Download BIO 420 – Mammalian Physiology

BIO 250 Genetics
Warner Pacific College
Modification of Mendelian Ratios
I. Intro
A. Mendelian Postulates
Paired unit factors
Dominance / recessiveness
Segregation
Independent Assortment
B. Non-classical ratios were observed by other researchers
II. Incomplete Dominance
A. Definition – Heterozygotes express a phenotype intermediate between the two
homozygote phenotypes
B. Example – Snapdragon flowers
Gene dosage
III. Co-dominance
A. Definition – Heterozygotes jointly express both alleles
B. Example – Human ABO blood typing
IV. Lethal Alleles
A. Definition – Alleles which cause death if expressed
Mutations to gene products essential to survival cause lethal alleles
Typically recessive; rarely dominant
B. Example – Yellow coat color in mice (recessive lethal)
C. Example – Huntington Disease (dominant lethal)
V. Dihybrid Crosses with Mendelian Deviations
A. Dihybrid crosses involving at least one non-classical ratio will result in F2 progeny with
altered ratios as well.
B. Example – Inheritance of albinism and blood type in the same individual
VI. Gene Interaction
A. Definition – phenotype may be affected by more than one gene
B. Epistasis – masking or modification of expression of one gene by another gene
Example – squash color is determined by 2 genes (dominant allele in first locus
masks expression of other gene)
Example – Bombay phenotype (recessive allele in one locus prevents expression
of gene product at second locus)
VII. Complementation
A. Setting – when two gene loci are involved in a single phenotype
Two different mutant strains can complement each other to produce a wildtype phenotype
BIO 250 Genetics
Warner Pacific College
B. Complementation Analysis
Cross two mutant strains; observe offspring to determine whether two mutant
strains are due to mutations in the same gene or in different genes
Example – Wingless Drosophila mutation analysis
IX. X-linked Traits / X-linked Genes
A. X-linked traits occur in species with pair of unlike sex chromosomes
Female: XX
Male: XY
B. Example: X-linked eye color in Drosophila
C. X-linked traits / diseases in humans
X. Sex-limited vs. sex-influenced traits
A. Characteristics
Both are autosomal, NOT on X or Y chromosome
An allele is dominant in one sex, but recessive in the other
Sex hormones influence gene expression
Sex-limited: only one sex or the other expresses a phenotype
Sex-influenced: sex influences expression, but is not completely restricted to
one sex
B. Example of Sex-influenced trait: pattern baldness
XI. Extranuclear Inheritance
A. Organelle heredity
B. Maternal effect
XII. Environmental Influences on Phenotype
A. Example - Temperature Effects
B. Pro12Ala mutation in PPAR-gamma
XIII. Biochemical Genetics
A. Biochemical pathways – often contain recessive gene products
“inborn errors of metabolism”
B. Example – wrinkled vs. round seeds
C. Expressivity – a particular genotype can produce variable degrees of phenotype in
different individuals
D. Penetrance – a phenotype predicted by genotype may not be expressed…penetrance
is a % expression in a population