Lecture Outline - Chapter 19

19.1 Genotype and Phenotype (p. 398, Fig. 19.2)
	1.	The genotype represents the actual genes of an individual. They can be homozygous dominant (EE) for a trait, heterozygous (Ee), or homozygous recessive (ee).
	2.	The phenotype refers to the outward expression of the genotype (e.g., red hair).
19.2 Dominant/Recessive Traits (p. 399)
	1.	Forming the Gametes (p. 399, Figs. 19.3, 19.4)
		During oogenesis and spermatogenesis (collectively, gametogenesis), chromosome number is reduced to half, and each gene pair for a trait is separated, so that the offspring receives one gene for each trait from each parent. 
	2.	Figuring the Odds (p. 400, Figs, 19.5, 19.6)
		a.	A parent who is homozygous dominant or homozygous recessive for a trait can pass on only one type of gamete in each case. Heterozygous parents can pass on either the dominant or the recessive gene for a given trait.
		b.	A Punnett square is useful for determining the possible outcomes of a genetic cross.
		c.	A cross of the gametes of two individuals who are both homozygous for a trait results in a 100% chance of having an offspring who is homozygous for the trait.
		d.	A cross of the gametes of two heterozygotes results in a 25% chance that the offspring will be homozygous recessive, a 50% chance that the offspring will be heterozygous, and a 25% chance that the offspring will be homozygous dominant. Each offspring has a 25% chance of being homozygous recessive or of being homozygous dominant.
	3.	Some Disorders Are Dominant (p. 401)
		a.	Neurofibromatosis (p. 401)
			Neurofibromatosis (von Recklinghausen disease) is inherited as an autosomal dominant. People with this condition develop benign neurofibromas under the skin and in various organs. The effects can range from mild to severe, and some neurological impairment if possible. The gene for this trait is a nested gene on chromosome 17.
		b.	Huntington Disease (p. 401, Fig. 19.7)
			Huntington disease is also inherited as an autosomal dominant and is characterized by progressive neurological degeneration of brain cells, resulting in personality disorders and muscle spasms. No treatment exists, and death occurs a decade or so after the symptoms appear.
	4.	Some Disorders Are Recessive (p. 402)
		a.	Tay-Sachs Disease (p. 402)
			i.	Tay-Sachs disease is inherited as an autosomal recessive. Between four and six months of age, an affected infant shows neurological impairment. The child gradually becomes blind, helpless, and paralyzed, and usually dies by age four.
			ii.	Tay-Sachs results from a lack of hexosaminidase A and the storage of its substrate, glycosphingolipid in lysosomes. This disease is most prevalent in Jewish people from central and eastern European descent.
		b.	Cystic Fibrosis (p. 402, Fig. 19.8)
			i.	Cystic fibrosis is the most common lethal genetic disease among U.S. Caucasians. The thick mucus in bronchial passageways and pancreatic ducts interferes with the functioning of these organs.
			ii.	The defect lies in a chloride ion transport protein within plasma membranes. When chloride passes through, water normally follows. In cystic fibrosis patients, a lack of water following through results in the thick mucus. The gene for the defect is on chromosome 7.
		c.	Phenylketonuria (PKU) (p. 402)
			Individuals with phenylketonuria lack an enzyme needed for the normal metabolism of phenylalanine. Phenylketone thus accumulates in the urine. If the infant is not put on a phenylalanine-restrictive diet in infancy until age seven, brain damage and severe mental retardation result. 
	5.	Pedigree Charts (p. 403, Figs. 19.9, 19.10)
		Pedigree charts are a way of making a family tree and indicate which individuals are affected by a trait. Since recessive and dominant traits exhibit different patterns of inheritance, pattern of inheritance can be partially determined by examining a pedigree chart. 
		HEALTH FOCUS: Genetic Counseling (p. 404, Table 19A)
		Genetic counselors use pedigree charts and a variety of other means to predict the likelihood of two parents producing an offspring with a genetic disorder. Some tests can be run during pregnancy.
19.3 Polygenic Traits (p. 405, Fig. 19.11)
	1.	Polygenic traits are those governed by more than one gene pair. Several pairs of genes may be involved in determining phenotype.
		2.	Skin Color (p. 405)
			The inheritance of skin color, determined by an unknown number of gene pairs, is a classic example of polygenic inheritance.
 		3.	Polygenic Disorders (p. 405)
			Many human traits, like allergies, schizophrenia, and cleft lip, appear to be inherited as polygenic traits. The expression of some genes is subject to environmental influences (e.g., a child allergic to ragweed will never express that trait while living in the Arctic).
19.4 Multiple Allelic Traits (p. 406)
	1.	In multiple alleles, more than two alternative types exist for a gene pair. ABO blood grouping is an example.
	2.	ABO Blood Types (p. 406)
		a.	The ABO blood grouping represents surface marker proteins on red blood cells. A person can have a gene for an A marker or a B marker, which are codominant, or lack an A or B marker, designated type O, which is recessive.
		b.	Human ABO blood types can then be type A (which can be AA or AO), type B (BB or BO), type AB (AB), or type O (OO). 
19.5 Dominance Has Degrees (p. 406, Fig. 19.12)
	1.	Patterns of dominance often go beyond simple dominant or recessive traits.
		a.	Codominance means that both alleles are expressed (type AB blood).
		b.	Incomplete dominance is exhibited when the heterozygote shows not the dominant trait but an intermediate phenotype, representing a sort of blending of traits (e.g., skin color or hair type).
	2.	Sickle-Cell Disease (p. 407, Fig. 19.13)
		Sickle-cell disease is an example of incomplete dominance. An individual with two genes for normal hemoglobin has normal hemoglobin. A heterozygote has a normal gene and a gene for sickled hemoglobin. An individual with two sickling genes has sickle-cell disease. What may have maintained this apparently detrimental gene in equatorial Africa is that heterozygotes for this trait have a marked resistance to the malarial parasite prevalent in the region.
19.6 Sex-Linked Traits (p. 408)
	1.	Sex-linked traits are genes (traits) carried most frequently on the X chromosome. (The Y chromosome is too small.)
	2.	X-Linked Alleles (p. 408, Fig. 19.14)
		a.	In X-linked traits, the gene is carried on the X chromosome. Since males have only one copy of the X chromosome, they show the phenotype for the allele they possess and are thus much more likely than females to show a recessive trait.
		b.	A female must have two copies of a recessive trait (one on each X chromosome) to display it. If a female has only one copy of a recessive gene, she is said to be a carrier and will pass the trait on to 50% of her sons, on average.
	3.	Some Disorders Are X-Linked (p. 409, Fig. 19.15)
		a.	Color Blindness (p. 409)
			Three types of cones are in the retina: those that detect red, those that detect green, and those that detect blue. Genes for blue cones are autosomal; those for red and green cones are on the X chromosome. Males are much more likely to have red/green colorblindness than are females.
		b.	Muscular Dystrophy (p. 409)
			Duchenne muscular dystrophy is X-linked and is characterized by progressive muscle deterioration during childhood.
		c.	Hemophilia (p. 410, Fig. 19.16)
			Hemophilia (bleeder's disease) can be traced to Queen Victoria of England and is characterized by the absence or minimal presence of one of two different clotting factors. Again, males are much more prone to this trait than females and often require blood transfusions.
	4.	Some Traits Are Sex-Influenced (p. 411, Fig. 19.17)
		Some traits carried on autosomes such as male-pattern baldness, can be influenced by gender. In this instance, the male hormone testosterone is the culprit.