Mendelian Genetics – inheritance of traits

Why Peas?? Many varieties (character, traits) Easy to control pollination Could choose distinct characters

First Experiments: True-breeding parent generation Why aren’t all the floweres light purple?

Mendel’s Law of Segregation White “heritable factor” did not disappear in F1 generation but only purple “heritable factor” was acting.  Alternative version of genes (heritable factors) account for variations in inherited characters.  For each character, an organism inherits two alleles (versions of the gene) one from each parent.  If the two alleles differ, one is fully expressed – DOMINANT; the other does not affect the organism’s appearance - RECESSIVE trait.  The two alleles segregate during gamete production

Mendelian Genetics Terms Homozygous Heterozygous Phenotype Genotype Punnett square Monohybrid cross Testcross

What happens if looking at two traits simultaneously? Dihybrid cross x True-breeding Parent (P) F1 F2: 9:3:3:1

Mendel’s Law of Independent Assortment Each character is independently inherited – segregate independently during gamete formation Practice dihybrid crosses Use rules of probability to predict outcomes of genetic crosses

ndelian_genetics/problem_sets/mo nohybrid_cross/monohybrid_cross.h tml

More Complicated Situations… 1.Incomplete dominance

Multiple Alleles I A I A = A; I A i = AI B I B =B; I B i = B I A I B = ABii = O A, B – codominant i - recessive

Other situations: Pleiotropy – one gene affects several phenotypes Epistasis – one gene affects the phenotypic expression of another gene Polygenetic inheritance – additive effect of two or more genes on a single phenotypic character

Human Disorders that follow Mendelian Patterns of Inhertitance Recessively inherited disorders: albinism, cystic fibrosis, Tay-Sachs disease, sickle-cell anemia Dominantly inherited disorders: Achondroplasia, Huntington’s disease

Cystic Fibrosis

Tay-Sachs Disease

Achondroplasia

Huntington Disease Late Acting – middle age Western Europeans

Screening for inherited disorders

Pedigrees

Pedigree – Huntington disease

Pedigree – Sickle cell anemia

Chromosomal Theory of Inheritance Thomas Morgan Fruitflies 4 pairs of chromosomes Wild type phenotype Mutant phenotype White-eyed males only

Sex-linked traits Located on Sex chromosomes: X or Y Females XX; Males XY X-linked recessive Hemophilia, color blindness