1. Observing Patterns in Inherited Traits
CHAPTER 11

4. 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

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

6. Crossing garden pea plants
Gregor Mendel
Crossing garden pea plants

8. F2 Dominant-to- Recessive Ratio
Trait Studied
Dominant Form
Recessive Form
F2 Dominant-to- Recessive Ratio
SEED SHAPE
5,474 round
1,850 wrinkled
2.96:1
SEED COLOR
6,022 yellow
2,001 green
3.01:1
POD SHAPE
882 inflated
299 wrinkled
2.95:1
POD COLOR
428 green
152 yellow
2.82:1
FLOWER COLOR
705 purple
224 white
3.15:1
FLOWER POSITION
651 long stem
207 at tip
3.14:1
STEM LENGTH
787 tall
277 dwarf
2.84:1

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

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

11. Allele Combinations Homozygous (purebred)
having two identical alleles at a locus
AA or aa
Heterozygous (hybrid)
having two different alleles at a locus Aa

12. A pair of homologous chromosomes
each in the unduplicated state
most often, one from a male parent and its partner from a female parent
gene locus (plural, loci),
the location for a specific gene on a chromosome
Alleles are at corresponding loci on a pair of homologous chromosomes
A pair of alleles may be identical or non-identical.
3 pairs of genes
at three loci on this pair of homologous chromosomes
same thing as three pairs of alleles

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

14. The Testcross
How can we tell the genotype of an individual with the dominant phenotype?
Such an 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 14

15. Monohybrid Cross
Testcross

16. Dominant phenotype, unknown genotype: PP or Pp? Recessive phenotype,
Technique
Dominant phenotype,
unknown genotype:
PP or Pp?
Recessive phenotype,
known genotype: pp
Predictions
If purple-flowered
parent is PP or
If purple-flowered
parent is Pp
Sperm
Sperm p p p p P P Pp Pp Pp Pp
Eggs
Eggs
Figure 11.7 Research method: the testcross P p Pp Pp pp pp
Results or
All offspring purple
½ offspring purple and
½ offspring white 16

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

18. Monohybrid Crosses AA X aa Aa (F1 monohybrids) Aa X Aa ?
Experimental intercross between two F1 heterozygotes
AA X aa
Aa (F1 monohybrids)
Aa X Aa ?

19. (true-breeding parents)
Experiment
P Generation
(true-breeding
parents)
Purple flowers
White flowers
Figure Inquiry: When F1 hybrid pea plants self- or cross-pollinate, which traits appear in the F2 generation? (step 1) 19

20. (true-breeding parents)
Experiment
P Generation
(true-breeding
parents)
Purple flowers
White flowers
F1 Generation
(hybrids)
All plants had purple flowers
Self- or cross-pollination
Figure Inquiry: When F1 hybrid pea plants self- or cross-pollinate, which traits appear in the F2 generation? (step 2) 20

21. All plants had purple flowers
Experiment
P Generation
(true-breeding
parents)
Purple flowers
White flowers
F1 Generation
(hybrids)
All plants had purple flowers
Self- or cross-pollination
Figure Inquiry: When F1 hybrid pea plants self- or cross-pollinate, which traits appear in the F2 generation? (step 3)
F2 Generation
705 purple-flowered
plants
224 white-flowered
plants 21

22. Punnett Squares a A aa female gametes male gametes a A aa Aa a A aa Aa
Stepped Art

23. Probability
The chance that each outcome of a given event will occur is proportional to the number of ways that event can be reached
Male
Female
9th
210
192
52%
48%
10th
198
184
11th
211
190
53%
47%
12th
213
227
TOTAL
832
793
51%
49%

24. 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

25. Mendel’s Theory of Segregation
Homozygous dominant parent
Homozygous recessive parent
Mendel’s Theory of Segregation
(chromosomes duplicated before meiosis)
meiosis I
meiosis II
(gametes)
(gametes)
fertilization produces heterozygous offspring

26. Law of Independent Assortment
The results of Mendel’s dihybrid experiments are the basis for the law of independent assortment
each pair of alleles segregates independently of each other pair of alleles during gamete formation
applies to genes on different, non-homologous chromosomes or those far apart on the same chromosome
Genes located near each other on the same chromosome tend to be inherited together 26

27. independent assortment
Experiment
P Generation
YYRR
yyrr
Gametes YR yr
F1 Generation
YyRr
Predictions
Hypothesis of
dependent assortment
Hypothesis of
independent assortment
Sperm or
Predicted
offspring in
F2 generation YR Yr yR yr
Sperm YR yr YR
YYRR
YYRr
YyRR
YyRr YR
YYRR
YyRr Yr
Eggs
YYRr
YYrr
YyRr
Yyrr
Figure 11.8 Inquiry: Do the alleles for one character segregate into gametes dependently or independently of the alleles for a different character?
Eggs yr
YyRr
yyrr yR
YyRR
YyRr
yyRR
yyRr yr
Phenotypic ratio 3:1
YyRr
Yyrr
yyRr
yyrr
9 16
3 16
3 16
1 16
Phenotypic ratio 9:3:3:1
Results
315
108
101 32
Phenotypic ratio approximately 9:3:3:1 27

28. The laws of probability govern Mendelian inheritance
Mendel’s laws of segregation and independent assortment reflect the rules of probability
When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss
In the same way, the alleles of one gene segregate into gametes independently of another gene’s alleles 28

29. Independent Assortment
Nucleus of a
diploid (2n)
reproductive cell
with two pairs of
homologous
chromosomes
Independent Assortment
Possible alignments
of the two homologous
chromosomes during
metaphase I of meiosis
The resulting alignments
at metaphase II
Allelic combinations
possible in gametes
1/4 AB
1/4 ab
1/4 Ab
1/4 aB

30. Segregation of alleles into eggs Segregation of alleles into spermRr  Rr
Segregation of
alleles into eggs
Segregation of
alleles into sperm
Sperm R r R R r R R
Figure 11.9 Segregation of alleles and fertilization as chance events
Eggs r r R r r 30

31. Solving Complex Genetics Problems with the Rules of Probability
We can apply the rules of probability to predict the outcome of crosses involving multiple characters
A dihybrid or other multi-character cross is equivalent to two or more independent monohybrid crosses occurring simultaneously
In calculating the chances for various genotypes, each character is considered separately, and then the individual probabilities are multiplied 31

32. For example, if we cross F1 heterozygotes of genotype YyRr, we can calculate the probability of different genotypes among the F2 generation
Probability of YYRR =
Probability of YyRR =
For example, for the cross PpYyRr  Ppyyrr, we can calculate the probability of offspring showing at least two recessive traits
ppyyRr =
ppYyrr =
Ppyyrr =
PPyyrr = 32

33. Mendels 4 principles:
Individual units, called genes, determine biological characteristics.
For each gene, an organism receives one allele from each parent.
The alleles separate from each other
a process called segregation
when reproductive cells are formed

34. Mendels 4 principles:
If an organism inherits different alleles for the same trait, one allele may be dominant over the other.
Some genes segregate independently.
Independent assortment
genes segregate independently and do not influence each others inheritance

35. Human Inheritance
Inheritance patterns are often more complex than predicted by simple Mendelian genetics
Not all heritable characters are determined as simply as the traits Mendel studied
However, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance 35

36. Human Inheritance
Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations
when alleles are not completely dominant or recessive
when a gene has more than two alleles
when a single gene influences multiple phenotypes 36

37. Degrees of Dominance Complete dominance Incomplete dominance
occurs when phenotypes of the heterozygote and dominant homozygote are identical
Incomplete dominance
the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties
Codominance
two dominant alleles affect the phenotype in separate, distinguishable ways 37

38. Incomplete Dominance
Incomplete dominance

39. P Generation Red CRCR White CWCW Gametes CR CW
Figure Incomplete dominance in snapdragon color (step 1) 39

40. P Generation Red CRCR White CWCW Gametes Pink CRCW F1 GenerationCR CW
Figure Incomplete dominance in snapdragon color (step 2) 40

41. P Generation Red CRCR White CWCW Gametes Pink CRCW F1 GenerationCR CW
Figure Incomplete dominance in snapdragon color (step 3)
Sperm CR CW
F2 Generation CR
CRCR
CRCW
Eggs CW
CRCW
CWCW 41

42. Multiple Alleles / co-dominance
Most genes exist in populations in more than two allelic forms
For example, the four phenotypes of the ABO blood group in humans are determined by three alleles of the gene: IA, IB, and i.
The enzyme (I ) adds specific carbohydrates to the surface of blood cells
The enzyme encoded by IA adds the A carbohydrate, and the enzyme encoded by IB adds the B carbohydrate; the enzyme encoded by the i allele adds neither 42

43. ABO Blood Type Range of genotypes: IAIA IBIB or or IAi IAIB IBi ii

44. (a) The three alleles for the ABO blood groups and their carbohydratesIA IB i
Carbohydrate A B
none
(b) Blood group genotypes and phenotypes
Genotype
IAIA or IAi
IBIB or IBi
IAIB ii
Figure Multiple alleles for the ABO blood groups
Red blood cell
appearance
Phenotype
(blood group) A B AB O 44

45. 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

46. Pleiotropy pleiotropy most genes have multiple phenotypic effects
alleles at a single locus may have effects on two or more traits
pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases
cystic fibrosis
mutation in a transporter gene
affects lungs, liver, pancreas, and intestines

49. Pleiotropy Marfan syndrome mutation in gene for fibrillin
affects skeleton, cardiovascular system, lungs, eyes, and skin

52. Pleiotropy sickle-cell disease abnormal red blood cells
affects brain, eyes, lungs, liver, heart, kidneys, penis, joints, bones, or skin.

54. Epistasis
Epistasis
a gene at one locus alters the phenotypic expression of a gene at a second locus
labrador retrievers and many other mammals, coat color depends on two genes
one gene determines the pigment color (with alleles B for black and b for brown)
the other gene (with alleles C for color and c for no color) determines whether the pigment will be deposited in the hair 54

55. Coat Color in Retrievers
Coat color in Labrador retrievers

56. ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ BbEe BbEe Sperm Eggs BBEE BbEE BBEe BbEe BbEE bbEE
Figure An example of epistasis be
BbEe
bbEe
Bbee
bbee 9
: 3
: 4 56

57. Comb Shape in Poultry
Comb shape in chickens

58. Comb Shape in Poultry rose comb pea comb walnut comb single comb X
RRpp
pea
rrPP F1
all walnut
RrPp
RrPp
RrPp F2
9/16 walnut
RRPP, RRPp,RrPP, or RrPp
3/16 rose
RRpp or Rrpp
3/16 pea
rrPP or rrPp
1/16 single
rrpp

59. Coat color in the Himalayan rabbit

60. Polygenic Inheritance
Quantitative characters
those that vary in the population along a continuum
Quantitative variation usually indicates polygenic inheritance
an additive effect of two or more genes on a single phenotype
the greater the number of genes and environmental factors that affect a trait, the more continuous the variation in versions of that trait
Human Traits that show continuous variation
Height
Weight
Eye color
Skin color 60

61. AaBbCc
AaBbCc
Sperm
1 8
1 8
1 8
1 8
1 8
1 8
1 8
1 8
1 8
1 8
1 8
1 8
Eggs
1 8
1 8
Figure A simplified model for polygenic inheritance of skin color
1 8
1 8
1 64
6 64
15 64
20 64
15 64
6 64
1 64
1 64
Phenotypes:
Number of
dark-skin alleles: 1 2 3 4 5 6 61

62. Continuous Variation
Variation in human eye color

63. Describing Continuous Variation

64. Continuous variation in height

65. Pleiotropic effects of Marfan syndrome
Pleiotropy
Pleiotropic effects of Marfan syndrome