1. CHAPTER 9 Patterns of Inheritance
Modules 9.11 – 9.23

2. VARIATIONS ON MENDEL’S PRINCIPLES
9.11 The relationship of genotype to phenotype is rarely simple
Mendel’s principles are valid for all sexually reproducing species
However, often the genotype does not dictate the phenotype in the simple way his principles describe

3. 9.12 Incomplete dominance results in intermediate phenotypes
When an offspring’s phenotype—such as flower color— is in between the phenotypes of its parents, it exhibits incomplete dominance
P GENERATION
Red RR
White rr
Gametes R r
Pink Rr
F1 GENERATION
1/2 R
1/2 r
1/2 R
1/2 R
Eggs
Sperm
Red RR
1/2 r
1/2 r
Pink Rr
Pink rR
F2 GENERATION
White rr
Figure 9.12A

4. Incomplete dominance in human hypercholesterolemia
GENOTYPES: HH
Homozygous for ability to make LDL receptors Hh
Heterozygous hh
Homozygous for inability to make LDL receptors
PHENOTYPES:
LDL
LDL receptor
Cell
Normal
Mild disease
Severe disease
Figure 9.12B

5. 9.13 Many genes have more than two alleles in the population
In a population, multiple alleles often exist for a characteristic
The three alleles for ABO blood type in humans is an example

6. The alleles for A and B blood types are codominant, and both are expressed in the phenotype
Blood Group (Phenotype)
Antibodies Present in
Blood
Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left
Genotypes O A B AB
Anti-A
Anti-B O ii
IA IA or
IA i A
Anti-B
IB IB or
IB i B
Anti-A AB
IA IB
Figure 9.13

7. 9.14 A single gene may affect many phenotypic characteristics
A single gene may affect phenotype in many ways
This is called pleiotropy
The allele for sickle-cell disease is an example

8. Individual homozygous for sickle-cell allele
Sickle-cell (abnormal) hemoglobin
Abnormal hemoglobin crystallizes, causing red blood cells to become sickle-shaped
Sickle cells
Clumping of cells and clogging of small blood vessels
Breakdown of red blood cells
Accumulation of sickled cells in spleen
Physical weakness
Heart failure
Pain and fever
Brain damage
Damage to other organs
Spleen damage
Anemia
Impaired mental function
Pneumonia and other infections
Kidney failure
Rheumatism
Paralysis
Figure 9.14

9. 9.15 Connection: Genetic testing can detect disease-causing alleles
Genetic testing can be of value to those at risk of developing a genetic disorder or of passing it on to offspring
Figure 9.15B
Dr. David Satcher, former U.S. surgeon general, pioneered screening for sickle-cell disease
Figure 9.15A

10. 9.16 A single characteristic may be influenced by many genes
This situation creates a continuum of phenotypes
Example: skin color

11. Fraction of population
P GENERATION
aabbcc (very light)
AABBCC (very dark)
F1 GENERATION
AaBbCc
AaBbCc
Eggs
Sperm
Fraction of population
Skin pigmentation
F2 GENERATION
Figure 9.16

12. THE CHROMOSOMAL BASIS OF INHERITANCE
9.17 Chromosome behavior accounts for Mendel’s principles
Genes are located on chromosomes
Their behavior during meiosis accounts for inheritance patterns

13. The chromosomal basis of Mendel’s principles
Figure 9.17

14. 9.18 Genes on the same chromosome tend to be inherited together
Certain genes are linked
They tend to be inherited together because they reside close together on the same chromosome

15. Figure 9.18

16. 9.19 Crossing over produces new combinations of alleles
This produces gametes with recombinant chromosomes
The fruit fly Drosophila melanogaster was used in the first experiments to demonstrate the effects of crossing over

17. A B a b B A a b A b a B Tetrad Crossing over Gametes
Figure 9.19A, B

18. Figure 9.19C

19. 9.20 Geneticists use crossover data to map genes
Crossing over is more likely to occur between genes that are farther apart
Recombination frequencies can be used to map the relative positions of genes on chromosomes
Chromosome g c l
17% 9%
9.5%
Figure 9.20B

20. Alfred H. Sturtevant, seen here at a party with T. H
Alfred H. Sturtevant, seen here at a party with T. H. Morgan and his students, used recombination data from Morgan’s fruit fly crosses to map genes
Figure 9.20A

21. A partial genetic map of a fruit fly chromosome
Mutant phenotypes
Short aristae
Black
body
(g)
Cinnabar
eyes
(c)
Vestigial
wings
(l)
Brown eyes
Long aristae (appendages on head)
Gray
body
(G)
Red
eyes
(C)
Normal
wings
(L)
Red eyes
Wild-type phenotypes
Figure 9.20C

22. SEX CHROMOSOMES AND SEX-LINKED GENES
9.21 Chromosomes determine sex in many species
A human male has one X chromosome and one Y chromosome
A human female has two X chromosomes
Whether a sperm cell has an X or Y chromosome determines the sex of the offspring

23. Parents’ diploid cells
(male)
(female)
Parents’ diploid cells X Y
Male
Sperm
Egg
Offspring (diploid)
Figure 9.21A

24. Other systems of sex determination exist in other animals and plants
The X-O system
The Z-W system
Chromosome number
Figure 9.21B-D

25. 9.22 Sex-linked genes exhibit a unique pattern of inheritance
All genes on the sex chromosomes are said to be sex-linked
In many organisms, the X chromosome carries many genes unrelated to sex
Fruit fly eye color is a sex-linked characteristic
Figure 9.22A

26. Their inheritance pattern reflects the fact that males have one X chromosome and females have two
These figures illustrate inheritance patterns for white eye color (r) in the fruit fly, an X-linked recessive trait
Female
Male
Female
Male
Female
Male
XRXR
XrY
XRXr
XRY
XRXr
XrY XR XR Xr Xr XR XR
XRXr Y
XRXR Y
XRXr Y Xr Xr
XRY
XrXR
XRY
XrXr
XRY
XrY
XrY
R = red-eye allele
r = white-eye allele
Figure 9.22B-D

27. 9.23 Connection: Sex-linked disorders affect mostly males
Most sex-linked human disorders are due to recessive alleles
Examples: hemophilia, red-green color blindness
These are mostly seen in males
A male receives a single X-linked allele from his mother, and will have the disorder, while a female has to receive the allele from both parents to be affected
Figure 9.23A

28. Czar Nicholas II of Russia
A high incidence of hemophilia has plagued the royal families of Europe
Queen Victoria
Albert
Alice
Louis
Alexandra
Czar Nicholas II of Russia
Alexis
Figure 9.23B