Chapter 4

Heredity and EvolutionHeredity and Evolution

Selective Breeding

Genetic PrinciplesDiscovered by Mendel

• Gregor Mendel (1822-1884) laid down the basic principles of heredity.

• Plant hybrids

Traits Mendel Studied: Peas

Results: One Trait at a Time

Principle of Segregation

Dominance and Recessiveness

• Homozygous vs. heterozygous• Recessive traits are not expressed in

heterozygotes.• Dominant traits are governed by an allele that

can be expressed in the presence of another, allele.

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.

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.

Results of Crosses When Traits Are Considered Simultaneously

Some Mendelian Traits in Humans: Dominant

Condition Manifestations

Achondroplasia 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 Manifestations

Cystic 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 disease

Most common among Ashkenazi Jews; degeneration of the nervous system

beginning at about 6 months of age; lethal by age 2 or 3 years.

Antigens

• Large molecules found on the surface of cells.

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

Codominance

• The expression of two alleles in heterozygotes.

• In this situation, neither allele is dominant or recessive so they both influence the phenotype.

Pedigree Chart

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, Heterozygotes, (carriers), or Homozygous recessives

X-linked traits

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.

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.

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

• Animals of both sexes inherit their mtDNA, and all mitochondrial traits, from their mothers.

• All the variation in mtDNA is caused by mutation.

The Modern Synthesis

Evolution is a two-stage process:1. The production and redistribution of

variation.2. Natural selection acting on this variation.– A current definition of evolution - a change in

allele frequency from one generation to the next.

Mutation

• Mutation is a molecular alteration in genetic material.

Gene Flow

• Gene flow is the exchange of genes between populations.

Genetic Drift

• Genetic drift occurs when some individuals contribute a disproportionate share of genes to succeeding generations.

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.

Bottleneck Effect

Recombination

• In sexually reproducing species both parents contribute genes to offspring and the genetic information is reshuffled every generation.

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 traits

Drift, gene flow Population Changes in allele frequencies between generations