Is it possible to determine the genotype of an individual with a recessive phenotype why

There are a couple of reasons for this. First off, you have two copies of most of your genes — one from mom and one from dad. And second, a dominant version of a gene can hide the presence of a recessive one. So if you have a dominant trait, you have two possible genotypes. You could either have two dominant versions of the gene that leads to the trait or one recessive and one dominant version.

With either genotype you show the dominant trait. Let's use eye color as a quick example. In eye color, brown is dominant over blue. Geneticists represent brown eyes with a B and blue eyes with a b.

Both the BB and the Bb genotypes give brown eyes. This means that if you have brown eyes, you can't know which of these two genotypes you have. What I want to do for the rest of the answer is expand on this to make it clearer why dominant traits have two possible genetic combinations. But I won't focus on tongue rolling since it is an open question whether it is even genetic let alone dominant. No, instead I'll focus on an old stand by - PTC tasting.

I also want to talk a little about situations where you can determine your genotype with a dominant trait. As you'll see, you can sometimes pull it off under the right circumstances. If you can taste it, you have the dominant trait. Well, we know for sure you have one copy of the dominant tasting version of the PTC gene. Otherwise you wouldn't be able to taste the PTC! But we can't figure out your other version with just this information.

People with two dominant PTC versions don't taste PTC any differently than do people with a dominant tasting version and a recessive non-tasting version. Both people can taste it. In genetics-speak, we know your phenotype — you can taste PTC. What we don't know is your genotype. You could either have two dominant copies or a dominant and a recessive copy of the PTC gene.

And this is true for other dominant traits as well. So now we know why you can't automatically figure out your genotype if you have a dominant trait. Of course that doesn't mean you can never find out. You can in certain situations. Sometimes a genetic test will give you your genotype. Sometimes you just need a bit of genetic luck in your family tree to figure it out. And sometimes you can tell the two genotypes apart just by looking at someone.

An obvious way to figure out you genotype is to have a genetic test done. Companies like 23andMe can do this pretty inexpensively nowadays. But the first formal genetic study was undertaken by a monk named Gregor Mendel in the middle of the 19th Century. Mendel bred peas and noticed he could cross-pollinate them in certain ways to get green or yellow seeds. Today, the field of genetics is breaking new ground searching for new ways to treat disease or develop crops more resistant to insects or drought.

Empower your students to learn about genetics with this collection of resources. Genetic variation is the presence of differences in sequences of genes between individual organisms of a species. It enables natural selection, one of the primary forces driving the evolution of life. Genes are units of hereditary information. A gene is a section of a long molecule called deoxyribonucleic acid DNA. Join our community of educators and receive the latest information on National Geographic's resources for you and your students.

Skip to content. Image Female portrait Many physical traits like hair color and texture, eye color, and skin color are determined by the genotypes that parents pass down to their children.

Photograph by Martin Schoeller. Twitter Facebook Pinterest Google Classroom. Encyclopedic Entry Vocabulary. Media Credits The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit.

Media If a media asset is downloadable, a download button appears in the corner of the media viewer. Text Text on this page is printable and can be used according to our Terms of Service. Interactives Any interactives on this page can only be played while you are visiting our website. Related Resources. View Collection. Empirical probabilities come from observations, like those of Mendel. Theoretical probabilities come from knowing how the events are produced and assuming that the probabilities of individual outcomes are equal.

A probability of one for some event indicates that it is guaranteed to occur, whereas a probability of zero indicates that it is guaranteed not to occur. An example of a genetic event is a round seed produced by a pea plant. When the F 1 plants were subsequently self-crossed, the probability of any given F 2 offspring having round seeds was now three out of four. In other words, in a large population of F 2 offspring chosen at random, 75 percent were expected to have round seeds, whereas 25 percent were expected to have wrinkled seeds.

Using large numbers of crosses, Mendel was able to calculate probabilities and use these to predict the outcomes of other crosses. Mendel demonstrated that the pea-plant characteristics he studied were transmitted as discrete units from parent to offspring. As will be discussed, Mendel also determined that different characteristics, like seed color and seed texture, were transmitted independently of one another and could be considered in separate probability analyses.

For instance, performing a cross between a plant with green, wrinkled seeds and a plant with yellow, round seeds still produced offspring that had a ratio of green:yellow seeds ignoring seed texture and a ratio of round:wrinkled seeds ignoring seed color. The characteristics of color and texture did not influence each other.

The product rule of probability can be applied to this phenomenon of the independent transmission of characteristics. The product rule states that the probability of two independent events occurring together can be calculated by multiplying the individual probabilities of each event occurring alone. To demonstrate the product rule, imagine that you are rolling a six-sided die D and flipping a penny P at the same time. The outcome of rolling the die has no effect on the outcome of flipping the penny and vice versa.

There are 12 possible outcomes of this action, and each event is expected to occur with equal probability. For example, consider how the product rule is applied to the dihybrid cross: the probability of having both dominant traits in the F 2 progeny is the product of the probabilities of having the dominant trait for each characteristic, as shown here:.

On the other hand, the sum rule of probability is applied when considering two mutually exclusive outcomes that can come about by more than one pathway. The sum rule states that the probability of the occurrence of one event or the other event, of two mutually exclusive events, is the sum of their individual probabilities.

What is the probability of one coin coming up heads and one coin coming up tails? This outcome can be achieved by two cases: the penny may be heads P H and the quarter may be tails Q T , or the quarter may be heads Q H and the penny may be tails P T.

Either case fulfills the outcome. You should also notice that we used the product rule to calculate the probability of P H and Q T , and also the probability of P T and Q H , before we summed them. Again, the sum rule can be applied to show the probability of having just one dominant trait in the F 2 generation of a dihybrid cross:.

To use probability laws in practice, it is necessary to work with large sample sizes because small sample sizes are prone to deviations caused by chance. The large quantities of pea plants that Mendel examined allowed him calculate the probabilities of the traits appearing in his F 2 generation. Alkaptonuria is a recessive genetic disorder in which two amino acids, phenylalanine and tyrosine, are not properly metabolized.

Affected individuals may have darkened skin and brown urine, and may suffer joint damage and other complications. In this pedigree, individuals with the disorder are indicated in blue and have the genotype aa. Unaffected individuals are indicated in yellow and have the genotype AA or Aa. For example, if neither parent has the disorder but their child does, they must be heterozygous.

Two individuals on the pedigree have an unaffected phenotype but unknown genotype. When true-breeding, or homozygous, individuals that differ for a certain trait are crossed, all of the offspring will be heterozygous for that trait. If the traits are inherited as dominant and recessive, the F 1 offspring will all exhibit the same phenotype as the parent homozygous for the dominant trait.

If these heterozygous offspring are self-crossed, the resulting F 2 offspring will be equally likely to inherit gametes carrying the dominant or recessive trait, giving rise to offspring of which one quarter are homozygous dominant, half are heterozygous, and one quarter are homozygous recessive. Because homozygous dominant and heterozygous individuals are phenotypically identical, the observed traits in the F 2 offspring will exhibit a ratio of three dominant to one recessive.

Mendel postulated that genes characteristics are inherited as pairs of alleles traits that behave in a dominant and recessive pattern. Alleles segregate into gametes such that each gamete is equally likely to receive either one of the two alleles present in a diploid individual. In addition, genes are assorted into gametes independently of one another. That is, in general, alleles are not more likely to segregate into a gamete with a particular allele of another gene. Punnett square: a visual representation of a cross between two individuals in which the gametes of each individual are denoted along the top and side of a grid, respectively, and the possible zygotic genotypes are recombined at each box in the grid.

Skip to content Chapter 8: Introduction to Patterns of Inheritance. The P cross produces F1 offspring that are all heterozygous for both characteristics. The resulting F2 phenotypic ratio is obtained using a Punnett square. Glossary allele: one of two or more variants of a gene that determines a particular trait for a characteristic dihybrid: the result of a cross between two true-breeding parents that express different traits for two characteristics genotype: the underlying genetic makeup, consisting of both physically visible and non-expressed alleles, of an organism heterozygous: having two different alleles for a given gene on the homologous chromosomes homozygous: having two identical alleles for a given gene on the homologous chromosomes law of dominance: in a heterozygote, one trait will conceal the presence of another trait for the same characteristic law of independent assortment: genes do not influence each other with regard to sorting of alleles into gametes; every possible combination of alleles is equally likely to occur law of segregation: paired unit factors i.

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