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

Victims of Cystic Fibrosis

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.


C) Later, seeds developinside pods of the cross-fertilized plant. An embryoin each seed develops intoa mature pea plant.


D) Every plant that arises fromthe cross has purple flowers.Predictable patterns such asthis are evidence of how inheritance works.


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

DNA replication

meiosis I

meiosis II

gametes (P) gametes (p)

zygote (Pp)

1 2

3

Segregation of Genes

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

Punnett Squares: A Monohybrid Cross

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

4

Dihybrid Cross: Second 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

Variation in Skin Color

ANIMATED FIGURE: Dihybrid cross



ANIMATED FIGURE: Independent assortment



ANIMATED FIGURE: Monohybrid cross



ANIMATED FIGURE: Test Cross



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

Codominance: Blood Type

Genotypes:

Phenotypes (blood type):

AAorAO AB

BBorBO OO

A AB

B O

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

Epistasis

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



ANIMATION: Chicken combs



ANIMATION: Dog color



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 Eye Color

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

Pedigree: Polydactyly

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



ANIMATION: Height Graph



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

Some Autosomal Dominant Traits

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


Stepped Art

affected child

normal child

Figure 9-17a p163

ANIMATED FIGURE: Pedigree diagrams



Some Autosomal Recessive Traits

carrier mother carrier father


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


normal child

affected child

carrier child

Stepped Art

Figure 9-18a p163

Victim of Tay–Sachs disease

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

Some X-Linked Recessive Disorders

carrier mother normal father


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


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



INTERACTION: Autosomal-recessive inheritance



INTERACTION: X-linked inheritance



VIDEO: Genetics, Sociology, and Breast Cancer

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

Some Disorders Caused by Aneuploidy

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

Down Syndrome

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

Three ways of imaging a fetus

C) Fetoscopy

B) 4D ultrasound

A) Conventional ultrasound

The amniotic sac

amniotic sac

chorion

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

Digging Into Data:Cystic Fibrosis and Typhoid Fever

Documents

BIOLOGY: Today and Tomorrow, 4e starr evers starr Chapter 9 Patterns of Inheritance