Genetics, Heredity, and Biotechnology

Unit 5

Genetic Variation• In asexual reproduction, there is zero genetic

variation; offspring are exact genetic copies of the parent.

• In sexual reproduction, there is much genetic variation but only before fertilization. The number of possible chromosome combinations in the gametes is 2n, where n = the haploid chromosome number and 2 is the number of chromosomes in a homologous pair. So, when n = 2, there are 4 possibilities; when n = 3, there are 8. Since humans have a haploid number of 23 (223), 8,388,608 combinations are possible.

Fertilization

• During fertilization, the two haploid gametes (sperm and egg) fuse to form a new diploid cell called a zygote. The zygote starts as a single cell with a set of 2n chromosomes, with each parent contributing one chromosome to each pair. To grow in size, the zygote begins mitosis and becomes an embryo. After 8 weeks of development, the embryo becomes a fetus until birth.

Zygote vs. Embryo

Cell Differentiation

• Stem Cells are the group of cells produced in the very early stages of embryonic growth; they are similar to the original zygote.

• When the embryo reaches 20 – 150 cells in size, this group begins to produce specialized cells that later become tissues.

• Stem cells can become any type of cell. This happens because genes within the cell can be “turned on” or “turned off” at specific times.

stem cells (cont’d)

• Every cell has the same genetic information that was present in the original zygote. Thus, cell differentiation occurs by the selective activation or inactivation of only some of these genes.

• Some cells become skin cells, while others might become liver cells, but both cells still contain genes for every other cell type within the organism.

• In summary, every stem cell has the capacity to become any type of cell found in that organism.

A Human Stem Cell…

Stem Cell Research

• Stem cells are “pre-cells”. They can become any type of cell with the proper instructions from DNA.

• Potential for using stem cells to help cure many human diseases or injuries is great; they could help people with nerve damage, Alzheimer’s, Parkinson’s, or arthritis.

stem cell research (cont’d)…

• Stem cells can be harvested from adult bone marrow, umbilical cord blood after delivery, or from human embryos.

• Harvesting from embryos usually kills the embryo.

• There are many ethical, political, and spiritual issues related to stem cell research. President Bush limited federal funds available for such research while President Obama supports using tax dollars to fund research.

What do YOU think?

Genetics

From Mendel

to the 21st Century

Principle of Dominance

• Mendel found that some forms of a gene or trait are dominant over other traits; these “weaker” traits are called recessive.

• Dominant traits mask, or hide, the presence of recessive traits.

Principle of Segregation

• Mendel discovered that when forming sex cells, the paired alleles separate so that each egg or sperm only carries one form of the allele.

• The two forms of the allele (one from each parent) come together again during fertilization.

Principle of Independent Assortment

• Mendel noticed this when doing dihybrid crosses.

• Says that each pair of alleles segregates independently during the formation of the egg or sperm.

• Leads to the 9:3:3:1 ratio of phenotypes in dihybrid crosses.

Modes of Inheritance

1. Sex-Linked Traits

• Sex chromosomes determine the sex of an organism.

• Males have the genotype XY; females XX.

• If a recessive trait is on the X chromosome it likely won’t be in the females phenotype.

• Females that have a recessive gene on one X chromosome are carriers for that trait. (ex. color blindness, baldness)

2. Incomplete Dominance

• When one trait is not completely dominant over the other.

• A blending, or mixing, of the two traits.

• If you cross a red flower and a white flower and get a pink flower, the traits mixed and neither was dominant.

• Bi-racial children are a classic example of incomplete dominance.

3. Co-Dominance

• When both traits contribute to the appearance of the offspring.

• Both alleles are completely expressed.

4. Multiple Alleles and Polygenic Traits

• Certain traits like blood type, hair color, and eye color, are determined by two genes from each parent for every trait (multiple alleles).

• Polygenic Traits are the result of interaction of multiple genes. Hypertension is genetically linked, but one gene doesn’t cause it. Weight, ability to process fats/cholesterol, ability to move salts through bloodstream (all controlled by genes) combined with lifestyle (addictive behaviors lie in genes). So…high blood pressure results from a combination of genes, or polygenic traits.

Genetic Pedigrees

• A graphical chart used to identify lineage of individuals.

• Similar to a family tree except it shows inheritance of genetic disorders within families.

• Used when breeding animals such as dogs or race horses.

• Males represented with a square and females with a circle; sufferers of the disorder are shaded.

Pedigree Graph

Mutations

• Mistakes in DNA replication.

• Some are harmful; some are beneficial.

• Play a significant role in creating diversity of life on Earth today.

• There are two groups of mutations – gene mutations and chromosomal mutations.

Gene vs. Chromosomal Mutations

Gene Mutations

• Mistakes that affect individual genes on a chromosome.

• One base substitutes for another on a DNA strand and leads to the wrong protein being made; this affects one or more functions within the organism.

Chromosomal Mutations

• Mistakes that affect the whole chromosome.

• There are four types of chromosomal mutations: duplication, deletion, inversion, and translocation.

• ALL MUTATIONS ARE CAUSED BY

MUTAGENS.

Genetic Diseases• Most mutations are caught and fixed before

any damage is done to the organism. Many that aren’t caught have no effect on the organism. Some, though, have negative effects on the organism and/or the organism’s offspring.

• Many diseases that are passed from parent to offspring are a result of mutations that were not corrected.

Genetic Disorders and Diseases• Sickle-cell Anemia. Result of two recessive genes

that causes the red blood cells to take on a sickle shape; keeps O2 and nutrients from reaching organs; leads to frequent infections and damage to major organs.

• Hemophilia. A sex-linked (comes from the mother) recessive condition involving failure of blood to clot properly. Mom will pass on the disease to half her sons, and the trait to half her daughters.

Genetic Diseases (cont’d)• Down’s Syndrome. When a person has

inherited an extra copy of chromosome 21, meaning they have a total of 47.

• Phenylketonuria (PKU). Inherited disease resulting from a missing enzyme; causes the amino acid phenylalanine to build up in an infant’s blood causing brain damage.

Still more genetic diseases…

• Cystic Fibrosis. Causes increased mucus in the lungs, lung infections, lung disease, poor growth rate, short life, and infertility. There is no treatment for CF.

• Tay-Sachs. Caused by a mutation to a gene found on chromosome 15. Results in a build up of fatty acids in nerve tissues; symptoms include blindness, deafness, difficulty swallowing, and death before age 3.