Download Chapter 12

Anu Singh-Cundy • Michael L. Cain
Discover Biology
FIFTH EDITION
CHAPTER 12
Pa#erns of Inheritance
© 2012 W. W. Norton & Company, Inc.
A genetic trait is any inherited characteristic of an organism that can be
observed or detected
Invariant genetic traits are the same in all individuals in a population
Essen1al Terms in Gene1cs
• The display of a par1cular version of a gene1c
trait in a specific individual is the phenotype
• Genotype – chromosomes
• Phenotype -­‐ display
Diploid Cells Have Two Copies
of Every Gene
• diploid two copies of each type of chromosome
homologous pair
• Humans have 23 pairs (total of 46) chromosomes
(Gametes from meiosis are haploid -­‐ only one set of
chromosomes
Genotype Directs Phenotype
• Alleles are different versions of genes
• Genotype is the allelic makeup
– The phenotype is the result of an individual’s
genotype
Genotype Directs Phenotype
• 2 copies of the same allele homozygous for
that gene
• 2 different alleles heterozygous for that gene
Some Phenotypes Are Controlled by
Dominant Alleles
• The allele that exerts a controlling influence
dominant (UPPER CASE leJer)
• An allele that has no effect when paired with a
dominant allele recessive (lower case leJer)
• Some traits are controlled by more than one
gene, whereas others are not controlled by
any one set of genes
Gene Muta1ons Are the
Source of New Alleles
• A mutaMon is a change in the DNA
• Harmful muta1ons are oRen recessive
(masked by the dominant allele) yet are
passed along to offspring by heterozygotes
• Carriers
Gene Muta1ons Are the
Source of New Alleles
• Muta1ons are oRen neutral or, on the rare
occasion, beneficial to the individual
• Gene muta1ons occur at random
• Only muta1ons that occur in the gametes or
the cells that produce gametes can be passed
to offspring
Controlled Crosses Help Us
Understand Pa#erns of Inheritance
• A geneMc cross is a controlled ma1ng
experiment performed to examine how a
par1cular trait is inherited
• The parents, or P generaMon, are crossed to
produce offspring, called the F1 generaMon
• Two individuals from the F1 genera1on are
then crossed to produce the F2 generaMon
Basic Pa#erns of Inheritance
• Before Mendel -­‐Blending (“in-­‐between”) ??
• Mendel proposed that offspring inherit two
separate units of gene1c informa1on, now
known as genes, with one from each parent
(each is an allele)
Also Pedigrees
Pedigrees are s1ll used to study heredity!
– Hemophilia -­‐ inherited condi1on where blood is slow
to clot or does not clot at all
– only expressed when individual has no copies of the normal allele
» Royal hemophilia -­‐ sex-­‐linked
Royal Hemophilia Pedigree
The field of
genetics
originated in
1866 after
Gregor Mendel
published a
paper on
inheritance in
pea plants
Mendel’s Gene1c Experiments Began
with True-­‐Breeding Pea Plants
• True-­‐breeding, or purebred, individuals have a
homogenous genotype
• Mendel crossed two lines of pure-­‐breeding
plants to produce two genera1ons of hybrid
plants and recorded the phenotypic data
Mendel’s Experiments
Second Filial Genera1on
Interpreta1on of Mendel’s Results
• Nota1onal conven1on
– P -­‐ dominant allele (purple)
– p -­‐ recessive allele (white)
• PP -­‐ homozygous dominant
• Pp -­‐ heterozygous
• pp -­‐ homozygous recessive
Interpreta1on of Mendel’s Results
• F1 genera1on
– PP x pp (parental genera1on) yielded all Pp
offspring
• F2 genera1on
– Pp x Pp yielded: (1:3:1) ra1o
• 1 PP
• 3 Pp
• 1 pp
• Punne# squares
Mendel Inferred That Inherited Traits
Are Determined by Genes
•
Mendel repeatedly observed a 3:1 ra1o of
dominant to recessive phenotypes
•
The basis of Mendel’s work can be
summarized as in five points:
1. Offspring inherit one copy of a gene from
each parent
2. Alterna1ve versions of genes cause
varia1on in inherited traits
3. An allele is dominant if, when paired with a
different allele, it has exclusive control over an
individual’s phenotype
4. The two copies (alleles) of a gene segregate
during meiosis and end up in different gametes
5. Gametes fuse randomly, without regard to the
par1cular alleles they carry
Extensions of Mendel’s Laws
• Mendel’s work was based on gene1c traits
controlled by a single gene with a dominant
and a recessive allele
• Mendel’s laws have been expanded to help
explain the more complex pa#erns of
inheritance
Transmission of Autosomal Dominant
Disorders
Huntington disease, degenerative
neural disorder.

Only single “faulty” allele of a gene
causes disease.

Even if only one parent has “faulty”
allele, 50% chance offspring have
condition

Father
Mother
Ss
H
sperm
Ss
Not “true breeding”
h
Hh
h
hh
The gene for
Huntington disease
is dominant.
h
eggs
Hh
hh
50% probability of
inheriting Huntington
disease.
The Alleles of Some Genes
Are Codominant
• Codominance occurs when the effect of both
alleles is equally visible in the phenotype of
the heterozygote
• Neither allele is diminished or diluted in a
heterozygote that displays codominance
• The AB blood type example of codominance
Many Alleles Display
Incomplete Dominance
• Incomplete dominance no single allele
completely dominates the other when the
two are paired in a heterozygote
• heterozygotes display an intermediate
phenotype
• Curly hair
Mendel’s Laws of Inheritance
(only 2)
• Mendel deduced the law of segregaMon from
breeding experiments in which he tracked a
single trait
• The law of independent assortment was
based on two-­‐trait breeding experiments that
Mendel conducted, in which he tracked two
completely different traits at the same 1me
Mendel’s Single-­‐Trait Crosses
Revealed the Law of Segrega1on
• The law of segrega1on states that the two
copies of a gene are separated during meiosis
and end up in different gametes
• The law of segrega1on can be used to predict
how a single trait will be inherited
Mendel’s Two-­‐Trait Experiments Led to
the Law of Independent Assortment
• Next Mendel crossed dihybrids, individuals that are
heterozygous for two traits,
• The law of independent assortment states that when
gametes form, the two copies of any given allele
segregate during meiosis, independently of any two
alleles of other genes
• The law of independent assortment applies to the
inheritance of two genes that are physically
separated on different chromosomes
Mendel’s Insights Rested on
Probability
• We can predict the probability that a par1cular
offspring will have a certain phenotype or
genotype, but we cannot predict the actual
phenotype or genotype of an individual
• The probability that a par1cular offspring will
display a specific phenotype is completely
unaffected by the number of offspring
A Pleiotropic Gene Affects
Mul1ple Traits
• When a single gene influences two or more
dis1nctly different traits –pleiotropy
• A muta1on in a pleiotropic gene can cause
changes in many different traits
• Albinism is an example of a pleiotropic
disorder
Alleles for One Gene Can Alter the
Effects of Another Gene
• The term epistasis -­‐phenotypic effect of the
alleles of one gene depends on the presence
of certain alleles for another, independently
inherited gene
• Epistasis can be seen in the coat color of
numerous animals, whose many genes code
for enzymes that convert the amino acid
tyrosine into melanin in a mul1step pathway
The Environment Can Alter the
Effects of a Gene
• Chemicals, nutri1on, sunlight, and other
internal and external environmental factors
can also alter the effects of certain genes
• The produc1on of melanin in Siamese cats is
sensi1ve to temperature—cooler
temperatures produce dark fur on the
extremi1es
• Most traits are considered polygenic because
they are governed by the ac1on of more than
one gene
• Skin color, running speed, blood pressure, and
body size are all polygenic traits in humans
• Gene1cists es1mate there are more than 12
genes that control melanin produc1on in our
skin, so an almost con1nuous varia1on in the
trait
• Complex traits are those that cannot be predicted
using Mendel’s laws of inheritance
• Most traits that are essen1al for survival are complex
traits
– Social skills, heart func1on,
• Phenotypic diversity is a substan1al evolu1onary
benefit
• Alzheimer's disease
Concept Quiz
A red carnation and a white carnation
produce offspring that are all pink. The type
of inheritance pattern occurring is:
A. Complete dominance
B. Incomplete dominance
C. Codominance
Concept Quiz
If an allele for tall plants (T) is dominant to
short plants (t), what offspring would you
expect from a TT x Tt cross?
A. ½ tall; ½ short
B. ¾ tall; ¼ short
C. All tall
Concept Quiz
Fur color in rabbits shows incomplete dominance.
FBFB individuals are brown, FBFW individuals are
cream, FWFW individuals are white. What is the
expected ratio of a FBFW x FW FW cross?
A. 3 white : 1 brown
B. 3 white : 1 cream
C. 2 white : 2 cream