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Mendelian Genetics Bio 100 Tri-County Tec. College Pendleton, S. C. Contributions of Mendel • Inherited characteristics are carried as discrete units. • These discrete units are parceled out in different ways with each generation. Mendel and Garden Peas • Mendel chose garden peas to work with. • Good Choice – abundantly available at his monastery – easily cultivated – varieties had clearly defined characteristics which bred true (“wild-type” trait) – pollen of the pea is contained within each flower--prevent accidental crosspollination Garden Pea Flower Genetic Terminology • Gene – a unit of heredity; a segment or locus on a chromosome influencing a single trait • Allele – alternate forms of the same gene – A plant has a gene for flower color with an allele for red flowers and an allele for white flowers. Terminology, ctd. • Phenotype – the visible characteristics of an organism; for example: tallness or shortness • Genotype – the genetic makeup of an organism that you cant see; for example TT for tallness or tt for shortness Terminology, ctd. • Homozygous – describing the genotypic condition present when members of a gene pair determining a single trait on homologous chromosomes are the same. TT for tallness is homozygous. tt for shortness is also homozygous Both are alleles of the gene for height. Terminology, ctd. • Heterozygous – Term that describes the genotypic condition present when members of a pair of alleles are different on homologous chromosomes. For example, Tt. • Dominant – describes a trait (allele) that is expressed whenever it is present Terminology, ctd. • Recessive – describes a trait (allele) that is expressed whenever the dominant allele is not a part of the genotype. • Please note*** – Dominant alleles are usually noted with a capital letter; while recessive alleles are noted in small caps. Terminology, ctd. • P1 (parental generation) – the generation that you start with in a genetic study • F1 (first filial generation) – offspring of the P1 parents • F2 (second filial generation) – offspring of the crossing of members of the F1 generation Law of Segregation of Alleles • It says-– a pair of alleles is segregated or separated during the formation of gametes during reduction division (meiosis) • For example: – “Aa” alleles will segregate with the A allele going into one gamete and the a going into another gamete. Traits Mendel Studied Probability vs. possibility • Weather forecast states probability of rain tomorrow is 30%….WRONG!!! • Probability is defined as the number of times an event occurs divided by total number of possible outcomes • Probability of rain on any given day is 50%…it will either rain or it will not rain • Time for an Estesism Explanation P1 Generation: F1 Generation: PP x pp Pp (all purple) Explanation Cross-pollination of members of the F1 Pp x Pp F2 PP Pp Pp pp Purple White Gametes • The genotype of the P1 is “PP” and “pp”. – the “PP” parent will produce gametes containing only the “P” allele – the “pp” parent will produce gametes containing only the “p” allele • The genotype of the F1 parents is “Pp”. – 50% of the gametes will contain “P” – 50% of the gametes will contain “p”. Making the cross • The cross between P1 is no problem – “P” combines with “p” to produce “Pp” • To do the cross between F1 members – Punnett square • Each space is 25% chance. P p P PP Pp p Pp pp Dated but still useful concept • Field of corn and we can see its all tall and we know tall is dominant trait • Do we know the genotype? • In old days, testcross would be used to determine genotype • TESTCROSS is mating of an unknown genotype with individual homozygous recessive for the trait • Do the Punnett square The Dihybrid Cross • Some characteristics are inherited together in pairs. • Mendel worked with seed color and seed surface texture. • Phenotypes – yellow and green seeds; smooth and wrinkled – yellow dominates green; smooth dominates wrinkled P1 pure breeding X purebreeding smooth-yellow F1 wrinkled-green all smooth-yellow smooth-yellow x smooth-yellow F2 315 smooth-yellow 101 wrinkled-yellow 108 smooth-green 32 wrinkled-green 9:3:3:1 ratio P1 Gametes • SSYY bred with ssyy • SSYY can produce only one kind of gamete: SY • ssyy can produce only one kind of gamete: sy • When these are combined you get the genotype for the F1 generation: SsYy Determining F2 Gametes • To determine the gametes for the F2 we have to segregate each pair of alleles independently of each other S s Y y Y y SY Sy sY sy Making The Cross • Use a 16 chambered Punnet Square SY Sy sY sy SY SSYY SSYy SsYY SsYy Sy SSYy SSyy SsYy Ssyy sY SsYY SsYy ssYY ssYy sy SsYy Ssyy ssYy ssyy Summarizing The Results • Since smooth dominates wrinkled and yellow dominates green – – – – S_Y_ : smooth yellow -- 9 of those S_ yy: smooth green -- 3 of those ssY_ : wrinkled yellow -- 3 of those ssyy : wrinkled green -- 1 of those Blood Types • Due to an antigen (protein) on the RBC • Due to multiple alleles • Three alleles for blood type – A-allele – B-allele – O-allele • Only two of these alleles per person Codominance of Blood Types • The A and B alleles dominate the O allele • When they are together, the A and B alleles show incomplete dominance (codominance) • All the possible genotypes for blood types – – – – type A genotypes: AA and AO type B genotypes: BB and BO type AB genotype: AB type O genotype: OO Incomplete or Lack of Dominance • Really does look like “blending” • Offspring is intermediate between the two parents • Red flower crossed with white flower giving all pink offspring • White hen and black rooster=all gray chicks • Red bull and white cow = roan calf Polygenic Inheritance • Prefix “poly” means many • Polygenic inheritance is controlled by the interaction of many genes • Human skin color – AA BB CC DD -- very dark skin – Aa Bb Cc Dd -- medium – aa bb cc dd -- very light • height is also polygenic controlled Pleiotropy • Opposite to polygenic inheritance • In pleiotropy, one gene results in the expression on many different phenotypes • Best example is sickle cell disease • Study 9.21 on page 157 Sex Determination • A human female always produces an ovum with an X-chromosome • 50% of sperm contain a X-chromosome • 50% of sperm contain a Y-chromosome • X-sperm + X-ovum = XX (female) • Y-sperm + X-ovum = XY (male) Punnet Square for Sex Determination X X X XX XX Y XY XY Sex Linkage • The X-chromosome carries alleles for various traits. • The Y-chromosome does not carry alleles for the traits we will discuss. • Some traits are sex-linked because their alleles are linked to the X-chromosome • We will look at colorblindness and hemophilia. Colorblindness • • • • • • XCY -- normal male XcY -- colorblind male XCXC-- normal female (homozygous) XCXc-- normal female (heterozygous) XcXc -- colorblind female (homozygous) Colorblind females are relatively rare. Recessive disorders • Majority of disorders affecting humankind are recessive in nature • Albinism • Cystic fibrosis • Galactosemia • Phenylketonuria (PKU) (care for diet soda?) • Sickle-cell disease • Tay-Sachs disease Dominant disorders • Achondroplasia • Alzheimer’s disease (this one is still kind of if’y) • Huntington’s disease • Hypercholesterolemia • ???markers for alcoholism, homosexuality, some mental disorders…???? Pedigree Analysis • Pedigree is family tree describing the occurrence of heritable characteristics in parents and offspring across many generations • Where would one find most of the pedigrees in this part of the country? • Useful in genetic counseling • Like it or not…”Roots” are important, especially as one gets older and more aware of his/her mortality Can’t choose your parents, yet!! • A carrier is an individual who is heterozygous for a recessive inherited disorder and who therefore DOES NOT show symptoms of that disorder • Carrier does have probability of passing the recessive gene on to offspring • Queen Elizabeth and the royal houses of Europe Fetal testing procedures • • • • • Ultrasound Amniocentesis Chorionic villus sampling (CVS) Fetoscopy All except ultrasound carry some risk for both mother and fetus • Big time difference in obtaining results • What choice would (should) one make???