1. Patterns of Inheritance
By observing how traits are passed to the next generation, how can the inheritance patterns be used to understand the principles of heredity?

2. Use of Garden Pea for Genetics Experiments
Stamens (male) produce pollen
Carpel (female) produces eggs
In the intact pea flower (left), the lower petals form a container enclosing the reproductive structures—the stamens (male) and carpel (female). Pollen normally cannot enter the flower from outside, so peas normally self-fertilize.
Flower dissected to show reproductive structures
Intact pea flower

3. Mendel’s Experiment With Peas Differing in a Single Trait
Parental: Smooth seed x Wrinkled seed
F1: All smooth seed coats
F1 smooth plants x F1 smooth plants
F2: 5474 smooth: 1850 wrinkled (3/4 smooth to 1/4 wrinkled)

4. Patterns of Inheritance
Mendel needed to explain
Why one trait seemed to disappear in the first generation.
2. Why the same trait reappeared in the second generation in one-fourth of the offspring.

5. Mendel’s Proposal
Each trait is governed by two factors – now called genes.
2. Genes are found in alternative forms called alleles.
3. Some alleles are dominant and mask alleles that are recessive.

6. Mendel’s Experiment With Peas Differing in a Single Trait
Parental: Smooth seed x Wrinkled seed SS ss
Homozygous Dominant
Homozygous Recessive
F1: All smooth seed coats Ss
Heterozygous
F1 smooth plants x F1 smooth plants Ss
Heterozygous Ss
Heterozygous F2

7. Homozygous parents can only pass one form of an allele to their offspring.

8. Heterozygous parents can pass either of two forms of an allele to their offspring.
Locus: Area on the chromosome where a gene is located.
For a heterozygote, homologous chromosomes will have different alleles at the same locus.

9. Additional Genetic Terms
Definition
Example
Genotype
Alleles carried by an individual
SS, Ss, ss
Phenotype
Physical characteristic or appearance of an individual
smooth or wrinkled

10. Mendel’s Principle of Genetic Segregation
In the formation of gametes, the
members of a pair of alleles separate (or segregate) cleanly from each other so that only one member is included in each gamete.
Each gamete has an equal probability of containing either member of the allele pair.

11. Genetic Segregation
Parentals:
SS x ss
F1 x F1:
Ss x Ss

12. Traits Studied by Mendel
Seed shape
Seed color
Pod shape
Pod color
Flower color
Traits of pea plants that Mendel studied
Flower location
Plant size

13. Mendel’s Experiment With Peas Differing in Two Traits
Parental: Smooth Yellow x Wrinkled Green
F1: All smooth yellow seed coats
F1 plants x F1 plants
1/16
32 wrinkled, green
3/16
101 wrinkled, yellow
108 smooth, green
9/16
315 smooth, yellow F2

14. Patterns of Inheritance
Mendel needed to explain
Why non-parental combinations appeared in the F2 offspring.
2. Why the ratio of phenotypes in the F2 generation was 9:3:3:1.

15. Mendel’s Principle of Independent Assortment
When gametes are formed, the alleles of one gene segregate independently of the alleles of another gene producing equal proportions of all possible gamete types.

16. Genetic Segregation + Independent Assortment
Parentals: SSYY x s s y y
SY SY SY SY
sy sy sy sy
F1:

17. Genetic Segregation + Independent Assortment
F1 x F1 : S s Y y x S s Y y
SY Sy sY sy
SY Sy sY sy
Four different types of gametes are formed in equal proportions.

18. F1 x F1 SsYy X SsYy Eggs SY Sy sY sy SY Sy Pollen sY sy 1 4 1 4 1 4
Figure: FIGURE 12.6
Title:
Predicting genotypes and phenotypes for a cross between gametes that are heterozygous for two traits
Caption:
Here we are working with both seed color and shape, with yellow (Y) dominant to green (y), and smooth (S) dominant to wrinkled (s). (a) Punnett square analysis. In this cross, both parents are heterozygous for each trait (or a single individual heterozygous for both traits self-fertilize). There are now 16 boxes in the Punnett square. In addition to predicting all the genotypic combinations, the Punnett square predicts 3/4 yellow seeds, 1/4 green seeds, 3/4 smooth seeds, and 1/4 wrinkled seeds, just as we would expect from crosses made of each trait separately. (b) Probability theory can be used to predict phenotypes that result from a cross between gametes that are heterozygous for two traits. The fraction of genotypes from each sperm and egg combination is illustrated within each box of the Punnett square. Adding the fractions for the same genotypes will give the genotypic ratios. Converting each genotype to a phenotype and then adding their numbers reveals that 3/4 of the offspring will be smooth and 1/4 will be wrinkled and that 3/4 will be yellow and 1/4 will be green. Multiplying these independent probabilities produces predictions for the phenotype of offspring. These ratios are identical to those generated by the Punnett square. sY
1 4 sy
1 4

19. F2 Genotypes and Phenotypes
Smooth
Yellow
Green
Wrinkled

20. Meiotic Segregation Explains Independent Assortment
Two possible orientations

21. Additional Genetic Patterns
Mendel’s peas
Alternative Pattern
Complete Dominance
Incomplete Dominance
Incomplete dominance: neither allele masks the other and both are observed as a blending in the heterozygote

22. Incomplete Dominance Four o’clock flowers R = red, R’ = white
Red x White RR R’R’
Four o’clock flowers
R = red, R’ = white
Pink RR’

23. Incomplete Dominance F1 x F1 Pink x Pink RR’ x RR’ Genotypic Ratio:
Phenotypic Ratio:

24. Additional Genetic Patterns
Mendel’s peas
Alternative Patterns
Complete Dominance
Codominance
Two alleles per gene
Multiple Alleles
Codominance: Neither allele masks the other so that effects of both alleles are observed in heterozygotes without blending
Multiple Alleles: Three or more alleles exist for one trait
Note: A diploid individual can only carry any two of these alleles at once.

25. Multiple Alleles and Codominance
ABO Blood Type in Humans
Blood Type
Allele
Type A A
Type B B
Type O o
A = B > o
A and B are codominant.
A and B are completely dominant over o.

26. Human ABO Blood Types
Type
Genotype
Antigen on RBCs
Anti- bodies
Re- ceives
Donates
Freq A
AA or Ao
Type A B
A or O
A or AB
40% B
BB or Bo
Type B A
B or O
B or AB
10% AB AB
A and B
Neither
AB, A, B, O (universal)
AB (universal) 4%
Figure: TABLE 12.1
Title:
Human blood group characteristics
Caption: O oo
Neither
Both O
O,AB, A,B (universal)
46%
Codominance is observed for Type AB Blood since the products of both the A and B alleles are found on the cells.

27. Inheritance of Rh Factor
Phenotype
Genotype*
Gene
Product
Antibodies Present
Rh Positive
RR or Rr
Rhesus Protein
None
Rh Negative rr
None unless exposed
*Although there are multiple R alleles, R1, R2, R3, etc. all are
completely dominant over all of the r alleles, r1, r2, r3, etc.
ABO Blood Type and Rh Factor are controlled by separate genes. They show independent assortment.

28. Multiple Alleles and Codominance
Type A, Rh positive x Type B, Rh negative
Phenotypic Ratio of Offspring

29. Additional Genetic Patterns
Mendel’s peas
Alternative Patterns
One gene affects one trait
Polygenic Inheritance
Polygenic Inheritance: Many genes affect one trait

30. Example of Polygenic Inheritance
Two genes affecting one trait
Number of Dominant
Alleles
Skin Color*
(Phenotype)
Genotypes
% Pigmentation*
White
aabb
0-11% 1
Light Black
Aabb or aaBb
12-25% 2
Medium Black
AAbb or AaBb or aaBB
26-40% 3
Dark Black
AABb or AaBB
41-55% 4
Darkest Black
AABB
56-78%
*Based on a study conducted in Jamaica.

31. Example of Polygenic Inheritance
Medium Black Woman X Darkest Black Man
(her mother is white)

32. Additional Genetic Patterns
Mendel’s peas
Alternative Patterns
One gene affects one trait
Pleiotropy
Pleiotropy: One gene affects many traits

33. Sickle-Cell Anemia One gene affects many phenotypic characteristics
Gene Product
Cell Shape
Disease Conditions SS
Hemoglobin A
Spherical, slightly
concave
No anemia
SS’
Hemoglobin S
Some sickling under extreme conditions
Sickle Cell Trait
Resistance to Malaria
S’S’
Sickled under low O2 tension
Sickle Cell Anemia