1. Chapter 10 Genetics

2. Heritable Traits
Heredity is the tendency for traits (features) to be passed from parent to offspring

3. Gregor Mendel Gregor Mendel performed experiments with garden peas
Peas are ideally suited to the study of heredity
White
Self-fertilize
Purple

4. Mendel’s Experimental Design
First he established true-breeding varieties and named these pure lines the P generation

5. Mendel’s Experimental Design
Next he crossed two different varieties and named the resulting offspring the F1 generation
The trait expressed in the F1 generation known as the dominant trait

6. Mendel’s Experimental Design
Finally he allowed the plants from the F1 generation to self-fertilize and named the resulting offspring the F2 generation
The trait not expressed in the F1 generation, but reemerges in the F2 generation is known as the recessive trait

7. Seven Characteristics Studied

8. Patterns Emerged
Mendel found a consistent proportions of expressed traits in the F2 generation
3/4 expressed the dominant trait while 1/4 expressed the recessive trait
the dominant : recessive ratio 3:1

9. F3 Generation
1/4 of plants from the F2 that were dominant were true-breeding dominant
1/2 of plants showed both traits
1/4 of the plants from the F2 that were recessive were true-breeding recessive

10. True Ratio
He determined that the 3:1 ratio that he observed in the F2 generation was, in fact, a disguised 1:2:1 ratio

11. Mendel Proposes Theories
Hypothesis 1
parents do not transmit traits directly to their offspring
parents transmit information about the trait in the form of genes
Hypothesis 2
each parent contains two copies of a gene
the two copies of the gene may or may not be the same

12. Mendel Proposes Theories
Hypothesis 3
alternative forms of a gene lead to alternative traits
the alternative forms of a gene are known as alleles
An individual with 2 identical alleles is said to be homozygous (homo = same)
An individual with 2 different alleles is said to be heterozygous (hetero = different)

13. Genotype vs Phenotype
Genotype indicates the alleles an individual possesses
a dominant trait is expressed as a capital letter
eg. P = purple flowers
a recessive trait is expressed as a lower case letter
eg. p = white flowers
Phenotype is the physical appearance of an individual depending on their genotype
a homozygous dominant individual has genotype PP, and phenotype purple flowers
a homozygous recessive individual has genotype pp, and phenotype white flowers
a heterozygous individual has genotype Pp, and phenotype purple flowers

14. Genotype vs Phenotype Genotype Phenotype PP Pp pp Homozygous Dominant
Heterozygous
Homozygous Recessive PP Pp pp
Genotype
Phenotype

15. Punnett Square
The results from a cross between a true-breeding, white-flowered plant (pp) and a true breeding, purple-flowered plant (PP) can be visualized with a Punnett square
Lists the possible gametes from one individual with the possible gametes from the other individual on the opposite side
The genotypes of potential offspring are represented within the square

16. A Punnett square analysis

17. Punnett Square of Flower Color

18. Testcross
Mendel devised the testcross with a homozygous recessive individual in order to determine the genotype of unknown individuals in the F2 generation
Heterozygous, ½ offspring dominant phenotype,
½ recessive phenotype
Homozygous dominant, all offspring dominant phenotype

19. Mendel’s Laws Mendel’s First Law: Segregation
the two alleles of a trait separate from each other during the formation of gametes, so that half of the gametes will carry one copy and half will carry the other copy

20. Dihybrid Cross
Investigates the inheritance pattern for 2 different characteristics
when crossing individuals who are true-breeding for two different characters, the F1 individual that results is a dihybrid
after the dihybrid individuals self-fertilize, there are 16 possible genotypes of offspring

21. Analysis of a dihybrid cross

22. Independent Assortment
Mendel’s Second Law: Independent Assortment
genes located on different chromosomes are inherited independently of one another

23. Why Some Traits Don’t Show Mendelian Inheritance
Continuous variation
Characteristics can show a gradation in phenotypes
This type of inheritance is called polygenic
The gradation in phenotypes is called continuous variation

24. Why Some Traits Don’t Show Mendelian Inheritance
Pleiotropic effects
an allele that has more than one effect on a phenotype is considered pleiotropic
these effects are characteristic of many inherited disorders, such as cystic fibrosis and sickle-cell disease

25. Pleiotropic effects of the cystic fibrosis gene, cf

26. Why Some Traits Don’t Show Mendelian Inheritance
Environmental effects
the degree to which many alleles are expressed depends on the environment
some alleles are heat-sensitive
Arctic foxes only produce fur pigment when temperatures are warm

27. Why Some Traits Don’t Show Mendelian Inheritance
Incomplete dominance
not all alternative alleles are either fully dominant or fully recessive
incomplete dominance produces a heterozygous phenotype that is intermediate between those of the parents
each allele is represented, but essentially only at 50%

28. Incomplete dominance

29. Why Some Traits Don’t Show Mendelian Inheritance
Codominance
a gene may have more than two alleles in a population
both alleles are expressed equally
these alleles are said to be codominant

30. Codominance and Blood Type
The gene that determines ABO blood type in humans exhibits more than one dominant allele
the gene that encodes blood type, designated I, has three alleles: IA,IB, and i
different combinations of the three alleles produce four different phenotypes, or blood types (A, B, AB, and O)
both IA and IB are dominant over i and also codominant

31. Multiple alleles controlling the ABO blood groups

32. Chromosomes Are the Vehicles
The chromosomal theory of inheritance –chromosomes carry the information for traits (genes)
Supported by evidence:
similar chromosomes pair with one another during meiosis
reproduction involves the initial union of only eggs and sperm
each gamete contains only copy of the genetic information and combines to form a diploid cell

33. Linkage
Linkage is defined as the tendency of close-together genes to segregate together
the farther apart two genes are from each other on the same chromosome, the more likely crossing over is to occur
this would lead to independent segregation
the closer that two genes are to each other on the same chromosome, the less likely that crossing over will occur between them

34. Linkage and Crossing Over
These two chromosomes could segregate independently into 2 separate cells as is, or….
During meiosis I, crossing over could occur, separating far genes from on another, but having no effect on genes close to each other
These two hybrid chromosomes could then segregate independently into 2 separate cells (gametes)

35. Human Chromosomes
Each human somatic cell normally has 46 chromosomes, or 23 pairs
22 of the 23 pairs are perfectly matched in both males and females and are called autosomes
1 pair are the sex chromosomes
females are designated XX while males are designated XY
the genes on the Y chromosome determine “maleness”
a female passes on an X to offspring, males may pass X (50%) or Y (50%)

36. Nondisjunction
Sometimes errors occur during meiosis
Nondisjunction is the failure of chromosomes to separate correctly during either meiosis I or meiosis II
this leads to aneuploidy, an abnormal number of chromosomes
most of these abnormalities cause a failure to develop or an early death before adulthood
in contrast individuals with an extra copy of chromosome 21

37. Nondisjunction

38. Nondisjunction OR meiosis Monosomy Trisomy Sperm Ovum (egg) Sperm M PM P
Monosomy OR M P
Ovum
Sperm
Trisomy

39. Trisomy 21 or Down Syndrome3 4 5 6 7 8 9 11 12 X Y 1 13 14 15 16 17 18 19 20 21 22
(b)
(a) 10

40. Nondisjunction of Sex Chromosomes
Nondisjunction of the X chromosome creates three possible viable conditions
XXX female (triple X)
usually taller than average but other symptoms vary
XXY male (Klinefelter syndrome)
sterile male with many female characteristics and diminished mental capacity
XO female (Turner syndrome)
sterile female with webbed neck and diminished stature

41. Nondisjunction of the X chromosome

42. Nondisjunction OR X meiosis Turner X XX XX Y XX XXY Klinefelter Sperm
Ovum X
Ova (eggs)
meiosis M P
Turner X XX OR M P
Ovum
Sperm XX Y XX
Klinefelter
XXY

43. Nondisjunction of Sperm
Nondisjunction of the Y chromosome also occurs
in such cases, YY gametes are formed, leading to XYY males
these males are fertile and may be aggressive

44. Studying Pedigrees
To study human heredity, scientists examine crosses that have already been made
they identify which relatives exhibit a trait by looking at family trees or pedigrees
often one can determine whether a trait is sex-linked or autosomal and whether the trait’s phenotype is dominant or recessive

45. A general pedigree

46. A pedigree of albinism Is the trait sex-linked or autosomal?V
Generation IV II
III
Unaffected
Carrier
Affected
Female
Male
KEY:
Is the trait sex-linked or autosomal?
Is the trait dominant or recessive?
Is the trait by a single gene or several?

47. Pedigree of color blindness
Generation II
III IV
Is the trait sex-linked or autosomal?
Is the trait dominant or recessive?
Is the trait by a single gene or several?

48. Mutations Accidental changes in genes are called mutations
mutations occur only rarely and almost always result in recessive alleles
in some cases, produce harmful effects called genetic disorders

49. Some Important Genetic Disorders

50. Hemophilia: Sex-linked Trait
Hemophilia is a recessive, blood-clotting disorder (do not make the protein clotting factor VIII)
This type of hemophilia is sex-linked

51. Sex-linked Genetic Disorder

52. Sickle-cell Trait: Recessive Trait
Affected individuals are homozygous recessive and carry two copies of mutated gene that produces a defective version of hemoglobin
the hemoglobin sticks together and forms rod-like structures that produce a stiff red blood cell with a sickle shape
the cells cannot move through the blood vessels easily and tend to clot
incomplete dominance occurs, affected individual makes 50% of defective gene, not enough to cause disease, but resistant to malaria, prevalent in African-decent

53. Sickle-cell disease

54. The sickle-cell allele confers resistance to malaria

55. Huntington’s Disease: Dominant Trait
it causes progressive deterioration of brain cells
every individual who carries the allele expresses the disorder but most persons do not know they are affected until they are more than 30 years old

56. Huntington’s disease is a dominant genetic disorder

57. Genetic Screening
Genetic screening can allow prenatal diagnosis of high-risk pregnancies
amniocentesis is when amniotic fluid is sampled and isolated fetal cells are then grown in culture and analyzed
1 in 600 tests may result in miscarriage
chorionic villus sampling is when fetal cells from the chorion in the placenta are removed for analysis
1 in 100 tests may result in miscarriage

58. Amniocentesis

59. Inquiry & Analysis
Is woolly hair sex-linked or autosomal? Is woolly hair dominant or recessive?
Why Woolly Hair Runs in Families