Transcript DNA
DNA And Genetics A. Definition = • The study of how genes are passed from parent to offspring (heredity) and how genes cause differences in organisms B. The Role of DNA and Chromosomes 1. During cell division, DNA segments coil to form chromosomes. 2. Each chromosome contains thousands of genes. 3. Each gene controls the production of a protein in the organism. 4. Proteins build your structure and create your traits or characteristics. 5. The chromosomes in a cell can be organized into homologous pairs or matched sets of chromosomes. (like socks) Human Chromosomes 6. The 46 chromosomes in a human cell create 23 homologous pairs of chromosomes. • The first 44 chromosomes or pairs 1 – 22 are referred to as autosomes. Autosomes • Autosomes = first 44 chromosomes or pairs 1-22 7. Pair # 23 is the gender determining chromosomes called sex chromosomes • May or may not match • Referred to as the X and Y sex chromosomes • The Y chromosome carries the genes for the male gender • If the 23rd pair of chromosomes matches, then the combination is called XX = female • If the 23rd pairs of chromosomes does not match, then the combination is called XY = male • Male or female? C. Genes 1. Are sections of DNA that code for specific proteins • Once a protein is produced, it creates a specific trait in the organism. • Examples of traits = eye color, hair texture, ear lobe attachment, eye shape 2. Most traits are controlled by just 2 genes • 1 gene for that trait comes from your mom • 1 gene for that trait comes from your dad 3. The genes for a single trait can be slightly different: • Different versions of the same gene for a trait are called alleles. • Different alleles can result in different effect on the trait • Examples: Trait for eye color • Alleles = blue, green, brown • Words to describe the genes that are present for a trait. • Homozygous = the alleles for the trait are the same or pure (purebred). • Examples of symbols for homozygous genes = HH, hh, BB, bb • Heterozygous = the alleles for the trait are different or hybrid. • Examples of symbols for heterozygous genes = Hh, Bb • Genotype = the exact genes that individual has for a trait • Examples: eye color = bb • height = Tt • disease = NN • Phenotype = a description of the effects of the genes on a trait; physical features • Examples: Blue eye color • Tall height • • Has cystic fibrosis Is nearsighted D. The Father of Genetics 1. The first individual to do extensive research on how genes work was Gregor Mendel. • Between 1856 and 1863 Mendel grew and • observed over 29,000 pea plants to discover how certain traits were passed from one generation to the next Some of the traits he studied were the height of the plants, color of the flowers, and the seed shape (peas). 2. Based on his research, Mendel proposed several genetic theories, one being the Law of Dominance and Recessiveness in genes for a traits. Complete Dominance • One of the two alleles for a trait hides the effects of the other allele = strong = dominant allele • The symbol for a dominant trait is written as a capital letter (T, R, B, W) • The other allele (the one that is hidden by the dominant allele) is weaker and is called the recessive allele. • The symbol for the recessive allele is written as a lower case letter (t, r, b, w) • When an individual has one of each of the alleles (Tt, Rr, Bb, Ww), only the dominant allele shows, recessive is hidden by the dominant allele. An example of a pea plant trait discovered by Mendel In pea plants, the trait for height is controlled by two alleles. • The tall allele is dominant over the short allele, which is recessive • Symbols: T for the tall allele, t for the short allele • Pea plants have the following gene combinations to control how high they grow • Bill Nye Video: Genes Possible 2 gene combinations a pea plant can have The height of the pea plant based on its genes TT (homozygous) Tall Tt (heterozygous) Tall tt (homozygous) Short The exact genes an individual has for a trait = genotypes The effects of the genes on an individual = phenotypes E. Punnett Squares • A Punnett Square is a chart used to figure out all of the possible gene combinations an offspring could get from its parents. • Example: • tt t T Tt t t Homozygous tall pea plant crossed with a homozygous short pea plant T_ __T__ Write the two genes from the other parent across the top __t_ _ Write the two genes from one parent along the side. _t___ • Fill in the boxes inside the Punnett Square with the two letters on the outside of each box (bring top one down and side one across). • The letters inside each smaller box represent the genes an offspring could POSSIBLY inherit. Example: One parent is heterozygous tall and the other is homozygous short Tt tt T _______ t ____ Tt Tt t _________ tt tt t ____ After filling in the Punnett Square: Determine the percent chance that an offspring will get certain genes (genotypes) and trait (phenotype) • 0 out of 4 boxes in square = 0% • 1 out of 4 boxes = 25% • 2 out of 4 boxes = 50% • 3 out of 4 boxes = 75% • 4 out of 4 boxes = 100% Possible genotypes and % chance each could occur Possible phenotypes and % chance each could occur Tt = 50% Tall = 50% Tt = 50% Short = 50% Practice problems • Do the 2 practice problems in your packet.