presentation for Ch. 10

1. Patterns of
Inheritance
BIOL1406
Ebeling

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Chromosomes are packets of genetic
information
Section 10.1
Recall that a chromosome is a
piece of DNA containing many
different genes. Each gene’s
locus is its location on a
chromosome.
When two haploid sex cells fuse
during fertilization, a diploid
zygote with two full sets of
chromosomes is formed.
Figure 10.1
(a): ©CNRI/Science
Source

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Chromosomes are paired
in diploid cells
Section 10.1
Each chromosome is a
member of a homologous pair
of chromosomes.
Humans have 22 homologous
pairs of autosomes and one
pair of sex chromosomes.
Figure 10.1
(a): ©CNRI/Science
Source

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Diploid cells have two alleles for each gene
Section 10.1
Members of a homologous
pair have the same genes
but might have different
versions (alleles) of those
genes.
These alleles might be
identical (gene A) or
different (gene B).
Figure 10.1
(a): ©CNRI/Science
Source

9. Genotype Phenotype

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Offspring gave Mendel clues about the
genes of the parents
Section 10.2 Figure 10.3
True-breeding plants
produce offspring
identical to themselves.
a.True-breeding: Self-
fertilization yields offspring
with same seed color as
parent plant
Hybrid plants outwardly
resemble true-breeders but
produce mixed offspring.
b.Hybrid:
Self-
fertilization
yields mix of
seed colors

15. Dominant Recessive

20. Homozygous Heterozygous

23. Reginald Crundall Punnett
1875-1967

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Punnett squares represent gamete
formation and fertilization
A Punnett square uses the
genotypes of the parents
to reveal which alleles the
offspring may inherit.
Figure 10.6

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Punnett squares show how the alleles
separate during meiosis
Section 10.3
When germ cells divide by
meiosis, the gametes receive
one allele per gene.
For the seed-color gene, there
is an equal chance of receiving
either allele Y or y.
Figure 10.6

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Punnett squares show expected
proportions of offspring
Section 10.3
This Punnett square is a
prediction that shows the
relative proportion of the
offspring phenotypes and
genotypes.
Figure 10.6

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Punnett squares help us answer Mendel’s
questions
Section 10.2
If yellow seed color is dominant,
why are some seeds green when
a yellow-seed plant is crossed
with a green-seed plant?
Figures 10.4, 10.5
Genotyp
e
Phenotyp
e
Homozygous dominant (YY)
Yellow
Heterozygous (Yy)
Yellow
Homozygous recessive
(yy) Green

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Offspring can reveal parental genotypes:
If a cross between a yellow-
seed pea plant (YY or Yy) and
a green-seed pea plant (yy)
yields all yellow seeds, the
yellow-seed parent is
homozygous dominant.
If a cross between a yellow-
seed pea plant (YY or Yy) and
a green-seed pea plant (yy)
yields some green seeds, the
yellow-seed parent is
heterozygous.

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Punnett squares summarize
meiosis and fertilization
Section 10.3
The two alleles for the seed color gene are packaged into
separate gametes, which then combine at random.
Figure 10.9

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Mendel’s law can be applied to
human traits
Section 10.3
Punnett squares are
also useful for
tracking the
inheritance of genetic
disorders, such as
cystic fibrosis.
Figure 10.10
Health noncarrier (FF): 25%
chance
Healthy carrier (Ff): 50% chance
Affected (ff): 25% chance©Paul Rodriguez/ZUMApress/Alamy

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Dihybrid crosses track the
inheritance of two genes at once
Section 10.4
Two genes on different chromosomes can be combined into
one large Punnett square.
Figure 10.11

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Not all genes follow Mendelian
inheritance patterns
Section 10.6
So far we’ve discussed genes
with two alleles, in which the
dominant allele masks the
recessive allele.
But alleles do not interact this
way for all genes, which leads
to alternative patterns of
inheritance.
Figure 10.17

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Incompletely dominant alleles create a
blended heterozygote phenotype
Section 10.6
When the red allele (r1) and white allele (r2) are both present,
the heterozygote (r1r2) phenotype is an intermediate pink.
The white allele encodes a
nonfunctional pigment protein.
Figure 10.17

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Codominant alleles do not mask each
other
Section 10.6
In codominance,
more than one
allele encodes a
functional protein.
Figure 10.18
Genotype
s
Phenotypes
Surface
molecules
Phenotypes
ABO blood
type
𝐼 𝐴
𝐼 𝐴
𝐼 𝐴
𝑖
Only A Type A
𝐼 𝐵
𝐼 𝐵
𝐼 𝐵
𝑖
Only B Type B
𝐼 𝐴
𝐼 𝐵 Both A and B Type AB
ii None Type O

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Codominant alleles create a double
phenotype
Section 10.6
If two dominant alleles
are present, both
proteins encoded by
those alleles will be
represented in the
phenotype.
Figure 10.18
Genotype
s
Phenotypes
Surface molecules
Phenotypes
ABO blood
type
𝐼 𝐴
𝐼 𝐴
𝐼 𝐴
𝑖
Only A Type A
𝐼 𝐵
𝐼 𝐵
𝐼 𝐵
𝑖
Only B Type B
𝐼 𝐴
𝐼 𝐵 Both A and B Type AB
ii None Type O

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Human blood type alleles A and B are
both dominant over the O allele
Section 10.6
The I gene also has a
recessive allele, i, which
encodes a
nonfunctional protein.
But the two dominant
alleles, 𝐼 𝐴 and 𝐼 𝐵, make
the I gene codominant.
Figure 10.18
Genotype
s
Phenotypes
Surface
molecules
Phenotypes
ABO blood
type
𝐼 𝐴
𝐼 𝐴
𝐼 𝐴
𝑖
Only A Type A
𝐼 𝐵
𝐼 𝐵
𝐼 𝐵
𝑖
Only B Type B
𝐼 𝐴
𝐼 𝐵 Both A and B Type AB
ii None Type O

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One gene, many phenotypes
Section 10.6
In pleiotropy, one gene has multiple effects on the phenotype.
For example, a gene might affect more than one biochemical
pathway.
Marfan syndrome is
an example of
pleiotropy.
Figure 10.19
Photo:©Roger Kisby/Getty
Images

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Products of different genes can
interact with each other
Section 10.6
Epistasis occurs
when one gene’s
product affects
the expression of
another gene.
Expression of the
h allele (gene 1)
affects the
expression of A
and B alleles
(gene 2).
Figure 10.20

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Sex-linked genes have unique
inheritance patterns
Section 10.7
In humans, females have two X
chromosomes (XX). Males have
one X chromosome and one Y
chromosome (XY).
This Punnett square shows
that each fertilization event
has a 50% chance of
producing a female and a
50% chance of producing a
male.
Figure 10.21
Girl (XX): 50%
chance
Boy (XY): 50%
chance
©Andrew Syred/Science
Source

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Sex-linked genes disorders can be dominant or recessive
TABLE 10.2 Some X-Linked Disorders in Humans
Disorder Genetic Explanation Characteristics
X-linked recessive
inheritance
Duchenne muscular dystrophy Mutant allele of gene encoding dystrophin Rapid muscle degeneration early in life
Fragile X syndrome Unstable region of X chromosome has unusually high
number of CCG repeats
Most common form of inherited intellectual
disability
Hemophilia A Mutant allele of gene encoding blood-clotting protein
(factor VIII)
Uncontrolled bleeding, easy brusing
Red-green color blindness Mutant alleles of gene encoding receptors for red or
green (or both) wavelengths of light
Reduced ability to distinguish between red
and green
Rett syndrome Mutant allele encoding DNA-binding protein expressed
in nerve cells
Multiple severe development problems;
occurs almost exclusively in females;
affected male fetuses rarely survive to birth
X-linked dominant
inheritance
Extra hairiness (congenital
generalized
hypertrichosis;some forms)
Mechanism unknown Many more hair follicles than normal
Hypophosphatemic rickets
(some forms)
Mutant allele of gene involved in phosphorus
absorption
Defective bones caused by low blood
phosphorus
Retinitis pigmentosa (some
forms)
Mutant allele of cell-signaling proteins; mechanism
unknown
Partial blindness caused by defects in retina

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X-inactivation prevents double dosing of gene
products and produces unique inheritance patterns
Section 10.7
Each cell in an XX
individual, such as these
female cats, randomly
inactivates one X
chromosome.
Figure 10.24
©Siede Preis/Getty Images RF
If one X chromosome
has an allele for orange
fur, and the other has an
allele for black fur, color
patterns emerge when X
chromosomes are
randomly inactivated.

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X-linked dominant disorders are less
severe in females
Section 10.7
In humans, an X-linked
disorder called Rett
syndrome is lethal to
boys and has varying
effects on girls,
depending on how
many cells inactivate
the X chromosome
carrying the Rett
allele.
Figure 10.25
(a) : ©Andy Cross/The Denver Post via Getty Images; (b) : ©Moof/Cultura/Getty Images RF

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A pedigree depicts family relationships
and phenotypes
Section 10.8 Figure 10.26
This pedigree tracks
achondroplasia, an
autosomal dominant
disorder.
(a) : ©Rick Wilking/Reuters/Corbis

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Pedigrees show modes of inheritance
Section 10.8 Figure 10.26
This pedigree tracks an
autosomal recessive disorder.
This pedigree tracks an X-
linked recessive disorder.
Note that more males are
affected than females

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The environment can alter phenotype
Section 10.9
Many genes are affected by the environment.
For example, the enzyme responsible for pigment production
in Siamese cat fur is active only in cool body parts.
In warm body parts, no color is produced.
Figure 10.27
©Carolyn A. McKeone/Science Source

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Some traits depend on multiple genes
Section 10.9
Skin color is a polygenic trait; it is affected by more than one
gene.
Figure 10.29
©Sarah Leen/National Geographic Stock