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What Is The Physical Basis Of Inheritance?
 Inheritance occurs when genes are
transmitted from parent to offspring.
A gene’s physical location on a chromosome is called its
locus.
Each member of a pair of homologous chromosomes carries
the same genes, located at the same loci.
Versions of a gene at a given locus are called alleles
- you have 2 possible alleles at each locus
a pair of
homologous
chromosomes
• The units of inheritance are genes
Gene eye color: Both genes have same
allele e.g., brown hair
Both alleles same = homozygous
• Gregor Mendel
gene loci
Gene Freckles: This locus contains
another gene for which
the organism is homozygous
Gene Hair Color: Brown allele and Blond
allele
2 different alleles for a gene = heterozygous
chromosome
from Padre
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chromosome
from Madre
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How Are Single Traits Inherited?
1. When two different alleles are present in an
organism, a dominant allele may mask the
expression of the recessive allele; however, the
recessive allele is still present.
Single Traits Inheritance?
 The distribution of alleles in gametes
homozygous parent
A
2. The two alleles of a gene segregate (separate)
from one another during meiosis; this is known as
Mendel’s law of segregation.
gametes
A
A
(a) Gametes produced by a homozygous parent
heterozygous parent
3. Which allele ends up in any given gamete is
determined by chance.
A
A
gametes
A
a
a
(b) Gametes produced by a heterozygous parent
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Copyright © 2009 Pearson Education Inc.
In pea plants, purple is dominant to white.
Letters are used to describe the alleles (P = dominant allele; p =
recessive allele).
Homozygous purple plants are PP; homozygous white plants are pp;
heterozygous purple plants are Pp
 Fusion of gametes from the F1 generation produces F2
offspring.
F2 offspring
purple parent
PP
sperm
P
+
eggs
P
+
P
PP
P
+
p
Pp
p
+
P
Pp
p
+
p
pp
P
all P sperm and eggs
white parent
pp
p
+
p
all p sperm and eggs
Gametes produced by homozygous parents
What gametes are produced by heterozygous parents?
Fusion of gametes from the F1 generation produces F2 offspring
Fig. 9-7c
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 Simple “genetic bookkeeping” can predict the
genotypes and phenotypes of offspring.
• The Punnett square method is a predicts the
genotypes and phenotypes of offspring.\
 Punnett square
Pp
self-fertilize
P
1
2
• Genotype = genetic makeup of individual
• Phenotype = outward expression of individual
1
2
1
2
p
P
sperm
1
2
eggs
1
4
PP
1
4
Pp
1
4
pP
1
4
pp
p
Fig. 9-8
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Copyright © 2009 Pearson Education Inc.
Homozygous x heterozygous
Heterozygous x Heterozygous
TT
T
Homozygous
TT x TT?
TT x tt?
T
Tt
Tt
Figure 29.4
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 Color:
Y = yellow
y = green
eggs
= wrinkly
sperm
S = smooth
S
 Crossing over can create new combinations of
linked alleles.
SsYy
self-fertilize
1 SY
4
1 Sy
4
1 sY
4
1 sy
4
Figure 29.4
Don’t forget about crossing over
Predicting multiple traits
 Skin:
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1 SY
4
1 Sy
4
1 sY
4
1
16 SSYY
1
16 SSYy
1
16 SsYY
1
16 SsYy
1
16 SSyY
1
16 SSyy
1
16 SsyY
1
16 Ssyy
1
16 sSYY
1
16 sSYy
1
16 ssYY
1
16 ssYy
1
16 sSyY
1
16 sSyy
1
16 ssyY
1
16 ssyy
1
4
sy
• Genes on the same chromosome do not always stay
together.
• During prophase I of meiosis, homologous
chromosomes sometimes exchange parts in the
process, called crossing over.
• Crossing over produces a new allele combination on
both homologous chromosomes.
• Therefore, the chromosomes of each haploid
daughter cell receives different combinations of
alleles from those of the parent cell.
Fig. 9-11
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9.6 How Is Sex Determined?
9.7 How Are Sex-Linked Genes Inherited?
 Offspring sex is determined by a special pair
of chromosomes called the sex
chromosomes.
 Genes that are found on one sex
chromosome but not on the other are called
sex-linked.
• In mammals, females have two X
chromosomes and males have an X
chromosome and a Y chromosome.
• While the X chromosomes look alike, the Y
chromosomes are much smaller than the X
chromosomes.
• Because females have two X chromosomes,
they can be either homozygous or
heterozygous for genes on the X
chromosome.
• Males only have one X chromosome, and
therefore express all the alleles they have on
their X chromosome.
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Copyright © 2009 Pearson Education Inc.
9.7 How Are Sex-Linked Genes Inherited?
 If a man inherits one defective, recessive
allele on his X chromosome, he will show
the defective phenotype.
 A female, however, may be phenotypically
normal because one of her two X
chromosomes may display a functional,
dominant allele.
incomplete dominance.
 When a heterozygous phenotype is
intermediate between the two homozygous
phenotypes
• Human hair texture is influenced by a gene
with two incompletely dominant alleles, C1 and
C2.
• A person with two copies of the C1 allele has
curly hair; two copies of the C2 allele produces
straight hair; heterozygotes with C1C2
genotype have wavy hair.
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Copyright © 2009 Pearson Education Inc.
9.8 Do Mendelian Rules Of Inheritance
Wavey mother
Apply To All Traits?
 Two wavy-haired
people could have
CC
the following
children: ¼ curly
C
C
eggs
(C1C1), ½ wavy
(C1C2), and ¼
Wavet father
C
straight (C2C2).
CC
CC
1
 A single gene may have multiple alleles.
• A single individual can have only two alleles
for any gene, one on each homologous
chromosomes.
• However, within all the members of a species
there could be dozens of alleles for every
gene.
2
1
2
sperm
1
C1C2
1
1
1
2
C2
C1C2
C2C2
Fig. 9-17
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 A single gene may have multiple alleles
• Human blood types A, B, and O arise as a
result of three different alleles of a single gene
on chromosome 9; this gene codes for an
enzyme that adds sugar molecules to
recognition proteins on the surfaces of red
blood cells.
• A person may have one of six genotypes: AA,
BB, AB, Ao, Bo, and oo.
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9.8 Do Mendelian Rules Of Inheritance
Apply To All Traits?
 A single trait may be influenced by several
genes.
• Many physical traits are governed not by
single genes, but by interactions among two or
more genes, a phenomenon called polygenic
inheritance.
• The more genes that contribute to a single
trait, the greater the number of phenotypes
and the finer the distinctions among them.
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How Is Sex Determined?
 Human eye color is controlled by at least
three genes.
 At least three, and possibly dozens, of
genes affect human skin pigmentation, and
exposure to sun further alters skin color.
 All animals have one pair
of sex chromosomes and
other chromosomes
(autosomes).
Females XX
Males XY
Sex chromosomes aren’t
exactly homologous
X has over 2500 genes
Y has ~75 genes
Photomicrograph of human sex chromosomes
Fig. 9-18
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 During gamete formation, the sex chromosomes segregate
so that each female gamete gets one X, but the male
gametes get either an X or a Y.
Y chromosome
X chromosome
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 Sex determination
in humans
female parent
X1
 Punnett square shows how offspring sex is determined
X2
eggs
from segregation of the sex chromosomes.
X2
X1
X1
male parent
Y
X2
Xm
female offspring
sperm
Xm
Xm
Xm
X1
Y
X2
Y
Y
male offspring
Fig. 9-15
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