With one locus and additive effects we have three phenotypic classes: AA, Aa and aa. With two loci and two alleles in a strictly additive model i. The more loci affecting the trait, the greater number of phenotypic classes.
Without genetic variation there will be no evolution. Thus, characterizing the genetic variation in natural populations is fundamental to the study of evolution. What kinds of variation are there? Discrete polymorphisms e. Examples: the carrots in the Burpee Catologue; human height. Continuously varying traits will have both a genetic and environmental components.
How much genetic variation is there? Historical debate: Classical school held that there was very little genetic variation, most individuals were homozygous for a "wild-type" allele. Rare heterozygous loci due to recurrent mutation; natural selection purges populations of their "load" of mutations. Balance school held that many loci will be heterozygous in natural populations and heterozygotes maintained by "balancing selection" heterozygote advantage. Selection thus plays a role in maintaining variation.
What are the possible phenotypes? There are 3 alleles for the gene that determines blood type. How many alleles are there? How do you calculate genotypes? How does DNA affect phenotype? Some mutations don't have any noticeable effect on the phenotype of an organism. This can happen in many situations: perhaps the mutation occurs in a stretch of DNA with no function, or perhaps the mutation occurs in a protein-coding region, but ends up not affecting the amino acid sequence of the protein.
How many gametes can be formed from AaBb? How many alleles per gene are found in haploid gametes? What is the difference between a gene and an allele? 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. 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. In the above cross between two mulatto genotypes AaBbCc x AaBbCc , each parent produces eight different types of gametes and these gametes combine with each other in 64 different ways resulting in a total of seven skin colors.
The skin colors can be represented by the number of capital letters, ranging from zero no capital letters to six all capital letters. The approximate shades of skin color corresponding to each genotype are shown in the above table.
Note: Skin color may involve at least four pairs of alleles with nine or more shades of skin color. Each term in the expression represents the number of offspring with a specific skin color phenotype based on the number of capital letters in the genotype.
For example, 20 offspring have three capital letters in their genotype and have a skin color that is intermediate between very dark with all caps AABBCC and very light with no caps aabbcc.
Questions 69 - These questions refer to the Rh types of Chrissy and John, and their baby boy named Cinco. Questions 63 - Questions 78 - Remember that the A and B alleles are dominant over the O allele. The type O blood phenotype must be homozygous for the O allele.
Type AB blood phenotype must be heterozygous for the A and B alleles. For these questions, use the process of elimination. Then eliminate the only parents that could have an AB baby, and so forth. Questions 87 - To determine the fractional probability for a taster boy with type B blood, you must make a cross between John and Mary using a genetic checkerboard Punnett square. Questions 91 - Place only decimal values in the squares of your checkerboard because you can't multiply percentages.
The total decimal value for gametes must add up to 1. In other words, 0. The total genotype values must also add up to 1. The following table using 5 coins illustrates these two questions.
Simply change the five coins to three coins or children. Remember that sex determination is much more complicated than tossing coins because many other factors are involved. There is only one permutation out of 32 refer to the top permutation, 3rd column from left.
In this example you must consider all possible permutations with 3 Heads and 2 Tails. The 3rd column from left in the above Pascal's Triangle shows 10 permutations out of 32 with 3 Heads and 2 Tails. This is also the probability of having 3 girls and 2 boys when all possible orders are considered. Thomas Hunt Morgan and Sex Linkage. Developing the Chromosome Theory. Genetic Recombination. Gregor Mendel and the Principles of Inheritance.
Mitosis, Meiosis, and Inheritance. Multifactorial Inheritance and Genetic Disease. Non-nuclear Genes and Their Inheritance. Polygenic Inheritance and Gene Mapping. Sex Chromosomes and Sex Determination. Sex Determination in Honeybees. Test Crosses.
Biological Complexity and Integrative Levels of Organization. Genetics of Dog Breeding. Human Evolutionary Tree.
Mendelian Ratios and Lethal Genes. Environmental Influences on Gene Expression. Epistasis: Gene Interaction and Phenotype Effects. Genetic Dominance: Genotype-Phenotype Relationships.
Phenotype Variability: Penetrance and Expressivity. Citation: Miko, I. Nature Education 1 1 Why can you possess traits neither of your parents have? The relationship of genotype to phenotype is rarely as simple as the dominant and recessive patterns described by Mendel. Aa Aa Aa. Complete versus Partial Dominance. Figure 1. Figure Detail. Multiple Alleles and Dominance Series.
Summarizing the Role of Dominance and Recessivity. References and Recommended Reading Keeton, W. Heredity 35 , 85—98 Parsons, P. Nature , 7—12 link to article Stratton, F. Article History Close. Share Cancel. Revoke Cancel. Keywords Keywords for this Article.
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