Modern Mendelian Genetics

Gene Expression Every organism has at least two alleles that govern every trait- one from the mother and one from the father- to the offspring. The genes encode information that is expressed as traits of the organism, which is called gene expression. A single gene (one set of alleles) may control one or several traits.

In any given cell within an organism, only some of the genes are expressed, while all other genes are inactivated. Genes that are “on” are expressed Genes that are “off” are not expressed. Mechanisms that can switch genes on and off include: 1. Intracellular chemicals 2. Enzymes 3. Environment

Heredity and the Environment The development and expression of inherited traits can be influenced by environmental factors such as: 1. Nutrients 2. Temperature 3. Sunlight

Ex: Temperature affects fur color in the Himalayan rabbit. 1. Under normal circumstances, these rabbits are white with black ears, feet, and nose. 2. If you shave the fur off its back and place an ice pack there, the hair will grow back in the color black-not white.

(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 This occurs during synapsis in the first meiotic division. The chromatids of homologous chromosomes often twist around each other, break, exchange segments and rejoin. This exchange of segments, called crossing-over, results in a rearrangement of linked genes and produces variations in offspring. Crossing-over is an important 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.

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 ¯).

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. (Colorblindness Test)

(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. al/bio/bio212/cytogene.html al/bio/bio212/cytogene.html

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.

Changes in Chromosome Structure: Changes in the make-up of chromosomes may result from random breakage and recombination of chromosome parts. Examples: Deletion Inversion Translocation

Deletion The deletion of a chromosome segment occurs when a portion of a chromosome breaks off and does not become attached to any other chromosomes.

Inversion & Translocation Inversion- A reversal in the order of genes on a chromosome segment. chromosome Translocation- Is a chromosomal rearrangement in which a section of a chromosome breaks off one chromosome.

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. Most gene mutations are recessive and hidden by the normal, dominant allele. However, if both parents carry the same recessive mutant gene, there is a chance that their offspring will be homozygous recessive and show the harmful trait.

(IV) Mutagenic Agents Causes mutations 1. Radiation- ultraviolet light, x-rays, radioactive substances. 2. Chemicals- asbestos fibers

Ultra-violet radiation from the sun strike cells in a person’s skin damaging the DNA in those cells. UV light, chemicals, tobacco, and x-rays can cause mutations in the DNA. All of these substances are known as mutagens.

What happens to your skin when you tan? When exposure to the sun occurs, our body has its own built-in repair system. This system consists of a series of repair enzymes that detect damaged pieces of DNA. The damaged pieces of DNA are removed, and the DNA is repaired. If exposure is too great, too much damage occurs. The body is unable to fix the damage and mutations occur. Mutations frequently produce cancers, although this disease may occur many years after exposure to the mutagen.

(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

Normal Sickle-cell

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.

3. Amniocentesis Removal of amniotic fluid for chemical and/or cellular analysis. Detection of sickle-cell anemia.