Which genotypes show dominant phenotypes

Even though all of the offspring have brown body color, they are heterozygous for the black allele. Figure 8: A Punnett square can help determine the identity of an unknown allele.

Brown flies can be either homozygous BB or heterozygous Bb - but is it possible to determine whether a female fly with a brown body has the genotype BB or Bb? To answer this question, an experiment called a test cross can be performed. Test crosses help researchers determine the genotype of an organism when only its phenotype i.

A test cross is a breeding experiment in which an organism with an unknown genotype associated with the dominant phenotype is mated to an organism that is homozygous for the recessive phenotype.

The Punnett square in Figure 8 can be used to consider how the identity of the unknown allele is determined in a test cross. Again, the Punnett squares in this example function like a genetic multiplication table, and there is a specific reason why squares such as these work.

During meiosis, chromosome pairs are split apart and distributed into cells called gametes. Each gamete contains a single copy of every chromosome, and each chromosome contains one allele for every gene.

Therefore, each allele for a given gene is packaged into a separate gamete. For example, a fly with the genotype Bb will produce two types of gametes: B and b.

In comparison, a fly with the genotype BB will only produce B gametes, and a fly with the genotype bb will only produce b gametes. Figure A monohybrid cross between two parents with the Bb genotype. Figure Detail The following monohybrid cross shows how this concept works.

The principle of segregation explains how individual alleles are separated among chromosomes. But is it possible to consider how two different genes, each with different allelic forms, are inherited at the same time?

For example, can the alleles for the body color gene brown and black be mixed and matched in different combinations with the alleles for the eye color gene red and brown? The simple answer to this question is yes. When chromosome pairs randomly align along the metaphase plate during meiosis I, each member of the chromosome pair contains one allele for every gene. Each gamete will receive one copy of each chromosome and one allele for every gene. When the individual chromosomes are distributed into gametes, the alleles of the different genes they carry are mixed and matched with respect to one another.

In this example, there are two different alleles for the eye color gene: the E allele for red eye color, and the e allele for brown eye color. The red E phenotype is dominant to the brown e phenotype, so heterozygous flies with the genotype Ee will have red eyes.

Figure The four phenotypes that can result from combining alleles B, b, E, and e. When two flies that are heterozygous for brown body color and red eyes are crossed BbEe X BbEe , their alleles can combine to produce offspring with four different phenotypes Figure Those phenotypes are brown body with red eyes, brown body with brown eyes, black body with red eyes, and black body with brown eyes.

Consider a cross between two parents that are heterozygous for both body color and eye color BbEe x BbEe. This type of experiment is known as a dihybrid cross. All possible genotypes and associated phenotypes in this kind of cross are shown in Figure The four possible phenotypes from this cross occur in the proportions Specifically, this cross yields the following:. Why does this ratio of phenotypes occur?

To answer this question, it is necessary to consider the proportions of the individual alleles involved in the cross. The ratio of brown-bodied flies to black-bodied flies is , and the ratio of red-eyed flies to brown-eyed flies is also This means that the outcomes of body color and eye color traits appear as if they were derived from two parallel monohybrid crosses. In other words, even though alleles of two different genes were involved in this cross, these alleles behaved as if they had segregated independently.

The outcome of a dihybrid cross illustrates the third and final principle of inheritance, the principal of independent assortment , which states that the alleles for one gene segregate into gametes independently of the alleles for other genes. To restate this principle using the example above, all alleles assort in the same manner whether they code for body color alone, eye color alone, or both body color and eye color in the same cross.

Mendel's principles can be used to understand how genes and their alleles are passed down from one generation to the next. When visualized with a Punnett square, these principles can predict the potential combinations of offspring from two parents of known genotype, or infer an unknown parental genotype from tallying the resultant offspring. An important question still remains: Do all organisms pass on their genes in this way?

The answer to this question is no, but many organisms do exhibit simple inheritance patterns similar to those of fruit flies and Mendel's peas. These principles form a model against which different inheritance patterns can be compared, and this model provide researchers with a way to analyze deviations from Mendelian principles. This page appears in the following eBook.

Aa Aa Aa. Genes come in different varieties, called alleles. Somatic cells contain two alleles for every gene, with one allele provided by each parent of an organism.

Often, it is impossible to determine which two alleles of a gene are present within an organism's chromosomes based solely on the outward appearance of that organism.

However, an allele that is hidden, or not expressed by an organism, can still be passed on to that organism's offspring and expressed in a later generation. Tracing a hidden gene through a family tree. 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. When discussing genotype, biologists use uppercase letters to stand for dominant alleles and lowercase letters to stand for recessive alleles.

An organism with two dominant alleles for a trait is said to have a homozygous dominant genotype. Using the eye color example, this genotype is written BB. An organism with one dominant allele and one recessive allele is said to have a heterozygous genotype. In our example, this genotype is written Bb. Finally, the genotype of an organism with two recessive alleles is called homozygous recessive. In the eye color example, this genotype is written bb.

Of these three genotypes, only bb, the homozygous recessive genotype, will produce a phenotype of blue eyes. The heterozygous genotype and the homozygous dominant genotype both will produce brown eyes, though only the heterozygous genotype can pass on the gene for blue eyes. The homozygous dominant, homozygous recessive, and heterozygous genotypes only account for some genes and some traits. Most traits actually are more complex, because many genes have more than two alleles, and many alleles interact in complex ways.

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If a media asset is downloadable, a download button appears in the corner of the media viewer. Therefore, the F 1 generation of offspring all are RrYy Figure 8.

In pea plants, purple flowers P are dominant to white p , and yellow peas Y are dominant to green y. What are the possible genotypes and phenotypes for a cross between PpYY and ppYy pea plants? How many squares would you need to complete a Punnett square analysis of this cross? The former two genotypes would result in plants with purple flowers and yellow peas, while the latter two genotypes would result in plants with white flowers with yellow peas, for a ratio of each phenotype.

The gametes produced by the F 1 individuals must have one allele from each of the two genes. For example, a gamete could get an R allele for the seed shape gene and either a Y or a y allele for the seed color gene. It cannot get both an R and an r allele; each gamete can have only one allele per gene. The law of independent assortment states that a gamete into which an r allele is sorted would be equally likely to contain either a Y or a y allele.

Thus, there are four equally likely gametes that can be formed when the RrYy heterozygote is self-crossed, as follows: RY , rY , Ry , and ry. From these genotypes, we find a phenotypic ratio of 9 round—yellow:3 round—green:3 wrinkled—yellow:1 wrinkled—green. These are the offspring ratios we would expect, assuming we performed the crosses with a large enough sample size. The physical basis for the law of independent assortment also lies in meiosis I, in which the different homologous pairs line up in random orientations.

Each gamete can contain any combination of paternal and maternal chromosomes and therefore the genes on them because the orientation of tetrads on the metaphase plane is random Figure 8. Probabilities are mathematical measures of likelihood. The empirical probability of an event is calculated by dividing the number of times the event occurs by the total number of opportunities for the event to occur. It is also possible to calculate theoretical probabilities by dividing the number of times that an event is expected to occur by the number of times that it could occur.

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.