1. Mendel and the Gene Idea
CHAPTER 14

2. What genetic principles account for the transmission of traits from parents to offspring?

3. One possible explanation of heredity is a “blending” hypothesis
The idea that genetic material contributed by two parents mixes in a manner analogous to the way blue and yellow paints blend to make green

4. An alternative to the blending model is the “particulate” hypothesis of inheritance: the gene idea
Parents pass on discrete heritable units, genes

5. Gregor Mendel
Documented a particulate mechanism of inheritance through his experiments with garden peas
DNA wasn’t known yet

6. Mendel used the scientific approach to identify two laws of inheritance
Mendel discovered the basic principles of heredity By breeding garden peas in carefully planned experiments

7. Mendel’s Experimental, Quantitative Approach
Mendel chose to work with peas because
They are available in many varieties
He could strictly control which plants mated with which
Self pollinating (able to start with pure plants)
Lots of offspring

8. CROSSING PEA PLANTS Removed stamens from purple flower1 5 4 3 2
Removed stamens
from purple flower
Transferred sperm-
bearing pollen from
stamens of white
flower to egg-
bearing carpel of
purple flower
Parental
generation
(P)
Pollinated carpel
matured into pod
Carpel
(female)
Stamens
(male)
Planted seeds
from pod
Examined
offspring:
all purple
flowers
First
offspring
(F1)
APPLICATION By crossing (mating) two true-breeding
varieties of an organism, scientists can study patterns of
inheritance. In this example, Mendel crossed pea plants
that varied in flower color.
TECHNIQUE
When pollen from a white flower fertilizes
eggs of a purple flower, the first-generation hybrids all have purple
flowers. The result is the same for the reciprocal cross, the transfer
of pollen from purple flowers to white flowers.
RESULTS

9. SOME GENETIC VOCABULARY
Trait: a heritable feature, such as flower color
Allele: a variant of a character, such as purple or white flowers

10. More Genetic Vocabulary
An organism that is homozygous for a particular gene
Has a pair of identical alleles for that gene (RR or rr)
Exhibits true-breeding (pure)
An organism that is heterozygous for a particular gene
Has a pair of alleles that are different for that gene (Rr)
Hybrid

11. An organism’s phenotype
Is its physical appearance
An organism’s genotype
Is its genetic makeup

12. Phenotype Versus Genotype3 1 2
Phenotype
Purple
White
Genotype PP
(homozygous) Pp
(heterozygous) pp
Ratio 3:1
Ratio 1:2:1

13. Mendel also made sure that
Mendel chose to track
Only those characters that varied in an “either-or” manner
Mendel also made sure that
He started his experiments with varieties that were “true-breeding”

14. In a typical breeding experiment
Mendel mated two contrasting, true-breeding varieties, a process called hybridization
The true-breeding parents
Are called the P1 generation
Cross RR x rr yielded all Rr (hybrids)

15. The hybrid offspring of the P1 generation
Are called the F1 generation
When F1 individuals self-pollinate
The F2 generation is produced
Hybrid x Hybrid Rr x Rr
Offspring 25% RR, 50% Rr, 25% rr
3:1 phenotypic ratio
1:2:1 genotypic ratio

16. The Law of Segregation Mendel derived the law of segregation
By following a single trait
The F1 offspring produced in this cross
Were monohybrids, heterozygous for one character

17. The Law of Segregation
The two alleles for a heritable trait separate (segregate) during gamete formation and end up in different gametes

18. Alternative Versions Of Genes
Account for variations in inherited characters, which are now called alleles
Capital letter (dominant allele)
Lowercase (recessive)
Allele for purple flowers
Locus for flower-color gene
Homologous
pair of
chromosomes
Allele for white flowers

19. For each character
An organism inherits two alleles, one from each parent
A genetic locus is the location where it is at one a chromosome

20. The Testcross or backcross
Allows us to determine the genotype of an organism with the dominant phenotype, but unknown genotype
Crosses an individual with the dominant phenotype with an individual that is homozygous recessive for a trait
RR x rr or Rr x rr

21. then 1⁄2 offspring purple
THE TESTCROSS 
Dominant phenotype,
unknown genotype:
PP or Pp?
Recessive phenotype,
known genotype: pp
If PP,
then all offspring
purple:
If Pp,
then 1⁄2 offspring purple
and 1⁄2 offspring white: p P Pp
APPLICATION An organism that exhibits a dominant trait,
such as purple flowers in pea plants, can be either homozygous for the dominant allele or heterozygous. To determine the organism’s
genotype, geneticists can perform a testcross.
TECHNIQUE In a testcross, the individual with the
unknown genotype is crossed with a homozygous individual
expressing the recessive trait (white flowers in this example). By observing the phenotypes of the offspring resulting from this cross, we can deduce the genotype of the purple-flowered parent.
RESULTS

22. The Law of Independent Assortment
Mendel derived the law of independent assortment
By following two traits
The F1 offspring produced in this cross
Were dihybrids, heterozygous for both characters

23. Phenotypic ratio approximately 9:3:3:1
A dihybrid cross
Produces four phenotypes in the F2 generation
YYRR
P Generation
Gametes YR yr 
yyrr
YyRr
Hypothesis of
dependent
assortment
independent
F2 Generation
(predicted
offspring)
1⁄2
1 ⁄2
3 ⁄4
1 ⁄4
Sperm
Eggs
Phenotypic ratio 3:1 Yr yR
9 ⁄16
3 ⁄16
1 ⁄16
YYRr
YyRR
Yyrr
YYrr
yyRR
yyRr
Phenotypic ratio 9:3:3:1
315
108
101 32
Phenotypic ratio approximately 9:3:3:1
F1 Generation
RESULTS
CONCLUSION The results support the hypothesis of independent assortment. The alleles for seed color and seed shape sort into gametes independently of each other.
EXPERIMENT Two true-breeding pea plants— one with yellow-round seeds and the other with green-wrinkled seeds—were crossed, producing dihybrid F1 plants. Self-pollination of the F1 dihybrids, which are heterozygous for both characters, produced the F2 generation. The two hypotheses predict different phenotypic ratios. Note that yellow color (Y) and round shape (R) are dominant.

24. Using the information from a dihybrid cross, Mendel developed the law of independent assortment
Each pair of alleles lines up independently during meiosis.

25. Probability Problem
If H horns is dominant to h hornless, and T fancy tail in dragons is dominant to t plain tail, what is the probability that results from a cross of Dragon #1 HHtt x HhTt Dragon #2:
Dragon #1 always contributes H & Dragon #2 contributes H half the time and h the other half SO the probability of having horns (HH x Hh) is equal to 1 and no horns is equal to 0

26. Probability Problem cont.)
Dragon #1 HHtt x HhTt Dragon #2
Dragon #1 always contributes a t while Dragon #2 contributes a T half the time and a t half the time
(tt x Tt)
SO the probability of having a fancy tail is ½ and the probability of having a plain tail is ½

27. Probability Problem cont.)
SO what’s the probability of having a baby dragon with:
Horns and fancy tail?
1 x ½ = ½
Horns and a plain tail?
Hornless and fancy tail?
0 x ½ = 0
Hornless and plain tail?

28. OTHER FACTORS AFFECTING INHERITANCE

29. Inheritance patterns are often more complex than predicted by simple Mendelian genetics
The inheritance of characters by a single gene may deviate from simple Mendelian patterns

30. The Spectrum of Dominance
Complete dominance
Occurs when the phenotypes of the heterozygote and dominant homozygote are identical
Codominance
Two dominant alleles affect the phenotype in separate, distinguishable ways
Both alleles are expressed
Human ABO blood type is an example of codominance

31. The ABO blood group in humans
Is determined by multiple alleles
A and B are dominant to 0

32. Incomplete Dominance
The phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties (blending).
P Generation
F1 Generation
F2 Generation
Red
CRCR
Gametes CR CW 
White
CWCW
Pink
CRCW
Sperm Cw
1⁄2
Eggs
CR CR
CR CW
CW CW

33. Pleiotropy In Pleiotropy 1 gene has multiple phenotypic effects
Frizzle gene in chickens causes feathers to curl outward, abnormal body temperature, and greater digestive capacity, lay fewer eggs

34. Extending Mendelian Genetics for Two or More Genes
Polygenic inheritance
May be determined by two or more genes (polygenic)
Many human traits are polygenic
In Epistasis
A gene at one locus alters the phenotypic expression of a gene at a second locus

35. Example of Epistasis - Fruit Color in Squash
Color is recessive to no color at one allelic pair
This recessive allele must be expressed before the specific color allele at a second locus is expressed.
At the first gene, white colored squash is dominant to colored squash, and the gene symbols are W=white and w=colored

36. Fruit Color in Squash cont.)
At the second gene, yellow is dominant to green, and the symbols used are Y=yellow, y=green
The presence of the dominant W allele masks the effect of either the Y or y alleles
W_Y_ & W_yy give white (12)
wwY_ is yellow (3)
wwyy is green (1)
12:3:1 ratio

37. Another Example Of EpistasisBC bC Bc bc
1⁄4
BBCc
BbCc
BBcc
Bbcc
bbcc
bbCc
BbCC
bbCC
BBCC
9⁄16
3⁄16
4⁄16 
Sperm
Eggs

38. Quantitative variation usually indicates Polygenic Inheritance
An additive effect of two or more genes on a single phenotype
Skin color
Eye Color
Hair color 
AaBbCc
aabbcc
Aabbcc
AaBbcc
AABbCc
AABBCc
AABBCC
20⁄64
15⁄64
6⁄64
1⁄64
Fraction of progeny

40. Nature and Nurture: The Environmental Impact on Phenotype
Another departure from simple Mendelian genetics arises
When the phenotype for a character depends on environment as well as on genotype
May inherit genes for height, but not receive the needed nutrition to grow tall
Heart disease & cancer
Simply put, you are a product of both your genes and your environment!

41. The Norm Of Reaction
Is the phenotypic range of a particular genotype that is influenced by the environment (iron in soil affects color)

42. GENETIC PATTERNS IN HUMANS

43. Pedigree Analysis A pedigree
Is a family tree that describes the interrelationships of parents and children across generations
Male
Female

44. Inheritance patterns of particular traits Can be traced and described using pedigreesWw ww WW or
First generation
(grandparents)
Second generation
(parents plus aunts
and uncles)
Third
generation
(two sisters) Ff ff
FF or Ff FF
Widow’s peak
No Widow’s peak
Attached earlobe
Free earlobe
(a) Dominant trait (widow’s peak)
(b) Recessive trait (attached earlobe)

45. Recessively inherited disorders
Pedigrees
Can also be used to make predictions about future offspring
Recessively inherited disorders
Show up only in individuals homozygous for the allele (albino)
Carriers
Are heterozygous individuals who carry the recessive allele but are phenotypically normal
Indicated by ½ shaded circle

46. Cystic Fibrosis Cystic fibrosis (CF)
Recessively inherited, defective gene
Carried by millions of people
Must inherit 2 genes (one from each parent)
Symptoms of cystic fibrosis include
Mucus buildup in the internal organs
Abnormal absorption of nutrients in the small intestine

47. Sickle-Cell Disease Sickle-cell disease 1/400 African Americans
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
*Heterozygous Advantage-higher frequency in malaria regions.

48. Mating of Close Relatives(Inbreeding)
Matings between genetically similar individuals
Can increase the probability of the appearance of a genetic disease
Hemophilia (free bleeders) once found in royal families that intermarried (female carriers)
Are called consanguineous matings
Prohibited in the United States

49. Dominantly Inherited Genetic Disorders
One example is achondroplasia
A form of dwarfism that is lethal when homozygous for the dominant allele

50. Dominantly Inherited Genetic Disorders
Huntington’s disease
Is a degenerative disease of the nervous system
Has no obvious phenotypic effects until about 35 to 40 years of age (adult onset)

51. Genetic Testing and Counseling
Genetic counselors
Can provide information to prospective parents concerned about a family history for a specific disease
Genetic counselors help couples determine the odds that their children will have genetic disorders

52. Tests for Identifying Carriers
For a growing number of diseases
Tests (1000’s) are available that identify carriers and help define the odds more accurately
Newborn screening (PKU)
Carrier screening (Tay-Sach)
Adult onset screeening (Huntington’s)
Estimating risk screening (Alzheimer’s)

53. Fetal Testing In amniocentesis In chorionic villus sampling (CVS)
The liquid that bathes the fetus is removed and tested
In chorionic villus sampling (CVS)
A sample of the placenta is removed and tested

54. (b) Chorionic villus sampling (CVS)
Fetal Testing
(a) Amniocentesis
Amniotic
fluid
withdrawn
Fetus
Placenta
Uterus
Cervix
Centrifugation
A sample of
amniotic fluid can
be taken starting at
the 14th to 16th
week of pregnancy.
(b) Chorionic villus sampling (CVS)
Fluid
Fetal
cells
Biochemical tests can be
Performed immediately on
the amniotic fluid or later
on the cultured cells.
Fetal cells must be cultured
for several weeks to obtain
sufficient numbers for
karyotyping.
Several
weeks
Biochemical
tests
hours
Chorionic viIIi
A sample of chorionic villus
tissue can be taken as early
as the 8th to 10th week of
pregnancy.
Suction tube
Inserted through
cervix
Karyotyping and biochemical
tests can be performed on
the fetal cells immediately,
providing results within a day
or so.
Karyotyping

55. 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
Phenylketonuria (PKU)