1. Observable Patterns of Inheritance
Chapter 14

2. Earlobe Variation
Whether a person is born with attached or detached earlobes depends on a single gene
Gene has two molecular forms (alleles)

3. Earlobe Variation
You inherited one allele for this gene from each parent
Dominant allele specifies detached earlobes
Recessive allele specifies attached lobes

4. Dominant & Recessive Alleles
If you have attached earlobes, you inherited two copies of the recessive allele
If you have detached earlobes, you may have either one or two copies of the dominant allele

5. Early Ideas About Heredity
People knew that sperm and eggs transmitted information about traits
Blending theory
Problem:
Would expect variation to disappear
Variation in traits persists

6. Gregor Mendel Strong background in plant breeding and mathematics
Using pea plants, found indirect but observable evidence of how parents transmit genes to offspring

7. The Garden Pea Plant Self-pollinating
True breeding (different alleles not normally introduced)
Can be experimentally cross-pollinated

8. Genes Units of information about specific traits
Passed from parents to offspring
Each has a specific location (locus) on a chromosome

9. Alleles Different molecular forms of a gene Arise by mutation
Dominant allele masks a recessive allele that is paired with it

10. Allele Combinations Homozygous Chromosomes Heterozygous Chromosomes
having two identical alleles at a locus
AA or aa
Heterozygous Chromosomes
having two different alleles at a locus Aa

11. Genetic Terms A pair of homologous chromosomes A gene locus
A pair of alleles
Three pairs of genes

12. Genotype & Phenotype
Genotype refers to particular genes an individual carries
Phenotype refers to an individual’s observable traits
Cannot always determine genotype by observing phenotype

13. Tracking Generations Parental generation P mates to produce
First-generation offspring F1
mate to produce
Second-generation offspring F2

14. F1 Results of One Monohybrid Cross

15. F2 Results of Monohybrid Cross

16. Mendel’s Monohybrid Cross Results
5,474 round
1,850 wrinkled
6,022 yellow
2,001 green
882 inflated
299 wrinkled
428 green
152 yellow
F2 plants showed dominant-to-recessive ratio that averaged 3:1
705 purple
224 white
651 long stem
207 at tip
787 tall
277 dwarf

17. Mendel’s Theory of Segregation
An individual inherits a unit of information (allele) about a trait from each parent
During gamete formation, the alleles segregate from each other

18. Probability
The chance that each outcome of a given event will occur is proportional to the number of ways that event can be reached

19. Punnett Square of a Monohybrid Cross
Female gametes
Male
gametes
A a A a Aa AA aa
Dominant
phenotype can
arise 3 ways,
recessive only
one

20. Test Cross
Individual that shows dominant (heterozygous or Homozygous dominant) phenotype is crossed with individual with recessive phenotype
Examining offspring allows you to determine the genotype of the dominant individual

21. Punnett Squares of Test Crosses
Homozygous
recessive
a a A a aa Aa
Homozygous
recessive
a a A Aa
Two phenotypes
All dominant phenotype

22. Dihybrid Cross
Experimental cross between individuals that are homozygous for different versions of two traits

23. A Dihybrid Cross - F1 Results
purple
flowers, tall
white
flowers,
dwarf
TRUE-
BREEDING
PARENTS:
AABB x
aabb
GAMETES: AB AB ab ab
AaBb
F1 HYBRID
OFFSPRING:
All purple-flowered, tall

24. F1 Results of Mendel’s Dihybrid Crosses
All plants displayed the dominant form of both traits
We now know:
All plants inherited one allele for each trait from each parent
All plants were heterozygous (AaBb)

25. Phenotypic Ratios in F2 Four Phenotypes: Tall, purple-flowered (9/16)
AaBb X
AaBb
Four Phenotypes:
Tall, purple-flowered (9/16)
Tall, white-flowered (3/16)
Dwarf, purple-flowered (3/16)
Dwarf, white-flowered (1/16)

26. Explanation of Mendel’s Dihybrid Results
If the two traits are coded for by genes on separate chromosomes, sixteen gamete combinations are possible
1/4
1/4
1/4
1/4 AB Ab aB ab
1/4
1/16
1/16
1/16
1/16 AB
AABB
AABb
AaBB
AaBb
1/4
1/16
1/16
1/16
1/16 Ab
AABb
AAbb
AaBb
Aabb
1/4
1/16
1/16
1/16
1/16 aB
AaBB
AaBb
aaBB
aaBb
1/4
1/16
1/16
1/16
1/16 ab
AaBb
Aabb
aaBb
aabb

27. 16 Allele Combinations in F2
1/4
1/4
1/4
1/4 AB Ab aB ab
1/4
1/16
1/16
1/16
1/16 AB
AABB
AABb
AaBB
AaBb
1/4
1/16
1/16
1/16
1/16 Ab
AABb
AAbb
AaBb
Aabb
1/4
1/16
1/16
1/16
1/16 aB
AaBB
AaBb
aaBB
aaBb
1/4
1/16
1/16
1/16
1/16 ab
AaBb
Aabb
aaBb
aabb

28. Independent Assortment
Mendel concluded that the two “units” for the first trait were to be assorted into gametes independently of the two “units” for the other trait
Members of each pair of homologous chromosomes are sorted into gametes at random during meiosis

29. Independent Assortment
Metaphase I OR A A a a A A a a B B b b b b B B
Metaphase II: A A a a A A a a B B b b b b B B
Gametes: B B b b b b B B A A a a A A a a
1/4 AB
1/4 ab
1/4 Ab
1/4 aB

30. (n is the number of gene loci at which the parents differ)
Tremendous Variation
Number of genotypes possible in offspring as a result of independent assortment and hybrid crossing is 3n
(n is the number of gene loci at which the parents differ)

31. Impact of Mendel’s Work
Mendel presented his results in 1865
Paper received little notice
Mendel discontinued his experiments in 1871
Paper rediscovered in 1900 and finally appreciated

32. Dominance Relations Complete dominance Incomplete dominance
Heterozygote phenotype is somewhere between that of two homozyotes
Codominance
Non-identical alleles specify two phenotypes that are both expressed in heterozygotes

33. Flower Color in Snapdragons: Incomplete Dominance
Red-flowered plant X White-flowered plant
Pink-flowered F1 plants
(homozygote)
(homozygote)
(heterozygotes)

34. Flower Color in Snapdragons: Incomplete Dominance
Pink-flowered plant X Pink-flowered plant
White-, pink-, and red-flowered plants
in a 1:2:1 ratio
(heterozygote)
(heterozygote)

35. Flower Color in Snapdragons: Incomplete Dominance
Red flowers - two alleles allow them to make a red pigment
White flowers - two mutant alleles; can’t make red pigment
Pink flowers have one normal and one mutant allele; make a smaller amount of red pigment

36. Genetics of ABO Blood Types: Three Alleles: Codominance
Gene that controls ABO type codes for enzyme that dictates structure of a glycolipid on blood cells
Two alleles (IA and IB) are codominant when paired
Third allele (i) is recessive to others

37. ABO Blood Type: Allele Combinations
Type A - IAIA or IAi
Type B - IBIB or IBi
Type AB - IAIB
Type O - ii

38. ABO Blood Type: Glycolipids on Red Cells
Type A - Glycolipid A on cell surface
Type B - Glycolipid B on cell surface
Type AB - Both glyocolipids A & B
Type O - Neither glyocolipid A nor B

39. ABO and Transfusions
Recipient’s immune system will attack blood cells that have an unfamiliar glycolipid on surface
Type O is universal donor because it has neither type A nor type B glycolipid

40. Pleitropy
Alleles at a single locus may have effects on two or more traits
Classic example is the effects of the mutant allele at the beta-globin locus that gives rise to sickle-cell anemia

41. Genetics of Sickle-Cell Anemia
Two alleles
1) HbA
Encodes normal beta hemoglobin chain
2) HbS
Mutant allele encodes defective chain
HbS homozygotes produce only the defective hemoglobin; suffer from sickle-cell anemia

42. Pleiotrophic Effects of HbS/HbS
At low oxygen levels, cells with only HbS hemoglobin “sickle” and stick together
This impedes oxygen delivery and blood flow
Over time, it causes damage throughout the body

43. Epistasis Interaction between the products of gene pairs
Common among genes for hair color in mammals

44. Genetics of Coat Color in Labrador Retrievers
Two genes involved
- One gene influences melanin production
Two alleles - B (black) is dominant over b (brown)
- Other gene influences melanin deposition
Two alleles - E promotes pigment deposition and is dominant over e

45. Allele Combinations and Coat Color
Black coat - Must have at least one dominant allele at both loci
BBEE, BbEe, BBEe, or BbEE
Brown coat - bbEE, bbEe
Yellow coat - Bbee, BbEE, bbee

46. Albinism
Phenotype results when pathway for melanin production is completely blocked
Genotype - Homozygous recessive at the gene locus that codes for tyrosinase, an enzyme in the melanin-synthesizing pathway

47. Alleles at two loci (R and P) interact
Comb Shape in Poultry
Alleles at two loci (R and P) interact
Walnut comb - RRPP, RRPp, RrPP, RrPp
Rose comb - RRpp, Rrpp
Pea comb - rrPP, rrPp
Single comb - rrpp

48. Campodactyly: Unexpected Phenotypes
Effect of allele varies:
Bent fingers on both hands
Bent fingers on one hand
No effect
Many factors affect gene expression

49. Continuous Variation
A more or less continuous range of small differences in a given trait among individuals
The greater the number of genes and environmental factors that affect a trait, the more continuous the variation in versions of that trait

50. Human Variation Some human traits occur as a few discrete types
Attached or detached earlobes
Many genetic disorders
Other traits show continuous variation
Height
Weight
Eye color

51. Describing Continuous Variation
Range of values for the trait
Number of individuals with
some value of the trait
(line of bell-shaped curve indicates continuous variation in population)
Range of values for the trait
Number of individuals with
some value of the trait

52. Temperature Effects on Phenotype
Himalayan rabbits are Homozygous for an allele that specifies a heat-sensitive version of an enzyme in melanin-producing pathway
Melanin is produced in cooler areas of body

53. Environmental Effects on Plant Phenotype
Hydrangea macrophylla
Action of gene responsible for floral color is influenced by soil acidity
Flower color ranges from pink to blue

54. Web Sites BSCI 124 Lecture Notes -- Mendelian Genetics
Biology Project - site map
Genome Project