Chapter 14&15 Notes

AP Biology Chapter 14 & 15 Chapter 14: Mendel and the Gene Idea Character-flower color Trait-specific: purple or white Hybridization-the mating or crossing of 2 true-breeding varieties P generation- parental generation F1- first filial generation F2- second filial generation 4 Concepts of Mendel’s Model: 1) Alternative versions of genes account for variations in inherited characters. 2) For each character, an organism inherits 2 allele, one from each parent. 3) If the 2 alleles at a locus differ, then one, the dominant allele, determines the organism’s appearance; the other, the recessive allele, has no noticeable effect on the organism’s appearance. 4) Law of segregation states that 2 alleles for heritable characters separate during gamete formation and end up in different gametes. Phenotype- physical traits Genotype- genetic makeup Testcross- breeding of a recessive homozygote with an organism of dominant phenotype but unknown genotype. Ex: Figure 14.7 Monohybrid- heterozygous for one character. Dihybrid- heterozygous for both characters. Law of Independent Assortment- each pair of alleles segregates independently of other pairs of alleles during gamete formation. Complete dominance- F2 plants always look like one of the 2 parental varieties. Codominance- 2 alleles both affect the phenotype in separate distinguishable ways. Incomplete dominance-the alleles for some characters fall in the middle of the spectrum of dominance. (ie. Pink snapdragons) Tay-Sachs disease- an inherited disorder in humans. The brain cells of a baby with TaySachs disease are unable to metabolize certain lipids because a crucial enzyme does not work properly. As these lipids accumulate in the brain cells, an infant begins to suffer seizures, blindness, and degeneration of motor and mental performances. An affected child dies within a few years. Only children who inherit 2 copies of this allele have the disease. Pleiotropy- multiple phenotypic effects. For example: pleiotropic alleles are responsible for the multiple symptoms associated with certain hereditary diseases in humans, such as cystic fibrosis and sick-cell disease.

Epistasis: a gene at one locus alters the phenotypic expression of a gene at a second locus. (example: fur in mice controls color) Norm of reaction- genotype that is generally not associated with a rigidly defined phenotype, but rather with a range of phenotypic possibilities due to environmental influences. (nature vs. nurture) Multifactorial- many factors, both genetic and environmental, collectively influence phenotype. (example: skin color) The term phenotype can refer not only to specific characters, such as flower color and blood group, but also to an organism in its entirety—all aspects of its physical appearance, internal anatomy, physiology, and behavior. Similarly, the term genotype can refer to an organism’s entire genetic makeup, not just its alleles for a single genetic locus. Carriers- heterozygotes may transmit the recessive allele to their offspring, which makes them a carrier. Cystic fibrosis- the normal allele for this gene codes for a membrane protein that functions in chloride ion transport between certain cells and the extracellular fluid. These chloride transport channels are defective or absent in the plasma membranes of children who inherit two recessive alleles for cystic fibrosis. Sickle-cell disease- caused by the substitution of a single amino acid in the hemoglobin protein of the red blood cells. Huntington’s disease- a degenerative disease of the nervous system, caused by a lethal dominant allele that has no obvious phenotypic effect until the individual is about 35 to 45 years old. Achondroplasia- dwarfism. Both Huntington’s disease and Achondroplasia are dominant. Amniocentesis- a physician inserts a needle into the uterus and extracts about 10 ml of amniotic fluid, the liquid that bathes the fetus. Chorionic villus sampling (CVS)- a physician inserts a narrow tube through the cervix into the uterus and suctions out a tiny sample of tissue from the placenta, the organ that transmits nutrients and fetal wastes between the fetus and the mother. In the ultrasound technique, sound waves are used to produce an image of the fetus by a simple noninvasive procedure. In fetoscopy, a needle-thin tube containing a viewing scope and fiber optics (to transmit light) is inserted into the uterus. Ultrasound has no known risk to either mother or fetus, but amniocentesis and fetoscopy cause complications, such as maternal bleeding or even fetal death, in about 1% of cases. One common screening program is for phenylketonuria (PKU), a recessively inherited disorder. Children with this disease cannot properly break down the amino acid phenylalanine. This compound and its by-product, phenylpyruvate, can accumulate to toxic levels in the blood, causing mental retardation.

Chapter 15: The Chromosomal Basis of Inheritance Chromosome theory of inheritance- Mendelian genes have specific loci (positions) on chromosomes, and it is the chromosomes that undergo segregation and independent assortment. Linked genes- genes located on the same chromosome that tend to be inherited together in genetic crosses. Genetic recombination- the production of offspring with combinations of traits differing from those found in either parent. Because these offspring have new combinations of seed shape and color, they are called recombinant types. General: Genetic map- an ordered list of the genetic loci along a particular chromosome.

Specific: Linkage map- a genetic map based on recombination frequencies.

Map units- the distances between genes, defining one map unit as equivalent to a 1% recombination frequency. Cytogenetic maps- locate genes with respect to chromosomal features, such as stained bands, that can be seen in microscopes. Sex-linked gene- a gene located on either sex chromosome. Fathers pass sex-linked alleles to all their daughters but to none of their sons. In contrast, mothers can pass sex-linked alleles to both sons and daughters. If a sex-linked trait is due to a recessive allele, a female will express the phenotype only if she is a homozygote. For this reason, far more males than females have sex-linked recessive disorders. <==Why? Duchenne muscular dystrophy- a sex-linked disorder which progressively weakens the muscles and loses coordination of the body. Barr body- the inactive X in each cell of a female condenses into a compact object which lies along the inside of the nuclear envelope. Most of the genes of the X chromosome that forms the Barr body are not expressed. In the ovaries, Barr-body chromosomes are reactivated in the cells that give rise to ova, so every female gamete has an active X. Thus, if a female is heterozygous for a sex-linked trait, about half her cells will express one allele, while the other will express the alternate allele. Nondisjuction- the members of a pair of homologous chromosomes do not move apart properly during meiosis I or sister chromatids fail to separate during meiosis II. a) deletion b) duplication c) inversion d) translocation aneuploidy- a condition where the offspring have an abnormal number of a particular chromosome. (Example: Down syndrome is a type of aneuploidy) polyploidy- chromosomal alteration. It is fairly common in the plant kingdom. Down syndrome- an extra chromosome 21, so that each body cell has a total of 47 chromosomes. Because the cells are trisomic for chromosome 21, Down syndrome is often

called trisomy 21. Down syndrome includes characteristic facial features, short stature, heart defects, susceptibility to respiratory infection, and mental retardation. Furthermore, people with Down syndrome can possibly get leukemia and Alzheimer’s disease. <==Why? Klinefelter syndrome: An extra X chromosome in a male, producing XXY. Males have abnormal feminine features and are sterile. Males with an extra Y chromosome. (XYY) do not exhibit any well-defined syndrome, but they tend to be somewhat taller than average.

Turner syndrome: Females with trisomy (XXX), cannot be distinguished from XX females except by karyotype. Monosomy X, called Turner syndrome.

Cri du chat- results from a specific deletion of chromosome 5. A child born with this deletion is mentally retarded, has a small head with unusual facial features. And has a cry that sounds like the mewing of a distressed cat. Such individuals usually die in infancy or early childhood. Chronic myelogenous leukemia- chromosomal translocations have been implicated in certain cancers. Genomic imprinting- variations in phenotype depending on whether an allele is inherited from male or female parent. Extranuclear genes-some genes are located in the organelles in the cytoplasm.