f)
