Other Patterns of Inheritance PDF

OTHER PATTERNS OF INHERITANCE 1  INTRO ▪ Incomplete dominance: heterozygote exhibits phenotype that is intermediate between homozygous phenotypes (pink flower offspring produced from red white parents) ▪ Codominance: simultaneous expressio...

OTHER PATTERNS OF INHERITANCE 1  INTRO ▪ Incomplete dominance: heterozygote exhibits phenotype that is intermediate between homozygous phenotypes (pink flower offspring produced from red white parents) ▪ Codominance: simultaneous expression of both alleles in heterozygote (human blood type) ▪ Common for more than 2 alleles of a gene (pumpkin sizes) to exist in a population ▪ Genes on X chromosomes are X-linked, males only inherit and express 1 allele making them more prone to displaying faults (hemophilia & color-blindness) ▪ Lethal alleles result in never observing that phenotype ▪ Punnett squares useful in applying rules of probability and meiosis to predict crosses ▪ Test crosses done to determine whether individual is hetero or homozygous by crossing with homozygous recessive. 2  INCOMPLETE DOMINANCE ▪ Mendel contradicted view of time that offspring exhibited a blend of parents’ traits ▪ Heterozygote phenotype does occasionally appear to display intermediate between 2 parents ▪ In Antirrhinum majus a cross between homozygous white flower (C^W C^W) + homozygous red (C^R C^R) yields (C^R C^W) that is pink, not red or white. ▪ Different genotypic abbreviations are used to differentiate this pattern from Mendel ▪ Incomplete dominance: denotes the expression of 2 contrasting alleles such that individual display an intermediate phenotype ▪ Ratio of cross can still be predicted just as with Mendelian dominant & recessive crosses. ▪ F2 generation will yield 1 Red, 1 White, 2 Pink 3  CODOMINANCE ▪ Codominance: both alleles for the same characteristic are simultaneously expressed in heterozygote. This is a variation of incomplete dominance ▪ In self-cross between heterozygotes expressing a codominant trait, the 3 possible offspring genotypes are phenotypically different. ▪ However, the same 1:2:1 genotypic ratio of Mendelian genetics is still observed 4  MULTIPLE ALLELES ▪ Mendel implied that only 2 alleles: 1 dominant & 1 recessive could exist for a given gene ▪ Individual diploid organism can only have 2 alleles for a given gene, but multiple alleles may exist at the population level so an array of allele combinations can be observed ▪ When many alleles exist for the same gene, the most common phenotype or genotype is called the wild type. It is often abbreviated as (“+”) ▪ Variants: all other phenotypes or genotypes that differ from standard wild type. ▪ It might be recessive or dominant to the wild-type allele. ▪ When multiple alleles involved, dominance hierarchies might be observed between all variants and wild type. Variety of phenotypes for heterozygote crosses. 5  MULTIPLE ALLELES ▪ Wild type dominant over all others ▪ Chinchilla incomplete dominant over Himalayan & Albino. ▪ Himalayan dominant over albino ▪ Complete dominance of a wild-type phenotype over all other mutants often occurs as an effect of dosage of specific gene product. In other words, wild-type has “correct” amount of gene product and mutants have less or none. ▪ One mutant gene can also be dominant over all other phenotypes. This can occur when mutant allele interferes with genetic message so that heterozygote expresses mutant phenotype. ▪ Mutant allele can interfere by enhancing function of wild-type gene or changing distribution in body 6  ALLELE LINK TO EVOLUTION ▪ Some pathogens have evolved resistance to commonly used treatments because of a wide gene pool. ▪ A population with variety in gene pool poses a threat to organism that pathogen preys upon ▪ A variety of alleles results in difficulties wiping out pathogen as increases likelihood of an individual in the population having a resistance to a drug ▪ From there, the organism that resisted treatment, can pass on these genes to a new generation that will make up most of the new gene pool ▪ Pathogens that multiply in large numbers within an infection cycle evolve relatively rapidly in response to the selective pressure of commonly used treatments. ▪ Must constantly develop new treatments to combat evolving pathogens. 7  EPISTASIS ▪ Mendel thought that each characteristic of an individual’s phenotype was controlled by a gene. ▪ Single observable characteristics are almost always under the influence of multiple genes acting in unison ▪ Polygenic Traits: characteristics that are encoded across multiple genes ▪ Many genes acting together can be responsible for the traits, or physical appearance of the organism ▪ Genes can contribute to a phenotype without directly interacting. They might be expressed sequentially or function complementary such that they need to be expressed simultaneously. ▪ Epistasis: an antagonistic interaction between genes, making it so that one masks or interferes with the expression of another. ▪ Hypostatic: the alleles that are masked or silenced by epistasis alleles 8  EPISTASIS ▪ Genotype can be for a particular fur color, but if other gene unable to produce pigmentation then will not get to express it. ▪ Can also occur when dominant allele masks expression at a separate gene ▪ It can also be reciprocal such that when either gene is present in dominant or recessive form they expresses the same phenotype. ▪ In shepherd’s purse plant, the characteristic of seed shape controlled by 2 genes. If both of them are recessive then the seed shape will be ovoid, but if either of them are heterozygous or homozygous dominant, then seed shape will always shift to triangular. 9  ALTERNATIVE TO DOMINANT/RECESSIVE ▪ Conclusions derived from Mendel’s experiment: ▪ Two units or alleles exist for every gene ▪ Alleles maintain their integrity in each generation (no blending) ▪ In the presence of dominant allele, recessive allele is “muted” and makes no contribution to phenotype ▪ Recessive alleles can be “carried” and not expressed by individuals ▪ Heterozygous individuals are sometimes called carriers. ▪ There are limitations to the fundamental principles of Mendelian genetics. An extension must be considered because some organisms exhibit more complexity. 10  PEDIGREE ▪ Many human diseases are genetically inherited ▪ A “healthy” person in a family in which some members suffer from a recessive genetic disorder (might be a carrier) may want to know if they have a risk of passing the disorder on to their offspring. ▪ Test crosses would be unethical and impractical. ▪ Geneticists use pedigree analysis to study inheritance pattern of human genetic disorder ▪ The information gained from pedigrees makes it possible to determine the nature of genes and alleles associated with inherited human traits 11  LETHALITY ▪ Large proportion of genes in an individual’s genome are essential for survival ▪ Sometimes, a nonfunctional allele for an essential gene can arise from a mutation and be transmitted in a population as long as individuals with this allele also have a wild-type, functional copy that is able to sustain life so it is dominant. ▪ Recessive lethal: 2 heterozygous organism that carry nonfunctional essential gene expected to pass down 25% of the time double (homozygous) recessive trait resulting in embryo death or producing an organism that will die sometime after birth. ▪ In other instances, recessive lethal allele might exhibit dominant, but not lethal phenotype in the heterozygote. (Example: the manifestation of a malformation) ▪ Dominant lethal: an allele is lethal in both homozygote and heterozygote. It can only be transmitted if lethality phenotype occurs after reproductive age. (Example: Huntington’s nervous system degeneration). 12  X-LINKED TRAITS ▪ In humans and many other animals, sex of individual determined by sex chromosomes ▪ Sex chromosomes: 1 pair of non-homologous chromosomes ▪ Autosomes: Non-sex chromosomes. In humans, pairs #1 through 22 ▪ Y chromosome has small regions of similarity so that it can pair with X during meiosis ▪ Y chromosome is much smaller. As a result, higher incidence of genetic diseases for males because do not have backup for faulty X genes ▪ X-linked: gene being examined is present on X chromosome, but not on Y ▪ Hemizygous: males have only 1 allele for any X-linked characteristic ▪ Reciprocal crosses do not produce same offspring ratios. 13  X-LINKED TRAITS ▪ Drosophila, also known as fruit flies encode the eye-color gene in the X chromosome ▪ The wild-type eye color is red (X^R) and is dominant to white eye color (X^r) ▪ Reciprocal crosses do not produce same offspring ratios: ▪ Genotype for males: X^R Y or X^r Y ▪ Genotype for females: X^R X^R, X^R X^r or X^r X^r ▪ When P male has white or red eyes and female is homozygous red-eyes, all offspring will display red eyes whether they are male or female ▪ Heterozygous female with red eye male, would produce only red eyes female and both red and white-eyed males ▪ Homozygous white-eyed female and red-eye male = F1 only heterozygous red females & only white-eyed males. F2 half females & males red-eye and half female & males white-eyed. 14  HUMAN SEX-LINKED DISORDERS ▪ When female parent is homozygous for a recessive X-linked trait, she will 100% pass on trait to offspring. ▪ The male offspring are 100% destined to express the trait as they only inherited the Y from the father. ▪ Alleles for conditions: color blindness and hemophilia are X-linked ▪ Heterozygous females can be “carriers” and not exhibit any phenotypic effect ▪ Pass on disease to half of their sons & pass on carrier status to half their daughters ▪ In birds, female is the sex with the non-homologous sex chromosomes so they exhibit most incidence of disease, instead of being carrier. 15  HUMAN SEX-LINKED DISORDERS ▪ X-linked recessive disorders are disproportionately observed in males because they only require 1 copy of the mutant allele to be affected. ▪ Females must inherit recessive X-linked alleles from both parents in order to express the trait ▪ When females inherit 1 recessive X-linked mutant allele and 1 dominant X-linked wild-type allele, they are carriers of the trait and are typically unaffected. ▪ Female carriers can contribute trait to their sons, resulting in son exhibiting trait or they can contribute recessive allele to their daughters, which if father was not sick, results in daughters being carriers of trait too. ▪ Y-linked disorders exist. Exclusive to males, but typically associated with infertility so they are not transmitted to subsequent generations. 16 HUMAN SEX-LINKED DISORDERS 17