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From Wikipedia

Quantitative trait locus

Quantitative traits refer to phenotypes (characteristics) that vary in degree and can be attributed to polygenic effects, i.e., product of two or more genes, and their environment. Quantitative trait loci (QTLs) are stretches of DNA containing or linked to the genes that underlie a quantitative trait. Mapping regions of the genome that contain genes involved in specifying a quantitative trait is done using molecular tags such as AFLP or, more commonly SNPs . This is an early step in identifying and sequencing the actual genes underlying trait variation.

Quantitative traits

Polygenic inheritance, also known as quantitative or multifactorial inheritance refers to inheritance of a phenotypic characteristic (trait) that is attributable to two or more genes, or the interaction with the environment, or both. Unlike monogenic traits, polygenic traits do not follow patterns of Mendelian inheritance (separated traits). Instead, their phenotypes typically vary along a continuous gradient depicted by a bell curve.

An example of a polygenic trait is human skin color. Many genes factor into determining a person's natural skin color, so modifying only one of those genes changes the color only slightly. Many disorders with genetic components are polygenic, including autism, cancer, diabetes and numerous others. Most phenotypic characteristics are the result of the interaction of multiple genes.

Examples of disease processes generally considered to be results of multifactorialetiology:

Congenital malformation

Adult onset diseases

Multifactorially inherited diseases are said to constitute the majority of genetic disorders affecting humans which will result in hospitalization or special care of some kind.

Multifactorial traits in general

Generally, multifactorial traits outside of illness contribute to what we see as continuous characteristics in organisms, such as height, skin color, and body mass. All of these phenotypes are complicated by a great deal of interplay between genes and environment. The continuous distribution of traits such as height and skin colour described above reflects the action of genes that do not quite show typical patterns of dominance and recessiveness. Instead the contributions of each involved locus are thought to be additive. Writers have distinguished this kind of inheritance as polygenic, or quantitative inheritance.

Thus, due to the nature of polygenic traits, inheritance will not follow the same pattern as a simple monohybrid or dihybrid cross. Polygenic inheritance can be explained as Mendelian inheritance at many loci, resulting in a trait which is normally-distributed. If n is the number of involved loci, then the coefficients of the binomial expansion of (a + b)2n will give the frequency of distribution of all n allele combinations. For a sufficiently high n, this binomial distribution will begin to resemble a normal distribution. From this viewpoint, a disease state will become apparent at one of the tails of the distribution, past some threshold value. Disease states of increasing severity will be expected the further one goes past the threshold and away from the mean.

Heritable disease and multifactorial inheritance

A mutation resulting in a disease state is often recessive, so both alleles must be mutant in order for the disease to be expressed phenotypically. A disease or syndrome may also be the result of the expression of mutant alleles at more than one locus. When more than one gene is involved with or without the presence of environmental triggers, we say that the disease is the result of multifactorial inheritance.

The more genes involved in the cross, the more the distribution of the genotypes will resemble a normal, or Gaussian distribution. This shows that multifactorial inheritance is polygenic, and genetic frequencies can be predicted by way of a polyhybrid Mendelian cross. Phenotypic frequencies are a different matter, especially if they are complicated by environmental factors.

The paradigm of polygenic inheritance as being used to define multifactorial disease has encountered much disagreement. Turnpenny (2004) discusses how simple polygenic inheritance cannot explain some diseases such as the onset of Type I diabetes mellitus, and that in cases such as these, not all genes are thought to make an equal contribution.

The assumption of polygenic inheritance is that all involved loci make an equal contribution to the symptoms of the disease. This should result in a normal curve distribution of genotypes. When it does not, then idea of polygenetic inheritance cannot be supported for that illness.

A cursory look at some examples

Examples of such diseases are not new to medicine. The above examples are well-known examples of diseases having both genetic and environmental components. Other examples involve atopic diseases such as eczema or genetics, a test cross, first introduced by Gregor Mendel, is used to determine if an individual exhibiting a dominant trait is homozygous or heterozygous for that trait. More simply test crosses determine the genotype of an individual with a dominant phenotype.

Test crosses involve breeding the individual in question with another individual that expresses a recessive version of the same trait. If all offspring display the dominant phenotype, the individual in question is homozygous dominant; if the offspring display both dominant and recessive phenotypes, then the individual is heterozygous.

In some sources, the "test cross" is defined as being a type of backcross between the recessive homozygote and F1 generation or F1 generation crossed with recessive parent is said to be a Test cross.

If the individual being tested produces any recessive offspring (except in cases of incomplete penetrance) its genotype is heterozygous. If all the offspring are phenotypically dominant, its genotype is homozygous.


Dihybrid cross

A dihybrid cross is a cross between F1 offspring (first generation offspring) of two individuals that differ in two traits of particular interest. For example: RRyy/rrYY or RRYY/rryy parents result in F1 offspring that are heterozygous for both R and Y (RrYy).

A dihybrid cross is often used to test for dominant and recessive genes in two separate characteristics. Such a cross has a variety of uses in Mendelian genetics.

Meiosis (cell reduction) is the cellular process of gamete creation. It is where sperm and eggs get the unique set of genetic information that will be used in the development and growth of the offspring of the mating. The rules of meiosis, as they apply to the dihybrid, are codified in Mendel's First Law and Mendel's Second Law, which are also called the Law of Segregation and the Law of Independent Assortment, respectively.

For genes on separate chromosomes, each allele pair shows independent segregation. If the first filial generation (F1 generation) produces four offspring, the second filial generation, which occurs by crossing the members of the first filial generation, shows a phenotypic (appearance) ratio of 9:3:3:1.

Mendel's Dihybrid Cross

Scientist Gregor Johann Mendel crossed pure breeding plants with round seeds and yellow albumen to pure breeding plants with wrinkled seeds and green albumen. The F1 generation plants all had round seeds and yellow albumen and Mendel predicted that they would be heterozygous for both traits (RrYy).



From Yahoo Answers

Question:C-curly antennae c-straight antennae S-spotted wings s-no spots on wings Could you tell me the difference between the genotypic and phenotypic ratios for a dihybrid cross if the first question is too much? thanks

Answers:Genotype means what the actual genetic composition is (so, ccSS or CcSs or CCss, etc) - phenotypic ratio is talking about what the APPEARANCE is - so CCss (curly non spotted) would be phenotypically the same as Ccss and CCSS (curly spotted) would be phenotypically the same as CcSs, CCSs, CcSS, etc. This cross should give you a phenotype ratio of 9:2:2:1 with 9 curly spotted, 2 straight spotted, 2 curly non spotted and 1 straight non spotted. The genotypes would be more varied - you can get those by setting up a punnett square. Not doing that for you :p **EDIT: Sorry; messed up my math at first - note corrected ratio. And as a result set up the punnett square to double check so here are your genotypes: ---------CS--------Cs-------cS-------cs CS | CCSS | CCSs | CcSS | CcSs Cs | CCSs | CCss | CCSs | Ccss cS | CcSS | CcSs | ccSS | ccSs cs | CcSs | Ccss | ccSs | ccss So: CCSS: 1 CcSs: 3 ccSs: 2 ccSS: 2 Ccss: 2 CcSS: 2 CCSs: 3 ccss: 1

Question:It is also usd to predict the possible gentic outcomes for those allele combination.

Answers:a monohybrid cross?

Question:16) CROSS I: Purebred lines of wild-type fruit flies (gray-body and normal wings) are mated to flies with black bodies and vestigial wings. All progeny (F1) are of the wild type phenotype. CROSS II: F1 offspring from above are crossed with flies recessive for both traits: Resulting offspring Number Percent Gray body; normal wings 965 42 Black body; vestigial wings 944 41 Black body; normal wings 206 9 Gray body; vestigial wings 185 8 key: a) cross 1 supports statement b) cross 1 gives evidence against the statement c) cross 2 supports d) cross 2 gives evidence against QUESTIONS (which letter choice is it???) an F1 cross should produce flies that will fall into a mendelian 9.3.3.1 ratio there are 17 map units between the genes for body color and wing shape

Answers:Well, working the problem I can see that the wild type is dominant. So let's write it out: G = gray g = black W = normal w = vestigial So what was done was: WwGg x wwgg A mendellian, non-linked ratio of 9:3:3:1 is clearly not what we have here. The only other information given is the "map units" between the two genes. Unfortunately I dropped out of genetics so this is as far as I can go without cracking open the textbook ;) I think it's clear, though, that the traits have a strong tendency to be transmitted together due to their close proximity on the chromosomes. EDIT: Alright I cracked open the textbook. God, All that boring Mendellian stuff in BIOL 1010 and we had to spend 5+ classes on it again in 2nd year genetics! So, we definitely don't have incomplete or co-dominance here. There are only two alleles, so that isn't fudging the phenotype, here. The example used to explain linkage uses the wild vs vestigial wing and wild vs black body example used in this question. This, plus the last bit of information in your question about map units, is strong evidence that we're dealing with a problem of linkage here. I'm not clear on what it is the question you've posted here is asking. Umm, perhaps what it's asking is which cross proves or disproves that the genes are linked. Well, it was the F2 generation, so I guess cross 2, which deviated from the expected results. With linked autosomal genes, the two parent types (in this case, homozygous recessive for both genes and heterozygous dominant for both genes) will occur more frequently than normally expected in a 9:3:3:1 ratio. As we can see, the cross 2 resulted in far black body vestigial wings, that is homozygous recessive for both genes than expected. Thus, cross 2 supports the statement that the two genes are linked.

Question:When reading a question...what hints should you look for to determine whether it's a: monohybrid cross, dihybrid cross, sex-linked, codomoninant, multiple alleles, complete/incomplete dominance.

Answers:Usually when you are reading this kind of questions they will give you the alleles involved and that will be your first clue, also you can see how many characteristics of the organism to be cross are they describing to you (very important). For example a mono-hybrid cross will be between genes with the following alleles Tt*tt, which will display only one characteristic of the organism (example size) and a dihybrid cross will display more than one characteristic of the organism example TYTY*tyty size and color. For co-dominance there are usually classic examples such as blood groups, but it depends on the text if two characteristics are both expressed with the same penetrance then it will probably be codominance. The same thing happens complete and incomplete dominance you will see that the phenotype of the offspring will combined instead of recessive and dominant. One classic example of this is will be a pink flower daughter of red and white parents. Sex linked characteristics are literally related to the sex genes they will talk about the disease that the mother or the father will carrie, you are suppose to know the classic examples which are hemophilia and color-blindness which are x-linked as well as baldness. Then the daughters of persons with these kind of diseases will almost never express the disease but they will be carriers of the gene.

From Youtube

Genetics: Dihybrid Cross: 04 :Basic genetics tutoring: solving a word problem involving a dihybrid cross, using a Punnett Square. Video 4 of a 4-part series.

Genetics: Dihybrid Cross: 02 :Basic genetics tutoring: solving a word problem involving a dihybrid cross, using a Punnett Square. Video 2 of a 4-part series.