I. The Origins of Genetics A. Heredity – the passing of traits from parents to offspring. B. Genetics – the study of heredity. C. Gregor Mendel – a monk whose experiments with pea plants gave us our basic understanding of heredity.
II. Mendel’s Theory A. Gene – is a segment of DNA that controls a hereditary trait. These are carried on the chromosomes. 1. Alleles – the different versions of a gene. a. dominant – allele that is the expressed form of the trait. Always shown by a capital letter. Ex: T b. recessive – allele that is not expressed when the dominant allele is present. Always shown by a lower case letter. Ex: t
B. homozygous – when the paired alleles for a trait are the same, either both dominant or both recessive. Ex: RR (homozygous dominant) or rr (homozygous recessive) C. heterozygous – when the paired alleles for a trait are different, one dominant and one recessive. Ex: Rr D. genotype – the gene combination present in an organism. Ex: RR, Rr, rr E. phenotype - the physical appearance resulting from the genes. Ex: round seeds, wrinkled seeds F. law of segregation – when the paired alleles for a trait are separated during meiosis and the formation of the gametes. G. Law of Independent Assortment - alleles for a trait separate when gametes are formed. EX: There are 4 ways the genes from DdRr can sort: DR, Dr, dR, dr
III. Studying Heredity A. Punnett square – a diagram used to predict the outcome of a genetic cross. B. Probability – the likelihood that a specific event will occur. Can be expressed in words, decimals, percentages, or as fractions.
C. Pedigree – a diagram that shows a family history and how a trait is inherited over several generations.
IV. Complex Patterns of Heredity A. Sex-linked – any trait whose allele is carried on the X chromosome. 1. Because males have just one X chromosome, any allele on the X chromosome is expressed. Ex: hemophilia, color blindness B. Polygenic trait – when several genes influence a trait. Ex: haircolor, eye color
C. Incomplete dominance – when an individual displays a trait that is intermediate (a blending) between the two parents. Ex: straight haired X curly haired = wavy haired red flower X white flower = pink flower D. Multiple alleles – genes with 3 or more alleles. Ex: blood types in humans (A, B, O) E. Codominance – two dominant alleles are expressed at the same time. Ex: blood type AB
F. Mutations – changes in the chromosome structure. The harmful effects produced by inherited mutations are called genetic disorders. 1. sickle-cell anemia – produces a defective form of the protein hemoglobin. Most common among African- Americans. 2. cystic fibrosis – most common fatal genetic disease among Caucasians. 3. hemophilia – sex-linked recessive disorder. Condition that prevents the blood from clotting properly. 4. Huntington’s disease – does not appear until age 30- 40. Loss of muscle control, physical spasms, severe mental illness, and finally death.