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Lecture 12

NSCI 1413 Lecture 12: Heredity

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Natural Science
NSCI 1413
Guy Robinson

Law of Dominance When two homozygous organisms for opposing traits mate, their offspring will be hybrid but exhibit only the dominant trait. Law of Segregation During the formation of gametes, the two traits carried by each parent separate. Monohybrid Cross A cross b/w two heterozygous individuals; 3:1 phentotypic ratio; 1:2:1 genotypic ratio. Backcross/Testcross Method of determining the genotype of an individual showing the dominant trait. Law of Independent Assortment ● A cross between two individuals hybrid for two or more traits that are not on the same chromosome (dihybrid cross); during gamete formation, ​the alleles of a gene for one trait (Aa) segregate independently from the alleles of another trait (Bb) ● The only factor that determines allele segregation is the random process of metaphase during meiosis I ● In contrast, if the gene for tall is linked to the gene for yellow seed color, the genes will no assort independently (If a plant is tall, it will have yellow seeds) ● Dihybrid Cross:​ A cross between two individuals that are hybrid for two different traits; 9:3:3:1 phenotypic ratio (9 round/yellow, 3 round/green, 3 wrinkled/yellow, 1 wrinkled/green) Incomplete Dominance ● Characterized by blending ● Red four o’clock flower crossed w/ a white four o’clock flower produces pink offspring ● Pink four o’clock flower crossed w/ another pink four o’clock flower produces red, pink, and white offspring w/ a phenotypic ratio of 1:2:1 Codominance ● Both traits are present ● Roan horse (has both red and white hairs, appearing pink) Multiple Alleles ● When there are more than two allelic forms of a gene ● Four different human blood types (A, B, AB, O) and three possible alleles (A, B, O) ● A and B are codominant, and O is recessvie ● Six different genotypes Gene Interactions ● Pleidotropy:​ The ability of one single gene to affect an organism in several or many ways ● Frizzle Trait in Chickens:​ Create a malformed feather, disallowing the feather to keep the chicken warm; causes changes in organ systems to compensate ● Marfan Syndrome in Humans:​ A single defective gene results in abnormalities of the eyes, skeleton, and great blood vessels ● Epistasis:​ When two separate genes control one trait, but one gene masks the expression of the other (the dominant, masking gene is consider “epistatic” to the gene being masked) ● Guinea Pig Coloration:​ The gene for production of melanin is epistatic to the gene for the deposition of melanin (C causes pigment to be produced whereas c does not); the gene for deposition of melanin is either B, causing full deposition of melanin, or b, causing only a moderate amount, resulting in a brown coat (however, neither B or b can deposit melanin if C isn’t present to make melanin in the first place) ■ P:​ C/C B/B (Black) X c/c b/b (Albino) ■ F1​:​ C/c B/b (Black) X C/c B/b (Black) ■ F2​:​ 9 C/_ B/_ (Black) : 3 C/_ b/b (Brown) : 3 c/c B/_ (Albino) : 1 c/c b/b (Albino) ● Polygenic Inheritance:​ Characteristics that result from a blending of separate genes that vary along a continuum ● Height:​ Two parents who are short carry more genes for shortness than tallness, however, a child could inherit mostly the genes for tallness and become taller than their parents ● Skin pigmentation ● Genes and the Environment: ​The environment can alter the expression of genes ● Fruit Flies:​ The mutation of vestigial wings can be Δ’d to almost long, normal wings if brought up in a hot environment ● Humans:​ The development of intelligence is a result of the interaction of genetic predisposition and the environment (nature vs. nuture) Sex-Influenced Inheritance ● Inheritance can be influenced by the sex of the individual carrying the traits ● These traits are not sex-linked, rather, ​being a certain sex can make you more or less likely to exhibit a certain trait ● Male-Pattern Baldness:​ Since the trait is dominant, a female will only exhibit male-pattern baldness if she is homozygous dominant for the trait, whereas a male will always exhibit the trait unless he is homozygous recessive Linked Genes ● Genes on the same chromosome are considered to be linked ● Since there are more genes than chromosomes, thousands of genes are linked ● Inherited together and do not assort independently Sex-Linkage ● Out of 46 chromosomes (23 pairs), only 2 are sex chromosomes ● Traits carried on the X chromosome are considered sex-linked; few genes are carried on Y ● Females inherit two X chromosomes, while men inherit one ● If a sex-linked trait is due to a ​recessive mutation​, a female will only exhibit the phenotype if she carries two mutated genes (X-X-); if she carries only one, she will be a carrier ● If a sex-linked trait is due to a ​dominant mutation​, a female will exhibit the phenotype even w/ only one copy (X-X) ● Since males only inherit one X chromosome, they will automatically express the mutated gene ● Recessive traits are more common than dominant sex-linked traits, therefore, more males suffer from sex-linked disorder than females do ○ Recessive Sex-Linked Traits: ● Color Blindness ● Hemophilia ● Duchenne Muscular Dystrophy Crossover and Linkage Mapping ● The further apart two genes are on a chromosome, the more likely they are to be recombined during crossovers in meiosis ● Chiasma:​ Site at which a crossover and recombination occurs ● Crossover and recombination are a major source of variation among sexually ● One map unit = distance w/I which recombination occurs 1% of the time (doesn’t give actual distance, which must be calculated, but does give the order of linked genes) The Pedigree ● Family tree that indicated phenotype of one trait being studied; used to determine how particular traits are inherited ● Patterns of Inheritance: ○ Autosomal:​ The gene responsible for phenotypes; located on autosomes ■ Dominant:​ Only one copy of the mutation; 50% chance of carrying ● Huntington Disease ● Acondroplasia (short-limbed dwarfism) ● Poly
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