Population Dynamics

Humans, Sickle-cell Disease, and Malaria

How does a population of humans become resistant to malaria?

Natural Selection• Overproduction• Environmental

pressure/competition• Pre-existing individual

variation• Heritable traits• Happens over generations

(time)• Happens in populations (not

single individuals)• Offspring must be viable and

fertile

The Origins of Genetic Variation

– Offspring of sexual reproduction are genetically different from their parents and from one another.

– Meiosis• Random mutations• Crossing over• Independent assortment of chromosomes

– Random fertilization

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The Origins of Genetic Variation

– Offspring of sexual reproduction are genetically different from their parents and from one another.

– Meiosis• Random mutations• Crossing over• Independent assortment of chromosomes

– Random fertilization

Intergenerational Mutation Rate• By how many

mutations does your genome differ from your parents genome?

• Roach, et al., Science (2010) found about 60 mutations, 30 from each parent, that occurred during meiosis.

Hemophilia in the Royal Family:Hypothesis - hemophilia allele arose through mutation in gamete of Queen Victoria’s mother or father.

Crossing Over

– In crossing over,• Homologous

chromosomes exchange genetic information.

• Genetic recombination occurs.

Independent Assortment of Chromosomes

– In independent assortment, every chromosome pair orients independently of the others during meiosis.

Random Fertilization

– The human egg cell is fertilized randomly by one sperm, leading to genetic variety in the zygote.

Natural Selection• Overproduction• Environmental

pressure/competition• Pre-existing individual

variation• Heritable traits• Happens over generations

(time)• Happens in populations (not

single individuals)• Offspring must be viable and

fertile

– Gregor Mendel• Was the first

person to analyze patterns of inheritance.

• Deduced the fundamental principles of genetics.

Heritable Variation and Patterns of Inheritance - Ch 9

Figure 9.6a

Monohybrid Crosses– A monohybrid cross is a cross between

parent plants that differ in only one characteristic.

– Mendel’s law of segregation

• The two members of an allele pair segregate (separate) from each other during the production of gametes.

• Shown using a Punnett square.

– Phenotype• An organism’s

physical traits; what it looks like.

– Genotype• An organism’s

genetic makeup; what genes it has.

Figure 9.7

Genetic Alleles and Homologous Chromosomes

Independent Assortment of Chromosomes

– In independent assortment, every chromosome pair orients independently of the others during meiosis.

– Mendel developed four hypotheses from the monohybrid cross:

• There are alternative forms of genes, called alleles.• For each characteristic, an organism inherits two

alleles, one from each parent.• Alleles can be dominant or recessive.• Gametes carry only one allele for each inherited

characteristic.

Figure 9.5

Is the mating of parental varieties differing in two characteristics.

Dihybrid cross

– Mendel’s law of independent assortment states that

• Each pair of alleles segregates independently of the other pairs during gamete formation.

Figure 9.23

Using a Testcross to Determine an Unknown Genotype

– A testcross is a mating between• An individual of unknown genotype and a

homozygous recessive individual.

Family Pedigrees•Shows the history of a trait in a family.•Allows geneticists to analyze human traits.

Human Disorders Controlled by a Single Gene

Variations On Mendel’s Laws– Some patterns of genetic inheritance are

not explained by Mendel’s laws.

• Incomplete dominance• Codominance• Pleiotropy• Polygenic Inheritance

Incomplete Dominance in Plants and People

– In incomplete dominance, F1 hybrids have an appearance in between the phenotypes of the two parents.

Incomplete Dominance in Plants and People

– In incomplete dominance, F1 hybrids have an appearance in between the phenotypes of the two parents.

ABO Blood Type: An Example of Multiple Alleles and

Codominance

– The ABO blood groups in humans are an example of multiple alleles.

– The immune system produces blood proteins

• That may cause clotting when blood cells of a different type enter the body.

Pleiotropy and Sickle-Cell Disease

– Pleiotropy is the impact of a single gene on more than one characteristic.

• Sickle-cell disease is an example.

Figure 9.20

Polygenic Inheritance

– Polygenic inheritance is the additive effects of two or more genes on a single phenotype.

Figure 9.21

The Role of Environment

– Many human characteristics result from a combination of heredity and environment.

Figure 9.22

The Chromosomal Basis of Inheritance

– The chromosome theory of inheritance states that

• Genes are located at specific positions on chromosomes.

• The behavior of chromosomes during meiosis and fertilization accounts for inheritance patterns.

Figure 9.23

Linked Genes

– Linked genes• Are located close together on a chromosome.• May be inherited together.

The Process of Science: Are Some Genes Linked?

– Using the fruit fly Drosophila melanogaster, Thomas Hunt Morgan determined

• That some genes were linked based on the inheritance patterns of their traits.

Figure 9.24

Genetic Recombination: Crossing Over

– Two linked genes• Can give rise to four different gamete

genotypes.• Can sometimes cross over.

Crossing Over

Figure 9.25

Figure 9.26

Linkage Maps

– Early studies of crossing over were performed using the fruit fly Drosophila melanogaster.

– Studies using Drosophila• Developed a method for mapping gene loci.• Resulted in linkage maps.

Figure 9.27

Sex Chromosomes and Sex-Linked Genes

– Sex chromosomes• Influence the inheritance of certain traits.

Sex Determination in Humans and Fruit Flies

– Sex chromosomes• Are designated X and Y.• Determine an individual’s sex.

Figure 9.28

Sex-Linked Genes

– Sex-linked genes• Are any genes located on a sex chromosome.• Were discovered during studies on fruit flies.

Figure 9.29

– Inheritance patterns of a sex-linked gene

Figure 9.30

Sex-Linked Disorders in Humans

– A number of human conditions result from sex-linked (X-linked) genes.

– Red-green color blindness• Is characterized by a malfunction of light-

sensitive cells in the eyes.

Figure 9.31

– Hemophilia• Is a blood-clotting disease.

Figure 9.32

– Hemophilia• Is a blood-clotting disease.

– Duchenne muscular dystrophy• Is characterized by a progressive weakening

and loss of muscle tissue.

The Rules of Probability

– The rule of multiplication states that• The probability of a compound event is the

product of the separate probabilities of the independent events.

Figure 9.11