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3.6: Phenotypes May Not Be As Expected from the Genotype - Biology

3.6:  Phenotypes May Not Be As Expected from the Genotype - Biology


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Environmental Factors

The phenotypes described thus far have a nearly perfect correlation with their associated genotypes; in other words an individual with a particular genotype always has the expected phenotype. However, many phenotypes are not determined entirely by genotype alone. They are instead determined by an interaction between genotype and non-genetic, environmental factors and can be conceptualized in the following relationship:

[mathrm{Genotype + Environment ⇒ Phenotype hspace{30px} (G + E ⇒ P)}]

Or:

[mathrm{Genotype + Environment + overset{Genetics: and: Environment}{Interaction_{GE}} ⇒ Phenotype hspace{30px} (G + E + I_{GE} ⇒ P)}]

This interaction is especially relevant in the study of economically important phenotypes, such as human diseases or agricultural productivity. For example, a particular genotype may pre-dispose an individual to cancer, but cancer may only develop if the individual is exposed to certain DNA-damaging chemicals. Therefore, not all individuals with the particular genotype will develop the cancer phenotype.

Penetrance and Expressivity

The terms penetrance and expressivity are also useful to describe the relationship between certain genotypes and their phenotypes.

  • Penetrance is the proportion of individuals (usually expressed as a percentage) with a particular genotype that display a corresponding phenotype (Figure (PageIndex{16})). Because all pea plants that are homozygous for the allele for white flowers (e.g. aa in Figure (PageIndex{3})) actually have white flowers, this genotype is completely penetrant. In contrast, many human genetic diseases are incompletely penetrant, since not all individuals with the disease genotype actually develop symptoms associated with the disease.
  • Expressivity describes the variability in mutant phenotypes observed in individuals with a particular phenotype (Figure (PageIndex{16})). Many human genetic diseases provide examples of broad expressivity, since individuals with the same genotypes may vary greatly in the severity of their symptoms. Incomplete penetrance and broad expressivity are due to random chance, non-genetic (environmental), and genetic factors (mutations in other genes).

Mastering Biology CH 14 homework

Diploid cells have two sets of chromosomes, one set inherited from each parent, that form homologous pairs.

The homologs of a chromosome pair contain the same genetic loci. Therefore, each genetic locus is represented twice in a diploid cell.

The white allele is the recessive allele.

the round allele (R) is dominant to the wrinkled allele (r) and

the yellow allele (Y) is dominant to the green allele (y).

The table below shows the F1 progeny that result from selfing four different parent pea plants.

Use the phenotypes of the F1 progeny to deduce the genotype and phenotype of each parent plant.

Parent phenotype plant 2: yellow round
Parent genotype plant 2: RrYy

Parent phenotype plant 3: yellow round
Parent genotype plant 3: RRYy

[round, yellow], [wrinkled, yellow], [round, green], and [wrinkled, green].

Use this information to deduce the genotypes of the parent plants.

Expected frequencies yellow wrinkled: 3/8
Progeny genotypes yellow wrinkled: rrYY, rrYy (x2)

Expected frequencies green round: 1/8
Progeny genotypes green round: Rryy

This Punnett square shows the results of a Yy x Yy cross to form F2 progeny.

Use your understanding of Mendel's law of segregation and the rules of probability to complete the Punnett square for this cross.

First identify the gametes. Use pink labels to identify the male and female gamete types and white labels to identify the gamete frequencies.

2. What is the probability that an F2 seed chosen at random from among the yellow seeds will breed true when selfed? 1/3

3. What is the probability that three F2 seeds chosen at random will include at least one yellow seed? 63/64

Forked: The mutant allele is dominant to its corresponding wild-type allele.

You designate the forked mutant allele as F (wild type = f+ ) and the pale mutant allele as p (wild type = P).

1. Consider the alleles for leaf color first. Drag the labels to the targets in Group 1 to identify the genotype of each F2 class. Remember that p (the pale mutant allele) and P (the wild-type allele) are incompletely dominant to each other.

2. Consider the alleles for leaf shape next. Drag the labels to the targets in Group 2 to identify the genotype of each F2 class. Remember that F (the forked mutant allele) is dominant to f + (the wild-type allele).


Abstract

To understand how evolving systems bring forth novel and useful phenotypes, it is essential to understand the relationship between genotypic and phenotypic change. Artificial evolving systems can help us understand whether the genotype-phenotype maps of natural evolving systems are highly unusual, and it may help create evolvable artificial systems. Here we characterize the genotype-phenotype map of digital organisms in Avida, a platform for digital evolution. We consider digital organisms from a vast space of 10 141 genotypes (instruction sequences), which can form 512 different phenotypes. These phenotypes are distinguished by different Boolean logic functions they can compute, as well as by the complexity of these functions. We observe several properties with parallels in natural systems, such as connected genotype networks and asymmetric phenotypic transitions. The likely common cause is robustness to genotypic change. We describe an intriguing tension between phenotypic complexity and evolvability that may have implications for biological evolution. On the one hand, genotypic change is more likely to yield novel phenotypes in more complex organisms. On the other hand, the total number of novel phenotypes reachable through genotypic change is highest for organisms with simple phenotypes. Artificial evolving systems can help us study aspects of biological evolvability that are not accessible in vastly more complex natural systems. They can also help identify properties, such as robustness, that are required for both human-designed artificial systems and synthetic biological systems to be evolvable.


Proverbs 18:18

18 Casting lots causes contentions to cease,
And keeps the mighty apart.

Probability and Inheritance

The same rules of probability in coin tossing apply to the main events that determine the genotypes of offspring. These events are the formation of gametes during meiosis and the union of gametes during fertilization.

Probability and Gamete Formation

How is gamete formation like tossing a coin? Consider Mendel’s purple-flowered pea plants again. Assume that a plant is heterozygous for the flower-color allele, so it has the genotype Bb (see Figure below). During meiosis, homologous chromosomes—and the alleles they carry—segregate and go to different gametes. Therefore, when the Bb pea plant forms gametes, the B and b alleles segregate and go to different gametes. As a result, half the gametes produced by the Bb parent will have the B allele and half will have the b allele. Based on the rules of probability, any given gamete of this parent has a 50 percent chance of having the B allele and a 50 percent chance of having the b allele.

Probability and Fertilization

Which of these gametes joins in fertilization with the gamete of another parent plant? This is a matter of chance, like tossing a coin. Thus, we can assume that either type of gamete—one with the B allele or one with the b allele—has an equal chance of uniting with any of the gametes produced by the other parent. Now assume that the other parent is also Bb. If gametes of two Bb parents unite, what is the chance of the offspring having one of each allele like the parents (Bb)? What is the chance of them having a different combination of alleles than the parents (either BB or bb)? To answer these questions, geneticists use a simple tool called a Punnett square.

Using a Punnett Square

A Punnett square is a chart that allows you to easily determine the expected percents of different genotypes in the offspring of two parents. An example of a Punnett square for pea plants is shown in Figure below.

In this example, both parents are heterozygous for flower color (Bb). The gametes produced by the male parent are at the top of the chart, and the gametes produced by the female parent are along the side. The different possible combinations of alleles in their offspring are determined by filling in the cells of the Punnett square with the correct letters (alleles). At the link below, you can watch an animation in which Reginald Punnett, inventor of the Punnett square, explains the purpose of his invention and how to use it.

An explanation of Punnett squares can be viewed in the video below:

Punnett Squares by Bozeman Science:

An example of the use of a Punnett square can be viewed at http://www.youtube.com/watch?v=nsHZbgOmVwg&feature=related (5:40), just in case you are totally not getting this yet. If you get it, move along and feel free to skip it!!

Predicting Offspring Genotypes

In the cross shown in Figure above (skip up past the 3 videos), you can see that one out of four offspring (25 percent) has the genotype BB, one out of four (25 percent) has the genotype bb, and two out of four (50 percent) have the genotype Bb. These percents of genotypes are what you would expect in any cross between two heterozygous parents. Of course, when just four offspring are produced, the actual percents of genotypes may vary by chance from the expected percents. However, if you considered hundreds of such crosses and thousands of offspring, you would get very close to the expected results—just like tossing a coin.

Predicting Offspring Phenotypes

You can predict the percents of phenotypes in the offspring of this cross from their genotypes. B is dominant to b, so offspring with either the BB or Bb genotype will have the purple-flower phenotype. Only offspring with the bb genotype will have the white-flower phenotype. Therefore, in this cross, you would expect three out of four (75 percent) of the offspring to have purple flowers and one out of four (25 percent) to have white flowers. These are the same percents that Mendel got in his first experiment.

Determining Missing Genotypes

A Punnett square can also be used to determine a missing genotype based on the other genotypes involved in a cross. Suppose you have a parent plant with purple flowers and a parent plant with white flowers. Because the b allele is recessive, you know that the white-flowered parent must have the genotype bb. The purple-flowered parent, on the other hand, could have either the BB or the Bb genotype. The Punnett square in Figure below shows this cross. The question marks (?) in the chart could be either B or b alleles.

Can you tell what the genotype of the purple-flowered parent is from the information in the Punnett square? No you also need to know the genotypes of the offspring in row 2. What if you found out that two of the four offspring have white flowers? Now you know that the offspring in the second row must have the bb genotype. One of their b alleles obviously comes from the white-flowered (bb) parent, because that’s the only allele this parent has. The other b allele must come from the purple-flowered parent. Therefore, the parent with purple flowers must have the genotype Bb.

Punnett Square for Two Characteristics

When you consider more than one characteristic at a time, using a Punnett square is more complicated. This is because many more combinations of alleles are possible. For example, with two genes each having two alleles, an individual has four alleles, and these four alleles can occur in 16 different combinations. This is illustrated for pea plants in Figure below. In this cross, both parents are heterozygous for pod color (Gg) and seed color (Yy).

How Mendel Worked Backward to Get Ahead

Mendel used hundreds or even thousands of pea plants in each experiment he did. Therefore, his results were very close to those you would expect based on the rules of probability. For example, in one of his first experiments with flower color, there were 929 plants in the F2 generation. Of these, 705 (76 percent) had purple flowers and 224 (24 percent) had white flowers. Thus, Mendel’s results were very close to the 75 percent purple and 25 percent white you would expect by the laws of probability for this type of cross. Of course, Mendel had only phenotypes to work with. He knew nothing about genes and genotypes. Instead, he had to work backward from phenotypes and their percents in offspring to understand inheritance. From the results of his first set of experiments, Mendel realized that there must be two factors controlling each of the characteristics he studied, with one of the factors being dominant to the other. He also realized that the two factors separate and go to different gametes and later recombine in the offspring. This is an example of Mendel’s good luck. All of the characteristics he studied happened to be inherited in this way. Mendel also was lucky when he did his second set of experiments. He happened to pick characteristics that are inherited independently of one another. We now know that these characteristics are controlled by genes on nonhomologous chromosomes. What if Mendel had studied characteristics controlled by genes on homologous chromosomes? Would they be inherited together? If so, how do you think this would have affected Mendel’s conclusions? Would he have been able to develop his second law of inheritance? To better understand how Mendel interpreted his findings and developed his laws of inheritance, you can visit the following link. It provides an animation in which Mendel explains how he came to understand heredity from his experimental results.

Non-Mendelian Inheritance

The inheritance of characteristics is not always as simple as it is for the characteristics that Mendel studied in pea plants. Each characteristic Mendel investigated was controlled by one gene that had two possible alleles, one of which was completely dominant to the other. This resulted in just two possible phenotypes for each characteristic. Each characteristic Mendel studied was also controlled by a gene on a different (nonhomologous) chromosome. As a result, each characteristic was inherited independently of the other characteristics. Geneticists now know that inheritance is often more complex than this.

Codominance and Incomplete Dominance

A characteristic may be controlled by one gene with two alleles, but the two alleles may have a different relationship than the simple dominant-recessive relationship that you have read about so far. For example, the two alleles may have a codominant or incompletely dominant relationship. The former is illustrated by the flower in Figure below, and the latter in Figure below.

Codominance

Codominance occurs when both alleles are expressed equally in the phenotype of the heterozygote. The red and white flower in the figure has codominant alleles for red petals and white petals.

Incomplete Dominance

Incomplete dominance occurs when the dominant allele is not completely dominant. Expression of the dominant allele is influenced by the recessive allele and an intermediate phenotype results. The pink flower in the figure has an incompletely dominant allele for red petals and a recessive allele for white petals.

Codominance. The flower has red and white petals because of codominance of red-petal and white-petal alleles.

Incomplete Dominance. The flower has pink petals because of incomplete dominance of a red-petal allele and a recessive white-petal allele.

Multiple Alleles

Many genes have multiple (more than two) alleles. An example is ABO blood type in humans. There are three common alleles for the gene that controls this characteristic. The allele for type A is codominant with the allele for type B, and both alleles are dominant to the allele for type O. Therefore, the possible phenotypes are type A, B, AB, and O. Do you know what genotypes produce these phenotypes?

Try out this worksheet to help you understand Mendelian inheritance:

Polygenic Characteristics

Polygenic characteristics are controlled by more than one gene, and each gene may have two or more alleles. The genes may be on the same chromosome or on nonhomologous chromosomes.

  • If the genes are located close together on the same chromosome, they are likely to be inherited together. However, it is possible that they will be separated by crossing-over during meiosis, in which case they may be inherited independently of one another.
  • If the genes are on nonhomologous chromosomes, they may be recombined in various ways because of independent assortment.

For these reasons, the inheritance of polygenic characteristics is very complicated. Such characteristics may have many possible phenotypes. Skin color and adult height are examples of polygenic characteristics in humans. Do you have any idea how many phenotypes each characteristic has?

Effects of Environment on Phenotype

Genes play an important role in determining an organism’s characteristics. However, for many characteristics, the individual’s phenotype is influenced by other factors as well. Environmental factors, such as sunlight and food availability, can affect how genes are expressed in the phenotype of individuals. Here are just two examples:

  • Genes play an important part in determining our adult height. However, factors such as poor nutrition can prevent us from achieving our full genetic potential.
  • Genes are a major determinant of human skin color. However, exposure to ultraviolet radiation can increase the amount of pigment in the skin and make it appear darker.

Lesson Summary

  • Probability is the chance that a certain event will occur. For example, the probability of a head turning up on any given coin toss is 50 percent.
  • Probability can be used to predict the chance of gametes and offspring having certain alleles.
  • A Punnett square is a chart for determining the expected percents of different genotypes and phenotypes in the offspring of two parents.
  • Mendel used the percents of phenotypes in offspring to understand how characteristics are inherited.
  • Many characteristics have more complex inheritance patterns than those studied by Mendel. They are complicated by factors such as codominance, incomplete dominance, multiple alleles, and environmental influences.

Lesson Review Questions

Recall

1. Define probability. Apply the term to a coin toss.

2. How is gamete formation like tossing a coin?

3. What is a Punnett square? How is it used?

4. What information must you know to determine the phenotypes of different genotypes for a gene with two alleles?

5. Based on the results of his experiments, what did Mendel conclude about the factors that control characteristics such as flower color?

Apply Concepts

6. Draw a Punnett square of an Ss x ss cross. The S allele codes for long stems in pea plants and the s allele codes for short stems. If S is dominant to s, what percent of offspring would you expect to have each phenotype?

7. What letter should replace the question marks (?) in this Punnett square? Explain how you know.

Think Critically

8. Explain how Mendel used math and probability to understand the results of his experiments.

9. Compare and contrast codominance and incomplete dominance.

10. Mendel investigated stem length, or height, in pea plants. What if he had investigated human height instead? Why would his results have been harder to interpret?

Points to Consider

Like most of the characteristics of living things, the characteristics Mendel studied in pea plants are controlled by genes. All the cells of an organism contain the same genes, because all organisms begin as a single cell. Most of the genes code for proteins.


3.6: Phenotypes May Not Be As Expected from the Genotype - Biology


Primer of Mendelian Genetics
The appearance of an organism ( phenotype ) is influenced by its heredity ( genotype ). Many individual characters (morphological, behavioral, biochemical, molecular, etc.) of organisms are influenced more or less directly by individual hereditary elements called genes . Genes are located on chromosomes , each at a particular physical location called a locus (plural, loci) .

Genetics is the science of analyzing phenotypes to infer the nature of their underlying genotypes . The basic principles were first described by Gregor Mendel in 1867. Genetics operated as a distinct science from the rediscovery of Mendel's work in 1900, without knowledge of the genetic material until 1953. Genetics is distinct from molecular biology , which analyzes genotypes (in a DNA molecule) to predict phenotypes (which are often direct or indirect products of proteins ) . For this reason, the so-called Central Dogma of molecular biology (DNA » RNA » Protein) is sometimes called " reverse genetics ."

1
. Alternative forms of genes are called alleles every individual possesses two alleles for each gene * .
An individual with two identical alleles is a homozygote and is described as homozygous
an individual with two dissimilar alleles is a heterozygote and is described as heterozygous .

2
. Some alleles (called dominant ) mask the phenotypic expression of other alleles (called recessive ).
Dominance is determined by comparison of the heterozygote phenotype with that of the two homozygotes
Dominant alleles are symbolized with a capital letter ( A )
recessive alleles with a lower-case letter ( a ).

For example, some people can taste the chemical phenylthiocarbimide ( PTC ) ("tasters"),
and some cannot ("non-tasters").
The character "PTC sensitivity" is influenced by a gene with two alleles,
one associated with " taster" and one with " non-taster" .
The "taster" allele masks the expression of the "non-taster" allele in heterozygotes :
Homozygous TT or heterozygous Tt individuals both show the "T" phenotype ("taster"):
only a homozygous tt individual show the " t " phenotype (" non-taster ").
Because the phenotype of the Tt individual resembles that of the TT individuals,
the T allele is described as dominant to the t allele.

3
. The two alleles separate (segregate) during the formation of gametes (eggs & sperm)
half of the germs cells carry one allele & half carry the other [Mendel's Law of Segregation] .

4
. Random union of gametes produces zygotes that develop into new individuals.
Zygotic genotypes occur in characteristic ratios , according to the genotypes of the parents.
For example, a cross between two heterozygotes (Aa x Aa)
produces an expected genotypic ratio of 1:2:1 among AA, Aa, & aa genotypes.

5
. The genotypic ratios produce characteristic phenotypic ratios ,
according to the dominance relationships of the alleles involved.
For example, if A is dominant to a, the cross between heterozygotes produces
an expected phenotypic ratio of 3:1 among "A" and "a" phenotypes.

6. Alleles at separate loci are inherited independently [ Mendel's Law of Independent Assortment ]
This produces characteristic genotypic and phenotypic ratios.
For example, in a dihybrid cross between two " double heterozygotes " ( AaBb x AaBb )
The genotypic ratios are 1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1
for the genotypes AABB AABb AAbb AaBB AaBb Aabb aaBB aaBb aabb
and the phenotypic ratios are 9 " AB " : 3 " Ab " : 3 " aB " : 1 " ab "

* Mendel was unaware t hat genes reside on chromosomes
Genes that occur on the same chromosome are said to be linked
Gene loci located near each other on a single chromosome will not assort independently.
T he characteristic ratios will be modified , according to how close they are.
The modified ratios can be used to create a genetic map of the chromosome

For example, sex in humans is determined by genes on sex chromosomes ( X and Y )
females are XX have two alleles (one on each X )
males are XY and have only one allele on the single X ( hemizygous )
Characters on the X (or Y ) chromosomes are sex-linked


Why do healthcare providers care about matching blood types during blood transfusions?

When a person receives a blood transfusion, it is very important that they receive the correct blood type. If a person receives the incorrect blood type, it can lead to a very dangerous immune response. Here is why that happens:

As you may already know, antibodies are part of your body’s defense system and they are used to help attack anything foreign found in the body. Antibodies function by destroying foreign bodies or marking them for destruction by some other component of the immune system. A healthy person would not make antibodies against anything that is normally found in their body. For example, if you have AB blood, your body would not make antibodies against A or B glycolipids. If you have O blood, however, your blood does not contain A or B glycolipids. As a result, your body would recognize the A and B glycolipids in A, B, and AB type blood as foreign and produce antibodies against them. More generally, a person will produce antibodies against any blood glycolipid that doesn’t already exist in their blood type.

This can create a problem during blood transfusions if you give someone blood that they have antibodies against, their immune system will work to destroy the donated blood and the process will lead to dangerous inflammation. As a result, people can only receive blood donations from blood types that they do not have antibodies against. This is why O is called the universal donor (O contains neither A nor B glycolipids so it does not induce an antibody response in anyone) and AB is called the universal recipient (AB blood contains both A and B glycolipids so a person with this blood type will not make antibodies against any other blood type).


Phenotypic Ratio: How To Find It.

Before finding out how to find the phenotypic ratio, it is worth brushing up on several terms used in the field of genetics.

    : a basic unit of inheritance that is the result of the genes of the parents. Genes are coded messages that produce specific proteins inside a cell, but only if a cell has been switched on to express it.
  • Allele: a version of a gene that comes from one of the two parents. When an allele from either parent is the same, it is called a homozygous gene. If two alleles inherited from the parents are different, the alleles of that organism’s offspring can also be heterozygous. (See below image).
  • Locus: a locus gives us the coordinates for the position of a specific gene on a chromosome. : the total set of genes in an organism that makes up a specific trait these genes do not have to be expressed they are present in every strand of DNA.
  • Phenotype: a trait that is observable or measurable in an organism at any point during that organism’s lifetime. A phenotype is an expressed gene.
  • Monohybrid: the offspring of two parents that only differ at a specific gene locus and for one specific trait. Which of these two heterozygous loci is expressed (dominant) decides the phenotype of the offspring.
  • Dihybrid: the offspring of two parents that only differ at two specific gene loci. Offspring can express different combinations of phenotypes.
  • Trihybrid: the offspring of two parents that only differ at three specific gene loci. Offspring can express a greater range of phenotypes than in dihybrids. : a pattern of inheritance in non-sex (autosomal) chromosomes. A dominant allele or gene will always be expressed as a phenotype when the corresponding allele from the other parent is recessive.
  • Autosomal recessive: an allele or gene that cannot override a dominant allele however, if the other parent also has a recessive allele at the same location, the recessive trait will be observed in the offspring. : a graphical representation of potential genotypes and phenotypes that predicts the probability of a specific trait in a breeding pair.

The phenotypic ratio is the number of times a specific combination of alleles appears in the predicted phenotypes of any offspring. Genetic information relating to the studied trait must be known. It is also possible to work out which parent alleles are dominant or recessive by studying the phenotypes of their offspring.

This is an example of Mendelian inheritance or inheritance patterns that occur in offspring after sexual reproduction between two organisms. The name comes from Gregor Mendel who – at first rather unwittingly – experimented with pea-plant crosses in his monastery’s garden. These observations led to our understanding of dominant and recessive traits.


Discussion

The change from an agrarian society to an industrial and postindustrial one has been well-noted (41). This change, along with others, resulted in dramatic shifts in the environments encountered by humans during the course of the 20th century. Expansion of schooling (42), medical improvements (43), increased longevity (44), and caloric abundance are just some of the changes that may influence not only relationships between important phenotypes but the underlying salience of their associated genotypes as well. In the present paper, we demonstrate that observed changes in mating preferences or fertility associations do not always correspond to shifts in the underlying genetic architecture. For example, whereas higher educational attainment has come to predict having fewer children in more recent birth cohorts, the same is not true of genetics correlated with educational attainment—i.e., the association between a person’s PGS for educational attainment and their fertility has remained constant across birth cohorts. Likewise, the spousal correlation in educational phenotype has been increasing even as the sorting on measured polygenic predictors of education has been flat (if not decreasing).

Contrary to worries about negative selection and/or increased polarization on education-related genetic measures, we predict—based on the results here—that any dysgenic dynamics with respect to education are not, in fact, increasing even as phenotypic associations strengthen. Although the environmental landscape of US society is certainly changing, the genetic makeup of the population may also be shifting along with it—although possibly in conflicting directions. Future researchers examining cohort trends in genetic influences—under the assumption that these are entirely driven by changing environmental conditions while genetic variance is constant—would be wise to reexamine that assumption (45 ⇓ ⇓ –48).

In addition to informing our knowledge about the changing genetic and phenotypic landscape of marriage and reproduction, knowing the degree to which spouses are correlated on phenotypically predictive genetic measures as opposed to merely observed phenotype has important implications for models of additive heritability that rely on an assumption of random mating (49). Namely, most heritability models—notably the classic twin or extended twin design—assume that siblings (i.e., fraternal twins) share, on average, 50% of the relevant genetic markers that are associated with the phenotype of interest. If there is significant positive assortative mating on the relevant, underlying genetic measures, then heritability is underestimated (even if overall genome-wide assortative mating is nil). Prior attempts to relax that assumption have operationalized assortative mating through the phenotypic correlation among parents, but this assumes that phenotypic correlations can act as accurate proxies for genotypic correlations, the true parameter of interest. Our results suggest the opposite: stable or potentially decreased genetic assortment on genetic measures linked to education in the face of increased phenotypic assortment. Likewise, differential fertility by genotype affects the likelihood of parents being included in any studies of intergenerational transmission—genetic or social—due to the practice of sampling on offspring in most retrospective studies or the exclusion of childless individuals from prospective studies of parent–child pairs. That is, if a given genetic measure is pleiotropic for affecting an outcome of interest—say education—as well as fertility, then estimating its effects based on a sample of living offspring (as is typically done) will yield substantially different results than sampling on the parental generation and allowing for nonfertile members of that generation to remain in the analysis and modeling offspring education conditional on being born at all.


Related Biology Terms

  • Mendelian Genetics – The set of theories proposed by Gregor Mendel, which attempt to explain the inheritance patterns of genetic characteristics based on simple breeding experiments involving single genes on chromosome pairs.
  • Genotype – The genetic makeup of an individual organism.
  • Phenotype – The physical and biological characteristics expressed in an individual as determined by their genotype.
  • Epistasis – The interactions between separate genes, in which one masks the effect of another.

1. In polygenic inheritance, traits are determined by:
A. Multiple alleles at a single locus
B. The interaction of multiple genes
C. Two dominant alleles on a gene
D. One gene being masked by another

2. How many different allele combinations can possibly be produced from two parents that are heterozygous for a polygenic trait controlled by three different genes with two allele pairs?
A. 5
B. 7
C. 54
D. 64

3. Which of the following phenotypes is unlikely to be a multifactorial trait affected by environment?
A. The risk of lung cancer
B. Down’s syndrome
C. High body Weight
D. Schizophrenia


7. Coda on Causality

The introduction noted that, if the genotype-phenotype relationship had been the focus of this entry, more attention would need to be given to philosophical arguments about causality and about abstraction as it relates to causal claims. Yet, realizing any of the programs of reintegration mentioned in this entry would entail rich causal analyses: networks of gene regulation linked to organized structures that branch into more organized structures, epigenetic modifications during and across lifespans, organisms shaping the dynamics of the ecological context in which they develop their traits, and frequencies of traits changing in populations over generations. This said, in realms of experimentally controlled biological materials and conditions, a simpler sense of causality may seem plausible, namely, a difference that makes a difference (see entry on causation and manipulability). (The serious debate about whether statistical analysis can distinguish causal from non-causal differences that &ldquomake&rdquo a difference should be noted Hernán et al. 2002.) The connection between an association within some population and causal mechanisms is susceptible to disconfirmation by experiments. At the same time, doing such experiments invites scrutiny of the relationship of experimentally altered dynamics to the original dynamics that generated the data that were analyzed to show the original statistical association (Taylor 2015).

Most importantly given the framing of this entry around control and reintegration: Any experimental as well as statistical association is also conditional on the subset of the population or species studied and the situations where they are observed (Lewontin 1974b). Understanding associations and formulating manipulations based on them requires attention to what has been experimentally or, at least, statistically held constant. In other words, in controlled conditions the direction of the arrow labeled identification in Figures 2 and 3 may be reversed and given a causal connotation, but the causality is conditional on the factors, including the rest of the organism, held constant. The understanding and manipulations may well extrapolate beyond the original, controlled population and situations (and thus match the general theory summarized in Figure 1), but, absent an actual program of reintegration, there is no basis for assuming that they will. While Waters (2007), Tabery (2014) and others would give greater status to differences that have actually been observed to make a difference (Griffiths & Stotz 2013), this entry has pointed to the control of biological materials and conditions that excludes many factors&mdashgenetic as well as environmental, structural as well as particulate&mdashfrom being seen to make a difference. Ironically, if appearances are not to mislead and obscure, or be spun into speculative theories (section 2), the science of heredity needs methods that bring back what was abstracted away under the experimental control that made Johannsen&rsquos original genotype-conception meaningful.


Watch the video: ΜΗ ΦΟΒΑΣΑΙ ΤΗ ΦΩΤΙΑ - (July 2022).


Comments:

  1. Kolten

    I had a similar situation. I soared for a long time over how to get out of the water dry. A friend said one decision, only something I rushed so abruptly to change everything that was acquired by back-breaking labor. Decided to be patient for now, to take a closer look? how it turns. What can I say? water wears away the stone. That's really, really so. I advise the author not to be sad. How is it in the song? "whole life ahead".

  2. Jovan

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  4. Alba

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  5. Mohamad

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  6. Damian

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