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Some immunology textbooks state that the urushiol allergen is engulfed by Langerhan's cells. The allergen and a small protein fragment called "major histocompatibility complex" MHC is then displayed on its membrane and presented to the effector T-cells.
Multiple Choice Questions 11 - 1. Mitosis Compared With Meiosis 2. Transposons: Jumping Genes 6. Red-Green Color Blindness 7. Polygenic Inheritance 9. Rh Factor: Polygenic Inheritance Genetics Extra Credit Problems Question The somatic body cells of a haploid organism typically contain only one set of chromosomes. In other words, the individual chromosomes do not occur in homologous pairs.
A male honey bee or drone is haploid because its somatic cells only contain maternal chromosomes from the queen bee. Question The illustrations for Question 12 apply to nondisjunction of sex chromosomes X and Y chromosomes during meiosis spermatogenesis. Nondisjunction can also occur with autosomes. These are all chromosomes excluding the X and Y. In other words, chromosome pairs 1 through 22 in human cells. The X and Y chromosomes are generally considered to be pair In Down's syndrome, one sex cell gets an extra autosome For example, lets say the egg carries two 21 autosomes a total of 24 chromosomes and the sperm carries one autosome 21 a total of 23 chromosomes.
During fertilization, the zygote gets three 21 autosomes a total of 47 chromosomes. See the following diagram and hyperlink for an explanation of human chromosomal anomalies: In the following diagram, normal spermatogenesis is compared with spermatogenesis with nondisjunction at meiosis I anaphase I and nondisjunction at meiosis II anaphase II. If the doubled X and Y chromosomes move to the same cell at meiosis I, the resulting gametes will each contain single X and Y chromosomes.
A study conducted at a men's prison many years ago revealed that a higher than normal percentage of the inmates carried an extra Y chromosome. It was concluded incorrectly that these men developed a criminal tendency because of the extra Y chromosome, perhaps by causing them to be more aggressive during their childhood and adolescent years. Subsequent studies have proven this conclusion to be false. Men with noncriminal behavior can also carry an extra Y chromosome.
See the following table and hyperlink for an explanation of human chromosomal anomalies: 1. A phenotypic male with one Barr body. XXY 2. A phenotypic female with zero Barr bodies. A phenotypic female with one Barr body. A phenotypic male with no Barr bodies. A phenotypic female with two Barr bodies. Simply place a 2 above each heterozygous gene pair and a one above each homozygous gene pair. Then multiply the numbers together to obtain the total number of different possible gametes.
Matching genes occurring at the same loci on homologous chromosomes are called alleles. If a pair of alleles are identical they are called homozgous. If the pair contains two different alleles one dominant and one recessive they are termed heterozygous.
In simple Mendelian genetics, alleles typically occur in two forms one dominant and one recessive. For example, there are two alleles one pair of alleles for seed coat in garden peas: The dominant allele for round R and the recessive allele for wrinkled r. In real life, there may be more than two alleles to choose from, and they are not always dominant and recessive.
In human blood types there are 3 alleles, A, B and O. They all occur at the same loci on homologous chromosome pair 9 autosome 9. Since there are more than two alleles to choose from, this type of inheritance is called "multiple allele inheritance. Sometimes a number of genes are involved in the inheritance of a trait. This may involve several pairs of alleles from several different loci on homologous chromosomes.
Since different loci are involved, you can't use the term multiple allele inheritance. So geneticists have devised the term "multiple gene" or "polygenic inheritance. Because there are different genes on different loci involved, numerous genotypes and phenotypes appearances are possible. The Rh factor is a good example of polygenic inheritance.
The disease sickle-cell anemia is a good example of a genetic mutation in which the gene for the vital protein hemoglobin has mutated. The sickle-cell gene has an altered DNA base pattern so that it codes for the amino acid valine instead of glutamic acid at a precise location in the hemoglobin molecule.
This results in a change in the structure of the molecule resulting in sickle-shaped rather than normal disk-shaped red blood cells. These abnormal cells do not flow as well through minute capillaries, forming painful "log jams" that impede blood circulation. A human male and female each have 23 pairs of homologous chromosomes per cell, a total of 46 chromosomes. A male or female with Down's syndrome has the 21st chromosome autosome in triplicate. Instead of the normal homologous pair, there are three 21 chromosomes.
In Klinefelter's syndrome, there are three 23 chromosomes X-Y chromosomes rather than the normal pair. In this case the individual has two X chromosomes and one Y chromosome. Because the Y chromosome carries the male-determining factor, the individual is a phenotypic male with a penis, although there may be some breast enlargement. In both of these syndromes, the total number of chromosomes per cell is raised by one compared with normal somatic cells.
See the following table: The Rh factor is an interesting example of polygenic inheritance. Unlike the A-B-O blood types where all the alleles occur on one pair of loci on chromosome pair 9, the Rh factor involves three different pairs of alleles located on three different loci on chromosome pair 1. Possible genotypes will have one C or c, one D or d, and one E or e from each chromosome. In order to determine how many different genotypes are possible, you must first determine how many different gametes are possible for each parent, then match all the gametes in a genetic checkerboard.
This number of gametes is based on all the total possible ways these genes can be inherited on each chromosome of homologous pair 1. The formula was actually devised by several of my general biology students. It may occur somewhere in a textbook, but the students came up with it independently.
See the following diagram showing one pair of homologous chromosomes, each with a single locus. Only one allele can occur at each locus, but there are 4 possible alleles per locus. Questions 32 - See the following table showing the number of different gametes due to independent assortment of chromosomes during meiosis and random combination of gametes.
Matching Questions 35 - 1. The following illustration shows a highly magnified cell membrane containing two kinds of embedded proteins, a carrier protein and a cell recognition protein. The cell recognition protein contains a carbohydrate "antenna" composed of polysaccharide subunits.
Tutorial Predicting the genotype of offspring There are four possible combinations of gametes for the AaBb parent. Half of the gametes get a dominant A and a dominant B allele; the other half of the gametes get a recessive a and a recessive b allele. Review problem 3's tutorial if necessary. Possible gametes for each AaBb parent Since each parent has four different combinations of alleles in the gametes, there are sixteen possible combinations for this cross.
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