BIOLOGY: Today and Tomorrow, 4e starr evers starr Chapter 9 Patterns of Inheritance.
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Transcript of BIOLOGY: Today and Tomorrow, 4e starr evers starr Chapter 9 Patterns of Inheritance.
BIOLOGY: Today and TomorrowBIOLOGY: Today and Tomorrow, 4e, 4estarr starr evers evers starrstarr
Chapter 9Patterns of Inheritance
9.1 Menacing Mucus
Cystic fibrosis, the most common fatal genetic disorder in the US, is caused by a deletion in the CFTR gene
The CF allele persists at high frequency despite devastating effects
Only those homozygous for the CF allele have the disorder
ANIMATED FIGURE: Crossing garden pea plants
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9.2 Tracking Traits
Mid-1800s: Genes and chromosomes were unknown; Gregor Mendel’s experiments with pea plants established principles of inheritance
anther
carpel
A) The flowers of garden pea plants have reproductive parts called anthers and carpels. Pollen grains that form in anthers produce male gametes; female gametes form in carpels.
Breeding Garden Peas
B) Experimenterscan control the transferof hereditary materialfrom one pea plant toanother by snipping offa flower’s anthers (toprevent the flower fromself-fertilizing), and thenbrushing pollen fromanother flower onto itscarpel. In this example,pollen from a plant thathas purple flowers isbrushed onto the carpelof a white-flowered plant.
Breeding Garden Peas
C) Later, seeds developinside pods of the cross-fertilized plant. An embryoin each seed develops intoa mature pea plant.
Breeding Garden Peas
D) Every plant that arises fromthe cross has purple flowers.Predictable patterns such asthis are evidence of how inheritance works.
Breeding Garden Peas
Inheritance in Modern Terms
Organisms breed true for a trait because they carry identical alleles of genes governing that trait
Homozygous Having identical alleles of a gene
Heterozygous Having two different alleles of a gene
Inheritance in Modern Terms
The particular set of alleles an individual carries is the individual’s genotype
Gene expression results in phenotype – an individual’s observable traits
An allele is dominant when its effect masks that of a recessive allele paired with it
genotype phenotype
PP (homozygous for dominant allele P)
pp (homozygous for recessive allele p)
Pp (heterozygous for alleles P and p)
Genotype gives rise to phenotype
9.3 Mendelian Inheritance Patterns
A cross (mating) between heterozygous individuals can reveal dominance relationships among the alleles under study
Monohybrid cross Cross in which individuals with different alleles of a gene
are crossed Dominant trait will have a 3:1 phenotype ratio
Segregation of Genes
When homologous chromosomes separate during meiosis, the gene pairs on those chromosomes separate
Each gamete that forms carries only one of the two genes of a pair
Stepped Art
gametes (p)
meiosis II
gametes (P)
DNA replication
meiosis I
1 2
zygote (Pp)
3
Figure 9-4 p153
Punnett Squares
Punnett squares are used to calculate the probability of the genotype and phenotype of offspring of crosses
In a testcross, an individual with a dominant trait (but an unknown genotype) is crossed with an individual known to be homozygous for the recessive allele
The pattern of traits among offspring can reveal whether the tested individual is heterozygous or homozygous
parent planthomozygous
for purpleflowers
parent planthomozygous
for whiteflowers
Pphybrid
two types of gametes
A) All of the F1 (first generation) offspring of a cross between two plants that breed true for different forms of a trait are identically heterozygous (Pp). These offspring make two types of gametes: P and p.
Monohybrid cross: First generation
B) A cross between two of the identically heterozygous F1 offspring is a monohybrid cross. In this example, the phenotype ratio among the F2 (second generation) offspring is 3:1 (three purple to one white).
Monohybrid cross: Second generation
Dihybrid Crosses
Mendel’s dihybrid crosses showed inheritance of one trait did not affect inheritance of other traits
Dihybrid cross Experiment in which individuals with different alleles of two
genes are crossed (9:3:3:1 ratio)
Independent assortment A gene tends to be distributed independently of how other
genes are distributed
parent plant homozygous for
white flowers and short stems
parent plant homozygous
for purple flowers and long stems
PpTt dihybrid
four types of gametes
1
2
3
Dihybrid Cross: First generation
The Contribution of Crossovers
Two genes located close together on the same chromosome tend to be inherited together
When two genes on the same chromosome are far apart, crossing over occurs more frequently between them; they tend to assort independently
A Human Example: Skin Color
Variations in skin color depend on the kinds and amounts of melanins produced
More than 100 gene products affect production and deposition of melanins
Independent assortment of these genes produces a wide variety of phenotypes
ANIMATED FIGURE: Dihybrid cross
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ANIMATED FIGURE: Independent assortment
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ANIMATED FIGURE: Monohybrid cross
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ANIMATED FIGURE: Test Cross
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9.4 Beyond Simple Dominance
Codominant Refers to two alleles that are both fully expressed in
heterozygous individuals
Incomplete dominance Condition in which one allele is not fully dominant over
another, so the heterozygous phenotype is between the two homozygous phenotypes
homozygous parent (RR)
homozygous parent (rr)
heterozygous offspring (Rr)X
A) Cross a red-flowered with a white-flowered snap-dragon, and all of the offspring will have pink flowers.
Incomplete Dominance
B) If two of the pink-flowered snapdragons are crossed, the phenotypes of their offspring will occur in a 1:2:1 ratio.
Incomplete Dominance
Epistasis
Some traits are affected by multiple gene products, an effect called polygenic inheritance or epistasis
Epistasis Effect in which a trait is influenced by the products of
multiple genes Example: Labrador retriever coat color
Pleiotropy
Products of pleiotropic genes influence two or more traits
Mutations in pleiotropic genes are associated with sickle cell anemia, cystic fibrosis, and Marfan syndrome
INTERACTION: Incomplete dominance
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ANIMATION: Chicken combs
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ANIMATION: Dog color
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9.5 Complex Variations in Traits
Mutations, interactions among genes, and environmental conditions can affect one or more steps in a metabolic pathway, and contribute to variation in phenotypes
Example: Seasonal changes affect production of pigments that color the skin and fur of many animals
Example: Water flea phenotypes depend on whether the aquatic insects that prey on them are present
Example: Genetically identical yarrow plants grow to different heights at different altitudes
Snowshoe Hare in Summer and Winter
A) The color of the snowshoe hare’s fur varies by season. In summer, the fur is brown (left ); in winter, it is white (right ). The variation offers seasonally appropriate camouflage from predators.
Water flea, with and without predators
B) The body form of the water flea on the top develops in environments with few predators. A longer tail spine and a pointy head (bottom) develop in response to chemicals emitted by predatory insects.
C) The height of a mature yarrow plant depends on the elevation at which it grows.
Elevation (meters above sea level)
Hei
gh
t (c
enti
met
ers)
3060 1400 30
60
0
Environmental Effects on Plant Phenotypes
Continuous Variation
Continuous variation A range of small increments of phenotype in a trait that is
influenced by the products of multiple genes The more genes and other factors that influence a trait,
the more continuous the distribution of phenotype
Bell curve Curve that results when range of variation in a continuous
trait is plotted against frequency in a population
Continuous Variation in Height
A) Male biology students at the University of Florida were divided into categories of one-inch increments in height and counted.
B) Graphing the resulting data produces a bell-shaped curve, an indication that height varies continuously.
Nu
mb
er o
f in
div
idu
als
20
15
10
5
063 64 65 66 67 68 69 70 71 72 73 74 75 76 77
Continuous Variation (Bell Curve)
9.5 Human Genetic Analysis
Inheritance patterns in humans are studied by following inherited genetic disorders in a family through generations and graphing results as a pedigree chart
Pedigree analyses shows whether a trait is associated with a dominant or recessive allele, and whether the allele is on an autosome or a sex chromosome
Types of Genetic Variation
Single genes on autosomes or sex chromosomes govern more than 6,000 genetic abnormalities and disorders
Genetic abnormality An uncommon version of a heritable trait that does not
result in medical problems
Genetic disorder A heritable condition that results in a syndrome of mild or
severe medical problems
ANIMATION: Coat color in the Himalayan rabbit
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ANIMATION: Height Graph
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9.6 Human Genetic Disorders
Some dominant or recessive alleles on autosomes or the X chromosome are associated with genetic abnormalities or disorders
An autosomal dominant allele is expressed in homozygotes and heterozygotes
An autosomal recessive allele is expressed only in homozygotes
normal mother
affected father
meiosis and gamete formation
affected child
normal child
disorder-causing allele (dominant)
X
A) A dominant allele on an autosome (red ) is fully expressed in heterozygous people
Autosomal Dominant Inheritance
disorder-causing allele (dominant)
affected father
normal mother
meiosis and gamete formation
Stepped Art
affected child
normal child
Figure 9-17a p163
ANIMATED FIGURE: Pedigree diagrams
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carrier mother carrier father
meiosis and gamete formation
normal child
carrier child
affected child
disorder-causing allele (recessive)
X
A) Only people homozygous for a recessive allele on an autosome have the trait associated with the allele.
In this example, both parents are carriers (red). Each of their children has a 25 percent chance of inheriting two alleles, and being affected by the trait.
Autosomal Recessive Inheritance
B) The albino phenotype is associated with autosomal recessive alleles that cause a deficiency in melanin.
disorder-causing allele (recessive)
carrier fathercarrier mother
meiosis and gamete formation
normal child
affected child
carrier child
Stepped Art
Figure 9-18a p163
X-Linked Recessive Disorders
Alleles on the X chromosome are inherited and expressed differently in males and females
Males cannot transmit a recessive X-linked allele to their sons
Females pass X-linked alleles to male offspring
Example: red-green color blindness
carrier mother normal father
meiosis and gamete formation
normal daughter or son
carrier daughter
affected son
recessive allele on X chromosome
X
A) In this example of X-linked inheritance, the mother carries a recessive allele on one of her two X chromosomes (red ).
X-Linked Recessive Inheritance
recessive allele on X chromosome
normal fathercarrier mother
meiosis and gamete formation
affected son
normal daughter or son
carrier daughter
Stepped Art
Figure 9-20a p165
B) A view of color blindness. The image on the left shows how a person with red–green color blindness sees the image on the right. The perception of blues and yellows is normal; red and green appear similar.
Red–green color blindness
C) Part of a standardized test for color blindness. A set of 38 of these circles is commonly used to diagnose deficiencies in color perception.
You may have another form of red–green color blindness if you see a 3 instead of an 8 in this circle.
You may have one form of red–green color blindness if you see a 7 instead of a 29 in this circle.
Red–green color blindness
INTERACTION: Autosomal-dominant inheritance
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INTERACTION: Autosomal-recessive inheritance
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INTERACTION: X-linked inheritance
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9.8 Changes in Chromosome Number
Many flowering plants, and some insects, fishes and other animals are polyploid – having three or more of each type of chromosome characteristic of the species
Chromosome number can change permanently, usually resulting from nondisjunction – the failure of chromosomes to separate normally during meiosis or mitosis
Metaphase I
Anaphase I
Telophase I
Metaphase II
Anaphase II
Telophase II
Nondisjunction During Meiosis
Figure 9-21 p166
Metaphase I
Stepped Art
Anaphase I
Telophase I
Metaphase II
Anaphase II
Telophase II
Aneuploidy
Aneuploidy A chromosome abnormality in which a cell has too many
or too few copies of a particular chromosome (trisomy, monosomy)
The most common aneuploidy in humans, trisomy 21, causes Down syndrome
Autosomal Change and Down Syndrome
Trisomy 21 (Down syndrome) The only autosomal trisomy that allows humans to survive
to adulthood Affected individuals tend to have certain physical features
and impairments
Nondisjunction leading to trisomy 21 increases with age of the mother
Change in Sex Chromosome Number
Usually associated with learning difficulties, speech delays, and motor skill impairment
Female sex chromosome abnormalities: Turner syndrome (XO) XXX syndrome
Male sex chromosome abnormalities: Klinefelter syndrome (XXY) XYY syndrome
9.9 Genetic Screening
Geneticists estimate the chance that a couple’s offspring will inherit a genetic abnormality or disorder
Potential parents who may be at risk of transmitting a harmful allele to offspring have screening or treatment options
Prenatal Diagnosis
Obstetric sonography may reveal defects associated with a genetic disorder
Other tests performed before birth carry risks of miscarriage or injury to fetus Amniocentesis Chorionic villi sampling (CVS) Fetoscopy
Preimplantation Diagnosis
A single cell taken from an embryo produced by in vitro fertilization is tested before implantation
9.10 Menacing Mucus (revisited)
The cystic fibrosis (CF) allele is very common in some populations
The CF allele is lethal in homozygotes, but offers heterozygotes some protection against bacterial diseases such as typhoid fever