Post on 01-Jan-2016
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Gregor Mendel’s Discoveries• Pre-Mendel Blending Theory of
Heredity– Hereditary material from each
parent mixes in the offspring• 2 problems• Individuals of a population should
reach a uniform appearance after many generations
• Once traits are blended, they can’t be separated
• Gregor Mendel Particulate Theory of Heredity
– Traits are inherited as separate factors
• Mendel used quantitative approach
• Studied peas for 3 reasons:– Many varieties– Self pollinating/cross pollinating– Each variety had 2 alternative
forms
• Used true breeding varieties • Used large sample sizes and
accurate observations• Used math to develop
probabilities and perform statistical analyses
• Used terms to define generations as: P, F1, F2
• Developed terms such as:– Alleles (factor)– Dominant/Recessive– Homozygous/ Heterozygous– Phenotype/Genotype– Testcross
• Derived 2 principles:– Law of segregation – two alleles for a character
separate when gametes are formed
Degrees of Dominance
• Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical
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
Fig. 14-16
Parents
Normal Normal
Sperm
Eggs
NormalNormal(carrier)
Normal(carrier) Albino
Aa Aa
A
AAA
Aa
a
Aaaa
a
• Dominant condition• Fatal• Only one Huntington’s
allele needed• Produces abnormal
protein that clumps up in cell nuclei – especially nerve cells in the brain
If an allele for tall plants (T) is dominant to short plants (t), what offspring would youexpect from a TT x Tt cross?
A. ½ tall; ½ shortB. ¾ tall; ¼ shortC. All tall
Concept Quiz
Identify vocab term:
• This allele gets masked in the phenotype
• Genetic Makup
• Alternate forms of a gene
• The allele that gets fully expressed
• Two identical alleles for a trait
• Physical appearance
If two heterozygotes are crossed for dimpled chin. What will be the expected genotypic ratio?
Phenotypic ratio?
• 1:2:1
• 3:1
Extending Mendelian Genetics for a Single Gene
• Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations:– When alleles are on the sex chromosomes– When alleles are not completely dominant or
recessive– When a gene has more than two alleles– When a gene produces multiple phenotypes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 14-10-2
Red
P Generation
Gametes
WhiteCRCR CWCW
CR CW
F1 GenerationPinkCRCW
CR CWGametes 1/21/2
In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties
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
Concept Quiz A red carnation and a white carnationproduce offspring that are all pink. The typeof inheritance pattern occurring is:
A. Complete dominanceB. Incomplete dominanceC. Codominance
Red is dominant to white in flower petal color. If a homozygous dominant is crossed
with a homozygous recessive and this inheritance is incomplete dominance. What
will be the phenotypic ratio of this cross?
• 1:2:1
Fur color in rabbits shows incomplete dominance.FBFB individuals are brown, FBFW individuals arecream, FWFW individuals are white. What is theexpected ratio of a FBFW x FWFW cross?
A. 3 white : 1 brownB. 3 white : 1 creamC. 2 white : 2 cream
Concept Quiz
Multiple Alleles
• Most genes exist in populations in more than two allelic forms
• For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: IA, IB, and i.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
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
What blood types would the offspring have if one parentIs type O and one parent is AB? Both parents AB?
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
Epistasis
• B = Black• b = Brown• C = Pigment• c = nonpig
A gene at one locusAlters a gene at another locus
Polygenic Inheritance
• Additive effect of 2 or more genes on a single phenotypic character
• Eye color, skin color
Identify each type of inheritance?• Both alleles are expressed in heterozygote• More than two alleles are possible for a trait• The dominant allele masks the recessive
allele• Additive effect of two or more genes• Intermediate phenotype in heterozygotes• A gene at one locus controls a gene at
another locus
Quantitative Genetics
• The environment plays a role – traits such as height, weight, musical ability, susceptibility to cancer,and intelligence
• Quantitative traits show continuous variation; we can see a large range of phenotypes in the population
• The amount of variation in a population is called variance
Genetically the same butPhenotypically different
Nutrition, exercise, and exposure to sun can cause differences in phenotypes
Diversity in Offspring
• Mutation, independent assortment, crossing over, and random fertilization result in unique combinations of alleles
• These processes produce the diversity of individuals found in humans and all other sexually reproducing biological populations
• You are one out of 64 trillion genetically different children that your parents could produce
Sex Determination andSex Linkage
• Some genes are on the X chromosome and are inherited in a specific manner
• In humans, sex determination involves the X and Y chromosomes
Sex Linkage• The genes on the X or Y
chromosomes are called sex-linked genes
• Genes on X are called “X-linked,” while those on Y are called “Y-linked”
• The X chromosome is much larger and carries far more genetic information
X-Linked Genes
• Since males only have one X chromosome, they are more likely to suffer from X-linked diseases– Hemophilia– red-green color
blindness– muscular dystrophy
• Since females get one X chromosome from each parent, and have two copies, they are less likely to suffer from X-linked diseases
7.3 Pedigrees
• A pedigree is a chart showing inheritance patterns in a family
• Pedigrees can be used to identify different types of inheritance patterns