Transcript Chapter 23

Lecture PowerPoint to accompany
Inquiry into Life
Twelfth Edition
Sylvia S. Mader
Chapter 23
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
23.1 Mendel’s Laws
• Gregor Mendel
– Investigated inheritance at
the organism level (1860’s)
– Concluded that plants
transmit distinct factors to
offspring
– Based on his studies, he
formulated the law of
segregation
23.1 Mendel’s Laws
• The Law of Segregation
– Each individual has two factors for each trait
– The factors segregate (separate) during the formation
of gametes
– Each gamete contains only one factor from each pair
of factors
– Fertilization gives each new individual two factors for
each trait
23.1 Mendel’s Laws
• Today we know genes determine characteristics of an
organism, genes are found on chromosomes
• Chromosomes that are homologous are members of a
pair and carry genes for the same traits in the same
order
• Alleles are alternate forms of a gene for the same trait
• Alleles are always at the same locus (location) on each
chromosome of a homologous pair
Homologous Chromosomes
23.1 Mendel’s Laws
• The Inheritance of a Single Trait
– Phenotype: physical appearance of the individual
with regard to a trait
– Genotype: Alleles responsible for a given trait
• Two alleles for a trait
• A capital letter symbolizes a dominant allele (W)
• A lower-case letter symbolizes a recessive allele (w)
• Dominant refers to the allele that will mask the expression
of the alternate (recessive) allele
23.1 Mendel’s Laws
23.1 Mendel’s Laws
• Gamete Formation
– During meiosis, homologous chromosomes separate
so there is only 1 member of each pair in a gamete
– There is one allele for each trait, such as hairline, in
each gamete
– No two letters in a gamete can be the same letter of
the alphabet
• If genotype is Ww, then gametes from this individual will
contain either a W or a w
23.1 Mendel’s Laws
• One-Trait Cross
– A homozygous man
with a widow’s peak
(X)
A woman with a
straight hairline
23.1 Mendel’s Laws
• One-Trait Cross
– Two individuals who
are both Ww
– A Punnett Square is
useful to solve this
problem
23.1 Mendel’s Laws
• One-Trait Crosses and Probability
– The chance of 2 or more independent events occurring together
is the product of their chance of occurring separately
– In the cross Ww X Ww, what is the chance of obtaining either a
W or a w from a parent?
• Chance of W = ½ and the chance of w = ½
– Therefore the probability of having these genotypes is as
follows
•
•
•
•
Chance of WW= ½ X ½ = ¼
Chance of Ww = ½ X ½ = ¼
Chance of wW= ½ X ½ = ¼
Chance of ww = ½ X ½ = ¼
23.1 Mendel’s Laws
• The One-Trait Test Cross
– Breeders of plants and animals may do a test cross to
determine the likely genotype of an individual with the
dominant phenotype
• Cross with a recessive individual-has a known genotype
• If there are any offspring produced with the recessive
phenotype, then the dominant parent must be heterozygous
One-Trait Testcross
23.1 Mendel’s Laws
• Practice Problems
– Both a man and a woman are heterozygous
for freckles. Freckles are dominant over no
freckles. What is the chance that their child
will have freckles?
23.1 Mendel’s Laws
• Practice Problems
– Both you and your sibling have attached ear
lobes, but your parents have unattached
lobes. Unattached earlobes (E) are dominant
over attached (e). What are the genotypes of
your parents?
23.1 Mendel’s Laws
• Practice Problems
– A father has dimples, the mother of his
children does not, and all 5 of their children
have dimples. Dimples (D) are dominant over
no dimples (d). Give the probable genotypes
of all persons concerned.
23.1 Mendel’s Laws
• The Inheritance of Two Traits
– The Law of Independent Assortment:
• Each pair of factors assorts independently (without
regard to how the others separate)
• All possible combinations of factors can occur in
the gametes
The Inheritance of Two Traits
Two-Trait Crosses (Dihybrid Cross)
23.1 Mendel’s Laws
• Two-Trait Crosses (Dihybrid Cross)
• WwSs (X) WwSs
– Phenotypic Ratio:
– 9 widow’s peak, short fingers
– 3 widow’s peak, long fingers
– 3 straight hairline, short fingers
– 1 straight hairline, long fingers
23.1 Mendel’s Laws
• Two-Trait Crosses and Probability
– Probability Laws
•
•
•
•
Probability of widow’s peak = ¾
Probability of short fingers= ¾
Probability of straight hairline= ¼
Probability of long fingers= ¼
– Using the Product Rule
•
•
•
•
Probability of widow’s peak and short fingers = ¾ X ¾ = 9/16
Probability of widow’s peak and long fingers = ¾ X ¼ = 3/16
Probability of straight hairline and short fingers = ¼ X ¾ = 3/16
Probability of straight hairline and long fingers = ¼ X ¼ = 1/16
23.1 Mendel’s Laws
• Practice Problems
– Attached earlobes are recessive, What
genotype do children have if one parent is
homozygous recessive for earlobes and
homozygous dominant for hairline, and the
other is homozygous dominant for unattached
earlobes and homozygous recessive for
hairline?
23.1 Mendel’s Laws
• Practice Problems
– If an individual from this cross reproduces
with another of the same genotype, what are
the chances that they will have a child with a
straight hairline and attached earlobes?
23.1 Mendel’s Laws
• Practice Problems
– A child who does not have dimples or freckles
is born to a man who has dimples and
freckles (both dominant) and a woman who
does not. What are the genotypes of all
persons concerned?
23.1 Mendel’s Laws
• Pedigrees
– A chart of family’s history with regard to a particular
genetic trait
Males =
Females =
– Affected individuals (for a given trait) are shaded
A Human Pedigree
23.2 Beyond Simple Inheritance
Patterns
• Incomplete Dominance
– Occurs when the heterozygote is intermediate
between the two homozygotes
• Codominance
– Occurs when alleles are equally expressed in a
heterozygote
– Blood type AB is an example
• Red blood cells have both Type A and Type B surface
antigens
Incomplete Dominance
23.2 Beyond Simple Inheritance
Patterns
• Multiple Allele Inheritance
– A trait is controlled by multiple alleles, the gene exists
in several allelic forms.
• Each person has only two of the possible alleles.
23.2 Beyond Simple Inheritance
Patterns
• Multiple Allele Inheritance
– ABO Blood Types
• IA = A antigens on red blood cells
• IB = B antigens on red blood cells
• i = has neither A nor B antigens on red blood cells
• Both IA and IB are dominant over i, IA and IB are codominant
Phenotype
Genotype
A
IAIA or IAi
B
IBIB or IBi
AB
IAIB
O
ii
23.2 Beyond Simple Inheritance
Patterns
• Multiple Allele Inheritance
– ABO Blood Types
– Both IA and IB are dominant over i, IA and IB are
codominant
– The Rh factor is inherited separately from ABO blood
types.
Inheritance of Blood Types
23.2 Beyond Simple Inheritance
Patterns
• Practice Problems
– If a person with straight hair marries someone
with wavy hair, can they have a child with
curly hair?
23.2 Beyond Simple Inheritance
Patterns
• Practice Problems
– A child with type O blood is born to a mother
with type A blood. What is the genotype of the
child? The mother? What are the possible
genotypes of the father?
23.2 Beyond Simple Inheritance
Patterns
• Practice Problems
– From the following blood types determine
which baby belongs to which parents:
Baby 1 type O
Baby 2 type B
Mrs. Doe type A
Mr. Doe type A
Mrs. Jones type A
Mr. Jones type AB
23.2 Beyond Simple Inheritance
Patterns
• Sex-Linked Inheritance
– In Humans:
• 22 pairs of autosomes, 1 pair of sex chromosomes
– X and Y
» In females, the sex chromosomes are XX
» In males, the sex chromosomes are XY
» Note that in males the sex chromosomes are not
homologous
• Traits controlled by genes in the sex chromosomes are called
sex-linked traits
• X chromosome has many genes, the Y chromosome does
not
23.2 Beyond Simple Inheritance
Patterns
• Sex-Linked Alleles
– Red-green colorblindness is X-linked
• The X chromosome has genes for normal color vision
– XB = normal vision
– Xb – colorblindness
– Genotypes
XBXB
XBXb
XbXb
XBY
XbY
Phenotypes
female with normal color vision
carrier female with normal color vision
colorblind female
male with normal color vision
colorblind male
Cross involving an X-linked Allele
23.2 Beyond Simple Inheritance
Patterns
• Practice Problems
– Both the mother and the father of a colorblind male
appear to be normal. From whom did the son inherit
the allele for colorblindness? What are the genotypes
of the mother, father, and the son?
23.2 Beyond Simple Inheritance
Patterns
• Practice Problems
– A woman is colorblind. What are the chances that her
son will be colorblind? If she is married to a man with
normal vision, what are the chances that her
daughters will be colorblind? Will be carriers?
23.2 Beyond Simple Inheritance
Patterns
• Practice Problems
– A husband and the wife both have normal vision. The
wife gives birth to a colorblind daughter. Is it more
likely the father had normal vision or was colorblind?
What does this lead you to deduce about the girl’s
parentage?
23.2 Beyond Simple Inheritance
Patterns
• Polygenic Inheritance
– Occurs when a trait is governed by two or more sets
of alleles.
– Each dominant allele codes for a product
– The effects of the dominant alleles are additive.
– The result is continuous variation.
– Examples of traits include size or height, shape,
weight, and skin color.
Polygenic Inheritance
23.2 Beyond Simple Inheritance
Patterns
• Practice Problems
– A certain polygenic trait is controlled by three
pairs of alleles: A vs a, B vs b, and C vs c.
What are the extreme genotypes for this trait?
23.3 Environmental Influences
• Environmental factors can influence the
expression of genetic traits.
– Examples:
• Primrose flowers are white at warmer temperatures and red
at cooler temperatures
• Siamese cats and Himalayan rabbits are darker in color
where body heat is lost to the environment.
Coat Color in Himalayan Rabbits
23.4 Inheritance of Linked Genes
• All the alleles on one chromosome form a linkage group.
• Recall that during meiosis crossing over sometimes
occurs
• If crossing over occurs between two alleles of interest,
then four types of gametes are formed instead of two
Linkage Groups
23.4 Inheritance of Linked Genes
• The occurrence of crossing-over can help determine the sequence
of genes on a chromosome
• Crossing-over occurs more often between distant genes than genes
that are close together
• In the example below, it is expected that recombinant gametes
would include G and z more often than R and s.
23.4 Inheritance of Linked Genes
• Practice Problems
– When AaBb individuals reproduce, the phenotypic
ratio is about 3:1. What ratio was expected? What
may have caused the observed ratio?
23.4 Inheritance of Linked Genes
• Practice Problems
– The genes for ABO blood type and for fingernails are
on the same homologous pair of chromosomes. In an
actual family, 45% of offspring have type B blood and
no fingernails, and 45% have type O blood and
fingernails; 5% have type B blood and fingernails, and
5% have type O blood and no fingernails. What
process accounts for the recombinant phenotypes?