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Chapter 4 – Patterns of Heredity 4.1 Living things inherit traits in patterns. Inherited Traits – traits you get from one or more parents. Acquired Traits – a trait that is developed during your life. Sexual Reproduction – A cell containing genetic information from the mother and a cell containing genetic information from the father combine into a completely new cell, which becomes the offspring. Genes and Chromosomes Gene- Unit of heredity that occupies a specific location on a chromosome and codes for a particular product. Heredity – passing of genes from parents to offspring. In most eukaryotes, cells contain pairs of chromosomes, with one chromosome of each pair coming from each of the two parents. Homologs – the chromosomes in a pair. The chromosomes have the same size and shape, and carry genetic information for particular traits. On each homolog are sites where specific genes are located. The genes may not always be identical. Alleles – The variations of the same gene. Chromosomes - Each organism has a specific number. - Humans have 23 pairs of chromosomes (total of 46). - Chromosomes are referred to by their pair number. - In humans the 23rd pair of chromosomes determines the sex. - 2 X chromosomes = a female - 1 Y and 1 X = a male Gregor Mendal (pg 104-105) Monk who examined traits in pea plants; 3 conlcusions: Concluded that some traits skipped a generation but came back in the next Each trait must have two factors: one from each parent. Some traits can be masked; only shown when both factors are for the trait. Phenotype vs. Genotype Phenotype – describes the actual characteristics that can be observed. Ex. Height, eye color, size of your feet, etc. Genotype – the name for genes that an organism has. Dominant and Recessive Alleles Dominant Alleles – one that is expressed in the phenotype even if only one copy is present in the genotype. Recessive Alleles – one that is expressed in the phenotype only when two copies of it are present on the homologs. * If one chromosome in the pair contains a dominant allele and the other contains a recessive allele, the dominant allele will be expressed in your phenotype. 4.2 Patterns of heredity can be predicted. Punnett Squares - Illustrate how parents’ alleles might combine for offspring traits. B b BB Bb bB bb - ¼ 2 dominant alleles - 2/4 or ½ 1 dominant + 1 recessive - ¼ 2 recessive alleles Ratios and Percentages Ratio – Shows a relationship between two quantities (4:4 or 4/4) Probability – chance that a specific outcome will happen Percentage – ratio that compares a number to 100. 4/4 x 100 = 100% ¼ x 100 = .25 x 100 = 25% 4.3 Meiosis is a special form of cell division. Most human cells (body cells) contain 46 chromosomes – the full number of chromosomes that is normal for a human being. Any cell that contains the full number of chromosomes (two sets) for a species is a 2n cell, also called a diploid cell. Gametes – cells that contain half the usual number of chromosomes – one chromosome from each pair. - Gametes are 1n cells, also called haploid cells. - Gametes are found in the reproductive organs of plants and animals. (Egg-female, sperm-male) - Human gametes contain 23 unpaired chromosomes Fertilization – the process that takes place when a sperm and an egg combine to form one new cell. (pg. 118) Meiosis- Special kind of cell division that produces haploid (1n) cells. - A cell goes through 2 cell divisions Meiosis I and Meiosis II. **- Meiosis takes place only in the reproductive tissues of an organism. - One cell in an organism’s reproductive system divides twice to produce four 1n cells or gametes Meiosis I diagram on page 121 Prophase I - The duplicated chromosomes pair up with their partners. - There are 2 sets of each of the chromosomes pairs in the parent cell. (twice as many) - The chromatids are attached together. - There are pairs of doubled homologs. Metaphase I - The chromosomes pairs line up in the middle of the cell. Anaphase I - The two copies of one homolog are pulled apart from the two copies of the other homolog. *Most significant step* Telophase I and Cytokinesis - A new cell membrane forms at the center of the cell, dividing the parent cell into 2 daughter cells. Meiosis II Prophase II - In each daughter cell, there are two copies of each of n chromosomes. - The copies are attached together. Metaphase II - Each duplicated chromosome lines up separately along each cell’s center. Anaphase II - The two attached copies of each chromosome separate and are pulled to opposite poles in each other. Telophase II and Cytokinesis - A new cell membrane forms in the center of each ell, as each cell divides into two 1n daughter cells, producing a total of four 1n cells. Mitosis vs. Meiosis Only cells that are to become gametes go through meiosis. A cell that divides by meiosis goes through two cell divisions, but the chromosomes are not copied before the second division. In mitosis, the chromosomes are always copied before division. Daughter cells produced by meiosis, which are haploid (1n), contain only half of the genetic material of the parent cell (one homolog from a chromosome pair). Daughter cells produced by mitosis, which are diploid (2n), contain exactly the same genetic material as the parent (pairs of chromosomes).