Presentation on theme: "Mendel and the Gene Idea Chapter 14. Gregor Mendel - monk - studied pea plants, looked at traits. Pea plants many varieties with distinct heritable features."— Presentation transcript:
Mendel and the Gene Idea Chapter 14
Gregor Mendel - monk - studied pea plants, looked at traits. Pea plants many varieties with distinct heritable features (characters) with different variants (traits). Pea plants self-fertilize; Mendel cross-fertilized to study traits.
Mendel cross-pollinated (hybridize) 2 contrasting, true-breeding pea varieties. True-breeding parents - P generation; hybrid offspring - F 1 generation. F 1 hybrids then self-pollinate to produce F 2 generation.
Thought genes blended - purple flower crossed with white flower result would be light purple flowers. All the flowers purple.
When flowers self-fertilized, white flower reappeared in next generation. Ratio of purple to white in F 2 generation was 3:1.
Developed hypothesis to explain process. 1 Alternative versions of genes (alleles) account for variations in inherited characters. 2 For each character organism inherits 2 alleles, 1 from each parent. Alleles can be same or different.
3 2 alleles differ - 1 (dominant allele) fully expressed in organism. Other (recessive allele) no noticeable effect on organism’s appearance. 4 2 alleles for each character segregate (separate) during gamete production.
Found that alleles not linked due to inheritance patterns. Independent assortment of each pair of alleles during gamete formation - law of independent assortment.
Therefore, Mendel had three different laws. The law of dominance and recessiveness states that one gene is dominant over the more recessive gene. The law of segregation states that alleles separate during meiosis. The law of independent assortment states that alleles organize in the gametes regardless of other alleles.
An organism with two identical alleles for a character is homozygous for that character. Organisms with two different alleles for a character is heterozygous for that character. A description of an organism’s traits is its phenotype. A description of its genetic makeup is its genotype.
A Punnett square predicts the results of a genetic cross between individuals of known genotype. A testcross, breeding a homozygous recessive with dominant phenotype, but unknown geneotype, can determine the identity of the unknown allele.
Mendel’s experiments focused on monohybrid crosses meaning that he looked at only one trait at a time. Later on he started looking at dihybrid crosses involving probabilities of two different traits.
Mendel’s ideas are based on probability. If you were to toss a coin 4 times, the coin has a ½ chance of coming up heads every time. Each toss is independent of the one done before. The probability of it coming up heads all four times is: ½ * ½ * ½ * ½ = 1/8. This is known as the rule of multiplication.
There is also a law of addition that determines the chances of an event happening in different ways. For example, there are two ways that F 1 gametes can combine to form a heterozygote. The dominant allele could come from the sperm and the recessive from the ovum (probability = 1/4). Or, the dominant allele could come from the ovum and the recessive from the sperm (probability = 1/4). The probability of a heterozygote is 1/4 + 1/4 = 1/2.
Incomplete dominance can also occur in offspring. In incomplete dominance, heterozygotes have a completely different phenotype than homozygotes. This happens in snapdragons. Homozygous recessive flowers are white; homozygous dominant flowers are red; heterozygotes are pink.
Another inheritance pattern is codominance in which two alleles affect the phenotype in separate, distinguishable ways. An example of this is blood type. If you inherit an A allele and a B allele, your blood type will be AB; if it is AA or AO, your blood type will be A. This means that A is dominant to O, B is dominant to O, but A is codominant to B.
Blood type is important because type A has anti-B antibodies. If exposed to B blood, it will clump together causing a transfusion reaction. People with blood type O have both antibodies and therefore can donate to any other blood type. On the other hand, AB has neither antibodies and therefore can receive from any blood type.
Dominant genes do not mean that they are more popular in a given population. Also, most genes do not control only one trait but are pleiotropic, affecting more than one phenotypic character. In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus.
In mice, one gene determines whether or not there will be a coat color. If that gene is turned off, the mouse will be white; if it is turned on, another locus will determine what the color is (brown or black).
Quantitative characters vary in a population along a continuum. This is because of polygenic inheritance which is when more than one gene controls a single trait. An example of this is skin color which is controlled by at least three different genes and is responsible for the variety of skin colors.
Phenotype also depends on environment. For humans, nutrition influences height, exercise alters build, sun-tanning darkens the skin, and experience improves performance on intelligence tests.
Genetic experiments cannot ethically be performed on humans, so geneticists use pedigrees to look at traits found in families. A family tree is then created showing the absence or presence of a specific trait to determine how it is passed.
Ethnicity plays a role in genetic disease patterns. For example, sickle-cell anemia is found predominately in African-Americans. This disease causes the red blood cells to be sickle shaped instead of the normal disk shape causing the cells to get stuck in the vessels.
Cystic fibrosis affects mostly Caucasians. Cystic fibrosis is a multi-system disease that causes mucous to build up in various organs, especially the lungs.
Tay-Sachs affects people of Jewish descent. Tay-Sachs affects the brains of small children, ultimately causing the death of the child prior to 5 years old.
Some genetic diseases, such as dwarfism, are dominant diseases. This means that a child has a 50% chance of inheriting the disease because one of the parents has the disease. Huntington’s disease is also a dominant disease that affects the nervous system. Most dominant diseases are not lethal (Huntington’s disease is).
Genetic counseling is a field of study that works with people that have a history of genetic disease in the family. A child with a recessive disease can be born to phenotypically normal parents. There are several tests that can be performed to determine a couple’s risk.
One technique is amniocentesis. Cells are extracted from the amniotic fluid surrounding the fetus and then analyzed to search for potential problems in a technique called karyotyping. Karyotyping is essentially mapping out the chromosomes of an individual.
A second technique, chorionic villus sampling (CVS) can allow faster karyotyping and extracts a sample of fetal tissue from the chorionic villi of the placenta.
A more routine test is an ultrasound which detects only physical abnormalities that are present.