Chapter Summary

The Emerging Role of Genetics and Genomics in Medicine (page 978)

Genetics is the study of trait transmission through DNA passed in sperm and egg cells from generation to generation. Genes, which are parts of chromosomes, encode proteins. The human genome consists of at least 40,000 protein-encoding genes plus many repetitive sequences. Somatic cells are diploid; sex cells are haploid. Genomics considers heredity in terms of many genes that interact with each other and the environment.

Modes of Inheritance (page 980)

1. Chromosomes and genes come in pairs a. Chromosome charts are called karyotypes.

b. Chromosomes 1 through 22, numbered in decreasing size order, are autosomes. They do not determine sex.

c. The X and Y chromosomes are sex chromosomes. They determine sex.

d. Chromosomes and the genes they carry are paired.

e. An alternate form of a gene is called an allele. An individual can have two different alleles for a particular gene. The gene itself can have many alleles, because a gene consists of many building blocks, any of which may be altered.

f. An individual with a pair of identical alleles for a particular gene is homozygous; if the alleles are different, the individual is heterozygous.

g. The combination of genes present in an individual's cells constitutes a genotype; the appearance of the individual is its phenotype.

h. A wild-type allele provides normal or the most common function. A mutant allele causes disease or an unusual trait; it is a change from the wild-type condition.

2. Dominant and recessive inheritance a. In the heterozygous condition, an allele that is expressed when the other is not is dominant. The masked allele is recessive.

b. Recessive and dominant genes may be autosomal or X-linked or Y-linked.

c. An autosomal recessive condition affects both sexes and may skip generations. The homozygous dominant and heterozygous individuals have normal phenotypes. The homozygous recessive individual has the condition. The heterozygote is a carrier. An affected individual inherits one mutant allele from each parent.

d. An autosomal dominant condition affects both sexes and does not skip generations. A person inherits it from one parent, who is affected.

e. Pedigrees and Punnett squares are used to depict modes of inheritance.

3. Different dominance relationships a. In incomplete dominance, a heterozygote has a phenotype intermediate between those of both homozygotes.

b. In codominance, each of the alleles in the heterozygote is expressed.

Gene Expression (page 985)

1. Penetrance and expressivity a. A genotype is incompletely penetrant if not all individuals inheriting it express the phenotype.

b. A genotype is variably expressive if it is expressed to different degrees in different individuals.

2. Pleiotropy a. A pleiotropic disorder has several symptoms, different subsets of which are expressed among individuals.

b. Pleiotropy reflects a gene product that is part of more than one biochemical reaction or is found in several organs or structures.

3. Genetic heterogeneity a. Genetic heterogeneity refers to a phenotype that can be caused by alterations in more than one gene.

b. The same symptoms may result from alterations in genes whose products are enzymes in the same biochemical pathway.

Complex Traits (page 986)

1. A trait caused by the action of a single gene is monogenic.

2. A trait caused by the action of more than one gene is polygenic.

3. A trait caused by the action of one or more genes and the environment is complex.

4. Height, skin color, eye color, and many common illnesses are complex traits.

5. A frequency distribution for a polygenic trait forms a bell curve.

Matters of Sex (page 987)

A female has two X chromosomes; a male has one X and one Y chromosome. The X chromosome has many more genes than the Y.

1. Sex determination a. A male zygote forms when a Y-bearing sperm fertilizes an egg. A female zygote forms when an X-bearing sperm fertilizes an egg.

b. A gene on the Y chromosome, called SRY, switches on genes in the embryo that promote development of male characteristics and suppresses genes that promote development of female characteristics.

2. Sex chromosomes and their genes a. Genes on the sex chromosomes are inherited differently than those on autosomes because the sexes differ in sex chromosome constitution.

b. Y-linked genes are considered in three groups: those with counterparts on the X; those similar to genes on the X; and genes unique to the Y, many of which affect male fertility. Y-linked genes pass from fathers to sons.

c. Males are hemizygous for X-linked traits; that is, they can have only one copy of an X-linked gene, because they have only one X chromosome.

d. Females can be heterozygous or homozygous for genes on the X chromosome, because they have two copies of it.

e. A male inherits an X-linked trait from a carrier mother. These traits are more common in males than in females.

f. A female inherits an X-linked mutant gene from her carrier mother, and/or from her father if the associated trait does not impair his ability to have children.

g. Dominant X-linked traits are rare. Affected males typically die before birth.

3. Gender effects on phenotype a. Sex-limited traits affect structures or functions seen in only one sex and may be autosomal.

b. Sex-influenced traits are dominant in one sex and recessive in the other.

c. In genomic imprinting, the severity, age of onset, or nature of symptoms varies according to which parent transmits the causative gene.

Chromosome Disorders (page 991)

Extra, missing, or rearranged chromosomes or parts of them can cause syndromes, because they either cause an imbalance of genetic material or disrupt a vital gene.

1. Polyploidy a. Polyploidy is an extra chromosome set.

b. Polyploidy results from fertilization involving a diploid gamete.

c. Human polyploids do not survive beyond a few days of birth.

2. Aneuploidy a. Cells with an extra or missing chromosome are aneuploid. Cells with the normal chromosome number are euploid.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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