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6.2: Inferring the Mode of Inheritance - Biology

6.2: Inferring the Mode of Inheritance - Biology


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Given a pedigree of an uncharacterized disease or trait, one of the first tasks is to determine which modes of inheritance are possible and then which mode of inheritance is most likely. This information is essential in calculating the probability that the trait will be inherited in any future offspring. We will mostly consider five major types of inheritance: autosomal dominant (AD), autosomal recessive (AR), X-linked dominant (XD), X-linked recessive (XR), and Y-linked (Y).

Autosomal Dominant (AD)

When a disease is caused by a dominant allele of a gene, every person with that allele will show symptoms of the disease (assuming complete penetrance), and only one disease allele needs to be inherited for an individual to be affected. Thus, every affected individual must have an affected parent. A pedigree with affected individuals in every generation is typical of AD diseases. However, beware that other modes of inheritance can also show the disease in every generation, as described below. It is also possible for an affected individual with an AD disease to have a family without any affected children, if the affected parent is a heterozygote. This is particularly true in small families, where the probability of every child inheriting the normal, rather than disease allele is not extremely small. Note that AD diseases are usually rare in populations, therefore affected individuals with AD diseases tend to be heterozygotes (otherwise, both parents would have had to been affected with the same rare disease). Achondroplastic dwarfism, and polydactyly are both examples of human conditions that may follow an AD mode of inheritance.

X-linked dominant (XD)

In X-linked dominant inheritance, the gene responsible for the disease is located on the X-chromosome, and the allele that causes the disease is dominant to the normal allele in females. Because females have twice as many X-chromosomes as males, females tend to be more frequently affected than males in the population. However, not all pedigrees provide sufficient information to distinguish XD and AD. One definitive indication that a trait is inherited as AD, and not XD, is that an affected father passes the disease to a son; this type of transmission is not possible with XD, since males inherit their X chromosome from their mothers.

Autosomal recessive (AR)

Diseases that are inherited in an autosomal recessive pattern require that both parents of an affected individual carry at least one copy of the disease allele. With AR traits, many individuals in a pedigree can be carriers, probably without knowing it. Compared to pedigrees of dominant traits, AR pedigrees tend to show fewer affected individuals and are more likely than AD or XD to “skip a generation”. Thus, the major feature that distinguishes AR from AD or XD is that unaffected individuals can have affected offspring.

X-linked recessive (XR)

Because males have only one X-chromosome, any male that inherits an X-linked recessive disease allele will be affected by it (assuming complete penetrance). Therefore, in XR modes of inheritance, males tend to be affected more frequently than females in a population. This is in contrast to AR and AD, where both sexes tend to be affected equally, and XD, in which females are affected more frequently. Note, however, in the small sample sizes typical of human families, it is usually not possible to accurately determine whether one sex is affected more frequently than others. On the other hand, one feature of a pedigree that can be used to definitively establish that an inheritance pattern is not XR is the presence of an affected daughter from unaffected parents; because she would have had to inherit one X-chromosome from her father, he would also have been affected in XR.

Y-linked and Mitochondrial Inheritance.

Two additional modes are Y-linked and Mitochondrial inheritance.

Only males are affected in human Y-linked inheritance (and other species with the X/Y sex determining system). There is only father to son transmission. This is the easiest mode of inheritance to identify, but it is one of the rarest because there are so few genes located on the Y-chromosome. An example of Y-linked inheritance is the hairy-ear-rim phenotype seen in some Indian families. As expected this trait is passed on from father to all sons and no daughters. Y-chromosome DNA polymorphisms can be used to follow the male lineage in large families or through ancient ancestral lineages. For example, the Y-chromosome of Mongolian ruler Genghis Khan (1162-1227 CE), and his male relatives, accounts for ~8% of the Y-chromosome lineage of men in Asia (0.5% world wide).

Mutations in Mitochondrial DNA are inherited through the maternal line (from your mother). There are some human diseases associated with mutations in mitochondria genes. These mutations can affect both males and females, but males cannot pass them on as the mitochondria are inherited via the egg, not the sperm. Mitochondrial DNA polymorphisms are also used to investigate evolutionary lineages, both ancient and recent. Because of the relative similarity of sequence mtDNA is also used in species identification in ecology studies.


Estimating the mode of inheritance in genetic association studies of qualitative traits based on the degree of dominance index

The biological justification for the choice of the genetic mode in genetic association studies (GAS) is seldom available. Then, the mode of inheritance is approximated by investigating a number of non-orthogonal genetic contrasts making the interpretation of results difficult.

Methods

We propose to define the mode of inheritance by the significance of the deviance of the co-dominant contrast and the degree of dominance (h), which is a function of two orthogonal contrasts (the co-dominant and additive). Non-dominance exists when the co-dominant contrast is non-significant and, hence, the risk effect of heterozygotes lies in the middle of the risk of the two homozygotes. Otherwise, dominance (including over- and under-dominance) is present and the direction of dominance depends on the value of h.

Results

Simulations show that h may capture the real mode of inheritance and it is affected by deviations from Hardy-Weinberg equilibrium (HWE). In addition, power for detecting significance of h when the study conforms to HWE rule increases with the degree of dominance and to some extent is related to the mutant allele frequency.

Conclusion

The introduction of the degree of dominance provides useful insights into the mode of inheritance in GAS.


Dominant inheritance

To show a trait is dominant, we need to find some evidence to show that it is not recessive. The most obvious way we see this is if two parents with the trait have one or more offspring without the trait. This would be impossible if the trait were recessive, as both parents would be homozygous recessive, so it would not be possible for a child to have the dominant allele.

Autosomal dominant

Once we have established that a trait is dominant, in order to show that it is autosomal, we need to rule out the possibility of X-linkage.

The pattern where a male with the trait and a female without the trait produce a male with the trait or a female without the trait is one that can rule out X-linkage. If the trait were X-linked, the male parent would have the dominant allele (XDY) and the female parent would be homozygous recessive (XdXd). Therefore all male offspring would not express the trait (XdY) and all female offspring would (XDXd). As this is not the case, it would be impossible for the trait to be X-linked dominant.

X-linked dominant

Once again, it is not usually possible to definitively rule out autosomal inheritance but there are some patterns that strongly imply X-linkage. For example, a male with the trait (XDY) and a female without (XdXd) who have 100% of female offspring expressing the trait (XDXd) and 100% of male offspring not expressing the trait (XdY) is strongly suggestive of X-linked dominant inheritance.


3 Main Types of Inheritance | Genetics

The following points highlight the three main types of inheritance with its characteristics. The types are: 1. Autosomal Dominant Inheritance 2. Autosomal Recessive Inheritance 3. Polygenic Disorders and Multifactorial Inheritance.

Type # 1. Autosomal Dominant Inheritance:

Farabee in 1905 gave the first description of a pedigree showing brachydactyly (short fingers and toes), an autosomal dominant trait. Some more examples are porphyria, Huntington’s chorea (a degenerative disease of nerve cells), Polydactyly, retinoblastoma (a malignant eye tumour of children) and others. The dominant transmission of a trait is also called manifest inheritance because whenever a gene is present its effect is produced.

Characteristics of Autosomal Dominant Inheritance:

a. The trait appears in every generation.

b. An affected child must have at least one affected parent.

c. About one half of the offspring of an affected person are affected the recurrence risk is 50% at each conception.

d. Both male and female persons are affected.

e. Individuals who appear normal do not transmit the trait to their offsprings.

Penetrance and Expressivity in Dominant Traits:

Penetrance is a quantitative term and denotes the fraction of individuals carrying a gene that show the specific phenotype. Sometimes a pedigree shows absence of an affected person expected in a generation. The trait has skipped a generation because a heterozygous person is showing the normal phenotype.

The gene though present was not penetrant. A gene is said to have low penetrance when it does not manifest in a large fraction of the individuals. In other cases of dominant inheritance the gene may manifest in all the heterozygotes, but the degree of clinical manifestation may be different. This is called variable expressivity.

Sex-Linked Dominant Inheritance:

The X-linked dominant trait shows itself in hemizygous males and heterozygous females. All the daughters of an affected male also show the trait. An affected mother produces normal and affected children, both male and female in the ratio 1:1.

In X-linked dominant inheritance the males are affected more severely than females. For example dermal hypoplasia is apparently lethal in males. In females it produces characteristics like cutaneous pigmentation and papillomas, a few more skin defects and atrophy. Hypophosphatemia is also an X-linked dominant disease.

Characteristics of X-Linked Dominant Inheritance:

a. The affected male transmits the trait to all his daughters but not to the sons.

b. When affected females are homozygous, they transmit the trait to all their children of both sexes.

c. When affected females are heterozygous, only 50% of their children of both sexes have a chance of being affected.

d. Affected females transmit the trait to their progeny in a manner similar to that in autosomal dominant inheritance.

Type # 2. Autosomal Recessive Inheritance:

In recessive inheritance the offspring inherits a trait from both parents. An albino offspring is born to parents who are normal in appearance, but each carries an albino gene (heterozygous Aa). If each person had married a normal (AA) person, no albino would appear in their progeny.

Abnormal recessive genes are thus transmitted for many generations through heterozygotes. Their existence is found out only when two heterozygotes marry and the homozygote appears, the ratio 1 normal: 1 affected.

The rare recessive conditions are more easily detected through consanguineous marriages which involve matings between blood relatives. In some parts of India consanguineous marriages are common, and the incidence of rare recessive disorders is also high. Microcephaly (small head), phenylketonuria, galactosemia and others are due to recessive genes.

Children affected with galactosemia are not able to metabolise galactose, which is a component of the milk sugar lactose. Normally, galactose is converted into glucose phosphate by the enzyme phospho-galactose uridyl transferase present in the liver. Homozygous children lack this transferase enzyme.

The heterozygotes have an enzyme level intermediate between that of the normal and affected homozygotes. Affected babies have severe vomiting and diarrhoea, and consequently suffer from undernourishment and fail to grow normally.

The condition can be treated if after birth the babies are kept on a lactose- and galactose-free diet, and are given specially formulated milk substitutes. Untreated children accumulate toxic amount of galactose- 1-phosphate which lead to cataract in the lens of the eye, damage to liver and kidney tubules and some mental retardation.

Characteristics of Autosomal Recessive Inheritance:

a. The trait is visible only in sibs, but not in their parents or other relatives.

b. The parents of an affected person may have been blood relatives (consanguineous).

c. About one fourths of the children of such parents are affected the recurrence risk at each birth is 25%.

d. Both male and female children have equal chance of being affected.

Sex-Linked Recessive Inheritance:

This type of inheritance is mostly X-linked and predominantly males are affected (due to hemizygous condition). Heterozygous females are carriers and are expected to produce affected and normal sons in the ratio 1: 1.

An affected male never produces an affected son. A famous example is haemophilia, the gene for which was passed on to the descendants of Queen Victoria. Some other examples are red green colour blindness, G6PD, Lesch-Nyhan syndrome and muscular dystrophy.

In haemophilia or bleeding disease, as it has been known for centuries, the blood is not able to clot within 4 to 8 minutes like the normal blood. Instead it takes an hour or more to clot. Failure to clot is due to the absence of a coagulation factor which is present in normal blood. Two different X-linked loci are involved in haemophilia.

One causing the more prevalent haemophilia A, the other giving rise to haemophilia B or Christmas disease (so called because it was first noted in a person named Christmas). The coagulation factor which is absent in the A form of haemophilia is called antihaemophilic globulin (AHG), while in the B form the deficient factor is called plasma thromboplastin component (PTC).

The affected individuals rarely live beyond the first decade, although with treatments available now they may live longer. The survivors still have problems due to internal bleeding in the joints. Being an X-linked recessive disorder, males are more frequently affected.

Haemophilia seems to have started in Britain’s royal family through a mutation in one of Queen Victoria’s parents. One of Victoria’s four sons was affected and produced a carrier daughter. Out of the two sons of this daughter, only one was a haemophiliac, the other was normal. Out of the 5 daughters of Victoria, two turned out to be carriers and produced in all 3 carrier daughters and 3 affected sons. The 3 carrier daughters further produced 6 carrier daughters and 5 haemophiliac sons.

Characteristics of X-linked Recessive Inheritance:

a. Males are affected more frequently than females.

b. When the female parent is carrying the trait then 50% of her sons have a chance of being affected, and 50 % of the daughters would be carriers but phenotypically normal.

c. The trait is transmitted through several generations by carrier females.

d. The affected male parent cannot transmit the trait directly to his sons.

Type # 3. Polygenic Disorders and Multifactorial Inheritance:

Some normal traits like height and intelligence, and disorders like cleft lip/palate, club foot, some allergies, diabetes mellitus, hydrocephalus, pyloric stenosis and others are inherited through polygenes and may be influenced by extraneous factors including drugs.

The polygenes have small additive effects. The clinical features are due to cumulative effects of all the polygenes as well as other factors. For this reason the term multifactorial inheritance is preferred. In most cases the exact number of genes involved is not known.

The congenital conditions of cleft lip (CL) as well as of cleft lip with cleft palate (CLP) are found to be associated with a large number of syndromes (listed in Nora and Fraser, 1974). Some of these are related to chromosomal aberrations, a few are caused by mutant genes, the rest appear to be multifactorially determined.

An individual may have a cleft lip (Fig. 21.4) due to defective closure of the primary palate in embryological development, or a cleft palate, due to faulty closure of the secondary palate or both cleft lip and cleft palate (CLP) may be present in the same person.

Studies on families have shown that cleft lip ± cleft palate are frequently associated with syndromes which are inherited as autosomal dominant, autosomal recessive, or X-linked conditions. It has also been revealed that some genes will cause cleft lip and cleft palate in some individuals, while in some other individuals they cause cleft palate.

The combined influence of genetic and environmental factors have established multifactorial inheritance for CLP. More males than females are affected by CLP. The recurrence risk is higher in sibs of female pro-bands. The condition is more common among Orientals. Rarely cleft palate occurs in absence of cleft lip. This is more common in females than males, and in children of older mothers.


Determining the mode of inheritance of genotypes

In pings, coat color may be sandy, red, or white. A geneticist spent several years mating true-breeding pigs of all different color combinations, even going so far as to obtain true-breeding lines from different parts of the country. For crosses 1 and 4 below, she encountered a major problem: her computer crashed and she lost the F2 data. She nevertheless persevered, and using the limited data shown here, she was able to predict the mode of inheritance, the number of genes involved, and assign genotypes to each coat color. Based on the available data generated from the crosses shown, attempt to duplicate her analysis.

Cross P1 F1 F2
1 sandy X sandy all red data lost
2 red X sandy all red 3/4 red: 1/4 sandy
3 sandy X white all sandy 3/4 sandy: 1/4 white
4 white X red all red data lost

Once you have formulated a hypothesis to explain the mode of inheritance and assigned genotype to the respective coat colors, predict the outcomes of the F2 generations where the data were lost.

After considering the above problem, concentrate on the following one.

Labrador retrievers may be black, brown, or golden in color. While each color may breed true, many different outcomes occur if numerous litters are examined from a variety of matings, where the parents are not necessarily true-breeding. Shown here are just some of the many possibilities. Propose a mode of inheritance that is consistent with these data, and indicated the corresponding genotypes of the parents in each mating. Indicated as well the genotypes of dongs that breed true for each color.

A. black X brown ---> all black

B. black X brown ---> 1/2 black 1/2 brown

C. Black X Brown ---> 3/4 black 1/4 golden

D. Black X Golden ---> all black

E. Black X golden ---> 4/8 golden 3/8 black 1/8 brown

F. Black X golden ---> 2/4 golden 1/4 black 1/4 brown

G Brown X Brown ---> 3/4 brown 1/4 golden

H. Black X Black ---> 9/16 black 4/16 golden 3/16 brown

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Solution Preview

Question 1
To start we must first establish the number of alleles and genes.
- 3 traits could mean one gene with 2 co-dominant alleles or 2 genes in a more complex relationships.

Co-dominance cannot explain the test crosses available so we will make a model that has two genes.

gene 1 (R,r) encodes for red such that
RR or Rr= red
rr= no color

gene 2 (S,s) encodes for sandy such that
Ss or Ss= sandy
ss= no color

Now we must look at the relationship between these two genes that contributes coat color.
-Since sandyx sandy= red and redx sandy or white = red .


Human Biology

The aim of the present study was to investigate the extent and mode of inheritance of hand osteoarthritis (OA) using a large sample of ethnically homogeneous pedigrees. Two types of segregation analysis (SA) models were examined. Type I models used the data adjusted for potential significant covariates, particularly age and sex, prior to genetic analysis. Type II models incorporated effects of the potential covariates into major gene penetrance functions, permitting an account of the genotype covariate-specific effect on study variables. The results of this study strongly supported the hypothesis of a major gene effect and additional multifactorial component. The best-fitting model was the Mendelian one with an additive type of inheritance. The estimates obtained using the standard three-factor variance decomposition analysis suggest that age (72.8%) and major gene (14.5%) are the main sources of interindividual differences in the development of hand OA. The contribution of the putative major gene on age- and sex-adjusted OA phenotype variation was 55% in the present study.


Pleiotropy

Some genes affect more than one phenotypic trait. This is called pleiotropy. There are numerous examples of pleiotropy in humans. They generally involve important proteins that are needed for the normal development or functioning of more than one organ system. An example of pleiotropy in humans occurs with the gene that codes for the main protein in collagen, a substance that helps form bones. This protein is also important in the ears and eyes. Mutations in the gene result in problems not only in bones but also in these sensory organs, which is how the gene’s pleiotropic effects were discovered.

Another example of pleiotropy occurs with sickle cell anemia. This recessive genetic disorder occurs when there is a mutation in the gene that normally encodes the red blood cell protein called hemoglobin. People with the disorder have two alleles for sickle-cell hemoglobin, so named for the sickle shape (Figure 8 ) that their red blood cells take on under certain conditions such as physical exertion. The sickle-shaped red blood cells clog small blood vessels, causing multiple phenotypic effects, including stunting of physical growth, certain bone deformities, kidney failure, and strokes.

Figure 8: The sickle-shaped red blood cell on the left is shown next to several normal red blood cells for comparison. (CC BY 3.0 OpenStax College via Wikimedia.org)


6. Epigenetics in a Dynamic Environment: Consequences of Rapid and Widely-Distributed Phenotype Switching

6.1. Epigenetically-Inherited Phenotypes: Neutral, Advantageous or Disadvantageous?

The consequences of an epigenetically-inherited phenotype depend in large part on what the effect of that phenotype is on the overall fitness of the individual bearing it. Just like genetically-inherited phenotypes, epigenetically-inherited phenotypes can be neutral, advantageous or disadvantageous. In the field of medicine, most focus is on disadvantageous epigenetically-inherited phenotypes that can lead to disease states. Advantageous epigenetically-inherited phenotypes have received less attention in human health, yet certainly exist [43]. Indeed, if they can be controlled and managed, advantageous phenotypes arising by transgenerational epigenetic inheritance may have potentially large impacts on medicine [44,45] and on agriculture [10,46,47,48,49,50]. To highlight a concrete example of beneficial epigenetic inheritance, consider the transgenerational epigenetic inheritance of hypoxia resistance in zebrafish [7]. As a result of parental (P0) exposure to 2, 3 or 4 weeks of hypoxia (15%), F1 larvae had greater hypoxic resistance than controls whose parents had not experienced hypoxia. Importantly, it was not just an individual larvae or two that were hypoxic resistant, but rather statistically, the entire F1 population had elevated hypoxic resistance. Similarly, we have recently observed that F1 of zebrafish parents exposed to polycyclic aromatic hydrocarbons (PAHs) showed enhanced resistance to these toxicants when compared to control larvae whose parents were not exposed (Martinez-Bautista N. and Burggren, W. unpublished data [51]). Again, most of the population, not just a few individuals, had resistance greater than the controls whose parents were not exposed. These two examples highlight that more experimentation of potentially advantageous epigenetically-inherited phenotypes is warranted.

6.2. Comparing the Time Courses of Genetic and Epigenetic Inheritance

One of the basic tenets of evolution is that natural selection shapes populations and species over evolutionary time. Natural selection acts on organisms with enhanced or diminished fitness, derived from the accumulation of mutations. The resulting phenotypic modifications are enhanced (or not) by these mutations, but the phenotypic switch at the population level and beyond typically occurs over hundreds or thousands of generations as the genotype leading to a modified phenotype of greater fitness slowly inserts itself into the general population or, alternatively, a genotype leading to lesser fitness is eliminated from the population [52]. Klironomos et al. [53] have provided a simple, but informative model of how increases in fitness in a population can derive from either epigenetic or genetic changes in a population over tens of thousands of generations. However, the effect of epigenetic inheritance may not only be potentially broad and sweeping, but may also be felt immediately in a population [52,53,54,55,56]. To underscore this point, consider a phenotype that is advantageous in an environment when a specific stressor that occurs intermittently. Unlike an advantageous gene mutation that affects an individual and then, perhaps, spreads slowly through the population and beyond over many generations, epigenetic inheritance can simultaneously affect many (if not most or all) of a single generation of an entire population. Why? While there is certainly some variation in epigenetic markers between individuals in a population (see below), whether they result in an advantageous or disadvantageous phenotype, epigenetic markers will arise in response to an environmental stressor far more broadly and quickly within a single generation of a population than will a single point mutation occurring in a single individual. Assuming that all individuals in a population of a species presumably experience an environmental stressor at the same time and to a similar extent and that many of the individuals in that population will as a consequence possess the same epigenetic markers, then an epigenetically-switched phenotype should affect many if not most individuals in the population.

The scenario described above is depicted in Figure 3 , which compares changes in a population of individuals with an advantageous phenotype arising by either epigenetic inheritance or by mutation. This scenario assumes firstly that the switched phenotype (either from genetic or epigenetic inheritance) is advantageous only in the presence of a deleterious environment, which persists over several generations (specifically, four generations in this scenario) before returning to normal, favorable environmental conditions. Second, this scenario assumes that upon return to the previous normal environment, the newly-switched phenotype is now disadvantageous and possibly lethal. Third, this scenario revolves around only a simple point mutation and, thus, ignores the complexities of pleiotropy, including antagonistic pleiotropy. Fourth, the scenario assumes that an epigenetically-inherited phenotype may persist over more than one generation. Indeed, abundant evidence now exists of epigenetically-inherited phenotypes persisting over multiple generations (e.g., [6,15,42,57,58,59]) before either suddenly disappearing or more slowly “washing out” [41].

A comparison of phenotype switching in a population occurring by inherited point mutation vs. inheritance through the effects of epigenetic markers. Events 1, 3 and 5 indicate proportional changes in a hypothetical population resulting from phenotype switching by point mutation that are advantageous during environmental stress, but otherwise disadvantageous (or at least energetically costly). Events 2, 4 and 6 indicate proportional changes in the population resulting from epigenetic phenotype switching. See the text for an additional explanation.

As Figure 3 illustrates, a mutation may result in an advantageous phenotype in only a single individual in a population (Event 1). Advantageous mutations occur at low frequency, become difficult to establish in the population and additionally may be easily lost to genetic drift [60,61]. Thus, this advantageous mutation (Event 1) is only slowly amplified by natural selection over numerous generations, at best. In contrast, an epigenetic phenotypic switch brought on by a deleterious environment can immediately aid in the survival of a potentially large proportion of a population (Event 2), since even allowing for the heterogeneity of epigenetic markers in a population, many in that population may have the epigenetic markers resulting in the modified phenotype. With dissipation of the deleterious environment, however, the individual(s) with the original mutation must cope with the newly disadvantageous phenotype, which cannot be eliminated from the gene pool, except by death of the individual or an unlikely second mutation back to the original gene form (Event 3). In contrast, however, the epigenetically-switched phenotype, now newly disadvantageous in the face of the return to the original environmental condition, is immediately lost by reversion to the original phenotype (Event 4). With a return of the deleterious environment after several generations, the mutant genotype and its phenotype (if they even survive the intervening return to the previous normal environment) will increase only slowly once again in the population at a rate enabled by natural selection (Event 5). Again in contrast, the epigenetically-inherited advantageous phenotype can result in the rapid re-appearance of the advantageous switched phenotype appearing in a large proportion of a population’s individuals (Event 6).

Important to acknowledge is that the scenario depicted in Figure 3 takes an 𠇎ither-or” approach for epigenetic or genetic inheritance. That is, that populations are shown in this figure to either persist by epigenetic inheritance or by genetic inheritance of an advantageous phenotype, but not necessarily both. We know this approach to be an oversimplification, because presumably, there are also genetic changes that occur in populations changing by epigenetic modification. In fact, it is difficult to separate out such simultaneous phenotypic changes caused by this duality [25,53,57,62,63,64].


Phenotypes and Genotypes

The observable traits expressed by an organism are referred to as its phenotype and its underlying genetic makeup is called its genotype.

Learning Objectives

Distinguish between the phenotype and the genotype of an organism

Key Takeaways

Key Points

  • Mendel used pea plants with seven distinct traits or phenotypes to determine the pattern of inheritance and the underlying genotypes.
  • Mendel found that crossing two purebred pea plants which expressed different traits resulted in an F1 generation where all the pea plants expressed the same trait or phenotype.
  • When Mendel allowed the F1 plants to self-fertilize, the F2 generation showed two different phenotypes, indicating that the F1 plants had different genotypes.

Key Terms

  • phenotype: the appearance of an organism based on a multifactorial combination of genetic traits and environmental factors, especially used in pedigrees
  • genotype: the combination of alleles, situated on corresponding chromosomes, that determines a specific trait of an individual, such as “Aa” or “aa”

Phenotypes and Genotypes

The observable traits expressed by an organism are referred to as its phenotype. An organism’s underlying genetic makeup, consisting of both physically visible and non-expressed alleles, is called its genotype. Johann Gregor Mendel’s (1822–1884) hybridization experiments demonstrate the difference between phenotype and genotype.

Mendel crossed or mated two true-breeding (self-pollinating) garden peas, Pisum saivum, by manually transferring pollen from the anther of a mature pea plant of one variety to the stigma of a separate mature pea plant of the second variety. Plants used in first-generation crosses were called P0, or parental generation one, plants. Mendel collected the seeds belonging to the P0 plants that resulted from each cross and grew them the following season. These offspring were called the F1, or the first filial (filial = offspring, daughter or son), generation. Once Mendel examined the characteristics in the F1 generation of plants, he allowed them to self-fertilize naturally. He then collected and grew the seeds from the F1 plants to produce the F2, or second filial, generation.

Mendelian crosses: In one of his experiments on inheritance patterns, Mendel crossed plants that were true-breeding for violet flower color with plants true-breeding for white flower color (the P generation). The resulting hybrids in the F1 generation all had violet flowers. In the F2 generation, approximately three-quarters of the plants had violet flowers, and one-quarter had white flowers.

When true-breeding plants in which one parent had white flowers and one had violet flowers were cross-fertilized, all of the F1 hybrid offspring had violet flowers. That is, the hybrid offspring were phenotypically identical to the true-breeding parent with violet flowers. However, we know that the allele donated by the parent with white flowers was not simply lost because it reappeared in some of the F2 offspring. Therefore, the F1 plants must have been genotypically different from the parent with violet flowers.

In his 1865 publication, Mendel reported the results of his crosses involving seven different phenotypes, each with two contrasting traits. A trait is defined as a variation in the physical appearance of a heritable characteristic. The characteristics included plant height, seed texture, seed color, flower color, pea pod size, pea pod color, and flower position. To fully examine each characteristic, Mendel generated large numbers of F1 and F2 plants, reporting results from 19,959 F2 plants alone. His findings were consistent. First, Mendel confirmed that he had plants that bred true for white or violet flower color. Regardless of how many generations Mendel examined, all self-crossed offspring of parents with white flowers had white flowers, and all self-crossed offspring of parents with violet flowers had violet flowers. In addition, Mendel confirmed that, other than flower color, the pea plants were physically identical.


The Mode of Inheritance and Gene Linkage for Drosophila

See the attached file.
7. Conclusion and discussion sections:

a. how many genes are involved in creating the eye pigment (based on the provided data). Mode of inheritance - are the genes autosomal or sex-linked and why, recessive or dominant and why, and are some genes linked and why.

b. Hypothesis statement based on conclusions about linkage from previous step (7a), and chi square test to accept or reject the hypothesis.

c. You should include punnett squares with genotypes to all generations of all 4 crosses!

I have connected the data table that we were given, and I have also given my explanation for crosses 1 and 3. However, I have no idea how 2 and 4 are linked, I have never seen a 14:1:1:4 ration before.

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OK. This is a bit complicated, so try and stay with me. This explanation will start out easy and get more complicated as we go. Read it slowly and carefully, making sure you understand as you go.

As you wrote, experiments 1 and 3 show conclusively that the gene in question in those experiments is located on the X chromosome. That particular gene has two alleles, one dominant allele which leads to a wt phenotype, and one recessive allele which leads to a white phenotype. So, that 1 gene (with 2 alleles) is X-linked (or sex-linked). It is not an autosomal gene.

So far, we've got 1 gene involved in eye color.

However, when we look at experiments 2 and 4, we see something weird going on. What can we learn? First of all, the results for experiments 2 and 4 are identical. The reciprocal crosses gave the same results. So, think. What does that tell us? It tells us that this gene (or genes) being investigated in these experiments must be autosomal. They cannot be located on the X-chromosome. That means that we now know that there are at least two genes involved -- the X-linked gene discovered in experiments 1 and 3, and at least one other gene on one of the autosomal chromosomes, discovered in experiments 2 and 4.

But, notice something else. Experiments 2 and 4 are shown with crosses between wt flies and "white" flies. They are not normal white-eyed flies although they have white eyes. That's why the "white" is in quotation marks. It's a flag to tell you to be on guard. Be alert! There's something more going on here than meets the eye!

In fact, if we look at the F2 from these crosses, we find two new eye colour types appearing out of nowhere. That should confirm to us that something unusual is indeed happening (as you figured out anyway). But, the question is: What's going on?

Solution Summary

The solution discusses when the mode of inheritance and gene linkage for drosophila crosses.


Watch the video: GCSE Biology - DNA Part 1 - Genes and the Genome #48 (July 2022).


Comments:

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  5. Fshd

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  6. Gardajin

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