1. Mendelian Genetics
AP Biology
Ch. 14
Ms. Haut

2. Pre-Mendelian Theory of Heredity
Blending Theory —hereditary material from each parent mixes in the offspring
Individuals of a population should reach a uniform appearance after many generations
Once traits are blended, they can no longer be separated out to appear in later generations
Problems —inconsistent with observations:
Individuals of a population don’t reach uniform appearance
Traits can skip generations

3. Modern Theory of Heredity
Based on Gregor Mendel’s fundamental principles of heredity
Parents pass on discrete inheritable factors (genes) to their offspring
These factors remain as separate factors from one generation to the next

4. Mendel’s Experimental, Quantitative Approach
Advantages of pea plants for genetic study:
There are many varieties with distinct heritable features, or characters (such as flower color); character variants (such as purple or white flowers) are called traits
Mating of plants can be controlled
Each pea plant has sperm-producing organs (stamens) and egg-producing organs (carpels)
Cross-pollination (fertilization between different plants) can be achieved by dusting one plant with pollen from another
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

5. Mendel’s Discoveries
Developed pure lines—populations that “breed true” (always produce offspring with the same traits as the parents when parents are self-fertilized)
Counted his results and kept statistical notes on experimental crosses

6. Useful Genetic Vocabulary
Homozygous —having 2 identical alleles for a given trait (PP or pp)
Heterozygous —having 2 different alleles for a trait (Pp); ½ gametes carry one allele (P) and ½ gametes carry the other allele (p)
Phenotype —an organism’s expressed traits (purple or white flowers)
Genotype —an organism’s genetic makeup (PP, Pp, or pp)

7. Combinations resulting from a genetic cross may be predicted by a Punnett square
This law predicts a 3:1 ratio observed in the F2 generation of a monohybrid cross

9. Mendel’s Principles of Heredity
First Law of Genetics: Law of Segregation
alternate forms of genes are responsible for variations in inherited traits
the gene for flower color in pea plants has two alleles, one for purple flowers and the other for white flowers
for each trait, an organism inherits 2 alleles, one from each parent
If 2 alleles differ, one is fully expressed (dominant allele); the other is completely masked (recessive allele)

10. Mendel’s Principles of Heredity
First Law of Genetics: Law of Segregation
Each gene resides at a specific locus on a specific chromosome
2 alleles for each trait segregate during gamete production

11. The Testcross
How can we tell the genotype of an individual with the dominant phenotype?
Such an individual must have one dominant allele, but the individual could be either homozygous dominant or heterozygous
The answer is to carry out a testcross: breeding the mystery individual with a homozygous recessive individual
If any offspring display the recessive phenotype, the mystery parent must be heterozygous
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

12. The Testcross CAUTION:
The cross of an individual with dominant phenotype to a homozygous recessive parent
Used to determine if the individual is homozygous dominant or heterozygous
CAUTION:
Must perform many, many crosses to be statistically significant

13. Mendel’s Principles of Heredity
Second Law of Genetics: Law of Independent Assortment
During gamete formation, the segregation of the alleles of one allelic pair is independent of the segregation of another allelic pair
Law discovered by following segregation of 2 genes

14. Dihybrid Cross

15. Mendelian Inheritance Reflects Rules of Probability
Rules of Multiplication:
The probability that independent events will occur simultaneously is the product of their individual probabilities.

16. Mendelian Inheritance Reflects Rules of Probability
Question: In a Mendelian cross between pea plants that are heterozygous for flower color (Pp), what is the probability that the offspring will be homozygous recessive?
Answer:
Probability that an egg from the F1 (Pp) will receive a p allele = ½
Probability that a sperm from the F1 will receive a p allele = ½
Overall probability that 2 recessive alleles will unite at fertilization: ½ x ½ = ¼

17. Mendelian Inheritance Reflects Rules of Probability
Works for Dihybrid Crosses:
Question: For a dihybrid cross, YyRr x YyRr, what is the probability of an F2 plant having the genotype YYRR?
Answer:
Probability that an egg from a YyRr parent will receive the Y and R alleles = ½ x ½ = ¼
Probability that a sperm from a YyRr parent will receive the Y and R alleles = ½ x ½ = ¼
Overall probability of an F2 plant having the genotype YYRR: ¼ x ¼ = 1/16

18. Mendelian Inheritance Reflects Rules of Probability
Rules of Addition: The probability of an event that can occur in two or more independent ways is the sum of the separate probabilities of the different ways.
Question: In a Mendelian cross between pea plants that are heterozygous for flower color (Pp), what is the probability that the offspring will being a heterozygote?
Answer:
There are 2 ways in which a heterozygote may be produced: the dominant allele may be in the egg and the recessive allele in the sperm, or the dominant allele may be in the sperm and the recessive allele in the egg.

19. Mendelian Inheritance Reflects Rules of Probability
Probability that the dominant allele will be in the egg with the recessive in the sperm is ½ x ½ = ¼
Probability that the dominant allele will be in the sperm with the recessive in the egg is ½ x ½ = ¼
Therefore, the overall probability that a heterozygote offspring will be produced is ¼ + ¼ = ½

21. Variations to Mendel’s First Law of Genetics
Incomplete dominance —pattern of inheritance in which one allele is not completely dominant over the other
Heterozygote has a phenotype that is intermediate between the phenotypes of the homozygous dominant parent and homozygous recessive parent

22. Incomplete Dominance in Snapdragon ColorF2
Genotypic ratio:
1 CRCR: 2 CRCW: 1 CWCW
Phenotypic ratio:
1 red: 2 pink: 1 white

23. Variations to Mendel’s First Law of Genetics
Codominance —pattern of inheritance in which both alleles contribute to the phenotype of the heterozygote

24. Codominance in MN Blood Groups
MN blood group locus codes for the production
of surface glycoproteins on the red blood cell
There are 3 blood types: M, N, and MN
Blood Type
Genotype M MM N NN MN
The MN blood type is the result of full phenotypic expression of both alleles in the heterozygote; both molecules, M and N, are produced on the red blood cell

25. Multiple Alleles
Some genes may have more than just 2 alternate forms of a gene.
Example: ABO blood groups
A and B refer to 2 genetically determined polysaccharides (A and B antigens) which are found on the surface of red blood cells (different from MN blood groups)
A and B are codominant; O is recessive to A and B

26. Multiple Alleles for the ABO Blood Groups
3 alleles: IA, IB, i

27. Pleiotropy The ability of a single gene to have multiple
phenotypic effects (pleiotropic gene affects
more than one phenotype)
Examples:
In tigers and Siamese cats, the gene that controls fur pigmentation also influences the connections between a cat’s eyes and the brain. A defective gene cause both abnormal pigmentation and cross-eye condition
Marfan’s syndrome—one gene causes the slender physique, hypermobility of the joints, elongation of the limbs, dislocation of the lens, and susceptibility to heart disease

28. Epistasis Interaction between 2 nonallelic genes in
which one modifies the phenotypic expression
of the other.
If epistasis occurs between 2 nonallelic genes,
the phenotypic ratio resulting from a dihybrid
cross will deviate from the 9:3:3:1 Mendelian
ratio

29. c = No Deposition (Albinism) B = Brown coat color b = Tan coat color
C = Melanin deposition
c = No Deposition (Albinism)
B = Brown coat color
b = Tan coat color
A cross between heterozygous brown horses for the 2 genes results in a 9:3:4 phenotypic ratio
9 Black (B_C_)
4 Albino (__cc)
3 Brown (bbC_)

30. Polygenic Traits
Mode of inheritance in which the additive effect of 2 or more genes determines a single phenotypic character
Skin pigmentation in humans
--3 genes with the dark-skin allele
(A, B, C) contribute one “unit” of
darkness to the phenotype.
These alleles are incompletely
dominant over the other alleles
(a, b, c)
--An AABBCC person would be
very dark; an aabbcc person
would be very light
--An AaBbCc person would have
skin of an intermediate shade

32. Pedigree Analysis
A pedigree is a family tree that describes the interrelationships of parents and children across generations
Inheritance patterns of particular traits can be traced and described using pedigrees
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

33. Pedigree Analysis Analysis of existing populations
Studies inheritance of genes in humans
Useful when progeny data from several generations is limited
Useful when studying species with a long generation time

34. Symbols: = female = male = affected individual = matingII
= offspring in birth order
I and II are generations
= Identical twins
= Fraternal twins

35. Dominant Pedigree: For dominant traits:II
III
For dominant traits:
Affected individuals have at least one affected parent
The phenotype generally appears every generation
2 unaffected parents only have unaffected offspring

36. Recessive Pedigree: For recessive traits:II
III
For recessive traits:
Unaffected parents can have affected offspring
Affected progeny are both male and female

37. Pedigree Analysis Is widow’s peak a dominant or recessive trait?

38. Pedigree Analysis Is attached earlobe a dominant or recessive trait?

39. Recessive Human Disorders
Parents are generally unaffected
Defective form of a normal trait. Generally, more serious phenotypic affect than dominant genes
2 Heterozygous normal, unaffected parents can have affected offspring
Probability the child of 2 carriers will be:
affected = ¼
Normal, but carriers = 1/2

40. Recessive Human Disorders
Cystic Fibrosis: autosomal recessive
Ineffective component of Na+/Cl-; causes mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine
Tay-Sachs: autosomal recessive
Usually fatal by 2 or 3 yrs
Developmental retardation, followed by paralysis, dementia, and blindness
Lack enzyme to breakdown lipids—accumulate in brain so cells lose function

41. Recessive Human Disorders
Sickle-cell anemia: autosomal recessive
Caused by single amino acid substitution in hemoglobin
Abnormal hemoglobin packs together to form rods creating crescent-shaped cells
Reduces amount of oxygen hemoglobin can carry

42. Dominant Human Disorders
Traits inherited in every generation
When there is 1 affected parent; ½ progeny are affected
2 affected parents can have unaffected offspring
If prevents survival, then gene is quickly eliminated from population
Usually more variable in its effects. If lethal, usually after reproductive age

43. Dominant Human Disorders
Huntington’s Disease: autosomal dominant
Average onset is 40 yrs.
Late acting, presents itself after reproductive age; lethal
Affects nervous system, muscle spasms
Destroys neurons
Located on chromosome 4
Children of an afflicted parent have a 50% chance of inheriting the lethal dominant allele

44. Nature versus Nurture Environmental conditions can influence the
phenotypic expression of a gene, so that a
single genotype may produce a range of
phenotypes
One may have a history of heart disease in
their family and thus be at risk of heart disease
themselves. If this person watches his/her
diet, exercises, doesn’t smoke, etc. his/her
risk of actually developing heart disease
decreases

45. Genetic Testing & Counseling
Genetic counselors can help determine probability of prospective parents passing on deleterious genes
Genetic screening for various known diseases alleles (gene markers)

46. Genetic Testing & Counseling
Fetal testing
Amniocentesis
needle inserted into uterus and amniotic fluid extracted
Test for certain chemicals or proteins in the fluid that are diagnostic of certain diseases
Karyotype-can see chromosome abnormalities

47. Genetic Testing & Counseling
Fetal testing
Chorion Villus Sampling
Suctions off a small amount of fetal tissue from the chorionic villus of placenta
Karyotype-can see chromosome abnormalities

48. Ultrasound at 12 weeks
--can see any physical abnormalities