Genetics and the Mechanics of Natural Selection ANTH A-103, Human Origins and Prehistory Larry J. Zimmerman, Ph.D., RPA Indiana University-Purdue University Indianapolis

Why study genetics? But it’s hard! Your life and that of offspring may depend on it! But it’s hard! Yes, it does involve some memorization and thought… but how much is your life or that of your child worth? And…

Of course there are questions of ethics…

Generalized Cell Structure The essentials Generalized Cell Structure DNA is in the nucleus; protein synthesis occurs in the ribosomes

The body is composed of cells of two basic types: Somatic cells and gametes. Somatic cells are the cellular components of body tissue: muscle, skin, bone, nerve, heart and brain. Gametes are sex cells specifically involved in reproduction and not important structural parts of the body.

The nucleus Contains two molecules or nucleic acids that contain the genetic information that controls the cell's function: DNA (deoxyribonucleic acid) RNA (ribonucleic acid).

Outside the nucleus The ribosomes are involved with translating the instructions coded in the material of inheritance into proteins or chemical messengers that are used by the body's cells.

Outside the nucleus Mitochondria are involved with energy transfer within the cell, and are interesting because they contain their own set of hereditary material independent of that contained in the nucleus. Mitochondrial DNA can be traced through females and dated back into time.

Chromosomes and DNA The material of inheritance within the nucleus of a cell is arranged in long strands called chromosomes. On the molecular level the chromosomes are actually nothing more than two long strings of DNA wound together in a spiral-like structure called a double helix.

The bases and genes Each of the two strands of DNA in a chromosome are composed of varying combinations of 4 simple molecules called bases. The four bases are adenine, cytosine, guanine, and thymine labeled A, C, G, and T. A single DNA strand is composed of a string of bases, each of which can be one of 4 types: A, C, G, or T. The order in which the bases occur on the DNA strand is not random. Genes are nothing more or less than unique, specific sequences of the 4 bases.

Allele "Gene" is a layman's term. Scientists avoid the use of the word gene, because it is not very specific. Locus (plural = loci) and allele are used instead.

Locus A locus is the section of a DNA strand that contains the instructions for one specific product (eye color, tongue rolling, blood serum protein albumin). A locus is simply a section of the chromosome that holds a sequence of bases and is like an address. The particular base sequence that resides at a given locus is called an allele. Only one allele can reside at a given locus in any one DNA strand.

Gene Expression Alleles code for proteins which are either the desired product or are messengers or controlling substances that produce the desired result. A protein is simple a chain of amino acids. Just like a DNA strand is a chain of bases, a protein is a chain of amino acids. There are 4 different bases that can be chained together to form a DNA strand There are about 20 different amino acids that can be chained together to form a protein.

Chromosome Number Each cell of the body (except sperm and ova) has two of each chromosome or diploid. One chromosome came from the mother and one came from the father. So, the reproductive cells, the sperm and ova, must have only one chromosome or haploid. Haploid reproductive cells are called gametes.

The process

Another way to diagram it

Human Genetic Structure Humans have 23 pairs of chromosomes, 46 altogether. For 22 of these chromosomes the two members of the pair are pretty much identical or autosomal chromosomes. 23rd pair of chromosomes is the sex chromosomes. The sex chromosomes come in two varieties, X and Y. Females have two X chromosomes Males have one X chromosome and one Y chromosome. Generally speaking, if you have a Y chromosome you are a male.

The Human Karyotype 46 chromosomes in 23 pairs Pair 23 is the sex chromosome, in this case a male

Detail on Chromosome Structure Chromosomes have two arms that are joined at the centromere. The short arm is called the p arm for the French word petit, which means small or short. The long arm is called the q arm because q comes after p in the alphabet. The ends of the chromosomes are called telomeres. Detail on Chromosome Structure

Sex and the single chromosome All the gametes produced by females carry only X chromosomes. But, half of the gametes produced by males carry the X chromosome and half carry the Y chromosome. Whether a given offspring is female or male depends on whether the father's sperm happens to have an X or a Y chromosome.

Autosomal and sex-linked chromosomes Most traits are autosomal traits, the loci carrying alleles for them are located on one of the autosomal chromosomes. A few traits are sex linked, carried on one of the sex chromosomes - usually the X. One sex-liked trait is hemophilia, caused by a recessive gene on the X chromosome. It occurs mostly in men because they only have one X chromosome. If they have the recessive gene, then they will have hemophilia.

Mendelian inheritance Gregor Mendel 1822-1884

Mendel’s Sweet Pea Experiments

Punnet Squares

Medel’s Peas Punnet Squares

Key Mendelian Terms Genotype: the genetic structure of the population Phenotype: the genetic structure of the individual Homozygous: both genes are the same (RR, YY) Heterozygous: the genes are different (Rr, Yy) Dominant: the characteristic shows (R, Y) Recessive: the characteristic normally is hidden (r, y) Dominant shows: RR, Rr, YY, Yy Recessive shows: rr, yy

Dominant & Recessive

Principle of Segregation Genes occur in pairs because chromosomes occur in pairs. During gamete production, the members of each gene pair separate so that each gamete contains one member of a pair. During fertilization, the full number of chromosomes is restored and members of a gene or allele pairs are reunited.

Principle of Independent Assortment The distribution of one pair of alleles into gametes does not influence the distribution of another pair. The genes controlling different traits are inherited independently of one another.

Genetic Disorders

Hereditary vs. acquired Mutations Gene mutations can be either inherited from a parent or acquired. A hereditary mutation is a mistake that is present in the DNA of virtually all body cells. Hereditary mutations are also called germline mutations because the gene change exists in the reproductive cells (germ cells) and can be passed from generation to generation, from parent to newborn. Moreover, the mutation is copied every time body cells divide. Acquired mutations, also known as somatic mutations, are changes in DNA that develop throughout a person's life. In contrast to hereditary mutations, somatic mutations arise in the DNA of individual cells; the genetic errors are passed only to direct descendants of those cells. Mutations are often the result of errors that crop up during cell division, when the cell is making a copy of itself and dividing into two. Acquired mutations can also be the byproducts of environmental stresses such as radiation or toxins.

Hereditary mutations are carried in the DNA of the reproductive cells Hereditary mutations are carried in the DNA of the reproductive cells. When reproductive cells containing mutations combine to produce offspring, the mutation will be present in all of the offspring's body cells.

Acquired mutations develop in DNA during a person's lifetime Acquired mutations develop in DNA during a person's lifetime. If the mutation arises in a body cell, copies of the mutation will exist only in descendants of that particular cell. Most mutations are benign, that is, the neither harm or help the individual. Severe mutations may be deleterious; others may eventually be adaptive.

In dominant genetic disorders, if one affected parent has a disease-causing allele that dominates its normal counterpart, each child in the family has a 50-percent chance of inheriting the disease allele and the disorder.

Genetic Testing Different types of genetic tests are used to hunt for abnormalities in whole chromosomes, in short stretches of DNA within or near genes, and in the protein products of genes.

Example: BRCA1 breast cancer/colon cancer susceptibility genes As many as 1 in 300 women may carry inherited mutations of breast cancer susceptibility genes, and approximately the same proportion of Americans carry mutations that make them susceptible to colon cancer. Inherited forms of cancer represent perhaps 5 or 10 percent of all cancers. The great majority of people who get breast cancer (or colon cancer) acquire mutations during their lifetimes.

Cancer usually arises in a single cell Cancer usually arises in a single cell. The cell's progress from normal to malignant to metastatic appears to follow a series of distinct steps, each controlled by a different gene or set of genes. Persons with hereditary cancer already have the first mutation.

Genetic testing gives you options! What does a predictive gene test tell you? Scientists looking for a disease gene often begin by studying DNA samples from members of 'disease families' that have numerous relatives, over several generations, who have developed an illness. Women who carry the BRCA1 breast cancer susceptibility gene have an 80-percent chance of developing breast cancer by the age of 65; their risk is high but not absolute. Genetic testing gives you options!

Not interested in genetics? Maybe you should be? In your lifetime, it’s just about 100% certain you will be!