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

Different dominant relationship

Two alleles with recessive lethal Some alleles may cause lethality

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.

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.

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

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.

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.)

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.

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

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

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

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

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

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

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

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

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

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

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

IA i IB

IA i IB IAIB

IA i IB IAIB IBi

IA i IB IAIB IBi IAi

IA i IB IAIB IBi IAi ii

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

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

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

IA IB i

IA IB i IAi

IA IB i IAi IBi

IA IB i IAi IBi

IA IB i IAi IBi

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

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)

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.

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

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.