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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
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1. Lethal alleles 2. MULTIPLE ALLELES 3. PENETRANCE AND EXPRESSIVITY
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 AYAYdoes 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.
½ ML½ m X ½ ML ½ m 2/3 tailless + 1/3 tails Lethal Alleles Example: Manx cat ML = tailless, lethal in homozygote m = tail Tailless male x Tailless female ML m x ML m ¼ ML ML ¼ ML m dies tailless ¼ mm ¼ ML m 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.
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. IA and IB are codominantand i (type O blood) is recessive.
Cross a heterozygous type A with a heterozygous type B. IAix ___
Cross a heterozygous type A with a heterozygous type B. IAix IBi
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. IAIBx ___
Cross a person with type AB blood with another person with type O blood. IAIBx ii
Genotypic Ratios: 2 IAi : 2 IBi 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.