2. Genes, Environment and Traits
Few, if any, genes act alone
Environmental factors and other genes may modify expression
Traits can be described as
- Mendelian = Caused by a single gene
- Polygenic = Caused by multiple genes
Both can be multifactorial or complex due to an interaction between genes and the environment

3. Genes, Environment and Traits
Single-gene traits are discrete or qualitative
- Often produce an “all-or-none” effect
Polygenic traits produce a continuously varying phenotype
- Also called quantitative traits
- DNA sequences involved are termed quantitative trait loci (QTLs) 3

4. Genes, Environment and Traits
Figure 7.1 4

5. Polygenic Traits
Are influenced by interaction of genes and by the environment
Examples:
- Height
- Skin color
- Body weight
- Fingerprint patterns
- Behavioral traits 5

6. Polygenic Traits
Individual genes follow Mendel’s laws, but their expression is hard to predict
Effect of genes is additive or synergistic
- However, input of genes is not necessarily identical
The frequency of distribution of phenotypes forms a bell-shaped curve! 6

7. Fingerprint Patterns Dermatoglyphics is the study of fingerprints
The number of ridges is largely determined by genes and prenatal contact
The average total ridge count is 145 in a male and 126 in a female 7

8. Fingerprint Patterns
Figure 7.2 8

9. Height
Figure 7.3 9

10. Height
The difference in height between the two sets of students is attributed to improved diet and better overall health
Genome-wide association studies have identified dozens of genes that affect height
- Also, certain SNPs patterns are seen in individuals with periods of rapid height increase 10

11. Skin Color
Melanin protects against DNA damage from UV radiation, and exposure to the sun increases melanin synthesis
We all have same number of melanocytes per unit area of skin
- However, we differ in melanosome number, size and density distribution 11

12. Skin Color
Figure 7.4 12

13. Skin Color
The definition of race based largely on skin color is a social construct more than a biological concept
Skin color is NOT a reliable indicator of ancestry
- Overall, 93% of varying inherited traits are no more common in people of one skin color than any other 13

14. Investigating Multifactorial Traits
Empiric risk measures the likelihood that a trait will recur based on incidence
Incidence is the rate at which a certain event occurs
Prevalence is the proportion or number of individuals who have a particular trait at a specific time 14

15. Investigating Multifactorial Traits
Cleft lip is more likely in a person who has a relative with the condition
Figure 7.5 15

16. Investigating Multifactorial Traits
Table 7.2 16

17. Heritability (H)
Estimates the proportion of the phenotypic variation in a population due to genetic differences
Figure 7.6 17

18. Heritability (H) 18

19. Heritability (H)
Researchers use several statistical methods to estimate heritability
One way is to compare the proportion of people sharing a trait to the proportion predicted to share the trait
The expected proportion is derived by knowing the blood relationships of the individuals 19

20. Coefficients of Relatedness
The proportion of genes shared between two people related in a certain way
Table 7.4 20

21. Tracing Relatives
Figure 7.7 21

22. Adopted Individuals
Similarities between adopted people and adopted parents reflect mostly environmental influences
Similarities between adoptees and their biological parents reflect mostly genetic influences
Therefore, information on both sets of parents can reveal how heredity and the environment both contribute to a trait 22

23. Twins Twin studies have largely replaced adoption methods
Concordance measures the frequency of expression of a trait in both members of monozygotic (MZ) or dizygotic (DZ) twins
- Twins who differ in a trait are said to be discordant for it
For a trait largely determined by genes, concordance is higher for MZ than DZ twins 23

24. Twins
Table 7.4 24

25. Separating Genetic and Environmental Influences
Dizygotic twins = Shared environment and 50% of genes
Monozygotic twins = Identical genotype and shared environment
Twins raised apart = Shared genotype but not environment
Adopted individuals = Shared environment but not genes 25

26. Twins
Figure 7.8 26

27. Table 7.5

28. SNPs
SNPs (single nucleotide polymorphisms) are sites in a genome where the DNA base varies in at least 1% of the population
In these studies, SNPs span the genome, rather than define a single gene
- A SNP can be anywhere among our roughly 3.2 billion base pairs 28

29. SNPs
Figure 7.9 29

30. Genome-Wide Association Studies
Older techniques search for known gene variants, typically in only a few people
Sequencing of the human genome and the HapMap project (which identifies SNPs) have led to a new tool
Genome-wide association studies seek correlations between SNP patterns and phenotypes in large groups of individuals 30

31. Genome-wide association studies seek SNPs that are shared with much greater frequency among individuals with the same trait than among others
Figure 7.10 31

32. A Step-wise Approach to Gene Discovery
Figure 7.11 32

33. Study Designs
In a cohort study, researchers follow a large group of individuals over time and measure many aspects of their health
In a case-control study, pairs of individuals are matched so that they share as many characteristics as possible
- SNP differences are then associated with the presence or absence of the disorder 33

34. Study Designs
The “affected sibling pair” strategy scans genomes of siblings for SNPs shared by those with the condition, but not by those who don’t have it
Homozygosity mapping is performed on families that are consaguineous
- The children in this case are more likely to inherit two copies of the mutation 34

35. Limitations of Genome-Wide Association Studies
1) They include so many data points and so are prone to error
2) They reveal associations between two types of information, not causes
3) Bias can be introduced in the way the patient population is selected
4) Their accuracy is affected by complicating factors, such as phenocopy and epistasis
5) They may miss extremely rare SNPs 35

36. Table 7.7 36

37. Body Weight
Body weight is a multifactorial trait that reflects energy balance
About 30% of all adults in the US are obese, and another 35% are overweight
Scientific studies use a measurement called body mass index (BMI)
= weight (kg)/height2 (m2) 37

38. Body Mass Index (BMI)
Figure 7.12 38

39. Body Weight
Studies on adopted individuals and twins indicate a heritability of 75% for obesity
Lifestyle, including diet and exercise, are environmental components impacting weight
Genes influence hunger and metabolism 39

40. Leptin and Associated Proteins
Leptin is a protein hormone produced by fat cells
- Acts on the brain to decrease appetite
Ghrelin is a peptide hormone produced in the stomach
- Responds to hunger by increasing appetite
These and other hormones are important for general weight regulation 40

41. Leptin and Associated Proteins
Table 7.8 41