1. NOTES: Ch 14, part 2 – Extending Mendelian Genetics

2. The Spectrum of Dominance
● Complete dominance occurs when the phenotypes of the heterozygote and dominant homozygote are identical

3. CODOMINANCE:
● In codominance 2 dominant alleles affect the phenotype in separate, distinguishable ways
● Example: the human blood group MN

4. CODOMINANCE:
● inheritance characterized by full expression of both alleles in the heterozygote
Example: in chickens, BB = black feathers, bb = white, Bb = “speckled” (both black and white feathers)

6. INCOMPLETE DOMINANCE:
● inheritance where one allele is not completely dominant over the other, so the heterozygote has a phenotype that is intermediate between the phenotype of the 2 homozygotes
Ex: RR = red flowers,
rr = white, Rr = pink

7. The Relationship Between Dominance and Phenotype
● Dominant and recessive alleles do not really “interact”
● they lead to synthesis of different proteins that produce a phenotype

8. Frequency of Dominant Alleles
● Dominant alleles are not necessarily more common in populations than recessive alleles

9. Multiple Alleles
● Most genes exist in populations in more than two allelic forms

10. ● The ABO blood group in humans is determined by multiple alleles
Table 14.2

12. Polygenic Inheritance
● Many human characters vary in the population along a continuum and are called quantitative characters

13. POLYGENIC INHERITANCE:
● mode of inheritance in which the additive effect of 2 or more genes determines a single phenotypic character
Examples: height
hair color (4 gene pairs)
eye color (2 gene pairs)
skin color

14. ● Quantitative variation usually indicates polygenic inheritance
(An additive effect of 2 or more genes on a single phenotype) 
AaBbCc
aabbcc
Aabbcc
AaBbcc
AABbCc
AABBCc
AABBCC
20⁄64
15⁄64
6⁄64
1⁄64
Fraction of progeny

16. PLEIOTROPY:
● the ability of a single gene to have multiple phenotypic effects
Examples: sickle cell anemia
Siamese cats & tigers

18. EPISTASIS:
● interaction between 2 nonallelic genes in which one modifies the phenotypic expression of the other
Example: gene for pigment deposition is epistatic to gene for melanin production in mice

20. ● An example of epistasis:BC bC Bc bc
1⁄4
BBCc
BbCc
BBcc
Bbcc
bbcc
bbCc
BbCC
bbCC
BBCC
9⁄16
3⁄16
4⁄16 
Sperm
Eggs

21. Nature and Nurture: The Environmental Impact on Phenotype
● Another departure from simple Mendelian genetics arises when the phenotype for a character depends on environment as well as on genotype

22. ● The norm of reaction is the phenotypic range of a particular genotype that is influenced by the environment
Figure 14.13

23. MULTIFACTORIAL INHERITANCE:
● A trait depends on many factors; a variety of genotypes as well as environmental influences (such as certain chemicals, medicines, or diet)
Examples of disorders that may be a result of multifactorial inheritance: diabetes, heart disease, neural tube defects, autism, Alzheimer disease, ALS, and many cancer syndromes

24. ~Same genotype
~Phenotype depends on acidity of soil
~height controlled by several genes
~height influenced by amount of nourishment

25. Integrating a Mendelian View of Heredity and Variation
● An organism’s phenotype includes its physical appearance, internal anatomy, physiology, and behavior
● the phenotype reflects its overall genotype and unique environmental history

26. Inheritance Patterns for Genetic Diseases in Humans

27. Pedigree Analysis
● A pedigree is a family tree that describes the interrelationships of parents and children across generations
George
Arlene
Sandra
Tom
Sam
Wilma
Ann
Michael
Daniel
Alan
Tina
Christopher
Carla

28. ● Inheritance patterns of particular traits can be traced and described using pedigreesWw ww WW or
First generation
(grandparents)
Second generation
(parents plus aunts
and uncles)
Third
generation
(two sisters) Ff ff
FF or Ff FF
Widow’s peak
No Widow’s peak
Attached earlobe
Free earlobe
(a) Dominant trait (widow’s peak)
(b) Recessive trait (attached earlobe)
Figure A, B

29. 1) Autosomal Recessive:
● recessive alleles that cause human disorders are usually defective versions of normal alleles
● defective alleles code for either a malfunctioning protein or no protein at all

30. ● heterozygotes can be phenotypically normal, if 1 copy of the normal allele is all that is needed to produce sufficient quantities of the “good” protein
Examples: cystic fibrosis, Tay-Sachs, sickle cell anemia

31. ● “Carriers” are heterozygous individuals who carry the recessive allele but are phenotypically normal

32. Cystic Fibrosis ● Symptoms of cystic fibrosis include:
-Mucus buildup in the some internal organs
-Abnormal absorption of nutrients in the small intestine

34. Sickle-Cell Disease ● Sickle-cell disease:
-Affects one out of 400 African-Americans
-Is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells
● Symptoms include:
-Physical weakness, pain, organ damage, and even paralysis

36. 2) Autosomal Dominant:
● only 1 dominant allele is needed in order to produce the effects of these diseases (heterozygous)
● Lethal homozygous dominant condition results in spontaneous abortion of fetus

37. ● homozygous recessives are of normal phenotype
Examples: achondroplasia, Huntington’s Disease

39. ● ACHONDROPLASIA: a form of dwarfism that is lethal when homozygous for the dominant allele
Figure 14.15

40. ● HUNTINGTON’S DISEASE: a degenerative disease of the nervous system
-it has no obvious phenotypic effects until about 35 to 40 years of age
Figure 14.16

42. Genetic Testing and Counseling
● Genetic counselors can provide information to prospective parents concerned about a family history for a specific disease

43. Counseling Based on Mendelian Genetics and Probability Rules
● Using family histories genetic counselors help couples determine the odds that their children will have genetic disorders

44. Tests for Identifying Carriers
● For a growing number of diseases tests are available that identify carriers and help define the odds more accurately

45. Fetal Testing
● AMNIOCENTESIS: the liquid that bathes the fetus is removed and tested
● CHORIONIC VILLUS SAMPLING (CVS): a sample of the placenta is removed and tested