Modification of Mendelian Ratios chapter 4 Non Mendelian genetic ratios these ratios occur when alleles of one gene do not exhibit simple dominance and recessiveness Mendels laws still govern the distribution of the alleles in the gametes not all genes are simply dominate and recessive Different alleles of the same gene alter phenotypes in different ways wild type allele so called normal version of the gene very often is the dominant over alleles but not always loss of function or in extreme case null allele is an allele causes a reduced function of the gene product gain of function alleles usually result from overexpression of a gene neutral mutations have changes that do NOT translate into any visible phenotypic difference Incomplete Partial Dominance where a cross between parents with contrasting traits results in an offspring with and intermediate phenotype ex lower color in snapdragon one functional gene does not give the full red color note on genetic notation different alleles can also be denoted by the letter with superscripts R1R1 red x R2R2 white R1R2 pink R1R2 pink x R1R2 pink R1R1 red R1R2 pink R1R2 pink R2R2 white QUESTION what is the genotypic ratio of offspring if I cross a pea plant with red flowers to one with pink flowers use same notation as text ANSWER 1 1 2R1R1 and 2R2R2 QUESTION what is the phenotypic ratio in the above cross ANSWER 1 1 2red 2pink Co Dominance where two alleles of a single locus produce two full functional but distinct products Ex MN blood type genotype LMLM LMLN LNLN phenotype M MN N At the molecular level these are two distinct glycoproteins proteins with sugar side groups there are different sugar side groups in each allele with both work well there are NO side effects unless one gets a blood transfusion ex LNLN individual receives LMLM blood and mounts an immune reaction against it Molecular basis of MN blood type Image shows the primary structure of glycophorin A a glycoproteins that spans the plasma membrane of human red blood cells each RBC has 500 000 copies of the molecule MN blood groups 40 alleles The M allele encodes Ser at position 1 Ser 1 and Gly at position 5 Gly 5 t The N allele encodes Leu 1 and Glu 5 So they are slightly different proteins and the change in the amino acids makes them a look like a threat to the immune system QUESTION A married couple that each have LMLN have children How many different blood types can their children have ANSWER 3 LMLM LNLN LMLN QUESTION Can all their children donate blood to their parents ANSWER Yes because the parents are heterozygous so their immune system in used to the glycoprotein QUESTION Can the parents donate blood to all the children ANSWER No only the heterozygous children Multiple Alleles Where there are more than two alleles of a single locus produce two fully functional but distinct products ex ABO blood type Genotype IAIA IAIO IBIB IBIO IAIB IOIO antigen A A B B A and B none phenotype A A B B AB O at the molecular level these are sugars attached to lipids on the end blood cell an O phenotype can also occur in the rare instance where the precursor to the membrane surface sugar is not made Molecular basis of ABO blood type the A antigen and the B antigen are derived from a common precursor known as The H antigen is a glycosphingolipid sphingolipid with carbohydrates linked with the H antigen or H substance ceramide moiety The I gene encodes a glycosyltransferase that is an enzyme that modifies the carbohydrate content of the RBC antigens The gene is located on the long arm of the ninth chromosome The IA allele gives type A IB gives type B and i nonfunctional enzyme gives I QUESTION A married couple that are IAIO and IBIO have kids How many of their offspring could be universal donors ANSWER you have kids that are IAIB IBIO IAIO IOIO only one child would be a universal donor IOIO and IAIB can get blood from anyone QUEISON what is the genotypic and phenotypic ratios ANSWER 1 1 1 1 because every child has a different combination Lethal Alleles where a mutation results in a recessive lethal one wild type copy is enough but to copies of the mutant allele result in a premature death ex the agouti gene in mice the agouti gene regulates hair color in mice the wild type mice are NOT black normal A allele gives the agouti color The AY allele called yellow is missing a regulatory part of a gene so the gene is always turned on leading to disruption of yellow pigment along the pari shaft a gain of function mutation AYAY homozygotes die early in development before birth AYA x AYA instead of 1 2 1 gives a 2 1 ration the AYAY are never born AA agouti x AA agouti AA agouti all survive AAY yellow x AAY yellow AA agouti AAY yellow AYA yellow AYAY lethal yellow agouti AA agouti x AAY yellow AA agouti AAYA yellow agouti yellow QUESTION Why do AYAY mouse fetuses die before birth ANSWER The deletion that causes the yell mutation extends into the next Merc gene which is essential for embryonic development QUESTION The rgh1 N1285 mutation of corn is embryo lethal What is the genotypic ratio of viable offspring in a Rgh1rgh1 x Rgh1rgh1 cross ANSWER have lethal gene so 1 2 for the living Gene Interaction Epistasis Epistasis is the greek work for stoppage and describes where the expression of one gene or gene pair can mask or modify the expression of another gene or gene pair the presence of zz stops expression of YY ex X could encode a transcription factor necessary for expression of the Y gene the Y gene is hypostatic to X ex Bombay Phenotype hh can affect AB blood type the presence of hh causes an A or B individual to show an O phenotype IAIB Hh x IAIB Hh gives a 3 6 3 4 modified ration Epistasis of locus over blood antigen locus IAIB Hh x IAIB Hh Consideration of blood types IAIB x IAIB IAIA IAIB IBIA IBIB consideration of H substance Hh x Hh HH Hh hH hh of all offspring type A for H substance and do not form H substance 3 16 2 4 type AB form H substance and do not form H substance type A and 1 16 type O 6 16 type AB and 2 16 type O type B and 1 16 type O type B form H substance and do not form H substance 3 16 QUESTION You cross a striped corn mutant from mexico with another striped mutant from Peru All the offspring are green Are these mutation in the same gene ANSWER No X Linkage The Y chromosome although acting as an homologous of the X chromosome during meiosis contains only a few genes none of which are contained on the X chromosome first documented by Thomas Hunt Morgan studying white eyed