Topic 4 Genes, Chromosomes • Offspring acquire genes from parents by inheriting chromosomes. • Each gene in an organism’s DNA exists at a specific locus on a certain chromosome. We inherit one set of chromosomes from our mother and one set from our father.
Comparison of Asexual and Sexual Reproduction • In asexual reproduction, a single parent produces genetically identical offspring by mitosis. • Sexual reproduction combines sets of genes from two different parents, forming genetically diverse offspring.
Fertilization and Meiosis alternate in sexual life cycles • Sets of chromosomes—Normal human somatic cells are diploid. They have 46 chromosomes made up of two sets of 23. --- one set from each parent. There are 22 pairs of autosomes, each with a maternal and paternal homolog. The 23rd pair , the sex chromosomes, determine gender (XX) or (XY).
Behavior of chromosome sets in the Human Life Cycle • At sexual maturity, ovaries and testes (the gonads) produce haploid gamete by meiosis, each gamete containing a single set of 23 chromosomes (n=23). During fertilization, an egg and sperm unite, forming a diploid (2n = 46) single celled zygote, which develops into a multicellular organism by mitosis.
Meiosis reduces the number of chromosome sets from diploid to haploid • The stages of meiosis produce four haploid daughter cells. The number of chromosome sets is reduced from two (diploid) to one (haploid) during meiosis I, the reductional division.
A comparison of Mitosis and Meiosis • Meiosis is distinguished from mitosis by three events of meiosis I • Prophase I: each homologous pair undergoes synapsis and crossing over. • Metaphase I: Chromosomes line up as homologous pairs on metaphase plate. • Anaphase I: Homologs separate from each other; sister chromatids remain joined at the cetromere.
Meiosis II What’s left? • Separation of sister chromatids.
Genetic variation produced in sexual life cycles contributes to evolution! • Three events in sexual reproduction contribute to genetic variation in a population
Independent assortment of chromosomes during meiosis. • Homologous chromosomes can line up in no particular order of paternal or maternal chromosomes.
Crossing over during meiosis I • In prophase I of meiosis I, the replicated homologous pair of chromosomes comes together in the process called synapsis, and sections of the chromosomes are exchanged. You can see that after crossing over, the resulting chromosomes are neither entirely maternal nor entirely paternal, but contain genes from both parents. Synapsis and crossing over occur only in meiosis I
Random fertilization of egg cells by sperm. • During random fertilization, any one of the 8,388,608 possible combinations of gametes .... give rise to sperm cells, which will fertilize an egg and result in the offspring.
Chiasmata • Do to sister chromatid cohesion, crossing over leads to chiasmata, which hold homologs together.
Evolutionary Significance of Genetic Variation Within Populations. • Genetic variation is the raw material for evolution by natural selection. • Mutations are the original source of this variation. • The production of new combinations of variant genes in sexual reproduction generates additional genetic diversity.
Mendel’s Law of Independent Assortment • States that when gametes are formed, the separation of one pair of alleles between the daughter cells is independent of the separation of another pair of alleles. • One allele does not follow another when it is passed on to a gamete—they will sort independently.
Independent Assortment and Meiosis • Why do traits get passed on independently of one another? • 1. The orientation of bivalents during metaphase 1