Chp 10
Patterns of Inheritance
✶ Dogs,
one of human’s
longest genetic experiments
Over 1,000’s of years, humans
have chosen and mated dogs
with specific traits. A process
called -artificial selection
The result – diverse breeds w/
distinct body types and
behaviors
✶ Sequencing
dog’s complete
DNA - genome - reveals
evolutionary relationships bt.
breeds
What breed of dog is this?
Wolf
Ancestral
canine
Chinese Shar-Pei
Akita
Siberian Husky
Basenji
Alaskan Malamute
Afghan hound
Saluki
Rottweiler
Sheepdog
Evolutionary relationships:
based on DNA similarities and differences
Retriever
2
Chromosomes Are
Packets of Genetic
Information
What we inherit
comes from two
parents
Mendel Uncovered Basic Laws of Inheritance
! Genetics
= scientific study of heredity
! Heredity = transmission of traits from one generation
to the next
✷ Gregor
– – – ✷ Mendel
Began field of genetics in 1860s,
Deduced principles of genetics
by breeding garden peas
Background in math, physics,
and chemistry
Eric Lander: The Human Genome Project
http://www.nytimes.com/video/2012/01/02/
science/100000001255558/eric-lander.html
4
5
Mendel determined
• Parents pass on to offspring discrete units
“heritable factors”
• Heritable factors (today called genes), retain
their individuality generation after generation
Genetic myths
– Characteristics
acquired in life can be
passed on
– Characteristics
of parents blend irreversibly
in offspring
6
Inheritance of a Single Character
✶ Mendel performed a
monohybrid cross
– ✶ Cross bt. 2 indivs
differing in a single
character
Crossed plant with
purple flowers and plant
with white flowers
• The F1 gen. – all w/
purple flowers
• F2 gen. - ¾ purple &
¼ white flowers
Crosses tracking one character (flower color)
7
Mendel developed 4 hypotheses
- (modern terms used below)
1. Alleles are alternative versions of genes
2. An organism inherits 2 alleles, one from each
parent. They can be the same, or different
A homozygous genotype has identical alleles
A heterozygous genotype has two different
alleles
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3. If alleles differ, then one determines organism’s
appearance = dominant. The other has no
noticeable effect on organism’s appearance =
recessive
- The phenotype is the appearance or expression
of a trait
- The genotype is the genetic makeup of a trait
- The same phenotype may be determined by
more than one genotype
4. A sperm or egg carries only 1 allele, b/c allele pairs
separate (segregate) from each other during the
production of gametes. This is called the law of
segregation
9
What are the genotypes
of these plants?
What are their
phenotypes?
What does the Punnet
square at the bottom of
the diagram show you?
Explanation of the
crosses in Figure 9.3A.
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Genotype, Phenotype and the Punnet Square
✶ Genotype
– – The genetic
makeup shown by
pair of alleles
PARENTS
Dd
Dd
Eggs
D
Sperm
DD
Normal
d
✶ Phenotype
- Ex: flower color, eye
color, deafness
Normal
D
Ex: AA, Aa, or aa
- The observable,
physical expression
of genotype
Normal
d
Dd
OFFSPRING
Dd
Normal
Normal
(carrier)
(carrier)
dd
Deaf
Punnet square showing offspring
produced by parents who are both
carriers for a recessive disorder
11
Homologous Chromosomes Have Alleles
✶ Every
individual has two alleles for each gene –
one from each parent
Three gene loci on homologous chromosomes
12
Mendel’s Laws Apply to Humans
✶ Segregation
and
fertilization are
chance events
✶ Each
allele
combination has a
given probability of
occurring
✶ Punnet
square
shows all possible
combinations of
______ in offspring
Segregation and fertilization as chance events
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Cystic Fibrosis Example
Lets say: Two healthy parents visit a
genetic counselor
✶ Counselor
tells them: Both of you are
heterozygous for Cystic Fibrosis
✶ What
is the probability of them having a
child with Cystic Fibrosis?
✶ CF
manifests itself in the homozygous
recessive individual
Question
Cystic fibrosis is caused by a
recessive allele. If a healthy carrier
and an affected individual have a
child, what is the chance the child
will be affected?
A. 1/4
B. 1/3
C. 1/2
D. 3/4
E. 1
Family Pedigrees Track Genetic Traits
✶ A pedigree
– shows
inheritance of a trait
through multiple
generations
– demonstrates
dominant or
recessive inheritance
– can
be used to deduce
genotypes of family
members
Examples of single-gene inherited traits in humans 16
Figure 9.8B Pedigree showing inheritance of attached versus free earlobe in a hypothetical
family.
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Many Inherited Disorders are Single Genes
✶ Inherited
– human disorders
Recessive inheritance
• 2 recessive alleles needed to show disease
• Heterozygous parents are carriers
• Probability of inheritance increases w/ inbreeding
– Dominant inheritance
• 1 dominant allele is needed to show disease
• Dominant lethal alleles usu. eliminated from a
population
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Many Genes Have More Than 2 Alleles
✶ Multiple
alleles
– More than 2 alleles found in the population
– A diploid individual can only carry _____ of these alleles
– Ex: ABO blood group has 3 alleles, leading to 4
phenotypes; type A, type B, type AB, and type O blood
✶ Codominance
– Neither allele is dominant
– Expression of both alleles is a distinct phenotype
– Ex. type AB blood
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In Codominance More Than One Allele
Encodes a Protein
The I gene has
3 possible
alleles:
IA, IB and i
Blood
Antibodies
Group
Present in
(Phenotype)
Blood
O
Anti-A
Anti-B
A
Anti-B
B
Anti-A
AB
—
Reaction When Blood from Groups Below Is Mixed
with Antibodies from Groups at Left
O
A
B
AB
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The Environment Can Alter Phenotype
✶ Many
phenotypic variations result
from combination of genes and
environment
– Nutrition during development (height,
weight)
– Skin color is affected by ________
– Exposure to drugs, toxins or
pollutants and risk of cancer
✶ For example, the enzyme responsible
for pigment production in Siamese cat
fur is active only in cool body parts.
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Some Traits Depend on Multiple Genes
Skin color is a polygenic trait; it is affected by
more than one gene. Section 10.9
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Figure 10.23
Sex-linked Genes Have Unique Inheritance
✶ Sex-linked
genes =
genes located on a
sex chromosome
✶ Which
gamete, the
sperm or the egg,
determines the sex
of the offspring?
✶ Examples:
Color
blindness,
Hemophilia
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X-linked Recessive Disorders Affect Mostly Males
WHY?
✶ Males
only need one recessive allele to be affected
✶ Female
must get a recessive allele on both X
chromosomes to be affected
The number 7
The number 96
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Punnet Square: X-linked Recessive Problem
If mom is a carrier of an X-linked disorder and dad is
normal, what are possible outcomes for
- a son? - a daughter?
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Question
Hemophilia is a X-linked
recessive disorder. If an affected
female and an unaffected male
have a boy, what is the chance
he will have hemophilia?
A. 0
B. 1/4
C. 1/2
D. 3/4
E. 1
Mastering Concepts
1. Distinguish between dominant and
recessive; heterozygous and
homozygous; phenotype and genotype.
2. Explain the meaning of: locus, multiple
alleles, pedigree, and codominance.
3. Describe the pattern of inheritance for
regular autosomal genes and sex-linked
genes.
4. Solve genetics problem, using punnet
squares involving monohybrid crosses
for autosomal and sex-linked traits
Investigating Life: Heredity and the
Hungry Hordes
Bollworm larvae devastate cotton crops. But some bollworms are
susceptible to Bt toxin. Biologists have inserted the gene encoding
this toxin into the cotton genome. Section 10.10
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Figure 10.24
Investigating Life: Heredity and the
Hungry Hordes
In a mating between two Bt-resistant bollworms, all of the
offspring will also be resistant. Section 10.10
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Figure 10.24
However, if a resistant bollworm mates with a
susceptible bollworm, only some—and sometimes
none—of the offspring will be resistant. Section 10.10
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Figure 10.24
To avoid 100% resistance among bollworms of
future generations, farmers must plant some crops
without the toxin gene. Crops with
the Bt toxin
Crops without the Bt toxin
Section 10.10
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Figure 10.24
This arrangement increases the chance that
some susceptible bollworms will remain in the
population. Resistance is
conferred by
a recessive
allele
Crops with
the Bt toxin
Crops without the Bt toxin
Section 10.10
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Figure 10.24
10.10 Mastering Concepts
Explain the logic of planting nonBt crop buffer strips around fields
planted with Bt crops. © 1996 PhotoDisc, Inc./Getty Images/RF
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.