Mendel and Genes Chapter 14. Fig. 14-1 The particulate hypothesis is the idea that parents pass on...
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Transcript of Mendel and Genes Chapter 14. Fig. 14-1 The particulate hypothesis is the idea that parents pass on...
Mendel and GenesChapter 14
Fig. 14-1
The “particulate” hypothesis is the idea that parents pass on discrete heritable units (genes)
•The “blending” hypothesis is the idea that genetic material from the two parents blends together (like blue and yellow paint blend to make green)
• How can we tell the genotype of an individual with the dominant phenotype?– The answer is to carry out a testcross:
Fig. 14-7TECHNIQUE
RESULTS
Dominant phenotype, unknown genotype:
PP or Pp?
Predictions
Recessive phenotype, known genotype: pp
If PP If Ppor
Sperm Spermp p p p
P
P
P
p
Eggs Eggs
Pp
Pp Pp
Pp
Pp Pp
pp pp
or
All offspring purple 1/2 offspring purple and1/2 offspring white
Concept 14.2: The laws of probability govern Mendelian inheritance
• Mendel’s laws of segregation and independent assortment reflect the rules of probability
• When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss
• In the same way, the alleles of one gene segregate into gametes independently of another gene’s alleles
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities
– Probability in an F1 monohybrid cross can be determined using the multiplication rule
• Segregation in a heterozygous plant is like flipping a coin: Each gamete has a chance of carrying the dominant allele and a chance of carrying the recessive allele
The Multiplication and Addition Rules Applied to Monohybrid Crosses
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
1
2
1
2
Fig. 14-9 Rr RrSegregation of
alleles into eggs
Sperm
R
R
R RR
R rrr
r
r
r1/2
1/2
1/2
1/2
Segregation ofalleles into sperm
Eggs1/4
1/4
1/41/4
• The rule of addition states that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities
• The rule of addition can be used to figure out the probability that an F2 plant from a monohybrid cross will be heterozygous rather than homozygous
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-9 Rr RrSegregation of
alleles into eggs
Sperm
R
R
R RR
R rrr
r
r
r1/2
1/2
1/2
1/2
Segregation ofalleles into sperm
Eggs1/4
1/4
1/41/4
Summary of Mendel’s Principles
• The inheritance of biological characteristics is determined by individual units known as genes. In organisms that reproduce sexually, genes are passed from parents to their offspring.
• In cases in which two or more forms of the gene for a single trait exist, some forms of the gene may be dominant and others may be recessive.
• In most sexually reproducing organisms, each adult has two copies of each gene—one from each parent. These genes are segregated from each other when gametes are formed.
• The alleles for different genes usually segregate independently of one another.
Concept 14.3: Inheritance patterns are often more complex than predicted by simple
Mendelian genetics
• The relationship between genotype and phenotype is rarely as simple as in the pea plant characters Mendel studied
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Beyond dominant/recessive
• incomplete dominance - situation in which one allele is not completely dominant over another
• There is no white no red: new phenotype pink
Fig. 14-10-3
Red
P Generation
Gametes
WhiteCRCR CWCW
CR CW
F1 GenerationPinkCRCW
CR CWGametes 1/21/2
F2 Generation
Sperm
Eggs
CR
CR
CW
CW
CRCR CRCW
CRCW CWCW
1/21/2
1/2
1/2
Beyond dominant and recessive alleles
• Codominance - situation in which both alleles of a gene contribute to the phenotype of the organism
Beyond dominant and recessive alleles
• multiple alleles - three or more alleles of the same gene
Fig. 14-11
IA
IB
i
A
B
none(a) The three alleles for the ABO blood groups and their associated carbohydrates
Allele Carbohydrate
GenotypeRed blood cell
appearancePhenotype
(blood group)
IAIA or IA i A
BIBIB or IB i
IAIB AB
ii O
(b) Blood group genotypes and phenotypes
Polygenic Inheritance
• Quantitative characters are those that vary in the population along a continuum
• Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype
• Height and Skin color in humans is an example of polygenic inheritance
Fig. 14-13
Eggs
Sperm
Phenotypes:
Number ofdark-skin alleles: 0 1 2 3 4 5 6
1/646/64
15/6420/64
15/646/64
1/64
1/8
1/8
1/8
1/8
1/8
1/8
1/8
1/8
1/81/8
1/81/8
1/81/8
1/81/8
AaBbCc AaBbCc
• Dominant alleles are not necessarily more common in populations than recessive alleles
• For example, one baby out of 400 in the United States is born with extra fingers or toes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Frequency of Dominant Alleles
Pleiotropy
• Most genes have multiple phenotypic effects, a property called pleiotropy
• For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as– cystic fibrosis
– PKU
– Sickle-cell disease
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Epistasis
• In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus– For example, in mice and many other mammals, coat
color depends on two genes
• One gene determines the pigment color (with alleles B for black and b for brown)
• The other gene (with alleles C for color and c for no color) determines whether the pigment will be deposited in the hair
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-12
BbCc BbCc
Sperm
EggsBC bC Bc bc
BC
bC
Bc
bc
BBCC
1/41/4
1/41/4
1/4
1/4
1/4
1/4
BbCC BBCc BbCc
BbCC bbCC BbCc bbCc
BBCc BbCc
BbCc bbCc
BBcc Bbcc
Bbcc bbcc
9 : 3 : 4
Nature and Nurture: The Environmental Impact on Phenotype
• The norm of reaction is the phenotypic range of a genotype influenced by the environment
• For example, hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity
• multifactorialCopyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Pedigree Analysis
• A pedigree is a family tree that describes the interrelationships of parents and children across generations
• Inheritance patterns of particular traits can be traced and described using pedigrees
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-15b
1st generation(grandparents)
2nd generation(parents, aunts,and uncles)
3rd generation(two sisters)
Widow’s peak No widow’s peak
(a) Is a widow’s peak a dominant or recessive trait?
Ww ww
Ww Wwww ww
ww
wwWw
Ww
wwWW
Wwor
Fig. 14-15c
Attached earlobe
1st generation(grandparents)
2nd generation(parents, aunts,and uncles)
3rd generation(two sisters)
Free earlobe
(b) Is an attached earlobe a dominant or recessive trait?
Ff Ff
Ff Ff Ff
ff Ff
ff ff ff
ff
FF or
orFF
Ff
(Start Next)The Behavior of Recessive Alleles
• Recessively inherited disorders show up only in individuals homozygous for the allele
• Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal (i.e., pigmented)
• Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-16
Parents
Normal Normal
Sperm
Eggs
NormalNormal(carrier)
Normal(carrier) Albino
Aa Aa
A
AAA
Aa
a
Aaaa
a
Autosomal Disorders
• Genes for these disorders are located on autosomes– Recessive disorder– Dominant disorders– Codominant disorders
Cystic Fibrosis
Fig. 14-17
Eggs
Parents
Dwarf Normal
Normal
Normal
Dwarf
Dwarf
Sperm
Dd dd
dD
Dd dd
ddDd
d
d
Multifactorial Disorders
• Many diseases, such as heart disease and cancer, have both genetic and environmental components– Little is
understood about the genetic contribution to most multifactorial diseases
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Genetic Testing and Counseling
• Genetic counselors can provide information to prospective parents concerned about a family history for a specific disease
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-18
Amniotic fluidwithdrawn
Fetus
Placenta
Uterus Cervix
Centrifugation
Fluid
Fetalcells
Severalhours
Severalweeks
Severalweeks
(a) Amniocentesis (b) Chorionic villus sampling (CVS)
Severalhours
Severalhours
Fetalcells
Bio-chemical
tests
Karyotyping
Placenta Chorionicvilli
Fetus
Suction tubeinsertedthroughcervix
Karyotype• Making a
Karyotype– Photograph
chromosomes during mitosis
– Cut chromosomes out of photograph
– Group them in order, in pairs
– Male 46XY– Female
46XX
Sex-Linked Genes
• Males have just one X chromosome. Thus, all X-linked alleles are expressed in males, even if they are recessive.
• Some disorders caused by recessive alleles on the X chromosome in humans:– Color blindness– Duchenne muscular dystrophy– Hemophilia
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-UN2
Degree of dominance
Complete dominanceof one allele
Incomplete dominanceof either allele
Codominance
Description
Heterozygous phenotypesame as that of homo-zygous dominant
Heterozygous phenotypeintermediate betweenthe two homozygousphenotypes
Heterozygotes: Bothphenotypes expressed
Multiple alleles
Pleiotropy
In the whole population,some genes have morethan two alleles
One gene is able toaffect multiplephenotypic characters
CRCR CRCW CWCW
IAIB
IA , IB , i
ABO blood group alleles
Sickle-cell disease
PP Pp
Example
Fig. 14-UN3
DescriptionRelationship amonggenes
Epistasis One gene affectsthe expression ofanother
Example
Polygenicinheritance
A single phenotypiccharacter isaffected bytwo or more genes
BbCc BbCc
BCBC
bC
bC
Bc
Bc
bc
bc
9 : 3 : 4
AaBbCc AaBbCc