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