For example, the gene responsible for the hair color trait has many alleles: an allele for brown hair, an allele for blonde hair, an allele for red hair, and so on. A gene is a portion of DNA that determines a certain trait. An allele is a specific form of a gene. Genes are responsible for the expression of traits. How many kinds of homozygotes are there? There are six different genotypes: the three homozygotes and three different types of heterozygotes.
In this allelic series, the alleles IA and IB each determine a unique antigen, which is deposited on the surface of the red blood cells. These antigens are two different forms of a single protein. What are allele combinations?
Allele Combination. MGI Glossary. A designation of the specific alleles present on the two homologous chromosomes for all relevant loci of a mouse i. An allele combination can be composed of one or more allele pairs.
Where do alleles come from? How are new alleles created? Figure 1: In this family pedigree, black squares indicate the presence of a particular trait in a male, and white squares represent males without the trait. White circles are females.
A trait in one generation can be inherited, but not outwardly apparent before two more generations compare black squares. Figure Detail. The family tree in Figure 1 shows how an allele can disappear or "hide" in one generation and then reemerge in a later generation. In this family tree, the father in the first generation shows a particular trait as indicated by the black square , but none of the children in the second generation show that trait.
Nonetheless, the trait reappears in the third generation black square, lower right. How is this possible? This question is best answered by considering the basic principles of inheritance. Mendel's principles of inheritance.
How do hidden genes pass from one generation to the next? Although an individual gene may code for a specific physical trait, that gene can exist in different forms, or alleles.
One allele for every gene in an organism is inherited from each of that organism's parents. In some cases, both parents provide the same allele of a given gene, and the offspring is referred to as homozygous "homo" meaning "same" for that allele. In other cases, each parent provides a different allele of a given gene, and the offspring is referred to as heterozygous "hetero" meaning "different" for that allele. Alleles produce phenotypes or physical versions of a trait that are either dominant or recessive.
The dominance or recessivity associated with a particular allele is the result of masking, by which a dominant phenotype hides a recessive phenotype. By this logic, in heterozygous offspring only the dominant phenotype will be apparent. The relationship of alleles to phenotype: an example. Dominance, breeding experiments, and Punnett squares. Figure 4: A brown fly and a black fly are mated. Figure 5: A Punnett square. Figure 6: Each parent contributes one allele to each of its offspring.
Thus, in this cross, all offspring will have the Bb genotype. Figure 7: Genotype is translated into phenotype. In this cross, all offspring will have the brown body color phenotype. The phenomenon of dominant phenotypes arising from the allele interactions exhibited in this cross is known as the principle of uniformity, which states that all of the offspring from a cross where the parents differ by only one trait will appear identical.
How can a breeding experiment be used to discover a genotype? Breeding the flies shown in this Punnett square will determine the distribution of phenotypes among their offspring. If the female parent has the genotype BB, all of the offspring will have brown bodies Figure 9, Outcome 1. In this way, the genotype of the unknown parent can be inferred. Figure 9. Figure The phenotypic ratio is brown body: black body. This observation forms the second principle of inheritance, the principle of segregation, which states that the two alleles for each gene are physically segregated when they are packaged into gametes, and each parent randomly contributes one allele for each gene to its offspring.
Can two different genes be examined at the same time? Figure The possible genotypes for each of the four phenotypes. The dihybrid cross: charting two different traits in a single breeding experiment. Figure These are all of the possible genotypes and phenotypes that can result from a dihybrid cross between two BbEe parents.
On the upper left, the female parent genotype is uppercase B lowercase b, uppercase E lowercase e. Uppercase B, uppercase E is labeled to the left of the top quadrant; lowercase b, lowercase e is labeled outside the second left quadrant; uppercase B, lowercase e is labeled outside the third left quadrant; and lowercase b, uppercase E is labeled outside the fourth left quadrant. On the upper right, the male parent genotype is also uppercase B lowercase b, uppercase E lowercase e.
Uppercase B, uppercase E is labeled to the right of the top quadrant; lowercase b, lowercase e is labeled to the outside the second right quadrant; uppercase B, lowercase e is labeled outside the third right quadrant, and lowercase b, uppercase E is labeled outside the fourth right quadrant. The offsprings' genotype and phenotype is represented in each of the cells of the Punnett square.
Nine of the 16 cells contain brown-bodied flies with red eyes. Of these nine flies, one has the genotype uppercase B, uppercase B, uppercase E uppercase E; four have the genotype uppercase B lowercase b, uppercase E lowercase e; two have the genotype uppercase B uppercase B, uppercase E lowercase e; and two have the genotype uppercase B lowercase b, uppercase E uppercase E.
Three cells contain brown-bodied flies with brown eyes. Of these three flies, one has the genotype uppercase B uppercase B, lowercase e lowercase e and two have the genotype uppercase B lowercase b, lowercase e lowercase e.
Three cells contain black-bodied flies with red eyes. Of these three flies, one has the genotype lowercase b lowercase b, uppercase E uppercase E and two have the genotype lowercase b lowercase b, uppercase E lowercase e.
The final cell contains a black-bodied fly with brown eyes; this fly has the genotype lowercase b lowercase b, lowercase e, lowercase e. The impact of Mendel's principles. Seminal experiments on inheritance. Key Questions What is non-nuclear inheritance? 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.
Vaccines Resulting In Active Immunity 2. Serums Resulting In Passive Immunity 3. Genetics Extra Credit Problems Questions 61 - Remember that the gene allele for taster T is dominant over the gene allele for nontaster t : Questions 63 - Human skin color is a good example of polygenic inheritance in people.
The offspring contain seven different shades of skin color based on the number of capital letters in each genotype. The words dominant and recessive are placed in quotation marks because these pairs of alleles are not truly dominant and recessive as in some of the garden pea traits that Gregor Mendel studied. A genotype with all "recessive" small case genes aabbcc has the lowest amount of melanin and very light skin. Each "dominant" capital gene produces one unit of color, so that a wide range of intermediate skin colors are produced, depending on the number of "dominant" capital genes in the genotype.
For example, a genotype with three "dominant" capital genes and three small case "recessive" genes AaBbCc has a medium amount of melanin and an intermediate skin color. This latter genotype would be characteristic of a mulatto.
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