Chapter 28 Modern Mendelian Genetics
(I) Gene-Chromosome Theory Genes exist in a linear fashion on chromosomes Two genes associated with a specific characteristic are known as alleles and are located on homologous chromosomes
Gene Linkage Genes for different traits are located on the same chromosome pair, and are said to be linked Therefore they are usually inherited together
Crossing-Over The chromatids of homologous chromosomes often twist around each other, break, exchange segments and rejoin. Crossing-over is a source of genetic variation in sexual reproduction
(II) Sex-Linked Traits 1. Each human cell contains 46 chromosomes 22 pairs of autosomes 1 pair of sex chromosomes a) sex chromosomes are designated as X and Y XX= female XY= male
2. The sex of a human is genetically determined at fertilization when a sperm cell containing either the X or Y chromosome unites with an egg cell containing the X chromosome
3. Sex-Linkage a) Morgan s work with Drosophila demonstrated that genes for certain traits are located on the X chromosome and do not appear on the Y chromosome b) genes found on the X chromosome are said to be sex-linked genes c) recessive traits that are sex-linked occur more frequently in males than in females. For the recessive trait to show in a female, she must be homozygous recessive. The gene must be present on both of her X chromosomes ( X` X`) d) for the recessive trait to show in males, the recessive gene on the X chromosome will be expressed since it does not appear on the Y chromosome ( X`Y)
Human Disorders associated with Sex-Linked Genes 1. Hemophilia- disease in which the blood does not clot properly 2. Colorblindness- inability to see certain colors, most commonly red and green * Both of these disorders are more common in males than in females because a female will not show the disorder as long as she has one normal gene. Females who are heterozygous for a sex-linked trait are said to be carriers for that trait
Sex-Linked Traits X H X h X H X H X H X H X H Y X H Y X h Y How many offspring have hemophilia?
Sex-Linked Traits X H X h X H X H X H X H X H Y X H Y X h Y How many offspring have hemophilia? 1
Sex-Linked Traits X H X h X h X H X h X h X h Y X H Y X h Y How many offspring have hemophilia? How many offspring are carriers?
Sex-Linked Traits X H X h X h X H X h X h X h Y X H Y X h Y How many offspring have hemophilia? 2 How many offspring are carriers? 1
Sex-Linked Traits X H X h X h X H X h X h X h Y X H Y X h Y How many offspring have hemophilia? How many offspring are carriers?
Human Pedigree
(III) Mutations Changes in genetic material are called mutations If a mutation occurs in the sex cell, it may be transmitted to the offspring Mutations occurring in body cells may be passed on to new cells of the individual due to mitosis, but will not be transmitted to the offspring by sexual reproduction Ex: cancer Mutations can be classified as chromosomal alterations or gene mutations
Chromosomal Alterations Is a change in the number of chromosomes or in the structure of the chromosome The effects of chromosomal alteration are often visible in the phenotype of an organism because many genes are usually involved An example of chromosomal alteration is nondisjunction
Nondisjunction One or more pairs of homologous chromosomes fail to separate. This results in gametes with more (or less) than their normal haploid chromosome number
If these gametes are involved in fertilization, the resulting zygote may have more (or less) than the normal diploid chromosome number Examples: 1. Down s Syndrome- results from the possession of an extra chromosome. This is due to the nondisjunction of chromosome #21 in one of the parents. Will result in mental retardation or physical abnormalities
2. Polyploidy- sometimes all 22 chromosomal pairs fail to separate. The resulting 2n gamete fuses with the normal n gamete, producing a 3n zygote. This is common in plants but rare in humans
Gene Mutations Involves a random change in the chemical nature of the genetic material (DNA) Some gene mutations, like albinism are obvious, while others are not noticeable
A woman with Abinism
Mutations
Mutations
Mutations
Polydactyl mutation
(IV) Mutagenic Agents Causes mutations 1. Radiation- ultraviolet light, x-rays, radioactive substances 2. Chemicals- asbestos fibers
(V) Human Genetic Disorders
1. Phenylketonuria (PKU) A disorder in which the body cannot synthesize an enzyme necessary for the normal metabolism of phenylalanine Results in mental retardation
2. Sickle-cell Anemia A gene mutation that results in the production of abnormal hemoglobin molecules and abnormal red blood cells African descent
3. Tay-Sachs Deterioration of the nervous system due to the accumulation of fatty material as a result of the inability to synthesize a specific enzyme Jewish people of Central Europe descent
(VI) Types of Genetic Disorder Detection Techniques
1. Screening Chemical analysis of body fluids such as blood and urine Detection of PKU and Tay-Sachs
2. Karyotyping The preparation of an enlarged photograph of chromosomes
2. Karyotyping
3. Amniocentesis Removal of amniotic fluid for chemical and/or cellular analysis Detection of sickle-cell anemia
3. Amniocentesis