AP Biology, Chapter 14
Mendel and the Gene Idea

Summary
Introduction

GREGOR MENDEL'S DISCOVERIES
	Intro
		1. Describe the favored model of heredity in the 19th century prior to Mendel.
			a. Called blending inheritance
			b. Traits of the parents mix like paints; irrevocably
	Mendel brought an experimental and quantitative approach to genetics: science as a 
	Process
		2. Explain how observations by Mendel and others and Mendel's hypothesis of 
		inheritance differed from the blending theory of inheritance.
			a. Mendel demonstrated the particulate hypothesis
				i. Quantitatively followed traits through multiple generations
				ii. Showed reappearance of hidden traits in expected proportions
			b. Genes are discrete heritable units
				i. Remain separate in offspring
				ii. Separate physical units  pieces of meat
				iii. Combinations give variation
		3. List several features of Mendel's methods that contributed to his success.
			a. Chose organism with many varieties and that gave thousands of offspring
			b. Strict control of pollination
			c. Carefully chose traits with distinct forms
			d. Started with "pure", true-breeding, parents
			e. Planned and executed experiments that took years
			f. Statistically analyzed his data
			g. Published and presented
	By the law of segregation, the two alleles for a character are packaged into separate 
	gametes
		4. Define true breeding, hybridization, monohybrid cross, P generation, F1 generation, 
		and F2 generation.
			a. True breeding: only gives parental phenotype when selfed
			b. Hybridization: cross between parents with contrasting characters
			c. Monohybrid cross: hybridization involving one trait
			d. P: parental
			e. F1: offspring of P
			f. F2: offspring of F1
		5. List and explain the four components of Mendel's hypothesis that led him to deduce 
		the law of segregation.
			a. Alleles are alternate versions of genes
				i. Genes are generally stably located on chromosomes
				ii. Alternate forms may be found at those loci
			b. Alleles generally come in pairs, one haploid set from each parent in gametes
			c. Mendel chose pairs of dominant and recessive alleles
				i. Pure dominant x pure recessive  all dominant
				ii. Recessive trait reappeared in F2
			d. Alleles segregate during meiosis
				i. Sets of chromosomes carry sets of alleles
				i. Each gamete gets one allele for each locus
		6. Explain how Mendel's law of segregation got its name.
			a. In meiosis, 50% of gametes receive one allele, 50% the other
			b. Separation (segregation) of alleles into gametes
		7. Use a Punnett square to predict the results of a monohybrid cross and state the 
		phenotypic and genotypic ratios of the F2 generation.
			a. Gametes on outside; copy down and across
			b. P: PP x pp
				i. Gametes: PP makes all P gametes, pp makes p
				ii. All offspring are Pp, purple
			c. F1: Pp x Pp
				i. Gametes: both make 1/2 P + 1/2 p
				ii. Cross: (1/2 P + 1/2 p)2 = 1/4 PP + 1/2 Pp + 1/4 pp
				iii. F2: 3/4 purple and 1/4 white
		8. Distinguish between the following pairs of terms: dominant and recessive; 
		heterozygous and homozygous; genotype and phenotype.
			a. Dominant and recessive
				i. Dominant allele completely hides recessive allele
				ii. That is, when they're in the same offspring
			b. Heterozygous and homozygous
				i. Homo-: have alleles that are the same, = pure
				ii. Hetero-: different alleles
			c. Genotype and phenotype
				i. Genotype: combination of alleles
				ii. Phenotype: outward expression
		9. Explain how a testcross can be used to determine if a dominant phenotype is 
		homozygous or heterozygous.
			a. Mendel hypothesized that the F2 purple were 1/3 PP + 2/3 Pp
			b. Randomly selected them and crossed them with pp
				i. PP x pp  all purple
				ii. Pp x pp  1/2 purple + 1/2 white
			c. Used statistics
	By the law of independent assortment, each pair of alleles segregates into gametes 
	Independently
		10. Use a Punnett square to predict the results of a dihybrid cross and state the 
		phenotypic and genotypic ratios of the F2 generation.
			a. See p. 245 for the square
			b. Phenotypes: 
				9/16 dom.-dom. +
				3/16 dom.-rec. +
				3/16 rec.-dom. +
				1/16 rec.-rec.
			c. Genotypes:
				1/16 homo. dom. - homo. dom.
				1/8 homo. dom. - hetero.
				1/8 hetero. - homo. dom.
				1/16 homo. dom. - homo. rec.
				1/4 hetero. - hetero.
				1/8 hetero. - homo. rec.
				1/16 homo. rec. - homo. dom.
				1/8 homo. rec. - hetero.
				1/16 homo. rec. - homo. rec.
		11. Define Medndel's law of independent assortment.
			a. = independent segregation of each pair of alleles during gametes formation
			b. Exception: linkage
	Mendelian inheritance reflects rules of probability
		12. Use the rule of multiplication to calculate the probability that a particular F2 individual 
		will be homozygous recessive or dominant.
			a. Complete: (1/2 dom. + 1/2 rec.)2
			b. Homo. rec.: 1/2 rec. x 1/2 rec. = 1/4 rec.-rec.
			c. Homo. dom.: 1/2 dom. x 1/2 dom. = 1/4 dom.-dom.
		13. Given a Mendelian cross, use the rule of addition to calculate the probability that a 
		particular F2 individual will be heterozygous.
			a. Two ways to become heterozygous
				i. 1/2 dom. x 1/2 rec. = 1/4 dom.-rec. or
				ii. 1/2 rec. x 1/2 dom. = 1/4 rec.-dom.
			b. The combined probability is the two added together
		14. Use the laws of probability to predict from a trihybrid cross between two individuals 			that are heterozygous for all three traits, what expected proportion of the offspring 
		would be: a) homozygous for the three dominant traits, b) heterozygous for all three 
		traits, and c) homozygous recessive for two specific traits and heterozygous for the third.
			a. For example: AaBbCc x AaBbCc
			b. Complete gametes: (1/8 ABC + 1/8 AbC + 1/8 aBC + 1/8 ABc. + 1/8 Abc + 
			1/8 aBc + 1/8 abC + 1/8 abc)2
			c. Cases
				i. Case a: 1/8 ABC x 1/8 ABC = 1/64 AABBCC
				ii. Case b:
					1/8 ABC x 1/8 abc +
					1/8 abc x 1/8 ABC +
					1/8 AbC x 1/8 aBc +
					1/8 aBc x 1/8 AbC +
					1/8 aBC x 1/8 Abc +
					1/8 Abc x 1/8 aBC +
					1/8 ABc x 1/8 abC +
					1/8 abC x 1/8 ABc = 1/8
				iii. Case c
					1/8 abc x 1/8 abC +
					1/8 abC x 1/8 abc +
					1/8 aBc x 1/8 abc +
					1/8 abc x 1/8 aBc +
					1/8 Abc x 1/8 abc +
					1/8 abc x 1/8 Abc = 3/32
		15. Explain why Mendel was wise to use large sample sizes in his studies.
			a. More likely to be closer to the expected average
	Mendel discovered the particulate behavior of genes: a review

EXTENDING MENDELIAN GENETICS
	The relationship between genotype and phenotype is rarely simple
		16. Give an example of incomplete dominance and explain why it is not evidence for the 
		blending theory of inheritance.
			a. Red x white  pink
			b. Pink x pink  1/4 red + 1/2 pink + 1/4 white
			c. White and red come back out in their original form in the expected ratios
		17. Explain how the phenotypic expression of the heterozygote is affected by complete 
		dominance, incomplete dominance, and co-dominance.
			a. Complete dominance: one allel completely hides another
			b. Incomplete dominance: heterozygous phenotype is intermediate
			c. Co-dominance: both alleles are expressed side-by-side in the heterozygote
		18. Explain why Tay-Sachs is considered recessive at the organismal level but co-
		dominant at the molecular level.
			a. Organismal level
				i. Gene for hexosaminidase A
				i. Disease is recessive
				ii. Must be passed by both parents
			b. Biochemical level
				i. Co-dominant
				ii. Heterozygotes have half the amount of enzyme
				iii. That is enough enzyme to prevent the disease
		19. Explain why genetic dominance does not mean that the dominant allele subdues a 
		recessive allele. Illustrate your explanation with the use of the round versus wrinkled pea 
		seed shape.
			a. Dominant round allele makes an enzyme that converts sugar to starch
			b. In the heterozygote there is enough enzyme
		20. Explain why dominant alleles do not necessarily mean that the allele is more common 
		in the population. Illustrate your explanation with the character polydactyly.
			a. Harmful and unusual dominants are selected against
			b. Polydactyly occurs in 1/400 births
		21. Describe the inheritance of the ABO blood system and explain why the IA and IB 
		alleles are said to be co-dominant.
			a. Three alleles: IA, IB, and i
			b. Genotypes and phenotypes
				i. IAIA and IAi give type A
				ii. IBIB and IBi give type B
				iii. ii gives type O
				iv. IAIB gives type AB
			c. Both are expressed in type AB, both cover up the I allele
		22. Define and give examples of pleiotropy and epistasis.
			a. Pleiotropy
				i. Most genes affect multiple phenotypes
				ii. Sickle-cell causes widespread tissue damage in addition to anemia
			b. Epistasis
				i. A combination of alleles at one locus affects expression at another
				ii. Example: coat color in Mus is overridden by albinism
		23. Describe a simple model of polygenic inheritance and explain why most polygenic 
		characters are described in quantitative terms.
			a. Many genes determine height, weight, and melanization
			b. Simplified example of melanization
				i. Three genes with two incompletely dominant alleles of each
				ii. Trihybrid cross gives six phenotypes in a normal distribution
		24. Describe how environmental conditions can influence the phenotypic expression of a 
		character. Explain what is meant by "a norm of reaction."
			a. Environment can have powerful effects on health especially
			b. There are simple examples like temperature sensitive genes
			c. "Norm of reaction" refers to the phenotypic range of a genotype depending on 
			environment 
		25. Distinguish between the specific and broad interpretations of the terms "phenotype" 
		and "genotype."
			a. Specific: outward expression of a specific combination of alleles
			b. Broad: total expression of all alleles

MENDELIAN INHERITANCE IN HUMANS
	Intro
		26. Explain why studies of human inheritance are not as easily conducted as Mendel's 
		work with his peas.
			a. Negatives
				i. Slow to mature
				ii. Few offspring
				iii. No controlled breeding
			b. Positives
				i. Inherently interesting
				ii. BIG MONEY
				iii. Ultimate control
	Pedigree analysis reveals Mendelian patterns in human inheritance
		27. Given a simple family pedigree, deduce the genotypes for some of the family 
		members.	
			a. Suggestions for analysis
				i. Consanguinity?  rare recessive
				ii. All affected are males?  sex-inked
				iii. Every affected has an affected parent?  dominant
			b. Warning: often too little data to make definite conclusions 
	Many human disorders follow Mendelian patterns of inheritance
		28. Explain how a recessive lethal gene can be maintained in population.
			a. Heterozygote advantage
			b. Examples
				i. Sickle-cell carriers are immune to malaria
				ii. Cystic fibrosis carriers may be resistant to cholera and typhoid fever 
		29. Describe the inheritance and expression of cystic fibrosis, Tay-Sachs disease, and 
		sickle-cell disease.
			a. CF
				i. Common autosomal recessive, half survive to 30
				ii. Altered trans-membrane conductance regulator, a chloride channel
				iii. High extracellular chloride  excess clogging mucus
			b. Tay-Sachs
				i. Autosomal recessive, lethal within a few years
				ii. Missing hexosaminidase, can't recycle a brain lipid
				iii. Severe neurodegeneration
			c. Sickle-cell
				i. Autosomal recessive, many successful new treatments
				ii. Point mutation in beta-hemoglobin, changes RBC shape
				iii. RBCs clog vessels and damage tissue
		30. Explain why consanguinity increases the probability of homozygosity in offspring.
			a. Everyone carries harmful recessives
			b. Fortunately they are rare
			c. Consanguinity makes it more likely that the same harmful recessive will 
			become homozygous
		31. Explain why lethal dominant genes are much rarer than lethal recessive genes.
			a. Lethality is potent selection
			b. Death before reproduction removes the alleles from the population
		32. Give an example of a late-acting lethal dominant gene in humans and explain how it 
		can escape elimination.
			a. Huntington's disease
				i. Autosomal dominant
				ii. Severe neurological degeneration

			b. Manifests itself at 45 years old; usually after reproduction
		33. Define and give examples of multifactorial disorders in humans. Explain what can 
		currently be done to reduce the frequency of these diseases.
			a. Multifactorial = caused by genes and the environment
			b. Examples
				i. Heart disease, diabetes, cancer, some mental illnesses
				ii. Some are polygenic
			c. Family history indicates susceptibility; environmental factors are isolated
	Technology is providing new tools for genetic testing and counseling
		34. Explain how carrier recognition, fetal testing, and newborn screening can be used in 
		genetic screening and counseling.
			a. Carrier recognition
				i. Tests alert prospective parents
				ii. Preparations and strategies can be defined
			b. Fetal testing
				i. Ultrasound, amniocenetesis, chorionic villus sampling
				ii. Risk of fetal death
				iii. Allows the choice of termination
			c. Newborn screening (NC List)
				i. Heel stick, drop on filter paper, mass spectrometry
				ii. Allows earliest treatment often with complete correction