Chapter 10 Genetics: Mendel and Beyond

1. Chapter 10 Genetics:Mendel and Beyond Biology 101 Tri-County Technical College Pendleton, SC

2. It’s a character trait • A character is a feature such as flower color • Best described as a gene • A trait is a particular form of a character, such as white flowers • Best described as an allele • A heritable character trait is one that is passed from parent to offspring • Before Mendel, blending was model of choice…and it was logical to a point

3. Law of Segregation • Mendel and it’s all about flowers and sex • In sexually reproducing organisms, an individual possesses 2 “factors” for each trait • One came from mommy and one from daddy • When that individual makes gametes, those “factors” (alleles) will separate (segregate) from each other and wind up in different gametes • **Gamete receives only ONE member of the pair of alleles

4. Defining Definitions • Genotype is genetic constitution of the organism • It’s what in the genes • Phenotype is the physical expression of the genotype • It’s what one sees—phenotype is the expression of genotype • Dominant means always expressed (in the phenotype) • Only one allele needed for its expression • DOES NOT MEAN “THE BEST TO HAVE”

5. Definitions, cont. • Recessive means “covered”, “hidden”, or “masked” by presence of dominant allele • Recessive allele only expressed when present in pairs • Homozygous means “same” or two copies of same allele • Can be homozygous dominant or homozygous recessive • TT or tt, respectively

6. Definitions, cont. • Heterozygous means “different” or “other” • Two different alleles of the “trait” (gene/allele) in question (Tt) • Monohybrid means differing in only ONE trait • tall or short; yellow or green, etc. • Dihybrid means differing in TWO traits • tall and yellow; short and green

7. Monohybrid Cross • P = parental generation • F1 = first filial generation (first generation of hybrid) • F2 = second filial generation (second generation of a hybrid) • Punnett square is an application that allows prediction of probability of genotypes/ phenotypes from a genetic cross

8. Test Cross • Test cross is dated concept that still serves useful purpose • Estes and the field of corn • Defined as mating an individual of unknown genotype with an individual homozygous recessive for the trait • Figure the Punnett square for this one…you will see it again!!

9. Law of Independent Assortment • In dihybrid crosses, parental traits appear in new combinations of 2 of the phenotypic classes • such new combinations called recombinant phenotypes • Alleles of different genes assort independently of one another during gamete formation • In other words, T does not always stay with Y and vice versa • Law of Independent Assortment NOT as universal as law of segregation

10. Independent Assortment, cont. • Applies to genes on separate chromosomes but not necessarily to those that lie on same chromosome • Simply states that chromosomes segregate independently during formation of gametes as do any two genes (alleles) on separate chromosome pairs

11. Dihybrid Crosses • Review: dihybrid means two contrasting traits • TTYY crosses with ttyy • Do the Punnett square and look for 9 genotypes and 4 phenotypes • Be sure and remember the phenotypic ratio…you will see it again and again!!!

12. Incomplete Dominance • When heterozygous phenotype is intermediate, gene is said to be governed by incomplete dominance/lack of dominance • Looks like blending to be sure • Time for a Wally World story…Yeah!!!! • Red and white snapdragon cross produces all pink offspring • All F1 to self-fertilize and predict F2 • Black rooster and white hen = all gray chicks • Red bull and white cow = all roan calves

13. To dominate…or Not • Complete dominance results when single allele produces enough protein to give maximum phenotypic response • Incomplete dominance results when heterozygotes show intermediate phenotype • Codominance results when two alleles at locus produce two different phenotypes that both appear in heterozygoes • Means both are fully expressed • Best example is ABO blood grouping

14. ABO Blood Grouping • Blood type determined by proteins on surface of RBCs • Protein coded for by info in DNA • Alleles are IA and IB • There are six genotypes and four phenotypes • This is ABO only, we will discuss Rh factor • This one always gets me into trouble • Chalk talk time on ABO and Rh typing

15. Linkage • Association between markers (genes/alleles) on same chromosome such that they do NOT show random assortment and seldom recombine • Closer the markers, lower frequency of recombination • Closer they are together, more likely they are to STAY together • MAP units

16. Determining Sex • Sex (gender) determined by sex chromosomes (23rd pair) • Females are XX and males are Xy • AFAWK, only thing on y chromosome is SRY (male-determining) gene and gene for hairy ears • Nature was not overly kind to males • Implications of XX and Xy

17. Sex-linked Disorders • Defined as being “tied” to X sex chromosome • Sex-linked disorders affect males more • One mutant recessive allele leads to red-green colorblindness, muscular dystrophy, and hemophilia • Human mutations inherited as sex-linked dominant must LESS common than recessives

18. Sex-linked, cont. • Dominants appear in every generation and individuals carrying harmful mutation (even as heterozygotes) often fail to survive and/or reproduce • Chalk talk time on hemophilia and/or color blindness