Transcript Heredity
Heredity Obj. 3d Heredity • The passing of traits from parent to offspring • Traits: physical characteristics of an organism –Example: eye color, hair color, & height Passing Traits to Offspring • Sex cells have 23 chromosomes and the two sex cells combine to form body cells with 46 chromosomes. • In fertilization, one sperm joins with one egg. • The offspring receives half of its genetic information from its mother and the other half from its father. Genetics • The study of how traits are passed from parent to offspring by looking at genes • Genes are small sections of DNA on a chromosomes that has information about a trait • Each chromosome has a gene for the same trait (eye color from mom & eye color from dad) • Traits are determined by alleles on the chromosomes • Each gene of a gene pair is called an allele • Inherited traits are determined by the alleles on the chromosome DOMINANT AND RECESSIVE • Dominant Alleles describe a genetic factor that is always expressed. – It prevents a recessive trait from showing up in offspring. – Represented by capital letters (B) • Recessive Alleles describe a genetic factor that is not always expressed. – It only expresses itself when both of the recessive traits are inherited – Represented by lowercase letters (b) Examining & Studying Traits • Two ways scientist study traits – Phenotype: Physical appearance or outside expression of a gene • Blue Eyes – Genotype: the two alleles a person has inherited that can only be seen on the DNA • BB, Bb, or bb • Two categories of genotypes – Homozygous: inherited two identical alleles • BB (pure dominant) or bb (pure recessive) – Heterozygous: inherited two different alleles • Bb (hybrid) INHERITANCE & PASSING TRAITS Punnett Squares • Shows all possible combinations of alleles that children can inherit from parents • Mom’s genotype for brown eyes (Bb) • Dad’s genotype for brown eyes (Bb) • Offspring’s Phenotype – 75% brown, 25% blue • Offspring’s Genotype – 25% BB, 50% Bb, 25% bb B b B BB Bb b Bb bb Punnett Square • A Punnett square is actually a way to show the events that occur at meiosis. A Punnett Square • The diagrams show how to make a Punnett square. In this cross, both parents are heterozygous for the trait of seed shape. R represents the dominant round allele, and r represents the recessive wrinkled allele. DOMINANT & RECESSIVE VARIATIONS Variations are the different ways that a certain trait appears. • These variations would allow one offspring to have light brown hair & another to have darker brown hair Genetic Advantages & Disadvantages • Selective Breeding organisms for a certain trait – Pure breeding • Advantages: allows breeder to have desirable traits expressed in offspring • Disadvantages: unfavorable traits can be passed & lack variety to survive diseases or climate changes – Hybrid • Advantages: has genetic variety to increase variations that allows species to adapt to climate changes or recover from diseases • Disadvantages: traits are uncertain in predicting Gregor Mendel • Known as the father of genetics • Studied several traits in pea plants and discovered patterns in the way traits are passed from one generation to another • Discovered dominate & recessive traits Mendel’s Work with Crossing Pea Plants Mendel’s Experiments • In all of Mendel’s crosses, only one form of the trait appeared in the F1 generation. However, in the F2 generation, the “lost” form of the trait always reappeared in about one fourth of the plants. Thomas Hunt Morgan • First person to definitively link trait inheritance to a specific chromosome • Studies fruit flies eye color