1. Mendel and the Gene Idea
Chapter 14
Mendel and the Gene Idea

2. 14.1 Blending vs. particulate ideas
Anecdotally many thought children inherited “_______” of their parents
Mendel showed that some traits are _____

3. Vocab
Character- varieties with distinct heritable features (such as flower color)
Trait- character variants (such as purple or white flowers)
**sometimes used interchangeably**

4. Why Peas? .

5. Fig. 14-2a
TECHNIQUE 1 2
Parental
generation
(P)
Stamens
Carpel 3 4

6. Mendel chose to track only those characters that varied in an either-or manner
What does that mean?
He also used varieties that were true-breeding

7. Vocab!!
In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a process called hybridization

8. EXPERIMENT P Generation (true-breeding parents) Purple flowers White
Fig
EXPERIMENT
P Generation
(true-breeding
parents) 
Purple
flowers
White
flowers
F1 Generation
(hybrids)
All plants had
purple flowers
F2 Generation
705 purple-flowered
plants
224 white-flowered
plants

9. What Mendel called a “heritable factor” is what we now call a gene
Mendel observed the same pattern of inheritance in six other pea plant characters, each represented by two traits

10. Reflect What did Mendel discover?
What’s the typical ratio for F2 generation?

11. Law of Segregation
1. Alternative versions of genes account for variations in inherited characters
Ex.

12. 2 Organisms inherits two alleles for every gene, one from each parent
Those two alleles:
May be

13. 3
If the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearance
In the flower-color example, the F1 plants had purple flowers because the allele for that trait is dominant

14. 4. Law of Segregation
The two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes
Thus, an egg or a sperm gets only one of the two alleles that are present in the somatic cells of an organism
Meiosis anyone??

15. Vocab Alleles- alternative versions of a gene
Locus- location on a specific chromosome Each gene resides at a specific locus
Dominant
Recessive

16. Allele for purple flowers
Fig. 14-4
Allele for purple flowers
Homologous
pair of
chromosomes
Locus for flower-color gene
Allele for white flowers

17. Punnett square- a diagram for predicting the results of a genetic cross between individuals of known genetic makeup

18. homozygous – having two _______alleles for a gene
heterozygous having two ________alleles for a gene
Not ___________

19. Genotype v Phenotype
Because of the different effects of dominant and recessive alleles, an organism’s traits do not always reveal its genetic composition
Genotype- genetic makeup .
Phenotype- physical appearance/expression of the gene

20. Phenotype Genotype PP Purple 1 (homozygous) 3 Purple Pp (heterozygous)
Fig. 14-6
Phenotype
Genotype PP
Purple 1
(homozygous) 3
Purple Pp
(heterozygous) 2
Purple Pp
(heterozygous) pp 1
White 1
(homozygous)
Ratio 3:1
Ratio 1:2:1

21. Testcross
How can we tell the genotype of an individual with the dominant phenotype?
testcross:

22. TECHNIQUE RESULTS Dominant phenotype, unknown genotype: PP or Pp?
Fig. 14-7
TECHNIQUE 
Dominant phenotype,
unknown genotype:
PP or Pp?
Recessive phenotype,
known genotype: pp
Predictions
If PP
If Pp or
Sperm
Sperm p p p p P P Pp Pp Pp Pp
Eggs
Eggs P p Pp Pp pp pp
RESULTS or
All offspring purple
1/2 offspring purple and
1/2 offspring white

23. Monohybrid
A cross between heterozygotes is called a monohybrid cross

24. Dihybrid dihybrids - heterozygous for two genes
Mendel used dihybrid crosses to figure out the law of independent assortment

25. EXPERIMENT RESULTS Fig. 14-8 P Generation F1 Generation Hypothesis of
YYRR
yyrr
Gametes YR  yr
F1 Generation
YyRr
Hypothesis of
dependent
assortment
Hypothesis of
independent
assortment
Predictions
Sperm or
Predicted
offspring of
F2 generation
1/4 YR
1/4 Yr
1/4 yR
1/4 yr
Sperm
1/2 YR
1/2 yr
1/4 YR
YYRR
YYRr
YyRR
YyRr
1/2 YR
YYRR
YyRr
1/4 Yr
Eggs
YYRr
YYrr
YyRr
Yyrr
Eggs
1/2 yr
YyRr
yyrr
1/4 yR
YyRR
YyRr
yyRR
yyRr
3/4
1/4
1/4 yr
Phenotypic ratio 3:1
YyRr
Yyrr
yyRr
yyrr
9/16
3/16
3/16
1/16
Phenotypic ratio 9:3:3:1
RESULTS
315
108
101 32
Phenotypic ratio approximately 9:3:3:1

26. Law of independent assortment
Each pair of alleles segregates independently of each other pair of alleles during gamete formation
Ex.
This law applies only to genes on different, nonhomologous chromosomes

27. 14.2
When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss
In the same way, the alleles of one gene segregate into gametes independently of another gene’s alleles

28. The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities

29. 1/2 1/2 1/2 1/4 1/4 1/2 1/4 1/4 Rr Rr  Segregation of Segregation of
Fig. 14-9 Rr Rr 
Segregation of
alleles into eggs
Segregation of
alleles into sperm
Sperm
1/2 R
1/2 r R R
1/2 R R r
1/4
1/4
Eggs r r
1/2 R r r
1/4
1/4

30. The rule of addition states that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities

31. The rule of addition can be used to figure out the probability that an F2 plant from a monohybrid cross will be heterozygous rather than homozygous

32. Fig. 14-UN1

33. 14.3 Inheritance patterns are often more complex than predicted by simple Mendelian genetics
Many heritable characters are not determined by only one gene with two alleles
However, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance

34. Non-Mendelian situations include
When alleles are not ______________
When a gene has more than ___________
When a gene produces _________________

35. Degrees of Dominance
Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical
Incomplete dominance, the phenotype of F1 hybrids
Codominance-two dominant alleles affect the phenotype ________________________

36. P Generation Red White CRCR CWCW Gametes CR CW Pink F1 Generation CRCW
Fig
P Generation
Red
White
CRCR
CWCW
Gametes CR CW
Pink
F1 Generation
CRCW
Gametes
1/2 CR
1/2 CW
Sperm
1/2 CR
1/2 CW
F2 Generation
1/2 CR
CRCR
CRCW
Eggs
1/2 CW
CRCW
CWCW

37. Tay-Sachs
Tay-Sachs disease is fatal; a dysfunctional enzyme causes an accumulation of lipids in the brain
At the organismal level, the allele is recessive
At the biochemical level, the phenotype (i.e., the enzyme activity level) is incompletely dominant
At the molecular level, the alleles are codominant

38. Dominance is NOT PREdominance!
Dominant alleles are not necessarily more common in populations than recessive alleles
For example, one baby out of 400 in the United States is born with extra fingers or toes

39. Multiple Alleles
Most genes exist in populations in more than two allelic forms
For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: IA, IB, and i.
The enzyme encoded by the IA allele adds the A carbohydrate, whereas the enzyme encoded by the IB allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither

40. (a) The three alleles for the ABO blood groups
Fig
Allele
Carbohydrate IA A IB B i
none
(a) The three alleles for the ABO blood groups
and their associated carbohydrates
Red blood cell
appearance
Phenotype
(blood group)
Genotype
IAIA or IA i A
IBIB or IB i B
IAIB AB ii O
(b) Blood group genotypes and phenotypes

41. Pleiotropy pleiotropy – genes that have multiple phenotypic effects
Ex. cystic fibrosis and sickle-cell disease have MULTIPLE SYMPTOMS

42. Epistasis
Epistasis-a gene at one locus alters the phenotypic expression of a gene at a second locus
Ex.

43. 1/4 1/4 1/4 1/4 1/4 1/4 1/4 1/4  BbCc BbCc Sperm Eggs BBCC BbCC BBCc
Fig 
BbCc
BbCc
Sperm
1/4
1/4
1/4
1/4 BC bC Bc bc
Eggs
1/4 BC
BBCC
BbCC
BBCc
BbCc
1/4 bC
BbCC
bbCC
BbCc
bbCc
1/4 Bc
BBCc
BbCc
BBcc
Bbcc
1/4 bc
BbCc
bbCc
Bbcc
bbcc 9
: 3
: 4

44. Polygenic Inheritance
Quantitative characters are those that vary in the population along a continuum
Indicate polygenic inheritance (an additive effect) of two or more genes on a single phenotype
Ex.

45. Fig 
AaBbCc
AaBbCc
Sperm
1/8
1/8
1/8
1/8
1/8
1/8
1/8
1/8
1/8
1/8
1/8
1/8
Eggs
1/8
1/8
1/8
1/8
Phenotypes:
1/64
6/64
15/64
20/64
15/64
6/64
1/64
Number of
dark-skin alleles: 1 2 3 4 5 6

46. Nature and Nurture: The Environmental Impact on Phenotype
For example, hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity
Sea Turtles?
Obesity/Heart Disease/ Diabetes

47. 14.4 Human Mendelian Traits
Humans are not good subjects for genetic research .
However, basic Mendelian genetics endures as the foundation of human genetics

48. Pedigree
A family tree that describes the interrelationships of parents and children across generations

49. (a) Is a widow’s peak a dominant or recessive trait?
Fig b
1st generation
(grandparents) Ww ww ww Ww
2nd generation
(parents, aunts,
and uncles) Ww ww ww Ww Ww ww
3rd generation
(two sisters) WW ww or Ww
Widow’s peak
No widow’s peak
(a) Is a widow’s peak a dominant or recessive trait?

50. (b) Is an attached earlobe a dominant or recessive trait?
Fig c
1st generation
(grandparents) Ff Ff ff Ff
2nd generation
(parents, aunts,
and uncles) FF or Ff ff ff Ff Ff ff
3rd generation
(two sisters) ff FF or Ff
Attached earlobe
Free earlobe
(b) Is an attached earlobe a dominant or recessive trait?

51. Recessively Inherited Disorders
Recessively inherited disorders show up only in individuals homozygous for the allele
Carriers -

52. Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair

53. Science and Society
Consanguineous matings (i.e., matings between close relatives) increase the chance of mating between two carriers of the same rare allele
Most societies and cultures have laws or taboos against marriages between close relatives

54. Cystic Fibrosis
The most common lethal genetic disease in the United States
The allele results in defective or absent chloride transport channels in plasma membranes
Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine

55. Sickle-Cell Disease
The disease is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells
Symptoms include physical weakness, pain, organ damage, and even paralysis

56. Dominantly Inherited Disorders
Some human disorders are caused by dominant alleles
Achondroplasia is a form of dwarfism caused by a rare dominant allele

57. Huntington’s A degenerative disease of the nervous system
The disease has no obvious phenotypic effects until the individual is about 35 to 40 years of age

58. Multifactorial Disorders
Many diseases, such as heart disease and cancer, have both genetic and environmental components
Little is understood about the genetic contribution to most multifactorial diseases

59. Genetic Testing and Counseling
Genetic counselors can provide information to prospective parents concerned about a family history for a specific disease
Use probability, Mendel to give families probability of having certain traits in an offspring

60. Fetal Testing
In amniocentesis, the liquid that bathes the fetus is removed and tested
In chorionic villus sampling (CVS), a sample of the placenta is removed and tested
Other techniques, such as ultrasound and fetoscopy, allow fetal health to be assessed visually in utero

61. Newborn Screening
Some genetic disorders can be detected at birth by simple tests that are now routinely performed in most hospitals in the United States
Do you remember PKU?