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

2. Overview: Drawing from the Deck of Genes
The “blending” hypothesis
genetic material from the two parents blends together
The “particulate” hypothesis
parents pass on discrete heritable units
Traits reappear after several generations
Mendel used experiments with garden peas

3. Figure 14.1
Figure 14.1 What principles of inheritance did Gregor Mendel discover by breeding garden pea plants?

4. Concept 14.1: Mendel identified two laws of inheritance
Discovered the basic principles of heredity by breeding garden peas

5. Mendel’s Experimental, Quantitative Approach
Advantages of pea plants for genetic study
There are many varieties of heritable features(characters)
Character variants (Traits)
Controlled Mating
Individual sex organs
Cross-pollination

6. Parental generation (P)
Figure 14.2
TECHNIQUE 1 2
Parental generation (P)
Stamens 3
Carpel 4
Figure 14.2 Research Method: Crossing Pea Plants
RESULTS 5
First filial generation offspring (F1)

7. What Mendel Used
Mendel chose to track only those characters that occurred in two distinct alternative forms
True-breeding (plants that produce offspring of the same variety when they self-pollinate)

8. Terms of the Experiment
Hybridization: mating two contrasting, true-breeding varieties
P generation: true-breeding parents
F1 generation: hybrid offspring of the P generation
F2 generation: F1 individuals self-pollinate or cross- pollinate with other F1 hybrids

9. The Law of Segregation
Crossing contrasting true-breeding white and purple pea plants: all of the F1 hybrids were purple
Crossing the F1 hybrids: many of the F2 plants had purple flowers, but some had white
Three to one ratio purple to white flowers in the F2 generation

10. (true-breeding parents) Purple flowers White flowers
Figure
EXPERIMENT
P Generation
(true-breeding parents)
Purple flowers
White flowers
F1 Generation (hybrids)
All plants had purple flowers
Self- or cross-pollination
Figure 14.3 Inquiry: When F1 hybrid pea plants self- or cross-pollinate, which traits appear in the F2 generation?
F2 Generation
705 purple- flowered plants
224 white flowered plants

11. Ideas that stemmed from observations
Only the purple flower factor was affecting flower color in the F1 hybrids
purple flower color: dominant trait
white flower color: recessive trait
Six other pea plant characters had same patterns
“Heritable factor”: we now call a gene

12. Table 14.1 The Results of Mendel’s F1 Crosses for Seven Characters in Pea Plants

13. Mendel’s Model
Alternative versions of genes: account for variations in inherited characters
Alternative versions of a gene are now called alleles
Each gene resides at a specific locus

14. Allele for purple flowers
Figure 14.4
Allele for purple flowers
Locus for flower-color gene
Allele for white flowers
Pair of homologous chromosomes
Figure 14.4 Alleles, alternative versions of a gene.

15. Mendel’s Model Organisms inherits two alleles, one from each parent
Dominant alleles determine the organism’s appearance
Recessive alleles have no noticeable effect on appearance
Law of segregation: two alleles for a heritable character separate during gamete formation

16. Punnett Square
Diagram for predicting the results of a genetic cross between individuals of known genetic makeup
A capital letter represents a dominant allele
A lowercase letter represents a recessive allele

17. P Generation Appearance: Purple flowers White flowers Genetic makeup:
Figure
P Generation
Appearance:
Purple flowers
White flowers
Genetic makeup: PP pp
Gametes: P p
F1 Generation
Appearance:
Purple flowers
Genetic makeup: Pp
Gametes:
1/2 P
1/2 p
Sperm from F1 (Pp) plant
F2 Generation P p
Figure 14.5 Mendel’s law of segregation. P
Eggs from F1 (Pp) plant PP Pp p Pp pp 3
: 1

18. Useful Genetic Vocabulary
Homozygous: An organism with two identical alleles for a character
Heterozygous: An organism that has two different alleles for a gene
Unlike homozygotes, heterozygotes are not true-breeding
Phenotype: physical appearance
Genotype: genetic makeup

19. PP (homozygous) Pp (heterozygous) Pp (heterozygous) pp (homozygous)
Figure 14.6
Phenotype
Genotype
Purple
PP (homozygous) 1 3
Pp (heterozygous)
Purple 2
Pp (heterozygous)
Purple
Figure 14.6 Phenotype versus genotype.
pp (homozygous) 1
White 1
Ratio 3:1
Ratio 1:2:1

20. The Testcross
How can we tell the genotype of an individual with the dominant phenotype?
Could be either homozygous dominant or heterozygous

21. Dominant phenotype, unknown genotype: PP or Pp?
Figure 14.7
TECHNIQUE
Dominant phenotype, unknown genotype: PP or Pp?
Recessive phenotype, known genotype: pp
Predictions
If purple-flowered parent is PP or
If purple-flowered parent is Pp
Sperm
Sperm p p p p P P Pp Pp Pp Pp
Eggs
Eggs
Figure 14.7 Research Method: The Testcross P p Pp Pp pp pp
RESULTS or
All offspring purple
1/2 offspring purple and 1/2 offspring white

22. The Law of Independent Assortment
Followed a single character
Monohybrids: individuals that are heterozygous for one character
A cross between such heterozygotes is called a monohybrid cross
Dihybrids: Two true-breeding parents differing in two characters produces
A dihybrid cross, a cross between F1 dihybrids

23. Hypothesis of dependent assortment
Figure 14.8
EXPERIMENT
YYRR
P Generation
yyrr
Gametes YR yr
F1 Generation
YyRr
Predictions
Hypothesis of dependent assortment
Hypothesis of independent assortment
Sperm
Predicted offspring of F2 generation or
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
Figure 14.8 Inquiry: Do the alleles for one character assort into gametes dependently or independently of the alleles for a different character?
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

24. Law of independent assortment
Pairs of alleles segregate independently of each other pair of alleles during gamete formation

25. Segregation of alleles into eggs Segregation of alleles into sperm
Figure 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
Figure 14.9 Segregation of alleles and fertilization as chance events.
1/4
1/4
Eggs r r R r
1/2 r
1/4
1/4

26. The Multiplication and Addition Rules Applied to Monohybrid Crosses
probability that two or more independent events will occur together is the product of their individual probabilities
Each gamete has a ½ chance of carrying the dominant allele and a ½ chance of carrying the recessive allele

27. The addition rule
Probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities
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

28. 1/4 (probability of pp)  1/2 (yy)  1/2 (Rr)  1/16 ppYyrr
Figure 14.UN01
Probability of YYRR 
1/4 (probability of YY) 
1/4 (RR) 
1/16
Probability of YyRR 
1/2 (Yy) 
1/4 (RR) 
1/8
ppyyRr
1/4 (probability of pp)  1/2 (yy)  1/2 (Rr)
 1/16
ppYyrr
1/4  1/2  1/2
 1/16
Ppyyrr
1/2  1/2  1/2
 2/16
PPyyrr
1/4  1/2  1/2
 1/16
Figure 14.UN01 In-text figure, p. 270
ppyyrr
1/4  1/2  1/2
 1/16
Chance of at least two recessive traits
 6/16 or 3/8

29. Extending Mendelian Genetics for a Single Gene
Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations:
When alleles are not completely dominant or recessive
When a gene has more than two alleles
When a gene produces multiple phenotypes

30. Degrees of Dominance
Complete dominance heterozygote and dominant homozygote phenotypes are identical
Incomplete dominance F1 hybrids are somewhere between the phenotypes of the two parental varieties
Codominance two dominant alleles affect the phenotype in separate, distinguishable ways

31. P Generation Red White CRCR CWCW Gametes CR CW F1 Generation Pink CRCW
Figure
P Generation
Red
White
CRCR
CWCW
Gametes CR CW
F1 Generation
Pink
CRCW
Gametes
1/2 CR
1/2 CW
Sperm
Figure Incomplete dominance in snapdragon color.
F2 Generation
1/2 CR
1/2 CW
1/2 CR
CRCR
CRCW
Eggs
1/2 CW
CRCW
CWCW

32. Dominance and Phenotype
A dominant allele does not subdue a recessive allele; alleles don’t interact that way
Alleles are simply variations in a gene’s nucleotide sequence
Frequency of Dominant Alleles
Dominant alleles are not necessarily more common
1/400 babies in the US is born with extra fingers or toes

33. Multiple Alleles
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.

34. (a) The three alleles for the ABO blood groups and their carbohydrates
Figure 14.11
(a) The three alleles for the ABO blood groups and their carbohydrates
Allele IA IB i
Carbohydrate A B
none
(b) Blood group genotypes and phenotypes
Genotype
IAIA or IAi
IBIB or IBi
IAIB ii
Figure Multiple alleles for the ABO blood groups.
Red blood cell appearance
Phenotype (blood group) A B AB O

35. Pleiotropy
Most genes have multiple phenotypic effects, a property called pleiotropy
For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease

36. Epistasis
A gene at one locus alters the phenotypic expression of a gene at a second locus
Coat color of certain animals depend on two genes
One gene determines the pigment color (with alleles B for black and b for brown)
The other gene (with alleles C for color and c for no color) determines whether the pigment will be deposited in the hair

37. BbEe BbEe Sperm 1/4 1/4 1/4 1/4 Eggs 1/4 1/4 1/4 1/4 9 : 3 : 4 BE bE
Figure 14.12
BbEe
BbEe
Sperm
1/4
1/4 BE
1/4
1/4 bE Be be
Eggs
1/4 BE
BBEE
BbEE
BBEe
BbEe
1/4 bE
BbEE
bbEE
BbEe
bbEe
1/4 Be
BBEe
BbEe
BBee
Bbee
Figure An example of epistasis.
1/4 be
BbEe
bbEe
Bbee
bbee 9
: 3
: 4

38. Polygenic Inheritance
Quantitative characters: vary in the population along a continuum
Quantitative variation usually indicates polygenic inheritance
an additive effect of two or more genes on a single phenotype
Skin color in humans is an example of polygenic inheritance

39. Number of dark-skin alleles: 1 2 3 4 5 6
Figure 14.13
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
Figure A simplified model for polygenic inheritance of skin color.
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

40. The effect of environment on phenotype
Figure The effect of environment on phenotype.

41. Integrating a Mendelian View of Heredity and Variation
An organism’s phenotype includes its physical appearance, internal anatomy, physiology, and behavior
An organism’s phenotype reflects its overall genotype and unique environmental history

42. Pedigree Analysis
A pedigree is a family tree that describes the interrelationships of parents and children across generations
Inheritance patterns of particular traits can be traced and described using pedigrees

43. Is a widow’s peak a dominant or recessive trait? b)
Figure 14.15
Key
Male
Female
Affected male
Affected female
Mating
Offspring
1st generation Ff Ff ff Ff
1st generation Ww ww ww Ww
2nd generation
2nd generation
FF or Ff ff ff Ff Ff ff Ww ww ww Ww Ww ww
3rd generation
3rd generation ff
FF or Ff
WW or Ww ww
Figure Pedigree analysis.
Widow’s peak
No widow’s peak
Attached earlobe
Free earlobe
(a)
Is a widow’s peak a dominant or recessive trait? b)
Is an attached earlobe a dominant or recessive trait?

44. Pedigrees Can also be used to make predictions about future offspring
Can use the multiplication and addition rules to predict the probability of specific phenotypes

45. Recessively Inherited Disorders
Many genetic disorders are inherited in a recessive manner
These range from relatively mild to life-threatening
Recessively inherited disorders show up only in individuals homozygous for the allele
Carriers are heterozygous individuals who carry the recessive allele

46. Parents Normal Aa Normal Aa Sperm A a Eggs Aa Normal (carrier) AA
Figure 14.16
Parents
Normal Aa
Normal Aa
Sperm A a
Eggs Aa
Normal (carrier) AA
Normal A
Figure Albinism: a recessive trait. Aa
Normal (carrier) aa
Albino a

47. Dominantly Inherited Disorders
Dominant alleles that cause a lethal disease are rare and arise by mutation
Achondroplasia is a form of dwarfism caused by a rare dominant allele

48. Parents Dwarf Dd Normal dd Sperm D d Eggs Dd Dwarf dd Normal d Dd
Figure 14.17
Parents
Dwarf Dd
Normal dd
Sperm D d
Eggs Dd
Dwarf dd
Normal d
Figure Achondroplasia: a dominant trait. Dd
Dwarf dd
Normal d

49. Multifactorial Disorders
Heart disease, diabetes, alcoholism, mental illnesses, and cancer have both genetic and environmental components

50. 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

51. Biochemical and genetic tests
Figure 14.19
(a) Amniocentesis
(b) Chorionic villus sampling (CVS) 1
Ultrasound monitor
Ultrasound monitor
Amniotic fluid withdrawn
Fetus 1
Placenta
Fetus
Suction tube inserted through cervix
Chorionic villi
Placenta
Cervix
Uterus
Cervix
Uterus
Centrifugation
Fluid
Several hours
Several hours
Biochemical and genetic tests 2
Fetal cells
Fetal cells
Several weeks
Figure Testing a fetus for genetic disorders. 2
Several weeks
Several hours 3
Karyotyping

52. 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