Observable Patterns of Inheritance

Chapter 11

Earlobe Variation

• Whether a person is born with attached or detached earlobes depends on a single gene

• Gene has two molecular forms (alleles)

Earlobe Variation

• You inherited one allele for this gene from each parent

• Dominant allele specifies detached earlobes

• Recessive allele specifies attached lobes

Dominant & Recessive Alleles

• If you have attached earlobes, you inherited two copies of the recessive allele

• If you have detached earlobes, you may have either one or two copies of the dominant allele

Early Ideas About Heredity

• People knew that sperm and eggs transmitted information about traits

• Blending theory

• Problem:

– Would expect variation to disappear

– Variation in traits persists

Gregor Mendel

• Strong background in plant breeding and mathematics

• Using pea plants, found indirect but observable evidence of how parents transmit genes to offspring

The Garden Pea Plant

• Self-pollinating

• True breeding (different alleles not normally introduced)

• Can be experimentally cross-pollinated

Genes

• Units of information about specific traits

• Passed from parents to offspring

• Each has a specific location (locus) on a chromosome

Alleles

• Different molecular forms of a gene

• Arise by mutation

• Dominant allele masks a recessive allele that is paired with it

Allele Combinations

• Homozygous

– having two identical alleles at a locus

– AA or aa

• Heterozygous

– having two different alleles at a locus

– Aa

Genetic Terms

Genotype & Phenotype

• Genotype refers to particular genes an individual carries

• Phenotype refers to an individual’s observable traits

• Cannot always determine genotype by observing phenotype

Tracking Generations

• Parental generation P

mates to produce

• First-generation offspring F₁

mate to produce

• Second-generation offspring F₂

F₁ Results of One Monohybrid Cross

F₂ Results of
Monohybrid Cross

Mendel’s
Monohybrid
Cross Results

Mendel’s Theory
of Segregation

• An individual inherits a unit of information (allele) about a trait from each parent

• During gamete formation, the alleles segregate from each other

Probability

The chance that each outcome of a given event will occur is proportional to the number of ways that event can be reached

Punnett Square of a Monohybrid Cross

Test Cross

• Individual that shows dominant phenotype is crossed with individual with recessive phenotype

• Examining offspring allows you to determine the genotype of the dominant individual

Dihybrid Cross

Experimental cross between individuals that are homozygous for different versions of two traits

A Dihybrid Cross - F₁ Results

F₁ Results of Mendel’s Dihybrid Crosses

• All plants displayed the dominant form of both traits

• We now know:

– All plants inherited one allele for each trait from each parent

– All plants were heterozygous (AaBb)

Phenotypic Ratios in F₂

Four Phenotypes:

– Tall, purple-flowered (9/16)

– Tall, white-flowered (3/16)

– Dwarf, purple-flowered (3/16)

– Dwarf, white-flowered (1/16)

Explanation of Mendel’s Dihybrid Results

If the two traits are coded for by genes
on separate chromosomes, sixteen gamete combinations are possible

16 Allele
Combinations in F₂

Independent Assortment

• Mendel concluded that the two “units” for the first trait were to be assorted into gametes independently of the two “units” for the other trait

• Members of each pair of homologous chromosomes are sorted into gametes at random during meiosis

Independent Assortment

Tremendous Variation

Number of genotypes possible in offspring as a result of independent assortment and hybrid crossing is

3ⁿ

(n is the number of gene loci
at which the parents differ)

Impact of Mendel’s Work

• Mendel presented his results in 1865

• Paper received little notice

• Mendel discontinued his experiments in 1871

• Paper rediscovered in 1900 and finally appreciated

Dominance Relations

• Complete dominance

• Incomplete dominance

– Heterozygote phenotype is somewhere between that of two homozyotes

• Codominance

– Non-identical alleles specify two phenotypes that are both expressed in heterozygotes

Flower Color in Snapdragons: Incomplete Dominance

Red-flowered plant X White-flowered plant

Pink-flowered F₁ plants

Flower Color in Snapdragons: Incomplete Dominance

Pink-flowered plant X Pink-flowered plant

White-, pink-, and red-flowered plants

in a 1:2:1 ratio

Flower Color in Snapdragons: Incomplete Dominance

• Red flowers - two alleles allow them to make a red pigment

• White flowers - two mutant alleles; can’t make red pigment

• Pink flowers have one normal and one mutant allele; make a smaller amount of red pigment

Genetics of ABO Blood Types: Three Alleles

• Gene that controls ABO type codes for enzyme that dictates structure of a glycolipid on blood cells

• Two alleles (A and B) are codominant when paired

• Third allele (O) is recessive to others

ABO Blood Type:
Allele Combinations

• Type A – AA or AO

• Type B – BB or BO

• Type AB - AB

• Type O - OO

ABO Blood Type:
Glycoproteins on Red Cells

ABO and Transfusions

• Recipient’s immune system will attack blood cells that have an unfamiliar glycolipid on surface

• Type O is universal donor because it has neither type A nor type B glycoprotein

Pleitropy

• Alleles at a single locus may have effects on two or more traits

• Classic example is the effects of the mutant allele at the beta-globin locus that gives rise to sickle-cell anemia

Genetics of Sickle-Cell Anemia

• Two alleles

1) Hb^A

Encodes normal beta hemoglobin chain

2) Hb^S

Mutant allele encodes defective chain

• Hb^S homozygotes produce only the defective hemoglobin; suffer from sickle-cell anemia

Pleiotrophic Effects of Hb^S/Hb^S

• At low oxygen levels, cells with only Hb^S hemoglobin “sickle” and stick together

• This impedes oxygen delivery and blood flow

• Over time, it causes damage throughout the body

Epistasis

• Interaction between the products of gene pairs

• Common among genes for hair color in mammals

Genetics of Coat Color in Labrador Retrievers

• Two genes involved

- One gene influences melanin production

• Two alleles - B (black) is dominant over b (brown)

- Other gene influences melanin deposition

• Two alleles - E promotes pigment deposition and is dominant over e

Allele Combinations
and Coat Color

• Black coat - Must have at least one dominant allele at both loci

– BBEE, BbEe, BBEe, or BbEE

• Brown coat - bbEE, bbEe

• Yellow coat - BBee, Bbee, bbee

Campodactyly:
Unexpected Phenotypes

• Effect of allele varies:

– Bent fingers on both hands

– Bent fingers on one hand

– No effect

• Many factors affect gene expression

Human Variation

• Some human traits occur as a few discrete types

– Attached or detached earlobes

– Many genetic disorders

• Other traits show continuous variation

– Height

– Weight

– Eye color

Continuous Variation

• A more or less continuous range of small differences in a given trait among individuals

• The greater the number of genes and environmental factors that affect a trait, the more continuous the variation in versions of that trait

Describing Continuous Variation

Environmental Effects on Plant Phenotype

• Hydrangea macrophylla

• Action of gene responsible for floral color is influenced by soil acidity

• Flower color ranges from pink to blue

Temperature Effects
on Phenotype

• Himalayan rabbits are Homozygous for an allele that specifies a heat-sensitive version of an enzyme in melanin-producing pathway

• Melanin is produced in cooler areas of body

Genes

• Units of information about heritable traits

• In eukaryotes, distributed among chromosomes

• Each has a particular locus

– Location on a chromosome

Homologous Chromosomes

• Homologous autosomes are identical in length, size, shape, and gene sequence

• Sex chromosomes are nonidentical but still homologous

• Homologous chromosomes interact, then segregate from one another during meiosis

Alleles

• Different molecular forms of a gene

• Arise through mutation

• Diploid cell has a pair of alleles at each locus

• Alleles on homologous chromosomes may be same or different

Sex Chromosomes

• Discovered in late 1800s

• Mammals, fruit flies

– XX is female, XY is male

• In other groups XX is male, XY female

• Human X and Y chromosomes function as homologues during meiosis

Karyotype Preparation - Stopping the Cycle

• Cultured cells are arrested at metaphase by adding colchicine

• This is when cells are most condensed and easiest to identify

Karyotype Preparation

• Arrested cells are broken open

• Metaphase chromosomes are fixed and stained

• Chromosomes are photographed through microscope

• Photograph of chromosomes is cut up and arranged to form karyotype diagram

Human Karyotype

Sex Determination

The Y Chromosome

• Fewer than two dozen genes identified

• One is the master gene for male sex determination

– SRY gene (Sex-determining region of Y)

• SRY present, testes form

• SRY absent, ovaries form

Effect of Y
Chromosome

The X Chromosome

• Carries more than 2,300 genes

• Most genes deal with nonsexual traits

• Genes on X chromosome can be expressed in both males and females

Discovering Sex Linkage

Discovering Linkage

Discovering Sex Linkage

• Morgan’s crosses showed relationship between sex and eye color

• Females can have white eyes

• Morgan concluded gene must be on the X chromosome

Autosomal Linkage Groups

• Genes on one type of chromosome

• Fruit flies

– 4 homologous chromosomes

– 4 linkage groups

• Indian corn

– 10 homologous chromosomes

– 10 linkage groups

Full Linkage

Incomplete Linkage

Crossover Frequency

Linkage Mapping in Humans

• Linkage maps based on pedigree analysis through generations

• Color blindness and hemophilia are very closely linked on X chromosome

– Recombination frequency is 0.167%

Genetic Abnormality

• A rare, uncommon version of a trait

• Polydactyly

– Unusual number of toes or fingers

– Does not cause any health problems

– View of trait as disfiguring is subjective

Genetic Disorder

• Inherited conditions that cause mild to severe medical problems

• Why don’t they disappear?

– Mutation introduces new rare alleles

– In heterozygotes, harmful allele is masked, so it can still be passed on to offspring

How Are Single-Gene Disorders Inherited?

• Some Human Genetic Disorders Are Sex-Linked

• Some Human Genetic Disorders Are Caused by Recessive Alleles

• Some Human Genetic Disorders Are Caused by Dominant Alleles

X-Linked Recessive Inheritance

• Males show disorder more than females

• Son cannot inherit disorder from his father

Examples of X-Linked Traits

• Color blindness

– Inability to distinguish among some of all colors

• Hemophilia

– Blood-clotting disorder

– 1/7,000 males has allele for hemophilia A

– Was common in European royal families

Autosomal Recessive Inheritance Patterns

• If parents are both heterozygous, child will have a 25% chance of being affected

Galactosemia

• Caused by autosomal recessive allele

• Gene specifies a mutant enzyme in the pathway that breaks down lactose

Autosomal
Dominant Inheritance

Trait typically appears in every generation

Huntington Disorder

• Autosomal dominant allele

• Causes involuntary movements, nervous system deterioration, death

• Symptoms don’t usually show up until person is past age 30

• People often pass allele on before they know they have it

Acondroplasia

• Autosomal dominant allele

• In homozygous form usually leads to stillbirth

• Heterozygotes display a type of dwarfism

• Have short arms and legs relative to other body parts

Pedigree

• Chart that shows genetic connections among individuals

• Standardized symbols

• Knowledge of probability and Mendelian patterns used to suggest basis of a trait

• Conclusions most accurate when drawn from large number of pedigrees

Aneuploidy

• Individuals have one extra or less chromosome

• (2n + 1 or 2n - 1)

• Major cause of human reproductive failure

• Most human miscarriages are aneuploids

Polyploidy

• Individuals have three or more of each type of chromosome (3n, 4n)

• Common in flowering plants

• Lethal for humans

– 99% die before birth

– Newborns die soon after birth

Nondisjunction

Down Syndrome

• Trisomy of chromosome 21

• Mental impairment and a variety of additional defects

• Can be detected before birth

• Risk of Down syndrome increases dramatically in mothers over age 35

Abnormal Numbers of Autosomes Cause Some Disorders
Trisomy 21 (Down Syndrome)

How Do Errors in Chromosome Number Affect Humans?

• Abnormal numbers of sex chromosomes cause certain disorders:

– Turner Syndrome (XO)

– Trisomy X (XXX)

– Klinefelter Syndrome (XXY)

– Jacob Syndrome (XYY

Turner Syndrome

• Inheritance of only one X (XO)

• 98% spontaneously aborted

• Survivors are short, infertile females

– No functional ovaries

– Secondary sexual traits reduced

– May be treated with hormones, surgery

Klinefelter Syndrome

• XXY condition

• Results mainly from nondisjunction in mother (67%)

• Phenotype is tall males

– Sterile or nearly so

– Feminized traits (sparse facial hair, somewhat enlarged breasts)

– Treated with testosterone injections

XYY Condition

• Taller than average males

• Most otherwise phenotypically normal

• Some mentally impaired

• Once thought to be predisposed to criminal behavior, but studies now discredit

Phenotypic Treatments

• Symptoms of many genetic disorders can be minimized or suppressed by

– Dietary controls

– Adjustments to environmental conditions

– Surgery or hormonal treatments

Genetic Screening

• Large-scale screening programs detect affected persons

• Newborns in United States routinely tested for PKU

– Early detection allows dietary intervention and prevents brain impairment

Prenatal Diagnosis

• Amniocentesis

• Chorionic villus sampling

• Fetoscopy

• All methods have some risks

Preimplantation Diagnosis

• Used with in-vitro fertilization

• Mitotic divisions produce ball of 8 cells

• All cells have same genes

• One of the cells is removed and its genes analyzed

• If cell has no defects, the embryo is implanted in uterus

Duplication

• Gene sequence that is repeated several to hundreds of times

• Duplications occur in normal chromosomes

• May have adaptive advantage

– Useful mutations may occur in copy

Duplication

Inversion

A linear stretch of DNA is reversed

within the chromosome

Translocation

• A piece of one chromosome becomes attached to another nonhomologous chromosome

• Most are reciprocal

• Philadelphia chromosome arose from a reciprocal translocation between chromosomes 9 and 22

Translocation

Deletion

• Loss of some segment of a chromosome

• Most are lethal or cause serious disorder

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