Download Chapter 11: Genetics

Patterns of Inheritance:
Mendelian Genetics
Chapter 11
Biology 1010
Gregor Mendel : Father of Genetics
A. History
1. Natural selection
a. evolution was due to differences in
variable traits.
b. variation improves chances of
survival.
2. Nineteenth century thought favored
blending
B. Mendel and the experimental approach
1. experienced in plant breeding and
mathematics
2. excellent observational abilities
3. used garden peas as the experimental
system
a. self-fertilize - ability to breed true
b. easily identifiable traits
c. easy to grow
4. the idea of genetics was unknown to
Mendel
Terminology
A. definitions
1. genes: the instructions for producing a
trait
2. locus: location of the gene on a
chromosome
3. alleles: variations of genes
a. variation is at the molecular level
4. homologous chromosomes: 2n organisms
have 2 copies of each chromosome
a. each chromosome has a copy of the
gene which may vary (alleles)
5. homozygous: both alleles are the same
6. heterozygous : alleles are different
7. dominant: one allele is expressed over
another
8. recessive: expression of the trait is
masked
9. genotype: genetic combination of alleles
a. homozygous dominant (AA)
b. homozygous recessive (aa)
c. heterozygous (Aa)
10. phenotype: observed traits
11. generations
a. P - parental generation
b. F1 - first generation
Mendel’s Theory of Segregation
A. Introduction
1. 2n organisms inherit two genes per trait
2. each gamete contains only one copy of
the gene
3. pea crosses
B. Monohybrid crosses - one trait
1. two parents that breed true for different
phenotypes
2. one form disappears in the first
generation, but reappears in the second
3. mathematical analysis
a. F2 offspring shows a 3:1 phenotypic
ratio
b. ratios represent probabilities
4. Punett squares predict outcome
5. Testcross
a. may be used to determine the
genotype of a parent
b. question - is the parent a AA or Aa
individual
c. constructing the cross
Independent Assortment
A. Dihybrid crosses - two traits
1. Does the ratio of one trait effect the ratio
of a second trait?
2. first generation traits show both dominant
traits
3. second generation traits are expressed in
a ratio of 9:3:3:1
4. genes on nonhomologous chromosomes
assort independently
B. Modern Interpretation
1. meiosis produces genes with only one
copy of each gene (haploid)
2. nonhomologous chromosomes assort
independently
C. Dominance
1. incomplete dominance - a dominant allele
can not completely mask a recessive allele
2. codominance - in heterozygotes, both
alleles are expressed
a. human AB blood
3. multiple allele systems
a. human blood groups
D. Pleiotropy
1. one gene may have several effects
a. sickle-cell anemia
E. Continuous variation in populations
1. phenotypes may vary by predictable
degrees
2. distribution of traits may follow a bellcurve
3. most traits are not qualitative
4. polygenetic inheritance controls many
traits such as skin color, height, body form,
intelligence
F. Environmental Effects
1. environmental factors may effect the
phenotype