Chapter 10: Sexual Reproduction and Genetics Fall 2011 Wood

Big Idea –Reproductive cells, which pass on genetic traits from the parents to the child, are produced by the pattern of meiosis. Sections –1) Meiosis –2) Mendellian genetics –3)Gene linkage and polyploidy

Section 1: Meiosis Each cell in the body has a specific number of chromosomes. For humans, our cells contain 46 chromosomes. We receive 23 chromosomes from the mother, and 23 chromosomes from the father

Human Cells Where do our 46 chromosomes come from? Having 2 sets of DNA is called having homologous chromosomes. –Same chromosome but carrying different versions of traits.

DadMom

Haploid Cells A cell that contains one set of chomosomes (n) is considered haploid (half-loid). –For humans these cells would only have 23 chromosomes –These cells are called gametes

Diploid Cells These are cells that contain 2 sets (2n) of chromosomes. –For humans, these would be normal cells with 46 chromosomes

Haploid & Diploid Cycle

Gamete formation Gametes, or haploid cells, are formed during a process called meiosis. Meiosis starts with one diploid cell, and ends up creating 4 haploid cells. Meiosis is split into 2 divisions: –Meiosis I –Meiosis 2

Interphase These cells still must go through interphase prior to meiosis. This allows the cell to make a copy if the DNA during the S-Phase

Meiosis I First phase is Prophase I –Chromosomes condense –Nuclear membrane dissolves

Metaphase I Homologous chromosomes align on the equator.

Anaphase I Homologous chromosomes separate and move to opposite poles.

Telophase I Chromosomes uncoil to form 2 nuclei The cell divides.

Meiosis II Prophase II –Repeat of prophase 1

Metaphase II Haploid number of chromosomes align on the midline.

Anaphase II Sister chromatids are pulled to opposite poles.

Telophase II Chromosomes reach the poles, and nuclear membranes form.

Cytokinesis II Meiosis results in 4 haploid cells each with n number of chromosomes.

Importance of Meiosis Creates 4 haploid daughter cells that are not identical. Results in genetic variation –Random creation of gametes –Ex) crossing over

Crossing over Occurs during prophase I to create genetic variation. Happens when parts of chromosomes are traded between a pair of homologous chromosomes.

Section 2: Mendellian genetics Overview –Start of genetics –Alleles –Dominant and recessive –Genotype and phenotype –2 laws of genetics –Punnett squares

The start of genetics In 1866, Greger Mendel published his findings on inheritance. He is now known as the “Father of Genetics” He was an Austrian monk who studied garden pea plants.

Mendel performed cross pollination in pea plants. He then studied these traits about the passing of traits through generations: –Seed color, flower color, seed shape or texture, and flower position.

Generations The parent generation is also known as the “P” generation.

The generation created by the parents is known as the “F 1 ” or first fillial generation. The second generation is called the “F 2 ” or second fillial generation.

Why was the second generation, or f1, all yellow? Why was there not green? This is due to the fact that genes always have different forms called allelles.

Alleles The alleles for our example are yellow and green. An allele is simply an alternate form of a gene. One allele will be dominant and the other will be recessive.

Where did the green seed come from in the third generation? They were not there in the second generation. Dominant alleles are shown, and recessive alleles are masked. Yellow seeds are dominant over green seeds.

Dominant vs Recessive Dominant alleles are always shown by a capital letter. Recessive genes are always shown by a lowercase letter. –Ex) yy YY Yy –Green yellow yellow

Homozygous vs heterozygous An individual can be one of 3 types: –Homozygous dominant –Heterozygous –Homozygous recessive

Genotype vs Phenotype A genotype is what genes an individual has. A phenotype is what characteristics are observed. Yy vs yy vs YY

Law of Segregation States that two alleles for a gene must separate during meiosis.

Law of Independent Assortment This law states that alleles occur in a random distribution. –Aka) the genes from one parent do not always stay together.

2 Laws of Genetics Mendel formulated 2 laws for genetics: –1) Law of Segregation –2) Law of Independent Assortment

Punnett Squares These predict the possible offspring of a cross between two known genotypes. Monohybrid cross

Section 3: Gene Linkage and Polyploidy Overview Genetic recombination Gene Linkage Polyploidy

Genetic Recombination The new combination of genes produced by crossing over and independent assortment is called genetic recombination. Human cells have a possible 2 23 combinations. Combine 2 cells and there are over 70 trillion possibilities.

Gene Linkage Genes that are located close to one another on a chromosome are said to be linked. This means that they usually travel together during meiosis.

Polyploidy This is the occurance of one of more extra sets of chromosomes in an organism. Ex) strawberries are 8n, coffee is 4n, and wheat is typically 6n.