1. Introduction to Genetics
Chapter 19
Human Biology, eighth edition
Cecie Starr et al

2. Impacts, Issues: The Color of Skin
Skin color comes from the pigment melanin
Produced by melanocytes in skin cells
More than 100 genes directly or indirectly influence amount of melanin in an individual’s skin
Lead to many variations in skin color

3. Video: ABC News: All in the family: Mixed race twins

4. 19.1 Basic Concepts of Heredity
Genes provide the instructions for all human traits, including physical features and how body parts function
Each person inherits a particular mix of maternal and paternal genes

5. Basic Concepts of Heredity (1)
Genes
Humans have ~21,500
Chemical instructions for building proteins
Locus: specific location on a chromosome
Diploid cells contain two copies of each gene on pairs of homologous chromosomes
Allele: each version of a gene

6. A Few Basic Genetic Terms

7. Many Genetic Traits Have Dominant and Recessive Forms

8. Basic Concepts of Heredity (2)
Homozygous condition: identical alleles
Heterozygous condition: different alleles
Dominant allele
Effect masks recessive allele paired with it

9. Basic Concepts of Heredity (3)
Genetic representations
Homozygous dominant (AA)
Homozygous recessive (aa)
Heterozygous (Aa)
Genotype
Inherited alleles
Phenotype
Observable functional or physical traits

10. Genotype and Phenotype Compared

11. 19.2 One Chromosome, One Copy of a Gene
We inherit pairs of a genes (alleles) on pairs of chromosomes, but a gamete receives only one gene from each pair

12. One Chromosome, One Copy of a Gene
Monohybrid cross
Learn more about genotypes
Segregation
Pairs of alleles separated during gamete formation

13. The Trait Called a Chin Fissure Arises from One Allele of a Gene

14. Animation: Chromosome segregation

15. Each Pair of Gene Alleles Is Separated and Two Alleles End Up in Different Gametes

16. 19.3 Genetic Tools: Testcrosses and Probability
When potential parents are concerned about passing a harmful trait to a child, genetic counselors must try to predict the likely outcome of the mating

17. Probability
Measure of the chance that some particular outcome will occur
Factor in the inheritance of single-gene traits
Cross CC x cc
All of the offspring will be heterozygous, Cc
Cross Cc x Cc
¼ CC, ½ Cc, and ¼ cc

18. A Punnett Square Can Be Used to Predict the Result of a Genetic Cross
Grid used to determine possible outcomes of genetic crosses
Rules of probability apply because fertilization is a chance event
Possibility can be expressed mathematically, e.g., between 0% and 100%
Most probable outcome does not have to occur
In a given situation, probability does not change

19. Making a Punnett Square Is One Way to Determine Likely Outcome of Genetic Cross

20. Different Genetic Results Possible in Second Generation after Monohybrid Mating

21. Use Multiplication to Figure the Probability of the Inheritance of Alleles

22. A Testcross Also Can Reveal Genotypes
Learn the genotype of a (nonhuman) organism
Cross organism with homozygous recessive organism (aa)
If all offspring are Aa, parent was probably AA
If some of the offspring have the dominant trait and some have the recessive trait, parent was Aa

23. 19.4 How Genes for Different Traits Are Sorted into Gametes
When we consider more than one trait, we see that the gene for each trait is inherited independently of the gene of other traits

24. How Genes for Different Traits Are Sorted into Gametes
Independent assortment
Occurs during meiosis
A given chromosome and its genes move randomly into gametes
Metaphase I
Metaphase II
Crosses between individuals heterozygous for two traits yields sixteen different gamete unions
Probability displayed using a Punnett square

25. Independent Assortment: Chromosomes Moved at Random into Forming Gametes

26. One of two possible alignments The only other possible alignment
a Initial chromo- some alignments
(at metaphase I):
b The resulting alignments at metaphase II:
Figure 19.8: In independent assortment chromosomes and the genes they carry are moved at random into forming gametes.
c Possible combinations of alleles in gametes: AB ab Ab aB
Stepped Art
Fig. 19-8, p. 378

27. Tracking Two Traits Shows the Results of Independent Assortment

28. meiosis, gamete formation meiosis, gamete formation
CcDd
CcDd
meiosis,
gamete formation
meiosis,
gamete formation
1/4 CD
1/4 Cd
1/4 cD
1/4 cd
1/4 CD
1/16 CCDD
1/16 CCDd
1/16 CcDD
1/16 CcDd
1/4 Cd
1/16 CCDd
1/16 CCdd
1/16 CcDd
1/16 Ccdd
1/4 cD
1/16 CcDD
1/16 CcDd
1/16 ccDD
1/16 ccDd
Figure 19.9: Animated! Tracking two traits shows the results of independent assortment.
Both parents are heterozygous for both genes. Rules of probability predict that certain combinations of phenotypes among offspring of this type of cross occur in a 9:3:3:1 ratio, on average.
1/4 cd
1/16 CcDd
1/16 Ccdd
1/16 ccDd
1/16 ccdd
Adding up the combinations possible:
9/16 or 9 chin fissure, dimples
3/16 or 3 chin fissure, no dimples
3/16 or 3 smooth chin, dimples
1/16 or 1 smooth chin, no dimples
Fig. 19-9, p. 379

29. Probability Rules Apply to Independent Assortment

30. 19.5 Single Genes, Varying Effects
Some traits have clearly dominant and recessive forms
For most traits, however, the story is not so simple

31. One Gene May Affect Several Traits
Pleiotropy
Wide-ranging effect of one gene
Sickle-cell anemia
One amino acid substitution in hemoglobin
Val instead of glu
Pleiotropic effects
Treatments

32. Single Genetic Change Leads to Many Physical Effects of Sickle-Cell Anemia

33. Figure 19.11: Animated! A single genetic change leads to the many physical effects of sickle-cell anemia.
Part (a) shows how an incorrect amino acid has been substituted in the chain of amino acids making up the hemoglobin protein. The inset shows how the shape of a sickled red blood cell differs from that of a normal red blood cell’s. (b) Melba Moore, celebrity spokesperson for sickle-cell anemia organizations. Right: The range of symptoms for a person who has inherited the mutated gene for hemoglobin’s beta chain from both parents.
Fig a, p. 380

34. Figure 19.11: Animated! A single genetic change leads to the many physical effects of sickle-cell anemia.
Part (a) shows how an incorrect amino acid has been substituted in the chain of amino acids making up the hemoglobin protein. The inset shows how the shape of a sickled red blood cell differs from that of a normal red blood cell’s. (b) Melba Moore, celebrity spokesperson for sickle-cell anemia organizations. Right: The range of symptoms for a person who has inherited the mutated gene for hemoglobin’s beta chain from both parents.
Fig b, p. 380

35. In Codominance, More Than One Allele of a Gene Is Expressed
Heterozygous for a trait, but both alleles are expressed
Example: alleles for blood type determine presence or absence of polysaccharides on surface of red blood cells
IA and IB; codominant when paired with each other
Multiple allele system
A gene that has three or more alleles

36. There Are Several Possible Allele Combinations for ABO Blood Types

37. 19.6 Other Gene Effects and Interactions
Many phenotypes, such as eye color, can’t be predicted with certainty
Biologists have uncovered several underlying causes for these variations

38. Other Gene Effects and Interactions
Penetrance
Probability that someone who inherits an allele will have the phenotype associated with it
Cystic fibrosis
Homozygous recessive
100% penetrant
Polydactyly
Dominant allele for extra digits
Incompletely penetrant

39. People with Polydactyly Have Extra Digits on Their Hands or Feet

40. Polygenic Traits Come from Several Genes Combined (1)
Combined expression of several genes
Skin and eye color; many variations due to the amount and distribution of melanin
Continuous variation
Populations show a range of continuous differences
Most evident in traits that can be measured, e.g., height

41. Polygenic Traits Come from Several Genes Combined (2)
Multifactorial trait
Phenotypes shaped by more than one gene and affected by the environment, e.g., height

42. Eye Color Is Just One of Many Human Polygenic Traits

43. The Environment Can Affect Phenotypes
Height
Diet low in protein
Disease or injury that prevents the normal release of growth hormone
Skin color
Tanning
Good lifestyle choices
May limit the chances that a harmful gene(s) will be expressed

44. “Tongue-Roller” Trait Is Due to a Dominant Allele