1. 2. MULTIPLE ALLELES 3. PENETRANCE AND EXPRESSIVITY
1. Lethal alleles
2. MULTIPLE ALLELES
3. PENETRANCE AND EXPRESSIVITY

2. Different dominant relationship

3. Two alleles with recessive lethal
Some alleles may cause lethality

4. Inheritance patterns
in three crosses
involving the
wild-type agouti allele (A) and the mutant yellow allele (AY) in mice. Note that the mutant allele behaves as a homozygous lethal allele, and the genotype AYAY does not survive.

5. Lethal Alleles
Many alleles that cause genetic diseases are called "dominant" because heterozygotes are affected. A common example is ACHONDROPLASIA, the most common form of dwarfism, with a normal length body trunk but shortened limbs. Another in the Manx cat, which doesn't have a tail.
In fact, these genes would be better described as partially dominant, because the homozygotes are quite different from the heterozygotes: homozygotes are lethal.

6. X Lethal Alleles ML m x ML m ¼ ML ML ½ ML ½ m ½ ML ½ m
Example: Manx cat
ML = tailless,
lethal in homozygote
m = tail
Tailless male x Tailless female
½ ML
½ m
½ ML ½ m
ML m x ML m
¼ ML ML
¼ ML m
2/3 tailless + 1/3 tails X
dies
tailless
¼ ML m
¼ mm
tailless
tail

7. More Lethal Alleles
Lethal alleles give an unusual inheritance ratio. Consider a mating between two Manx cats. Each is heterozygous Tt, with T the dominant tailless allele and t the recessive normal tail allele.
Using Mendel's Law of Segregation, we see that zygotes form in the ratio of 1/4 TT, 1/2 Tt, and 1/4 tt.
However, all the TT embryos die at a very early stage, and only the Tt (tailless) and tt (tailed) cats are born.
Because there are twice as many Tt as tt, the ratio of offspring in the Tt x Tt cross is 2/3 Tt (tailless) to 1/3 tt (tailled).
Note that pure breeding lines of Manx cats (and achondroplastic dwarves) can't exist, because 1/3 of their offspring are of the incorrect type.

8. when more than two different alleles exist for the same trait.
Multiple Alleles:
when more than two different alleles exist for the same trait. *
(Remember: each individual will only have two alleles for a trait but there are several alleles to choose from.)

9. In humans, blood type has multiple alleles
In humans, blood type has multiple alleles. IA and IB are codominant and i (type O blood) is recessive.

10. Blood Type
Type of Antigen
Possible Genotypes
Type A
Type B
Type AB
Type O

11. Blood Type
Type of Antigen
Possible Genotypes
Type A
A antigen
Type B
Type AB
Type O

12. Blood Type
Type of Antigen
Possible Genotypes
Type A
A antigen
IAIA, IAi
Type B
Type AB
Type O

13. Blood Type
Type of Antigen
Possible Genotypes
Type A
A antigen
IAIA, IAi
Type B
B antigen
Type AB
Type O

14. Blood Type
Type of Antigen
Possible Genotypes
Type A
A antigen
IAIA, IAi
Type B
B antigen
IBIB, IBi
Type AB
Type O

15. Blood Type
Type of Antigen
Possible Genotypes
Type A
A antigen
IAIA, IAi
Type B
B antigen
IBIB, IBi
Type AB
A and B antigens
Type O

16. Blood Type
Type of Antigen
Possible Genotypes
Type A
A antigen
IAIA, IAi
Type B
B antigen
IBIB, IBi
Type AB
A and B antigens
IAIB
Type O

17. Blood Type
Type of Antigen
Possible Genotypes
Type A
A antigen
IAIA, IAi
Type B
B antigen
IBIB, IBi
Type AB
A and B antigens
IAIB
Type O
No antigens

18. Blood Type
Type of Antigen
Possible Genotypes
Type A
A antigen
IAIA, IAi
Type B
B antigen
IBIB, IBi
Type AB
A and B antigens
IAIB
Type O
No antigens ii

19. Cross a heterozygous type A with a heterozygous type B. IAi x ___

20. Cross a heterozygous type A with a heterozygous type B. IAi x IBi

21. IA i IB

22. IA i IB
IAIB

23. IA i IB
IAIB
IBi

24. IA i IB
IAIB
IBi
IAi

25. IA i IB
IAIB
IBi
IAi ii

26. Genotypic Ratios: 1 IAi : 1 IAIB : 1 IBi : 1 ii Phenotypic Ratios: 1 Type A: 1 Type AB: 1 Type B: 1Type O

27. Cross a person with type AB blood with another person with type O blood. IAIB x ___

28. Cross a person with type AB blood with another person with type O blood. IAIB x ii

29. IA IB i

30. IA IB i
IAi

31. IA IB i
IAi
IBi

32. IA IB i
IAi
IBi

33. IA IB i
IAi
IBi

34. 2 IAi : 2 IBi Genotypic Ratios: Phenotypic Ratios:
2 Type A: 0 Type AB: 2 Type B: 0 Type O

35. Penetrance and Expressivity
Penetrance = percentage of individuals with a given genotype who exhibit the phenotype
Expressivity = extent to which genotype is expressed at the phenotypic level (may be due to allelic variation or environmental factors)

36. Penetrance and Expressivity
Expression of many genes is affected by the environment or by "background" genetic influences. Two closely related concepts are used to describe this.
Penetrance is the percentage of offspring with the mutant genotype that express the mutant phenotype.
Expressivity is the degree to which the mutant phenotype is expressed.
Example. Polydactyly is having extra fingers and toes. There are several forms of this condition. For one form, polydactyly is 65% penetrant: 65% of those who carry the dominant polydactyly allele have extra digits. Examining these people, there is a range of expression: some have 1 extra digit, some have 2, etc. Also, some of the digits are functional: have proper bones, muscles and nerves, while others are missing vital components or connections.

37. Polydactyly
Alfredo Alfonseca "The Six Shooter", former Chicago Cubs relief pitcher.
Six fingers and toes on each hand, all functional.

38. Environmental Effects
Many traits are affected by the environment as well as by genetics.
For example, the hydrangea flower color is controlled first by flower color genes similar to those in the pea: purple vs. white with complete dominance. But, pink vs. purple is controlled by the acidity of the soil in which the plants grow.