Rebecca Davenport BIOL 1406 11 29 16 Ch 14 blueprint 1 Mendel and the Gene Idea a Who s Mendel i A monk ii Bred pea plants b What did Mendel do i Bred pea plants to study inheritance ii Why pea plants 1 Many varieties 2 Short generation time 3 Large number of offspring 4 Self fertilization c What did Mendel find i ii P generation Parental true breeding parents purple and white 1 Either PP or pp both are capitalized or both are lower case iii F1 generation all purple ALL F1 s are heterozygous iv F2 generation 3 1 ratio of purple white 1 Phenotypic ratio is 3 1 2 Genotypic ratio 1 2 1 PP 1 Pp 2 pp 1 3 Testcrossa Always mate the unknown with a homozygous recessive b How can you determine if the genotype of a purple flower is PP or Pp i You perform a test cross breeding an organism of unknown genotype with a recessive homozygote ii The allele from the mystery plant will determine the appearance of the offspring iii All off spring purple PP iv Both purple and white Pp v 1 Your possible gametes a MUST KNOW HOW TO GET POSSIBLE GAMATES FROM PARENTS i You can only have one of each letter ii EX TTRr x Ttrr 1 2 What if you have four letters each YYRR x yyrr a Can only have one of each letter in each box b Must find gametes first then complete punnet square vi Led to the Law of Segregation and the Law of Independent Assortment 1 Law of Segregationa During meiosis the alleles of a gene pair segregate each going to a separate gamete 2 Law of independent Assortmenta Two or more genes on different chromosomes assort independently i Each pair of alleles segregates independently of other pair of alleles during gamete formation b c F1 YyRr plant will produce four classes of gametes in equal quantitites YR Yr yR yr d Results in formation of F2 offspring with a phenotypic ratio of 9 3 3 1 d What did he observe i Alternative versions of genes account for variations in inherited characters ii For each character an organism inherits two copies of a gene one from each parent iii If the two alleles at a locus differ then the dominant allele determines the organism s appearance iv The two alleles for a heritable character segregate during gamete formation and end up in different gametes e Extending Medelian Genetics for a Single Genei Degree of dominance Alleles can show different degrees of dominance or recessiveness in relation to each other 1 Complete dominance phenotype of Rr same as RR the capital R has dominance over either another capital R or a lowercase r 2 Incomplete Dominance F1 hybrids have phenotype between those of the parents i e RR red Rr pink rr white a 1 2 1 genotypic AND phenotypic ratio b 3 Codominance the two alleles each affect the phenotype in different ways i e MN blood group a EX blood types A B AB O i Alleles IA IB I or IO ii iii iv Complete dominance IAIO v Co Dominance IAIB 4 An allele is said to be dominant because it is seen in the phenotype NOT because it subdues a recessive allele f Relation between dominance and Phenotype i A dominant allele does not subdue a recessive allele alleles don t interact that way ii Alleles are simply variations in a gene s nucleotide sequence iii For any character dominance recessiveness relationships of alleles depend on the level at which we examine the phenotype g Frequency of dominant Alleles i Dominant alleles are not necessarily more common in populations than recessive alleles ii For example one baby out of 400 in the United States is born with extra fingers or toes The allele for this unusual trait is dominant to the allele for the more common trait of five digits per appendage Thus the recessive allele is more common h Multiple Alleles i Most genes exist in populations in more than two allelic forms ii Ex four phenotypes of the ABO blood groups are determined by three alleles for the enzyme I that attaches A or B carbohydrates to red blood cells IA IB and i iii IA allele adds the A carbohydrate the IB allele adds the B carbohydrate the i allele adds neither iv Blood types A B AB both carbs O no carbs i Pleiotropy when a gene has many phenotypic effects i Pleiotropy when one gene has multiple phenotypic effects ii For example pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases such as cystic fibrosis and sicklecell disease j Extending Mendelian Genetics to 2 Genes i Epistasis one gene affects the phenotype of another because the two gene products interact 1 Example Dog coat color color coat of labs ii Polygenic Inheritance multiple genes independently affect a single trait 1 Examples height and skin color k Epistasis i One gene affects the phenotype of another because the two gene products interact ii The phenotypic expression of a gene at one locus alters that of a gene at a second locus 1 Ex Fur coat color of Labrador retrievers a b c d B Black b brown with complete dominance E deposits pigment e no pigment deposited ee results in yellow coat regardless of Black brown locus B or b determines color but E or e determines whether or not to deposit said pigment e iii Gene for pigment deposition is epistatic to gene that codes for black or brown pigment l Polygenic Inheritance i Multiple genes affect a single trait ii Quantitative characters heritable feature that varies continuously over a range rather than an either or fashion iii Ex Height at least 180 genes affect height iv EX Skin color three genes A B C contribute a unit of darkness to the phenotype and being incompletely dominant to the other allele a b or c 1 AABBCC very dark aabbcc very light AaBbCc intermediate shade m Environment Impact on Phenotype i The phenotype for a character can depends on environment as well as genotype ii The phenotypic range is broadest for polygenic characters iii Traits that depend on multiple genes combined with environmental influences are called multifactorial iv EX tanning it effects your physical appearance not your genetic makeup n Pedigree Analysis i A pedigree is a family tree that describes the interrelationships of parents and children across generations ii Inheritance patterns of particular traits can be traced and described using pedigrees o Behavior of Recessive Alleles i Recessively inherited disorders show up only in individuals homozygous for the allele ii Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal p Cystic Fibrosis i Cystic fibrosis is the most common lethal genetic disease in the U S ii The cystic fibrosis allele results in defective
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