1. Chapter 13 Observing Patterns in Inherited Traits
(Sections ) 1

2. 13.4 Mendel’s Theory of Independent Assortment
When homologous chromosomes separate during meiosis, either one of the pair can end up in a particular nucleus
Thus, gene pairs on one chromosome get sorted into gametes independently of gene pairs on other chromosomes
Punnett squares can be used to predict inheritance patterns of two or more genes simultaneously

3. Dihybrid Cross
In a dihybrid cross, individuals identically heterozygous for alleles of two genes (dihybrids) are crossed, and the traits of the offspring are observed
dihybrid cross
Breeding experiment in which individuals identically heterozygous for two genes are crossed
The frequency of traits among the offspring offers information about the dominance relationships between the paired alleles

4. A Dihybrid Cross
Start with one parent plant that breeds true for purple flowers and tall stems (PPTT ) and one that breeds true for white flowers and short stems (pptt)
Each plant makes only one type of gamete (PT or pt)
All F1 offspring will be dihybrids (PpTt) and have purple flowers and tall stems

5. A Dihybrid Cross (cont.)
Then cross two F1 plants: a dihybrid cross (PpTt X PpTt)
Four types of gametes can combine in sixteen possible ways
In F2 plants, four phenotypes result in a ratio of 9:3:3:1
9 tall with purple flowers
3 short with purple flowers
3 tall with white flowers
1 short with white flowers

6. Law of Independent Assortment
Mendel discovered the 9:3:3:1 ratio in his dihybrid experiments – and noted that each trait still kept its individual 3:1 ratio
Each trait (gene pair) sorted into gametes independently of other traits (gene pairs)
law of independent assortment
During meiosis, members of a pair of genes on homologous chromosomes get distributed into gametes independently of other gene pairs

7. Independent Assortment

8. Independent Assortment
A This example shows just two pairs of homologous chromosomes in the nucleus of a diploid (2n) reproductive cell. Maternal and paternal chromosomes, shown in pink and blue, have already been duplicated.
B Either chromosome of a pair may get attached to either spindle pole during meiosis I. With two pairs of homologous chromosomes, there are two different ways that the maternal and paternal chromosomes can get attached to opposite spindle poles. or
meiosis I
meiosis I
C Two nuclei form with each scenario, so there are a total of four possible combinations of parental chromosomes in the nuclei that form after meiosis I.
Figure 13.7 Independent assortment.
meiosis II
meiosis II
D Thus, when sister chromatids separate during meiosis II, the gametes that result have one of four possible combinations
of maternal and paternal chromosomes.
gamete genotype: pt PT pT Pt
Fig. 13.7, p. 194

9. Independent Assortment
A This example shows just two pairs of homologous chromosomes in the nucleus of a diploid (2n) reproductive cell. Maternal and paternal chromosomes, shown in pink and blue, have already been duplicated.
B Either chromosome of a pair may get attached to either spindle pole during meiosis I. With two pairs of homologous chromosomes, there are two different ways that the maternal and paternal chromosomes can get attached to opposite spindle poles. or
meiosis I
C Two nuclei form with each scenario, so there are a total of four possible combinations of parental chromosomes in the nuclei that form after meiosis I.
Figure 13.7 Independent assortment.
meiosis II
D Thus, when sister chromatids separate during meiosis II, the gametes that result have one of four possible combinations
of maternal and paternal chromosomes.
gamete genotype: Pt pT PT pt
Stepped Art
Fig. 13.7, p. 194

10. A Dihybrid Cross

11. A Dihybrid Cross parent plant homozygous4 PT Pt pT pt PP
PPTT
PPTt
PpTT
PpTt
parent plant homozygous
for purple flowers and long stems
parent plant homozygous
for white flowers
and short stems Pt
PPTt
PPtt
PpTt
Pptt
PPTT
pptt PT pt pT
PpTT
PpTt
ppTT
ppTt 1
Figure A dihybrid cross between plants that differ in flower color and plant height. P and p stand for dominant and recessive alleles for flower color. T and t stand for dominant and recessive alleles for height. 2
PpTt
dihybrid pt
PpTt
Pptt
ppTt
pptt PT Pt pT pt 3
four types of gametes
Fig. 13.8, p. 195

12. A Dihybrid Cross
Figure A dihybrid cross between plants that differ in flower color and plant height. P and p stand for dominant and recessive alleles for flower color. T and t stand for dominant and recessive alleles for height.
Fig , p. 195

13. A Dihybrid Cross
Figure A dihybrid cross between plants that differ in flower color and plant height. P and p stand for dominant and recessive alleles for flower color. T and t stand for dominant and recessive alleles for height.
Fig , p. 195

14. A Dihybrid Cross parent plant homozygous4 PT Pt pT pt PP
PPTT
PPTt
PpTT
PpTt
parent plant homozygous
for purple flowers and long stems
parent plant homozygous
for white flowers
and short stems Pt
PPTt
PPtt
PpTt
Pptt
PPTT
pptt PT pt pT
PpTT
PpTt
ppTT
ppTt 1
Figure A dihybrid cross between plants that differ in flower color and plant height. P and p stand for dominant and recessive alleles for flower color. T and t stand for dominant and recessive alleles for height. 2
PpTt
dihybrid pt
PpTt
Pptt
ppTt
pptt PT Pt pT pt 3
four types of gametes
Stepped Art
Fig. 13.8, p. 195

15. ANIMATION: Dihybrid cross
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16. The Contribution of Crossovers
Genes that are far apart on a chromosome tend to assort into gametes independently because crossing over occurs between them very frequently
Genes that are very close together on a chromosome are linked, they do not assort independently because crossing over rarely happens between them
linkage group
All genes on a chromosome

17. Key Concepts Insights From Dihybrid Crosses
Pairs of genes on different chromosomes are typically distributed into gametes independently of how other gene pairs are distributed
Breeding experiments with alternative forms of two unrelated traits can be used as evidence of such independent assortment

18. ANIMATION: Crossover review
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19. 13.5 Beyond Simple Dominance
Mendel studied inheritance patterns that are examples of simple dominance, in which a dominant allele fully masks the expression of a recessive one
Other patterns of inheritance are not so simple:
Codominance
Incomplete dominance
Epistasis
Pleiotropy

20. Codominance
Codominant alleles are both expressed at the same time in heterozygotes, as in multiple allele systems such as the one underlying ABO blood typing
codominant
Refers to two alleles that are both fully expressed in heterozygous individuals
multiple allele system
Gene for which three or more alleles persist in a population

21. Codominance: ABO Blood Types
Which two of the three alleles of the ABO gene you have determines your blood type
The A and the B allele are codominant when paired
Genotype AB = blood type AB
The O allele is recessive when paired with either A or B
Genotype AA or AO = blood type A
Genotype BB or BO= type B
Genotype OO = type O

22. Codominance: ABO Blood Types

23. Codominance: ABO Blood Types
AA or AO
BB or BO
Genotypes: AB OO
Figure Combinations of alleles that are the basis of human blood type.
Phenotypes (blood type): A AB B O
Fig. 13.9, p. 196

24. ANIMATION: Codominance: ABO blood types
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25. Incomplete Dominance
With incomplete dominance, the heterozygous phenotype is between the two homozygous phenotypes
incomplete dominance
Condition in which one allele is not fully dominant over another, so the heterozygous phenotype is between the two homozygous phenotypes

26. Incomplete Dominance: Snapdragons
In snapdragons, one allele (R) encodes an enzyme that makes a red pigment, and allele (r) makes no pigment
RR = red; Rr = pink; rr = white
A cross between red and white (RR X rr) yields pink (Rr)
A cross between two pink (Rr X Rr) yields red, pink, and white in a 1:2:1 ratio

27. Incomplete Dominance: Snapdragons

28. Incomplete Dominance: Snapdragons
homozygous (RR) x homozygous (rr) heterozygous (Rr)
A Cross a red-flowered with a white-flowered plant, and all of the offspring will be pink heterozygotes.
Figure Incomplete dominance in heterozygous (pink) snapdragons. An allele that affects red pigment is paired with a ‘white’ allele.
B If two of the pink heterozygotes
are crossed, the phenotypes
of the resulting offspring will occur in a 1:2:1 ratio.
Fig , p. 196

29. Incomplete Dominance: Snapdragons
Figure Incomplete dominance in heterozygous (pink) snapdragons. An allele that affects red pigment is paired with a ‘white’ allele.
Fig a, p. 196

30. Incomplete Dominance: Snapdragons
Figure Incomplete dominance in heterozygous (pink) snapdragons. An allele that affects red pigment is paired with a ‘white’ allele.
homozygous (RR) x homozygous (rr) heterozygous (Rr)
A Cross a red-flowered with a white-flowered plant, and all of the offspring will be pink heterozygotes.
Fig a, p. 196

31. Incomplete Dominance: Snapdragons
Figure Incomplete dominance in heterozygous (pink) snapdragons. An allele that affects red pigment is paired with a ‘white’ allele.
Fig b, p. 196

32. Incomplete Dominance: Snapdragons
B If two of the pink heterozygotes are crossed, the phenotypes of the resulting offspring will occur in a 1:2:1 ratio.
Figure Incomplete dominance in heterozygous (pink) snapdragons. An allele that affects red pigment is paired with a ‘white’ allele.
Fig b, p. 196

33. ANIMATION: Incomplete dominance
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34. Epistasis
Some traits are affected by multiple gene products, an effect called polygenic inheritance or epistasis
epistasis
Effect in which a trait is influenced by the products of multiple genes

35. Epistasis: Labrador Retriever
Labrador retriever coat color, can be black, brown, or yellow
Figure Epistasis in dogs. Epistatic interactions among products of two gene pairs affect coat color in Laborador retrievers. Left, all dogs with an E and B allele have black fur. Those with an E and two recessive b alleles have brown fur. All dogs homozygous for the recessive e allele have yellow fur. Right: black, chocolate, and yellow Laborador retrievers.

36. Epistasis: Labrador Retriever
A dominant allele (B) specifies black fur, and its recessive partner (b) specifies brown fur
A dominant allele of a different gene (E ) causes color to be deposited in fur, and its recessive partner (e) reduces color
A dog with an E and a B allele has black fur
A dog with an E allele and homozygous for b is brown
A dog homozygous for the e allele has yellow fur regardless of its B or b alleles

37. Epistasis: Labrador Retriever
Figure Epistasis in dogs. Epistatic interactions among products of two gene pairs affect coat color in Laborador retrievers. Left, all dogs with an E and B allele have black fur. Those with an E and two recessive b alleles have brown fur. All dogs homozygous for the recessive e allele have yellow fur. Right: black, chocolate, and yellow Laborador retrievers.

38. Animation: Coat Color in Labrador Retrievers

39. Pleiotropy A pleiotropic gene influences multiple traits
Mutations in pleiotropic genes are associated with complex genetic disorders such as sickle-cell anemia, cystic fibrosis, and Marfan syndrome
pleiotropic
Refers to a gene whose product influences multiple traits

40. Pleiotropy: Marfan Syndrome
In Marfan syndrome, mutations affect elasticity of tissues of the heart, skin, blood vessels, tendons, and other body parts
Haris Charalambous died when his aorta burst – he was 21

41. ANIMATION: Pleiotropic effects of Marfan syndrome
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42. Animation: Comb Shape in Chickens

43. 13.6 Complex Variation in Traits
Phenotype often results from complex interactions among gene products and the environment
Many traits show a continuous range of variation

44. Continuous Variation
Some traits appear in two or three forms; others occur in a range of small differences (continuous variation)
The more genes and environmental factors that influence a trait, the more continuous is its variation
continuous variation
In a population, a range of small differences in a shared trait

45. Continuous Variation (Cont.)
If a graph line drawn around the top of the bars showing the distribution of values for a trait is bell-shaped (a bell curve) the trait varies continuously
bell curve
Bell-shaped curve
Typically results from graphing frequency versus distribution for a trait that varies continuously

46. Continuous Variation (Cont.)
Human height and eye color are traits that vary continuously

47. Continuous Variation (Cont.)
Figure Continuous variation in height among male biology students at the University of Florida. The students were divided into categories of one-inch increments in height and counted (bottom). A graph of the resulting data produces a bell-shaped curve (top), an indication that height varies continuously.

48. Environmental Effects on Phenotype
Environmental factors often affect gene expression, which in turn affects phenotype:
Seasonal change in animal fur colors
Spines grow in presence of predators
Different plant heights when grown at different altitudes

49. Environmental Effects on Phenotype
In summer, the snowshoe hare’s fur is brown; in winter, white – offering seasonal camouflage from predators
Figure Example of environmental effects on animal phenotype. The color of the snowshoe hare’s fur varies by season. In summer, the fur is brown (left); in winter, white (right). Both forms offer seasonally appropriate camouflage from predators.

50. Animation: Continuous Variation in Height

51. Environmental Effects on Phenotype
Daphnia at right developed a longer tail spine and a pointy head in response to chemicals emitted by predatory insects
Figure Environmental effects on the body form of Daphnia. In this animal, the form of the individual on the left develops in environments with few predators. The Daphnia on the right has a longer tail spine and a pointy head—traits that develop in response to chemicals emitted by predatory insects.

52. Environmental Effects on Phenotype
Yarrow plant (Achillea millefolium) grew to different heights at three different elevations
Figure Environmental effects on plant phenotype. Cuttings from the same yarrow plant (Achillea millefolium) grow to different heights at three different elevations.

53. Environmental Effects on Phenotype
Figure Environmental effects on plant phenotype. Cuttings from the same yarrow plant (Achillea millefolium) grow to different heights at three different elevations.
A Plant grown at high elevation (3,060 meters above sea level)
B Plant grown at mid-elevation (1,400 meters above sea level)
C Plant grown
at low elevation (30 meters above sea level)
Fig , p. 199

54. Key Concepts Variations on Mendel’s Theme
Not all traits appear in Mendelian inheritance patterns
An allele may be partly dominant over a nonidentical partner, or codominant with it
Multiple genes may influence a trait; some genes influence many traits
The environment also influences gene expression

55. Menacing Mucus (revisited)
The ΔF508 allele that causes cystic fibrosis in homozygotes may persist because it offers heterozygous individuals a survival advantage against certain deadly infectious diseases
People who carry it may have a decreased susceptibility to typhoid fever and other bacterial diseases that begin in the intestinal tract