1. Concept 14.2: The laws of probability govern Mendelian inheritance
Mendel’s laws of segregation and independent assortment reflect the rules of probability
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
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2. Probability Line

3. Probability Line

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

6. Parents Normal Normal Aa Sperm A a Eggs Aa AA A Normal (carrier)
Fig
Parents
Normal
Normal Aa  Aa
Sperm A a
Figure Albinism: a recessive trait
Eggs Aa AA A
Normal
(carrier)
Normal Aa aa a
Normal
(carrier)
Albino

8. Parents Dwarf Normal Sperm Eggs Dwarf Normal Dwarf Normal Dd dd D d Dd
Fig
Parents
Dwarf
Normal Dd  dd
Sperm
Figure Achondroplasia: a dominant trait D d
Eggs Dd dd d
Dwarf
Normal Dd dd d
Dwarf
Normal

9. Huntington’s Disease

10. Huntington’s Disease
Click image to view video

11. Effects of HD

12. Concept 14.3: Inheritance patterns are often more complex than predicted by simple Mendelian genetics
The relationship between genotype and phenotype is rarely as simple as in the pea plant characters Mendel studied
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

13. 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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

14. Degrees of Dominance
Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical
Gene Linkage – Genes found on same chromosome and stay together during meiosis (unless cross-over occurs)
In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties
In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

15. Dihybrid Cross EXPERIMENT RESULTS Fig. 14-8 P Generation F1 Generation
YYRR
yyrr
Gametes YR  yr
Dihybrid Cross
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
Figure 14.8 Do the alleles for one character assort into gametes dependently or independently of the alleles for a different character?
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

16. Gene Linkage: Drosophilia melanogaster –has
only 4 pairs of chromosomes.
Therefore must have large
# of genes on each.
Dr. Thomas Hunt Morgan (1910)
mapped these genes.
Genes stay together during
process of meiosis unless
crossing over occurs.
This does not support Mendel’s
Principle of Independent Assortment
Chromosome #2 has genes controlling
Eye color, wing type, body color, etc.
Crosses involving linked genes do not give
The same phenotypic ratio as unlinked.

17. Figure 15.3 Morgan’s first mutant

19. Morgan’s Choice of Experimental Organism
Several characteristics make fruit flies a convenient organism for genetic studies:
They breed at a high rate
A generation can be bred every two weeks
They have only four pairs of chromosomes
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20. Gene map of a fruit fly (Drosophila melanogaster)

21. Degrees of Dominance
Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical
Gene Linkage – Genes found on same chromosome and stay together during meiosis (unless cross-over occurs)
In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties
In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

23. Degrees of Dominance
Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical
Gene Linkage – Genes found on same chromosome and stay together during meiosis (unless cross-over occurs)
In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties
In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

25. Most genes exist in populations in more than two allelic forms
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
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

26. Degrees of Dominance
Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical
Gene Linkage – Genes found on same chromosome and stay together during meiosis (unless cross-over occurs)
In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties
In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

28. The approximate distribution of blood types in the U. S
The approximate distribution of blood types in the U.S. population is as follows:
O-positive: 38 percent.
O-negative: 7 percent.
A-positive: 34 percent.
A-negative: 6 percent.
B-positive: 9 percent.
B-negative: 2 percent.
AB-positive: 3 percent.
AB-negative: 1 percent.

30. Red blood cell appearance Phenotype (blood group)
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
Figure Multiple alleles for the ABO blood groups
IAIA or IA i A
IBIB or IB i B
IAIB AB ii O
(b) Blood group genotypes and phenotypes