Chapter 4

Heredity and Evolution

Chapter Outline• The Genetic Principles Discovered by

Mendel• Mendelian Inheritance in Humans• Non-Mendelian Inheritance • Genetic and Environmental Factors• Modern Evolutionary Theory

Chapter Outline• A Current Definition of Evolution Factors that

Produce and Redistribute Variation• Natural Selection Acts on Variation• Review of Genetics and Evolutionary Factors• New Frontiers in Research: Molecular

Applications in Forensic Anthropology

Selective Breeding• A practice whereby animal and plant

breeders choose which animals will be allowed to mate based on traits (such as coat color, body size, shape of face) they hope to produce in offspring.

• Animals that don’t have the desirable traits aren’t allowed to breed.

Genetic PrinciplesDiscovered by Mendel• Gregor Mendel (1822-1884) laid down the

basic principles of heredity.• He explored the ways physical traits could

be expressed in plant hybrids. A hybrid is the offspring of individuals

that differ with regard to certain traits or certain aspects of genetic makeup.

Gregor Mendel• A portrait of Gregor

Mendel.

Traits Mendel Studied: Peas

Results: One Trait at a Time

Principle of Segregation• Genes occur in pairs because

chromosomes occur in pairs. • During gamete production, members of

each gene pair separate so each gamete contains one member of a pair.

• During fertilization, the full number of chromosomes is restored and members of a gene or allele pairs are reunited.

Dominance and Recessiveness• Recessive traits are not expressed in

heterozygotes. For a recessive allele to be expressed, there

must be two copies of the allele.• Dominant traits are governed by an allele that

can be expressed in the presence of another, allele. Dominant alleles prevent the expression of

recessive alleles.

Homozygous• Having the same allele at the same locus

on both members of a pair of chromosomes.

Heterozygous • Having different alleles at the same locus

on members of a pair of chromosomes.

Punnett square• 1⁄4 of the plants can

be expected to be homozygous tall (TT).

• 1/2 can be expected to be tall but will be heterozygous (Tt).

• The remaining 1⁄4 are homozygous for the recessive “short” allele (tt).

Principle of Independent Assortment• The distribution of one pair of alleles

into gametes does not influence the distribution of another pair.

• The genes controlling different traits are inherited independently of one another.

Phenotype• The observable or detectable physical

characteristics of an organism.• The detectable expressions of genotypes,

frequently influenced by environmental factors.

Phenotypic Ratio• The proportion of one phenotype to other

phenotypes in a group of organisms. For example,Mendel observed that

there were approximately three tall plants for every short plant in the F2 generation.

This is expressed as a phenotypic ratio of 3:1.

Question• Traits that have a range of phenotypic

expressions and show a continuum of variation are termed

a) co-dominant.b) polygenic.c) polymorphic.d) sex-linked.

Answer: b• Traits that have a range of phenotypic

expressions and show a continuum of variation are termed polygenic.

Question• Genes exist in pairs in individuals; during

the production of gametes, the pairs are separated so that a gamete has only one of each kind. This is known as the

a)principle of segregation.b)principle of independent assortment.c)mitosis.d)unification theory.

Answer: a• Genes exist in pairs in individuals; during

the production of gametes, the pairs are separated so that a gamete has only one of each kind. This is known as the principle of segregation.

Mendelian Traits • Characteristics that are influenced by

alleles at only one genetic locus.• Examples include many blood types, such

as ABO.• Many genetic disorders such as sickle-

cell anemia and Tay-Sachs disease are also Mendelian traits.

Mendelian Inheritance in Humans• Over 4,500 human trains are known to be

inherited according to Mendelian principles. • The human ABO blood system is an example of

a simple Mendelian inheritance. The A and B alleles are dominant to the O

allele. Neither the A or B allele are dominant to one

another; They are codominant and both traits are expressed.

Results of Crosses When Traits Are Considered Simultaneously

Some Mendelian Traits in Humans: Dominant

Condition ManifestationsAchondroplasia Dwarfism due to growth defects involving

the long bones of the arms and legs; trunk and head

size usually normal.Brachydactyly Shortened fingers and toes.

Familial hyper-cholesterolemia

Elevated cholesterol levels and cholesterol plaque deposition;

a leading cause of heart disease, with death frequently occurring

by middle age.

Some Mendelian Traits in Humans: Recessive

Condition ManifestationsCystic fibrosis Among the most common genetic disorders

among European Americans; abnormal secretions of the exocrine glands, with

pronounced involvement of the pancreas; most patients develop obstructive lung

disease. Tay-Sachs

diseaseMost common among Ashkenazi Jews;

degeneration of the nervous system beginning at about 6 months of age; lethal by

age 2 or 3 years.

Question• Mendel used the term dominant for

a) plants that were larger than others of the same variety.

b) a trait that prevented another trait from appearing.

c) a variety of pea plants that eliminated a weaker variety.

d) a trait that "skipped" a generation.

Answer: b• Mendel used the term dominant for a

trait that prevented another trait from appearing.

Antigens• Large molecules found on the surface of

cells.• Several different loci govern various

antigens on red and white blood cells. Foreign antigens provoke an immune

response.

Codominance• The expression of two alleles in

heterozygotes. • In this situation, neither allele is dominant

or recessive so they both influence the phenotype.

ABO Genotypes and Associated Phenotypes

Genotype Antigens onRed Blood Cells

ABO Blood Type(Phenotype)

AA, AO A A

BB, BO B B

AB A and B AB

OO None O

Pedigree Chart• A diagram showing

family relationships; it’s used to trace the hereditary pattern of particular genetic traits.

Autosomal Dominant Trait

• Inheritance of an autosomal dominant trait: a human pedigree for brachydactyly. How can individuals 5, 11, 14, 15, and 17 be unaffected?

Pattern of Inheritance of Autosomal Dominant Traits

• (a) Diagram of possible gametes produced by one parent with brachydactyly and another with normal hands and fingers.

• (b) Punnett square depicting possible genotypes in the offspring of one parent with brachydactyly (Bb) and one with normal hands and fingers (bb).

Partial Pedigree for Albinism

• Individuals 6 and 7, children of unaffected parents, are affected. Four individuals are definitely unaffected carriers. Which ones are they?

An African Albino• An African albino.

This young man has a greatly increased likelihood of developing skin cancer.

Phenotypically Normal Parents, Both Carriers of the Albinism Allele

• Offspring:• Homozygous dominants

(AA) with normal phenotype, 25%

• Heterozygotes, (carriers) (Aa) with normal phenotype, 50%

• Homozygous recessives (aa) with albinism, 25%.

Mendelian Disorders Inherited as X-Linked Recessive Traits

Condition Manifestations

G-6-PD deficiency

Lack of an enzyme in red blood cells; produces severe anemia in the presence of

certain foods and/or drugs.Muscular dystrophy

One form; other forms can be inherited as autosomal recessives; progressive weakness and atrophy of muscles beginning in early

childhood; continues to progress

Mendelian Disorders Inherited as X-Linked Recessive Traits

Condition Manifestations

Red-green color blindness

Two separate forms, one involving the perception of red and the other affecting the

perception of green.

Lesch-Nyhan Impaired motor development noticeable by 5 months; progressive motor impairment,

disease diminished kidney function, self-mutilation, and early death.

Mendelian Disorders Inherited as X-Linked Recessive Traits

Condition Manifestations

Hemophilia In hemophilia A, a clotting factor is missing; hemophilia B is caused by a defective clotting

factor. Both produce abnormal internal and external bleeding from minor injuries; severe

pain is a frequent accompaniment; without treatment, death usually occurs before

adulthood.

Mendelian Disorders Inherited as X-Linked Recessive Traits

Condition Manifestations

Ichthyosis A skin condition due to lack of an enzyme; characterized by scaly, brown lesions on the

extremities and trunk. In the past, people with this condition were sometimes exhibited in

circuses and sideshows as “the alligator man.”

Polygenic Inheritance• Polygenic traits are continuous traits governed

by alleles at more than one genetic locus.• Continuous traits show gradations, there is a

series of measurable intermediate forms between two extremes.

• Skin color is a common example of a polygenic trait it is governed by 6 loci and at least 12 alleles.

Discontinuous Distribution of Mendelian Traits

• Shows the discontinuous distribution of ABO blood type in a hypothetical population.

• The expression of the trait is described in terms of frequencies.

Continuous Expressionof a Polygenic Trait• Represents the continuous expression of

height in a large group of people.

Height

• Male students arranged according to height. The most common height is 70 inches, which is the mean, or average, for this group.

Pleiotropy• Pleiotropy is a situation where a single gene

influences more than one phenotypic expression. Example: The autosomal recessive disorder

phenylketonuria (PKU). • Individuals who are homozygous for the PKU

allele don’t produce phenylketonurase, the enzyme involved in the conversion of the amino acid phenylalanine to the amino acid, tyrosine.

Mitochondrial Inheritance• All cells contain mitochondria that convert

energy into a form that can be used by the cell.• Each mitochondrion contains several copies of

a ring-shaped DNA molecule, or chromosome. • Animals of both sexes inherit their mtDNA, and

all mitochondrial traits, from their mothers. • All the variation in mtDNA is caused by

mutation.

Heredity and Evolution• Evolution works at four levels:

Molecular Cellular Individual Population

• The levels reflect different aspects of evolution.

The Modern SynthesisEvolution is a two-stage process:1. The production and redistribution of variation

(inherited differences between individuals).2. Natural selection acting on this variation

(whereby inherited differences, or variation, among individuals differentially affect their ability to reproduce successfully).

A Current Definition Of Evolution• From a modern genetic perspective, we

define evolution as a change in allele frequency from one generation to the next.

• Allele frequencies are indicators of the genetic makeup of an interbreeding group of individuals known as a population.

Mutation• Mutation is a molecular alteration in genetic

material: For a mutation to have evolutionary

significance it must occur in a gamete (sex cell).

Such mutations will be carried on one of the individual's chromosomes.

During meiosis the chromosome carrying the mutation will assort giving a 50% chance of passing the allele to an offspring.

Gene Flow• Gene flow is the exchange of genes between

populations.• If individuals move temporarily and mate in the

new population (leaving a genetic contribution), they don’t necessarily remain in the population.

• Example: The offspring of U.S. soldiers and Vietnamese women represent gene flow, even though the fathers returned to their native population.

Genetic Drift• Genetic drift is directly related to population

size. • Genetic drift occurs when some individuals

contribute a disproportionate share of genes to succeeding generations.

• Drift may also occur solely because the population is small: Alleles with low frequencies may simply not

be passed on to offspring, so they eventually disappear from the population.

Genetic Drift

Founder Effect• Genetic drift in which allele frequencies are

altered in small populations that are taken from, or are remnants of, larger populations.

• A new population will be established, and as long as mates are chosen only within this population, all the members will be descended from the founders.

• An allele that was rare in the founders’ parent population but is carried by even one of founders can become common.

Question• When alleles are introduced into a

population from another population, this is known as

a) genetic drift.b) gene flow/migration.c) founder effect.d) bottleneck effect.

Answer: b • When alleles are introduced into a

population from another population, this is known as gene flow/migration.

Recombination• In sexually reproducing species both

parents contribute genes to offspring. • The genetic information is reshuffled every

generation. • Recombination doesn’t change allele

frequencies, however, it does produce different combinations of genes that natural selection may be able to act on.

Natural Selection• Natural selection provides directional change in

allele frequency relative to specific environmental factors.

• If the environment changes, selection pressures also change.

• If there are long-term environmental changes in a consistent direction, then allele frequencies should also shift gradually each generation.

Genetic Variation• Cheetahs, like many

other species, have passed through a genetic bottleneck.

• As a species, they have little genetic variation.

Sickle-cell Distribution inthe Old World

Life Cycle of the Parasite That Causes Malaria

Levels of Organization in the Evolutionary Process

EvolutionaryFactor

Level Evolutionary Process

Mutation DNA Storage of genetic information; ability to replicate; influences

phenotype by production of proteins

Mutation Chromosomes A vehicle for packaging and transmitting DNA

Levels of Organization in the Evolutionary Process

EvolutionaryFactor

Level Evolutionary Process

Recombination(sex cells only)

Cell Basic unit of life, contains chromosomes, divides for

growth and production of sex cells

Natural selection Organism The unit that reproduces and which we observe for

phenotypic traitsDrift, gene flow Population Changes in allele frequencies

between generations