Meiosis

General Overview Goal is to produce 4 unique gametes Occurs in germ cells in testes and ovaries. Genetic information is exchanged between maternally and paternally inherited traits. Contributes to variability.

Homologous Chromosomes Same size/length Same centromere position Carry the same information

Meiosis I: The Reduction Division  Results in reduction of chromosome number from diploid (cell has two copies of each chromosome, one maternal and one paternal chromosome) …to haploid (cell has only one copy of each chromosome, either maternally-derived or paternally-derived chromosome) Chromosomes are two sister chromatids at this point

Interphase DNA is replicated. Each chromosome is copied There is now the equivalent of four copies of each chromosome in the cell

Prophase I Chromatin condenses Homologous chromosomes pair up

Crossing Over Occurs in Prophase I Paternal and Maternal homologous chromosomes are paired up and intertwine, exchanging information to create new and unique chromosomes.

Metaphase I Chromosomes attach to spindle fibers at the centromere. Homologous chromosomes line up at the equator.

Anaphase I Homologous chromosomes move to opposite poles of the cell

Telophase Spindle fibers break down. Chromosomes uncoil into nuclei Cell divides into two intermediate cells each with 46 chromosomes.

Prophase II Spindle fibers form. Chromosomes condense

Metaphase II Haploid chromosomes line up on the equator

Anaphase II Sister chromatids are pulled apart at the centromere and moved to opposite poles of the cell.

Telophase II Chromosomes reach the poles Spindle fibers disappear and nuclear membranes reform.

Finally… Meiosis results in four genetically unique haploid cells, each with n chromosomes.

Bell work 12/7 Suppose that for an organism, 2N = 24. How many chromosomes do the organism’s gametes contain? Explain.

Importance of Meiosis Genetic diversity. Genetic issues possessed by one parent may not be passed on to offspring. Depending on how the chromosomes line up at the equator, four gametes with four different combinations of chromosomes can result. Genetic variation also is produced during crossing over and during fertilization, when gametes randomly combine

Mendelian Genetics Gregor Mendel: Austrian, Monk, Awesome. “Father of genetics” (he kind of only discovered simple genetics that aren’t really the common type in humans but whatev) Published his findings on pea plants. I am really into the genetics of peas.

Inheritance of Traits Mendel selectively bred yellow and green pea plants, removing male organs of the yellow plants to prevent self-fertilization. The yellow and green plants were known as the parent or P generation- first generation. Pea plants are true-breeding: they produce offspring with only one form of a trait.

First Filial Generation (F1) The offspring of the P generation is the first filial generation. When yellow and green pea plants are crossed, the resulting seeds were always yellow. Mendel crossed the F1 generation with itself to get the F2 generation….

F2 Generation Mendel noticed that in the F1 generation, the green trait seemed to disappear. To test this, he crossed F1 plants together. These offspring were called the F2 generation. These seeds showed a near perfect 3:1 yellow:green ratio. The trait was not gone, simply masked in the generation before.

Dominance The yellow trait in this case was dominant and masks the green color trait. Only individuals with homozygous green genotype (yy) would display a green phenotype. Homozygous= 2 of the same alleles Any heterozygous (Yy) or homozygous yellow (YY) individuals would appear yellow. Heterozygous= 2 different alleles

Genotype and Phenotype Genotype: the pairs of alleles (alternative forms of a gene) that an individual possesses. Phenotype: the outward expression of the allele pairs an individual possesses.

Mendel’s Law of Segregation every individual possesses a pair of alleles for any particular trait; each parent passes a randomly selected copy (allele) of only one of these to its offspring. Alleles separate during meiosis.

Mendel’s Law of Independent Assortment Allele pairs separate independently during the formation of gametes. This means that traits are transmitted to offspring independently of one another.

Gene Linkage Genes that are close together will not assort independently. They are often inherited together. The farther apart the two genes are, the less likely they are to be inherited together. This is due to crossing over.

Monohybrid Cross Crossing two hybrid (heterozygous) individuals Yy x Yy: Phenotypic ratio is 3:1

Dihybrid Cross Crossing two individuals hybrid for two traits. AaBb x AaBb Phenotypic ratio is 9:3:3:1