Alberts • Bray • Hopkin • Johnson • Lewis • Raff • Roberts • Walter Essential Cell Biology FOURTH EDITION Chapter 19 Sexual Reproduction and the Power of Genetics Copyright © Garland Science 2014

Most multicellular organisms reproduce sexually. fertilization of egg by sperm Fig. 19-4

After fertilization, the diploid zygote then undergoes rounds of mitosis to generate a new multicellular adult. Fig. 19-5

Meiosis differs from mitosis in also having a reductive division. non-reductive division reductive division non-reductive division Fig. 19-6

Physical Basis of reductive division: separation of chromosome homologues Fig. 19-7

holds homologues together Paired chromosome homologues after duplication (one from each parent) Cohesin holds sister chromatids together Synaptonemal Complex holds homologues together Fig. 19-9

Physical Basis of reductive division: separation of chromosome homologues Fig. 19-8

Meiosis generates genetic diversity in two ways. Law of Independent Assortment Fig. 19-15

Recombination by strand invasion/copying mechanism Fig. 19-10

Cohesin holds sister chromatids together through Meiosis I. Fig. 19-13

Cohesin cleavage allows sister chromatid separation during Meiosis II. Fig. 19-13

Failure to separate in Meiosis I (or II) generates aneuploid gametes, like those responsible for Down’s Syndrome. Fig. 19-16

Mendel used pea plants to uncover the laws of genetics. Fig. 19-20

Law of Segregation (alleles separate during meiosis) He started with true-breeding plants. They were homozygous for the genes of interest. These crosses revealed two versions of genes (alleles); some are dominant and some are recessive. Law of Segregation (alleles separate during meiosis) Fig. 19-21

Crosses of F1 progeny showed that recessive trait is still present in F1 progeny. It reappeared in F2 progeny. Fig. 19-23

Alleles for two different genes can segregate independently Law of Independent Assortment (multiple chromosome homologue pairs segregate independently) Fig. 19-27

The same laws apply to other diploid multi-cellular organisms, including us. A gene encodes enzyme needed for melanin production. Fig. 19-25

Examples of how mutations can generate recessive vs. dominant phenotypes Example: G protein lost ability to bind GTP retains GTP binding but lost GTPase GTP-binding & GTPase domains recessive dominant

Frequency of recombination between two genes depends on distance between them. will appear to segregate independently will almost always co-segregate Fig. 19-29

Recombination frequency used to map genes on chromosomes T.H. Morgan and students responsible for establishing this principle Panel 19-1

Morgan established fruit flies as model system for mapping genes on chromosomes. Visible phenotypes (such as eye color) provided a whole array of genetic markers that can be used to map the positions of new genes.

Single Nucleotide Polymorphisms (SNPs) provide genetic markers for mapping mutations in human genes that cause disease. Fig. 19-36

SNPs inherited in chromosomal blocks (Haplotypes) Karp, Cell and Molecular Biology, Wiley & Sons

Haplotype blocks reflect our evolutionary history. - Closely linked SNPs co-segregate into populations. - Size of haplotype block reflects # generations since emergence of SNPs in the block. - Information used to map histories of different human populations after exiting Africa. larger haplotype blocks smaller haplotype blocks larger haplotype blocks Fig. 19-37

Genome-Wide Association Studies (GWAS) identify SNPs associated with disease phenotypes. Fig. 19-38 disease-associated mutation located in this haplotype block SNP and disease gene in linkage disequilibrium