1. Chapter 13 Observing Patterns in Inherited Traits

2. Mendel’s Experimental Approach
Gregor Mendel was a monk with training in plant breeding and mathematics
He studied the garden pea (Pisum sativum), which breeds true for a number of traits
Mendel discovered that traits of offspring of cross-fertilized pea plants often appear in predictable patterns
Mendel’s work led him to conclude that hereditary information passes from one generation to the next in discrete units

3. Gregor Mendel
Figure 13.2 Animated Breeding garden pea plants (Pisum sativum),
which can self-fertilize or cross-fertilize.

4. Garden Pea Plant: Self Fertilization and Cross-FertilizationB C A
carpel
anther D
Figure 13.2 Animated Breeding garden pea plants (Pisum sativum),
which can self-fertilize or cross-fertilize.
A Gregor Mendel systematically bred pea plants and documented traits of their offspring through many generations.
B Garden pea flower, cut in half. Male gametes form in pollen grains produced by anthers, and female gametes form in carpels. Experimenters can control the transfer of hereditary material from one flower to another by snipping off a flower’s anthers (to prevent the flower from self-fertilizing), and then brushing pollen from another flower onto its carpel.
C In this example, pollen from a plant that has purple flowers is brushed onto the carpel of a white-flowered plant.
D Later, seeds develop inside pods of the cross-fertilized plant. An embryo in each seed develops into a mature pea plant.
E Every plant that arises from the cross has purple flowers. Predictable patterns such as this are evidence of how inheritance works. E

5. Terms Used in Modern Genetics
Genes are heritable units of information about traits
Each gene has a specific locus on a chromosome
Diploid cells (chromosome number 2n) have pairs of genes on homologous chromosomes
Alleles are different molecular forms of a gene

6. Loci of Some Human Genes

7. (coronary artery disease)
LDL receptor
(coronary artery disease)
insulin receptor
brown hair color
green/blue eye color
(Warfarin resistance)
Figure 13.3 Loci of a few human genes. Genetic diseases that result from mutations in the genes are shown in parentheses. The number or letter below each chromosome is its name; the characteristic banding patterns appear after staining. Appendix VI has a similar map of all 23 human chromosomes.
HCG, β chain 19
LH, β chain
Figure 13-3c p205

8. (anhidrotic ectodermal dysplasia)
dystrophin
(muscular dystrophy)
(anhidrotic ectodermal dysplasia)
IL2RG (SCID-X1)
XIST X chromosome
inactivation control
Figure 13.3 Loci of a few human genes. Genetic diseases that result from mutations in the genes are shown in parentheses. The number or letter below each chromosome is its name; the characteristic banding patterns appear after staining. Appendix VI has a similar map of all 23 human chromosomes.
(hemophilia B)
(hemophilia A)
(red-deficient color blind) X
(green-deficient color blind)
Figure 13-3e p205

9. Terms Used in Modern Genetics
The particular set of alleles that an individual carries is the individual’s genotype
An individual with two identical alleles of a gene is homozygous for that gene
An individual with nonidentical alleles of a gene is heterozygous for that gene

10. Terms Used in Modern Genetics
An allele is dominant if its effect masks the effect of a recessive allele paired with it
Capital letters (P) signify dominant alleles; lowercase letters (p) signify recessive alleles
Homozygous dominant (PP)
Homozygous recessive (pp)
Heterozygous (Pp)

11. Genotypes Give Rise to PhenotypesPP
(homozygous for
dominant allele P) pp
(homozygous for
recessive allele p) Pp
(heterozygous at
the P gene locus)
genotype:
phenotype:
Figure 13.4 Genotype gives rise to phenotype. In this example, the dominant allele P specifies purple flowers; the recessive allele p, white flowers.

12. Take-Home Message: How do alleles contribute to traits?
Gregor Mendel discovered the role of alleles in inheritance by breeding pea plants and tracking traits of their offspring
Genotype refers to the particular set of alleles carried by an individual’s somatic cell; phenotype refers to the individual’s set of observable traits; genotype is the basis of phenotype
A homozygous individual has two identical alleles at a particular locus; a heterozygous individual has nonidentical alleles at the locus
Dominant alleles mask the effects of recessive ones in heterozygous individuals

13. 13.3 Mendel’s Law of Segregation
Pairs of genes on homologous chromosomes separate during meiosis, so they end up in different gametes
Mendel showed that garden pea plants inherit two “units” of information for a trait, one from each parent

14. Gene Segregation
Homologous chromosomes (and all the alleles they carry) segregate into separate gametes during meiosis
Plants homozygous for the dominant allele (PP) can only make gametes that carry the allele P
Plants homozygous for the recessive allele (pp) can only make gametes that carry the allele p
Heterozygous plants produce both type of gametes

15. Calculating Probabilities
Probability
A measure of the chance that a particular outcome will occur
Punnett square
A grid used to calculate the probability of genotypes and phenotypes in offspring

16. DNA replication meiosis I gametes (p) meiosis II gametes (P)1 2
gametes (p)
meiosis II
gametes (P)
zygote (Pp) 3
Figure 13.5 Gene segregation. Homologous chromosomes separate during meiosis, so the pairs of genes they carry separate too. Each of the resulting gametes carries one of the two members of each gene pair. For clarity, only one set of chromosomes is shown.
All gametes made by a parent homozygous for a dominant allele carry that allele.
All gametes made by a parent homozygous for a recessive allele carry that allele.
If these two parents are crossed, the union of any of their gametes at fertilization produces a zygote with both alleles. All offspring of this cross will be heterozygous.
4. This outcome is easy to see with a Punnett square. Parental gametes are listed in circles on the top and left sides of a grid. Each square is filled with the combination of alleles that would result if the gametes in the corresponding row and column met up.
female gametes
male gametes 4
Stepped Art
Figure 13-5 p206

17. female gametes male gametes
Figure 13.5 Gene segregation. Homologous chromosomes separate during meiosis, so the pairs of genes they carry separate too. Each of the resulting gametes carries one of the two members of each gene pair. For clarity, only one set of chromosomes is shown.
All gametes made by a parent homozygous for a dominant allele carry that allele.
All gametes made by a parent homozygous for a recessive allele carry that allele.
If these two parents are crossed, the union of any of their gametes at fertilization produces a zygote with both alleles. All offspring of this cross will be heterozygous.
4. This outcome is easy to see with a Punnett square. Parental gametes are listed in circles on the top and left sides of a grid. Each square is filled with the combination of alleles that would result if the gametes in the corresponding row and column met up.
Figure 13-5b p206

18. Testcrosses
A testcross is a method of determining if an individual is heterozygous or homozygous dominant
An individual with unknown genotype is crossed with one that is homozygous recessive (PP x pp) or (Pp x pp)

19. Generations in a Monohybrid Cross
P stands for parents, F for filial (offspring)
F1: First generation offspring of parents
F2: Second generation offspring of parents

20. Mendel’s Monohybrid Crosses
Mendel used monohybrid crosses to find dominance relationships among pea plant traits
When he crossed plants that bred true for white flowers with plants that bred true for purple flowers, all F1 plants had purple flowers
When he crossed two F1 plants, ¾ of the F2 plants had purple flowers, ¼ had white flowers

21. Table 13-1 p207

22. Mendel’s Dihybrid Cross4 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 2
PpTt
dihybrid pt
PpTt
Pptt
ppTt
pptt PT Pt pT pt 3
four types of gametes
Stepped Art

23. Offspring of Mendel’s Monohybrid Cross

24. Mendel’s Law of Segregation
Mendel observed a phenotype ratio of 3:1 in the F2 offspring of his monohybrid crosses
Consistent with the probability of the pp genotype in the offspring of a heterozygous cross (Pp x Pp)
This is the basis of Mendel’s law of segregation
Diploid cells have pairs of genes on pairs of homologous chromosomes
The two genes of each pair separate during meiosis, and end up in different gametes

25. Take-Home Message: What is Mendel’s law of segregation?
Diploid cells carry pairs of genes, on pairs of homologous chromosomes
The two genes of each pair are separated from each other during meiosis, so they end up in different gametes
Mendel discovered patterns of inheritance in pea plants by tracking the results of many monohybrid crosses

26. 13.4 Mendel’s Law of Independent Assortment
During meiosis, members of a pair of genes on homologous chromosomes get distributed into gametes independently of other gene pairs

27. Dihybrid Crosses
Dihybrid crosses test for dominance relationships between alleles at two loci
Individuals that breed true for two different traits are crossed (PPTT x pptt)
F2 phenotype ratio is 9:3:3:1 (four phenotypes)
Individually, each dominant trait has an F2 ratio of 3:1 – inheritance of one trait does not affect inheritance of the other

28. The Contribution of Crossovers
Independent assortment also occurs when the genes are on the same chromosome, but far enough apart that crossing over occurs between them very frequently
Genes that have loci very close to one another on a chromosome tend to stay together during meiosis and not assort independently

29. Linkage Groups All genes on one chromosome are called a linkage group
The farther apart two genes are on a chromosome, the more often crossing over occurs between them
Linked genes are very close together; crossing over rarely occurs between them
The probability that a crossover will separate alleles of two genes is proportional to the distance between those genes

30. Codominance Codominance
Two nonidentical alleles of a gene are both fully expressed in heterozygotes, so neither is dominant or recessive
May occur in multiple allele systems
Multiple allele systems
Genes with three or more alleles in a population
Example: ABO blood types

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

32. Incomplete Dominance Incomplete dominance
One allele is not fully dominant over its partner
The heterozygote’s phenotype is somewhere between the two homozygotes, resulting in a 1:2:1 phenotype ratio in F2 offspring
Example: Snapdragon color
RR is red
Rr is pink
rr is white

33. heterozygous (Rr) homozygous (RR) homozygous (rr)
Figure Animated Incomplete dominance in heterozygous (pink) snapdragons. An allele that affects red pigment is paired with a “white” allele.
heterozygous (Rr)
homozygous (RR)
homozygous (rr)
Figure p210

34. Figure Animated Incomplete dominance in heterozygous (pink) snapdragons. An allele that affects red pigment is paired with a “white” allele.
Figure 13-10b p210

35. Epistasis Epistasis Two or more gene products influence a trait
Typically, one gene product suppresses the effect of another
Example: Coat color in dogs
Alleles B and b designate colors (black or brown)
Two recessive alleles ee suppress color

36. Coat Colors in Labrador Retrievers
Figure Epistasis in dogs. Epistatic interactions among products of two gene pairs affect coat color in Laborador retrievers. 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.

37. Figure 13. 11 Epistasis in dogs
Figure Epistasis in dogs. Epistatic interactions among products of two gene pairs affect coat color in Laborador retrievers. 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.
Figure 13-11b p211

38. Pleiotropy A pleiotropic gene influences multiple traits
Example: Some tall, thin athletes have Marfan syndrome, a potentially fatal genetic disorder

39. Environment and Epigenetics
Environmentally driven changes in gene expression patterns can be permanent and heritable
Such changes are implemented by gene controls such as chromatin modifications and RNA interference that act on DNA itself
Example: Many environmental factors affect DNA methylation patterns, enhancing or suppressing gene expression

40. Effects of Temperature on Gene Expression
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).

41. a Mature cutting at high elevation (3,060 meters above sea level)
b Mature
cutting at
mid-elevation
(1,400 meters
above sea
level)
Figure Experiment showing environmental effects on phenotype in yarrow (Achillea millefolium). Cuttings from the same parent plant were grown in the same kind of soil at three different elevations.
c Mature
cutting at low
elevation (30
meters above
sea level)
Figure 13-14a p212 41

42. 13.7 Complex Variations in Traits
Individuals of most species vary in some of their shared traits
Many traits (such as eye color) show a continuous range of variation

43. Continuous Variation Continuous variation
Traits with a range of small differences
The more factors that influence a trait, the more continuous the distribution of phenotype
Bell curve
When continuous phenotypes are divided into measurable categories and plotted as a bar chart, they form a bell-shaped curve

44. Regarding the Unexpected Phenotype
Phenotype results from complex interactions among gene products and the environment
Enzymes and other gene products control steps of most metabolic pathways
Mutations, interactions among genes, and environmental conditions may result in unpredictable traits
Example: Camptodactyly can affect any fingers on either or both hands