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Genetics Chapters 14-15
Introduction to Genetics
Chapter 14 Section 1 will review:
1. Law of Independent Assortment
2. Genes and alleles
3. Generations
4. Genotype vs. phenotype
5. Dominant vs. recessive
6. Heterozygous vs. homoszgous
7. Monohybrid crosses vs. dihybrid cross (Punnett squares!)
**Test crosses, incomplete dominance, codominance, sex-linked**
Types of
Crosses
Monohybrid Cross
Single trait cross
Test cross
Used to determine an unknown dominant genotype
Unknown is always crossed with recessive; two squares
Incomplete Dominance
Neither trait is completely dominant
Mixing of traits in heterozygous genotype
Codominance
Both traits are equally expressed; heterozygous affected
Sex-linked cross
Sex-linked traits: Genes located on sex chromosomes
Both X-linked and Y-linked genes
Determine more than the sex of an organism
Hemophilia, muscular dystrophy, patterned baldness
X-Linked Genes
Females have two X’s= twice as many proteins as males?
One X is mostly inactive in females
Same “dose” of x-linked genes in both genders
Barr body- Condensed inactive X chromosome from females;
Rules of
Probability
Multiplication Rule
Multiply the probability of one event by the probability of another.
Ex. a coin landing heads up twice= .5 x .5 = .25
Ex. Probabiliy of offspring YYRr from cross YyRr x YyRr
Yy x Yy = YY = 25% or .25Rr x Rr = Rr = 50% or .5
.25 x .5 = .125 or 12.5% YYRrr
Multiplication Rule
.125 or 12.5% = 2/16
Addition Rule
Determine the probability of two or more exclusive events occurring by adding their individual probabilities.
Multiplication rule gives individual probability for addition rule
Ex. PpYyRr x Ppyyrr —> probability of showing at least two of the three traits recessive?
Addition Rule
PpYyRr x Ppyyrr
PPyyrr = .25 x .5 x .5
Ppyyrr = .5 x .5 x .5
ppYyrr = .25 x .5 x .5
ppyyRr = .25 x .5 x .5
ppyyrr = .25 x .5 x .5
.0625 .125
.0625 .0625 + .0625
.375 or 6/16
Genetic Variations
Multiple Alleles
Genes that exist in two or more allelic forms
Ex. blood type (IA, IB, i)
Blood Typing
Pleiotropy
A property where a gene can have multiple phenotypic effects
Ex. A hereditary disease causing many other symptoms; sickle cell anemia
Epistasis
Expression of a gene at one locus (location) alters expression of a gene at another.
Ex. Retriever coat color - color and pigment uptake expression
Polygenic Inheritance
Additive effect of two or more genes on a single phenotypic characteristic; creates a gradient
Ex. skin color and eye color
Eye Color