1. Heredity and Evolution
Chapter 4
Heredity and Evolution

2. Genetic Principles Discovered by Mendel
Gregor Mendel ( )
basic principles of heredity.
Crossed different strains of purebred plants and studied their progeny.
Studied common garden peas (only one trait at a time).
His work illustrates the basic rules of inheritance.

3. Principle of Segregation
Genes occur in pairs (like chromosomes).
During gamete production, members of each gene pair separate.
During fertilization, the full number of chromosomes is restored (allele pairs are reunited).
Homozygous- same allele at same locus on both members of a chromosome pair. (i.e.TT, tt)
Heterozygous- two different alleles at the same locus on a chromosome pair.

4. 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, different allele.
Dominant alleles prevent the expression of recessive alleles in heterozygotes.

5. 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.

6. Mendelian Inheritance in Humans
Mendelian principles apply to over 4,500 human traits.
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 (codominant and both traits are expressed).

7. ABO Genotypes and Associated Phenotypes
Antigens on
Red Blood Cells
ABO Blood Type (Phenotype)
AA, AO A
BB, BO B AB
A and B OO
None O

8. Polygenic Inheritance
Polygenic traits are continuous traits governed by alleles at more than one genetic locus.
Continuous traits show gradations.
Skin color is a common example of a polygenic trait it is governed by 6 loci and at least 12 alleles.

9. Mitochondrial Inheritance
Convert energy into a form that can be used by the cell.
Animals inherit their mtDNA (and mt traits) from their mothers.
All the variation in mtDNA is caused by mutation, which makes it very useful for studying genetic change over time.

10. Heredity and Evolution
Evolution works at four levels:
Molecular
Cellular
Individual
Population
The levels reflect different aspects of evolution and are integrated in a way that produces evolutionary change.

11. The Modern Synthesis Evolution is a two-stage process:
The production and redistribution of variation.
Natural selection acting on this variation (affect ability to reproduce & survive)
Evolution = Mutation or Natural Selection

12. A Current Definition Of Evolution
Modern Genetics: evolution= 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.
Small changes + Time + Natural Selection= Evolution

13. 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.

14. 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.

15. 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.

16. Founder Effect
Genetic drift in which allele frequencies are altered in small populations from larger populations.
A new population will be established.
(all members will be descended from the founders)
An allele that was rare in the founders’ parent population but is carried by even one of the founders can eventually become common.
Example: French Canadians in Quebec

17. 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.

18. 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.

19. Levels of Organization in the Evolutionary Process
Factor
Level
Evolutionary Process
Mutation
DNA
Storage of genetic information; ability to replicate; influences phenotype by production of proteins
Chromosomes
A vehicle for packaging and transmitting DNA

20. Levels of Organization in the Evolutionary Process
Factor
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

21. Quick Quiz

22. 1. Mendel used the term dominant for
plants that were larger than others of the same variety.
a trait that prevented another trait from appearing.
a variety of pea plants that eliminated a weaker variety.
a trait that "skipped" a generation.

23. Answer: b
Mendel used the term dominant for a trait that prevented another trait from appearing.

24. 2. 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.
principle of independent assortment.
mitosis.
unification theory.

25. correct: 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.

26. 3. Traits that have a range of phenotypic expressions and show a continuum of variation are termed
co-dominant.
polygenic.
polymorphic.
sex-linked.

27. Answer: b
Traits that have a range of phenotypic expressions and show a continuum of variation are termed polygenic.

28. 4. When alleles are introduced into a population from another population, this is known as
genetic drift.
gene flow/migration.
founder effect.
bottleneck effect.

29. Answer: b
When alleles are introduced into a population from another population, this is known as gene flow/migration.

30. 5. The most complete definition of biological evolution is
change.
mutation.
survival of the fittest.
a change in allele frequency from one generation to the next.

31. Answer: d
The most complete definition of biological evolution is a change in allele frequency from one generation to the next.