4 Chapter 14~ Mendel & The Gene Idea

Mendelian genetics 4 Character – heritable feature that varies among individuals: fur color, flower color, height 4 Trait – variant for a character: brown/white, tall/short 4 True-bred (self-pollination) – all offspring of same variety 4 Hybridization (cross-pollination) – crossing of 2 different true-breds 4 P generation (parents) 4 F1 generation (first filial generation) –Offspring of P generation 4 F2 generation – offspring of F1

Leading to the Law of Segregation 4 Alternative versions of genes (alleles) account for variations in inherited characteristics 4 For each character, an organism inherits 2 alleles, one from each parent 4 If the two alleles differ, then one, the dominant allele, is fully expressed in the organism’s appearance; the other, the recessive allele, has no noticeable effect on the organism’s appearance 4 Mendel’s Law of Segregation –The alleles for each character segregate (separate) during gamete production (meiosis).

Genetic vocabulary……. 4 Punnett square: predicts the results of a genetic cross between individuals of known genotype 4 Homozygous: pair of identical alleles for a character 4 Heterozygous: two different alleles for a gene 4 Phenotype: an organism’s traits 4 Genotype: an organism’s genetic makeup 4 Testcross: breeding of a recessive homozygote X dominate phenotype (but unknown genotype)

The Law of Independent Assortment 4 Law of Segregation involves 1 character. What about 2 (or more) characters? 4 Monohybrid cross (cross of 1 character) vs. dihybrid cross (cross of 2 characters) 4 Mendel’s Law of Independent Assortment –The two pairs of alleles segregate independently of each other.

Non-single gene genetics, I 4 Spectrum of Dominance 4 Complere dominance: dominant only expressed 4 Incomplete dominance: appearance between the phenotypes of the 2 parents. Ex: snapdragons 4 Codominance: two alleles affect the phenotype in separate, distinguishable ways. –Ex: Tay-Sachs disease – enzyme to metabolize lipids in brain doesn’t work: AA – all normal, Aa – ½ normal, ½ dysfunctional, aa – all dysfunctional 4 Multiple alleles: more than 2 possible alleles for a gene. –Ex: human blood types, rabbit fur color

Non-single gene genetics, II 4 Pleiotropy: genes with multiple phenotypic effect. –Ex: sickle-cell anemia, cystic fibrosis 4 Epistasis: a gene at one locus (chromosomal location) affects the phenotypic expression of a gene at a second locus. –Ex: mice coat color – 1 gene determines black/brown, 1 gene determines if there is pigment 4 Polygenic Inheritance: an additive effect of two or more genes on a single phenotypic character –Ex: human skin pigmentation (3 genes) and height 4 Nature vs Nurture

Human disorders 4 The family pedigree –Circle – female; Square – male –Shaded – have trait; Not shaded – trait isn’t expressed 4 Recessive disorders: –Albinism –Cystic fibrosis – increase of concentration in extracellular chloride causes mucus build up in lungs/digestive tract Most common Lethal 1/2500 of European descent –Tay-Sachs

Human Disorders cont. –Sickle-cell – amino acid substitution in hemoglobin protein Shape causes clumping 1/400 of African descent Large # of heterozygous - Malaria 4 Dominant disorders –Achondroplasia - dwarfism –Huntington’s – degenerative disease of nervous system Appears btw Fatal

Human Disorders cont. 4 Testing –Amniocentesis – insert a needle into uterus Test on amniotic fluid 14 th -16 th week –Chorionic villus sampling (CVS) – insert tube through cervix into uterus Take tissue from placenta

Chromosomal Linkage 4 Thomas Hunt Morgan – work proved that genes are on specific chromosomes 4 Worked with Drosophilia melanogaster (fruit fly) 4 XX (female) vs. XY (male) 4 Sex-linkage: genes located on a sex chromosome 4 Linked genes: genes located on the same chromosome that tend to be inherited together

Human sex-linkage 4 SRY gene: gene on Y chromosome that triggers the development of testes 4 Fathers= pass X-linked alleles to all daughters only (but not to sons) 4 Mothers= pass X-linked alleles to both sons & daughters 4 Sex-Linked Disorders: Color-blindness; Duchenne muscular dystropy (MD); hemophilia 4 4 X-inactivation: 2nd X chromosome in females condenses into a Barr body (randomly) – genes are only expressed from one 4 Females may be a mosaic 4 Barr body is reactivated in ovaries 4 Ex. Tortoiseshell cats & Calico cats (white determined by another gene)

Chromosomal errors, I 4 Nondisjunction: members of a pair of homologous chromosomes do not separate properly during meiosis I or sister chromatids fail to separate during meiosis II 4 Aneuploidy: chromosome number is abnormal –Monosomy~ missing chromosome–Turner syndrome –Trisomy ~ extra chromosome - Down syndrome (pair 21), Kleinfelter syndrome –Polyploidy~ extra sets of chromosomes – common in plants

Chromosomal errors, II 4 Alterations of chromosomal structure: –Deletion: removal of a chromosomal segment –Duplication: repeats a chromosomal segment –Inversion: segment reversal in a chromosome –Translocation: movement of a chromosomal segment to another