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How do we account for genetic variation?
*Independent assortment *Crossing over *Random fertilization
Independent Assortment:
Cross over:
Chi-Square Analysis:
Mendel’s monohybrid and dihybrid ratios are predictions based on the following assumptions:
1. Each allele is dominant or recessive
2. Random segregation of alleles
3. Independent assortment
4. Fertilization is random
Chi-Square Analysis:p value (probability): consider as a percentage (i.e. 0.05 = 5%)
Example from Table 3.1:
Human PedigreesPedigree
= Female
= Male
=Unknown
*Proband (p)
Chapter 4: Modification of Mendelian RatiosAllele
*Wild-type allele
*Mutant allele
Conventional symbols for alleles:
recessive allele- initial letter of the name of the recessive trait, lowercased and italicized
dominant allele- same letter in uppercase
Genetic nomenclature is extremely diverse!
Incomplete or Partial DominanceCross between parents
with contrasting traits: Red flowers or white
flowers
Offspring with an intermediate
phenotype: pink flowers
Codominance:
Example:
MN Blood group- red blood cells contain a transmembrane glycoprotein (glycophorin); two different forms of this protein exist, M and N
Examples:
*Table 4.1: over 100 alleles at a given locus in Drosophila
*ABO Blood group in humans
Multiple Alleles:
*Characterized by the presence of glycoprotein antigens on the surface of red blood cells
*Distinct from the M and N antigens
*Also exhibits codomiance
Lethal Alleles:
Example: Coat color in mice
*A = agouti = wild-type allele
*AY = yellow = mutant allele
Dominant Lethal: Huntington’s disease (H); heterozygous individuals (Hh) have late onset
Combining modified modes of inheritance:
Gene interaction:
*Epistasis
Example:
In Drosophila, the recessive gene eyeless (when homozygous) prevents the expression of eye color genes present in genome
*Novel phenotypes due to gene interaction
Example:
disc-shaped fruit (AABB) X long fruit (aabb)
Genes on the X Chromosome:
*Sex chromosomes*Autosomes
Example: In Drosophila and all mammals
sex chromosomes designated as X and Y
XX=female
XY=male
*1909 Thomas Hunt Morgan
II III IV
XX
XY
or
*X-linkage
X-linkage in Drosophila: white mutation (eyes)
Genes on the X Chromosome con’t:
½ red
½ white
Genes on the X Chromosome con’t:
½ red
Sex-limited Inheritance:*Sex-limited trait
*holandric genes: genes on the Y chromsome
Example: hypertrichosis (ear hair)
*autosomal genes
Example: milk production in mammals; L=lots, l=little
*Sex-influenced trait
Sex-influenced Inheritance:
Examples:
*cleft palate in humans*horns in sheep *pattern baldness in
humans
Summary:
Sex-linked on X or Y sex-chromosome
Sex-limited all or none expression by sex
Sex-influenced genotype + sex determines phenotype
Phenotypic Expression:
*Penetrance
*Expressivity
Gene expression often governed by genotype and environment
Example:
if 9/10 of individuals carrying an allele express the trait, the trait is said to be 90% penetrant
*Onset of genetic expression
*Temperature