1. MENDEL AND THE GENE IDEA
CHAPTER 14

2. Heredity Genetics is the branch of biology that studies heredity
characteristics that are inherited are traits
What is heredity?
The passing on of characteristics from parents to offspring
Mendel was the first to succeed in predicting how traits would be transferred from one generation to the next

3. Gregor Mendel Austrian monk
Brought experimental and quantitative approach to genetics
Bred pea plants to study inhertance
Why peas?
Control mating (self- vs. cross- pollination)
Many varieties available
Short generation time

4. P (parental) generation = true breeding plants
F1 (first filial) generation = offspring
F2 (second filial) generation = F1 offspring

5. 7 characters in pea plants
Dominant vs. Recessive (expressed) or (hidden)

6. MENDEL’S 4 PRINCIPLES
Inheritance of traits are determined by "units" or "factors" that are passed on to offspring (called genes)
Different forms of a gene are alleles
For each character, every organism inherits one allele from each parent.
If 2 alleles are different, the dominant allele will be fully expressed; the recessive allele will have no noticeable effect on offspring’s appearance.
Law of Segregation: the 2 alleles for each character separate during gamete formation.

7. Alleles: alternate versions of a gene

8. Law of Segregation
The two alleles separate when gametes are formed

9. Dominant and Recessive Genes
Gene that prevents the other gene from “showing” – dominant
Gene that does NOT “show” even though it is present – recessive
Symbol – Dominant gene – upper case letter – T
Recessive gene – lower case letter – t
(Always use the same letter for the same alleles— No S = straight, h = hitchhiker’s)
* Must have 2 recessive alleles for a recessive trait to “show”

10. dominant (P), recessive (p)
homozygous (purebred) = 2 same alleles (PP or pp)
heterozygous (hybrid) = 2 different alleles (Pp)
Dominant trait observed

11. Phenotype: expressed physical traits, observable characteristics
Genotype: genetic make-up

12. Ratios Genotypes: Phenotypes:
Homozygous dominant:Hetergozygous:Homozygous recessive
AA:Aa:aa
Phenotypes:
Dominant:recessive
Tall:short

13. Punnett Square Genotypic Ratio: Phenotypic Ratio:
Device for predicting offspring from a cross
Example: Pp x Pp (P=purple, p=white)
Genotypic Ratio:
Phenotypic Ratio:

14. Practice
Label the following genotypes as homozygous or heterozygous. AA Bb Ff gg Yellow seeds are dominant over green seeds. What would the phenotypes of the following genotypes look like? Gg gg GG

15. Yellow seeds are dominant over green seeds
Yellow seeds are dominant over green seeds. Cross a homozygous green seed with a homozygous yellow seed. What are the genotypic ratios? What are the phenotypic ratios?

16. Yellow seeds are dominant over green seeds
Yellow seeds are dominant over green seeds. Cross two heterozygous seeds. What are the genotypic ratios? What are the phenotypic ratios?

17. Testcross: used to determine if dominant trait is unknown (homozygous or heterozygous?) by crossing with recessive (pp)

18. Law of Independent Assortment:
Each pair of alleles segregates (separates) independently during gamete formation
If they are on separate chromosomes
Eg. color is separate from shape

19. Monohybrid Cross: study 1 character
eg. flower color
Dihybrid Cross: study 2 characters
eg. flower color & seed shape

20. Dihybrid Cross
Example: AaBb x AaBb

21. The laws of probability govern Mendelian inheritance
Multiplication Rule:
Probability that 2+ independent events will occur together in a specific combination  multiply probabilities of each event
Ex. 1: probability of throwing 2 sixes
1/6 x 1/6 = 1/36
Ex. 2: probability of having 5 boys in a row
½ x ½ x ½ x ½ x ½ = 1/32
Ex. 3: If cross AABbCc x AaBbCc, probability of offspring with AaBbcc is:
Answer: ½ x ½ x ¼ = 1/16

22. The laws of probability govern Mendelian inheritance
Addition Rule:
Probability that 2+ mutually exclusive events will occur  add together individual probabilities
Ex. 1: chances of throwing a die that will land on 4 or 5?
1/6 + 1/6 = 1/3
Chance of getting at least two recessive traits in the following cross?
PpYyRr x PpYyRr

23. Segregation of alleles and fertilization as chance events

24. Extending Mendelian Genetics
The relationship between genotype and phenotype is rarely simple
Complete Dominance: heterozygote and homozygote for dominant allele are indistinguishable
Eg. YY or Yy = yellow seed

25. Purebred red snapdragons were crossed with purebred white snapdragons all offspring were pink

26. Eg. red x white = pink flowers
Incomplete Dominance: F1 hybrids have appearance that is between that of 2 parents
Combined (blended) phenotype
one allele for a trait is not completely dominant over the other allele
Eg. red x white = pink flowers
R = allele for red flowers W = allele for white flowers
red x white ---> pink RR x WW ---> 100% RW

27. Codominance: phenotype of both alleles is expressed together in the heterozygous individual
Eg. red hair x white hairs = roan horses

28. In humans, straight hair and curly hair are incompletely dominant traits that result in hybrids that have wavy hair.
Cross a straight hair with a wavy hair.
What are the chances of having a curly haired child?
What are the chances of having a straight hair child?

29. Increases possible number of genotypes & phenotypes
Multiple Alleles: gene has 2+ alleles
Eg. human ABO blood groups
Increases possible number of genotypes & phenotypes
Alleles = IA, IB, i
IA,IB = Codominant

30. Blood Typing Phenotype (Blood Group) Genotype(s) Type A IAIA or IAi
IBIB or IBi
Type AB
IAIB
Type O ii

31. Blood Transfusions Blood transfusions must match blood type
Mixing of foreign blood  clumping  death
Rh factor: protein found on RBC’s (Rh+ = has protein, Rh- = no protein)

32. Practice Problem #1:
A man who is heterozygous with type A blood marries a woman who is homozygous with type B blood. What possible blood types might their children have?
A woman with Type O blood and a man who is Type AB have are expecting a child.  What are the possible blood types of the kid?
What are the possible blood types of a child who's parents are both heterozygous for "B" blood type?

33. Babies Jane (blood type B), John (blood type O), and Joe (blood type AB) were mixed up in the hospital. Who are their parents?
Couple #1: A, A
Couple #2: A,B
Couple #3: B,O

34. Chromosome Theory of Inheritance
Genes have specific loci (positions) along chromosomes
Chromosomes undergo segregation and independent assortment
Chromosomes tagged to reveal a specific gene (yellow).

35. Chromosomal basis of Mendel’s laws

36. Thomas Hunt Morgan Drosophila melanogaster – fruit fly
Fast breeding, 4 prs. chromosomes (XX/XY)
Sex-linked gene: located on X or Y chromosome
Red-eyes = wild-type; white-eyes = mutant
Specific gene carried on specific chromosome

37. Morgan’s Conclusions
Gene for eye color is carried on the X chromosome = eye color is an X- linked trait
Y chromosome does not carry a gene for eye color
Red-eyed = XRXR, XRXr , XRY
White-eyed = XrXr, XrY

38. Sex Determination
Observed that one pair of chromosomes was different between males and females
Large one named “X” chromosome
Smaller one named “Y” chromosome
XX = female; XY = male
Fathers pass X-linked genes to daughters, but not sons
Males express recessive trait on the single X (hemizygous)
Females can be affected or carrier

39. Sex Linkage
Sex Linkage: the presence of a gene on a sex chromosome (X or Y)
X-linked genes: genes found on the X chromosome
X chromosome carries more genes
Y-linked genes: genes found on the Y chromosome

40. In humans colorblindness (b) is an example of a sex-linked recessive trait. A male without colorblindness marries a female who isn’t colorblind but carries the allele.
How many females will be colorblind?
What sex will any colorblind children be?
What percent will be male and colorblind?

41. In fruit flies red eye color (R) is dominant to white eyes (r) and is a sex linked trait.
A heterozygous red eye female mates with a red eye male.
How many will have red eyes?
What percent will have white eyes?
How many will be female and red eyed?

42. In fruit flies red eye color (R) is dominant to white eyes (r) and is a sex linked trait. A homozygous red eye female mates with a white eye male. How many males will have white eyes?

43. Epistasis
Epistasis – one allele hides/suppresses another allele

44. Polygenic Inheritance: the effect of 2 or more genes acting upon a single phenotypic character (eg. skin color, height)

45. Another example of a polygenic trait:
Hair Color
Hair color is controlled by alleles on chromosomes 3, 6, 10, and 18
more dominant alleles = the darker the hair!

46. Nature and Nurture: both genetic and environmental factors influence phenotype
Hydrangea flowers vary in shade and intensity of color depending on acidity and aluminum content of the soil.

47. Environmental Factors
Inherit tendency of getting heart diseases
Diet/exercise can also influence
can have genes to be tall, but without proper nutrition, they will not reach their optimal height
Water/ temperature/ sunlight effect plants

48. Mendelian Inheritance in Humans
Pedigree: diagram that shows the relationship between parents/offspring across 2+ generations
Woman =
Man =
Trait expressed:

49. Pedigree Analysis: Widow’s Peak Trait

50. Pedigree Analysis: PTC Tasting

51. Practice Problem
The pedigree below traces the inheritance of alkaptonuria, a biochemical disorder. Affected individuals are shaded. Does alkaptonuria appear to be caused by a dominant or recessive allele?

52. Genetic Disorders Major types of genetic disorders: Autosomal
Single genes
Multiple genes
Sex-linked
Chromosome abnormalities

53. Levels of Genetic Disorders
Level 1 Single gene
mutation affecting a single gene
Level 2 Chromosomes
entire chromosomes, or large segments of them, are missing, duplicated, or otherwise altered
Level 3 Multifactor
mutations in multiple genes, often coupled with environmental causes

55. Genetic Disorders Autosomal Recessive Autosomal Dominant Albinism
Cystic fibrosis (CF)
Tay-Sachs disease
Sickle-cell disease
Phenylketonuria (PKU)
Achondroplasia
Huntington’s disease (HD)
Lethal dominant allele

56. Multifactorial Disorders
Heart disease
Diabetes
Cancer
Alcoholism
Mental illnesses (schizophrenia, bipolar disorder)

57. Chromosome Abnormalities
Autosomal and sex-linked genetic disorders are both caused by certain alleles
Other genetic disorders result from chromosome abnormalities caused by mistakes made during meiosis.
May change the number or structure of chromosomes within gametes

58. Chromosome Abnormalities
Translocation is when a piece of one chromosome breaks off and attaches to a different chromosome
Often happens to 2 chromosomes at once
Deletion – chromosome fragment is lost
Duplication – extra segment of DNA
Inversion – reverse orientation of DNA

59. Chromosomal Mutations

60. Diagnosing Genetic Disorders
There are several ways to determine whether a child will have a genetic disorder
Two main ways to diagnose:
Analysis of fetal cells
Amniocentesis
Chorionic villus biopsy
Imaging techniques
Ultrasonography (computerized image)
Fetoscopy (direct observation)

61. How is genetic testing done?
blood, hair, skin, amniotic fluid, or other tissue
Heel prick on newborns
Look for changes in chromosomes, DNA, proteins

62. Amniocentesis
a procedure a pregnant woman can have in order to detect some genetics disorders

63. Why we find disorders? Genetic tests use blood and other tissue
Doctors use genetic tests for:
Find possible genetic diseases in unborn babies
Find out if people carry a gene
Screening embryos for disease
Test for genetic diseases before symptoms occur
Confirming a diagnosis

64. Genetic Counseling

65. Practice Problems
Cystic Fibrosis is an autosomal recessive disorder. What are the chances that 2 carriers for this disease will have a child with CF?
Huntington’s Disease is an autosomal dominant disorder. If a woman with this disease marries a normal man, what are the chances that their children will have the disease?