Mendelism

Genetics - the biological science of heredity and variation

A common delight is to wonder why we humans appear as we do. Why is it that members of a family or community tend to look more alike than outsiders? Why do some individuals express unusual traits such as color blindness or cystic fibrosis? Why do some of these traits skip a generation or tend to be more common in one sex than the other? What types of children can a couple expect? What kinds of traits can be passed on to our offspring - appearance, disease, behaviour, intelligence? Can we, or should we, try and modify the inheritance or expression of human traits?

These are difficult questions that often have only partial answers. When considered in our own species finding answers to these questions pose many practical difficulties. Consider the second most frequent type of cancer in women (breast) and man (prostate).

Are all cases of breast and prostate cancer known to be heritable ? Which of these two diseases is most clearly understood to be heritable?? Which of the predominant genetic mechanisms are responsible for breast ? and prostate cancers ? What are some of the problems in understanding these cancers and how could information on other species be useful ?

2. Before Mendel - variations of pangenesis

The failure of contemporary science to completely determine the inheritance of breast or prostate cancer indicates the difficulties associated with genetic analysis. It is hardly surprizing that early fanciful notions about heredity were found wanting. Hippocrates (ca. 400 B.C.) believed that body parts donated reproductive material ("humors") that were passed to the progeny. These reproductive "humors" determined the offspring to be like the body parts from whence they came.

Aristotle (ca. 350 B.C.) found fault with the Hippocratic view since plants or animals with missing parts could still give rise to progeny with these parts! Thus he proposed that "nutritive substances" were not derived from body parts but rather from nutritive elements on their way to body parts but diverted to a reproductive path. He further belived that the male and female contributions were not the same. In the spirit of male chauvinism he suggested that the seminal "vital heat" shaped the menustral "physical substance" much like a sculpturer molding clay.

In 1859 Charles Darwin proposed in The Origin of Species that selection of biological variants could form the basis for the the origin of new species. His theory required a genetic mechanism to explain the origin and inheritance of the biological variation. He resorted to a Hippocrates-like theory called the theory of pangenesis. Thus "humours" and "nutritive substances" became "gemmules." "Gemmules" were thrown off from cells and tissues and passed to reproductive cells. If a variant was superior in some way, superior "gemmules" were thus passed to offspring to be favoured by selection in the process of speciation.

Although pangenesis and other theories such as preformation provided explanations for some features of inheritance, they were not based on rigorous scientific experimentation. The definitive treatise on genetics was produced by an Augustinian monk in 1865 entitled Experiments in Plant Hybridization,.

Johann Gregor Mendel

One of the strangest facts of scientific history is that the father of genetics was a monk trained in theology. A group photo shows that the 'flower monk' was amongst colleagues who were supportive of his work even though they, and the rest of humanity, did not grasp its significance.

The Flower Monk

Johann Mendel, the son of a poor peasant family, entered the Augustinian monastry in Brno in 1843 where he took the name of Gregor. This provided the only opportunity for a gifted mind to pursue his education in a centre of learning and scientific activity. He studied phyics, mathematics and biology at the University of Vienna. Although he failed his exams he returned to Brno and began a series of experiments in a small garden that laid the foundation for modern genetics.

The Most Famous Garden in the World - Ever

The Mendel Web

Students of genetics and the history of science are indebted to Roger Blumberg for his construction of The Mendel Web.

In addition to translation and annotation of Mendel's classic paper , The Mendel Web has Essays and Commentary on the many interpretations and evaluations of Experiments in Plant Hybridization,

Gregor Mendel has the unusual distinction of being responsible for the foundation of a new branch of science.

"The British evolutionist, Sir Gavin de Beer, had no doubt on the matter. In 1965, the centenary of Mendel's famous paper, he declared on the radio: 'There is not known another example of a science which sprang fully formed from the brain of one man.' To an audience at the Royal Society that year he delivered an address with the title, 'Genetics: The Centre of Science', in which he explained himself more fully: It is not often possible to pinpoint the origin of a whole new branch of science accurately in time and place . . . But genetics is an exception, for it owes its origin to one man, Gregor Mendel, who expounded its basic principles at Brno on 8 February and 8 March 1865" - (De Beer cited Robert C. Olby in The Mendel Web).

 

Unlike most scientific documents, this powerful and elegant paper can ellict an emotional response;

"Gregor Mendel's short treatise "Experiments on Plant Hybrids" is one of the triumphs of the human mind. It does not simply announce the discovery of important facts by new methods of observation and experiment. Rather, in an act of highest creativity, it presents these facts in a conceptual scheme which gives them general meaning. Mendel's paper is not solely a historical document. It remains alive as a supreme example of scientific experimentation and profound penetration of data. It can give pleasure and provide insight to each new reader-and strengthen the exhilaration of being in the company of a great mind at every subsequent study. "(Curt Stern and Eva Sherwood cited by Jan Sapp in The Mendel Web)

and move others to philosophical prose.

"Mendel is a curious wraith in history. His associates, his followers, are all in the next century. That is when his influence began. Yet if we are to understand him and the way he rescued Darwinism itself from oblivion we must go the long way back to Brunn in Moravia and stand among the green peas in a quiet garden. Gregor Mendel had a strange fate: he was destined to live one life painfully in the flesh at Brunn and another, the intellectual life of which he dreamed, in the following century. His words, his calculations were to take a sudden belated flight out of the dark tomblike volumes and be written on hundreds of university blackboards, and go spinning through innumerable heads." (Loren Eisely cited by Jan Sapp in The Mendel Web)

Our blackboard may be an e-page but our heads welcome

Mendelian controversy

As Loren Eisley noted it was some 35 years after Mendel's presentation of Experiments in Plant Hybridization before the significance of his work finally permeated the scientific community. Athough Mendel's paper is clearly regarded as giving birth to genetics it has been the source of much controversy. Foremost among these issues is the intent of Mendel's research. What did Mendel think he discovered? Was he trying to find a mechanism for Darwinian evolution, was he a non-Darwinian or was he not at all interested in evolution? Why was his paper neglected for such a long time? Did he falsify his data, or even more extreme, were his experiments completely fictious!

To gain some appreciation of the import of a revered century-old document on contemporary science identify the paragraph in Introductory Comments of Experiments in Plant Hybridization that:
(i) describes his experimental approach and
(ii) indicates his intent

20th century Mendelism

Three scientists, de Vries, Correns and Tschermak, independently rediscovered Mendelian genetics in 1900. There is evidence that the 'rediscovery' included a reluctant admission of the earlier work of Mendel. The rapid widespread application of genetics to basic and applied research resulted in the development of contemporary terminology and a focus on his so-called Laws of Segregation and Independent Assortment. Following is a development of the modern application with extensive reference to his original work.

Selection of experimental system

Mendel chose the garden pea Pisum sativm as his experimental material. Why did Mendel use the garden pea {see Experiments in Plant Hybridization} ?

He obtained 34 varieties from local seedmen and grew them out for two years to identify true-breeding stock. That is, the strains (lines) of peas remained unchanged from parent to offspring. Rather than trying to identify all the subtle variations in appearance between the different lines he identified seven traits that differed in appearance amongst the lines. For example, a pure-breeding line produced seeds that were round and another line produced seed that was wrinkled . Thus the seed shape appearance (phenotype) was round or ? . Some true-breeding lines had yellow seed endosperm and another pure-breeding line had green seed endosperm.The two phenotypes for seed endosperm color are ? and ? . What were the other four paired phenotypes that Mendel selected{see Experiments in Plant Hybridization, } ?

 

F1 Monohybrids From Reciprocal Crosses

The true-breeding parental lines P1 and P2 were crossed in a reciprocal manner to produce a F1 hybrid. Thus the pollen from a yellow-seeded line was dusted onto the stigma of a green-seeded line. In the reciprocal cross the pollen from a green-seeded line was dusted onto the stigma of a yellow-seeded line.When the seed bearing plants matured, the seed was harvested and scored for the yellow vs green phenotypes. Reciprocal crosses always gave an identical F1.

Note that the F1(hybrid) seed phenotypes of round vs wrinkled shape and yellow vs green endosperm are expressed directly in the harvested seed. For the other five traits the F1 seed has to be grown out and the F1 phenotype determined on the mature F1 plant. Thus the seed traits, shape and endosperm color, are particularly convenient to work with and were used extensively by Mendel.

Only one of the two trait phenotypes was expressed in the F1. This occurred regardless of which parent was the pollen parent; Mendel thereby demonstrated that reciprocal crosses gave identical results. The phenotype expressed in the F1 was said to be dominant and the trait not expessed was defined as being recessive.

 

P1 round by P2 wrinkled gave a round F1; round was ? and wrinkled was ? .

P1 yellow by P2 green gave a yellow F1; green was ? and yellow was ? .

 

 

The dominant phenoype can only be known by crossing true-breeding parents with contrasting trait phenotypes and observing the F1. Consult Experiments in Plant Hybridization to find the dominant phenotype for each of the other five traits ?

At this stage Mendel didn't know if the dominant round or yellow F1 was identical to the true breeding round or yellow parents - or differed in some aspects. He determined the nature of the F1 by self pollination of the F1 to produce the F2. That is, the F1 was known by the F2!

F1 hybrids were allowed to naturally self pollinate and produce F2 progeny. The F1 produced 6022 yellow and 2001 green F2 seeds. Mendel noted that this represented the dominant : recessive trait in a 3:1 ratio.

The 3:1 F2 ratio was observed for all seven traits including length of stem.

Mendel's summary comment was "In this generation there reappear, together with the dominant characters, also the recessive ones with their peculiarities fully developed, and this occurs in the definitely expressed average proportion of 3:1, so that among each 4 plants of this generation three display the dominant character and one the recessive. "

Rock of Gibraltor He also stated, "Transitional forms were not observed in any experiment " - Mendel's emphasis. For example, he observed only yellow and green seeds in the F2 . No new forms, such as orange, red, purple etc. were observed. This indicates that dominant and recessive phenotypes are stable and are passed from parents to offspring in an unaltered form. The notion that heredity elements are passed through the generations unchanged is one of the most fundamental of Mendels discoveries. Oddly enough, he states, what to him was the obvious, but does not dwell on this seminal discovery. Constancy of heredity elements over the generations underlies the modern application of Mendelism.

 

As the F1 as known by the F2, the F2 would be known by the F3. One can do no better than Mendel's own clear and vivid text.

"Those forms which in the first generation exhibit the recessive character do not further vary in the second generation as regards this character; they remain constant in their offspring. "

"It is otherwise with those which possess the dominant character in the first generation. Of these two-thirds yield offspring which display the dominant and recessive characters in the proportion of 3:1, and thereby show exactly the same ratio as the hybrid forms, while only one-third remains with the dominant character constant. "

 

Notation: We assume that a Mendelian gene is responsible for a trait and refer to that trait by a genetic symbol. For the seed shape trait we could arbitrarily choose lower case w to stand for the recessive wrinkled gene determinant. It would be equally valid to select upper case R to stand for the alternate dominant round gene determinant. Any other designation would be valid as long as it was clearly defined; e.g., wri or w^ri for wrinkled and Rnd or R_nd for round. Genotypes are usually, but not always, italicized. Net browsers don't support italics or superscripts, so you can't interact with the courseware in these modes. It's not a big loss. Mendel avoided confusion by using only A and a to refer to dominant or recessive determinants and then used them within the context of different traits. We'll follow Mendel's lead and carefully define and use notation within specific contexts.

Wild-type: The concept of wild-type is that of a species as it would exist in nature. Designation of wild-type is obviously arbitrary since the appearance of individuals in nature is variable. Another way of thinking about wild-type is that it is that form the geneticist calls wild-type for reasons of convenience or convention. It is common to use + to indicate the wild-type determinant. For example, if wild type is deemed to be round, rather than using R, the wild-type gene may be indicated by +, R+, or R⁺. However, as noted above, we'll define wild-type as appropriate - with or without + notation.

If we used w to designate the recessive wrinkled gene then a common sense designation for the dominant round determining gene would be W. Since w and W result in different phenotypes we refer to them as alternative forms or alleles of the same gene.

We further assume that every individual contains two genetic determinants. By this notation, the round P1 genotype (gene designation) would be WW; P2 genotype is ww; and F1 genotype is ? or ? The dominant phenotype can have two different designations. WW is said to be a homozygous dominant genotype and Ww is referred to as a heterozygous genotype. ww is a ? recessive genotype

Using the same seed color notation for the P1 yellow by P2 green cross above: the designated (phenotypes/genotypes) are P1 (yellow / ? ); P2 (green / ? ); and F1 (yellow / ? )

The 3:1 F2 ratio is explained by assuming that the dominant and recessive alleles from the F1 hybrid pass into different gametes with equal frequency and combine at random to form F2 progeny.

Fill in for the seed shape trait.

P1/P2	W
?	?

F1 gametes combine at random to form the F2

F1 gametes	1/2 ?	1/2 ?
1/2 ?	1/4 ?	1/4 ?
1/2 ?	1/4 ?	wrinkled

The fraction ? of the dominant F2 phenotype is homozygous and breeds true whereas ? of the dominant F2 phenotype is heterozygous and, when self pollinated produces dominant : recessive phenotypes in a ? ratio. The F2 phenotypic ? ratio is seen to be a ? genotypic ratio.

Mendel did not summarize these observations by stating a specific law or theorem. However, with the 20th century rediscovery of Experiments in Plant Hybridization, and the widespread application of Mendelian genetics, the Law of Segregation was formulated and became widely used. Practice application of the stated Law of Segregation below to the two above ^ tables^.

The Law of Segregation states that a monohybrid "Aa" produces haploid "A" and "a " gametes with equal frequency which then combine at random to form diploid progeny.

 

Geneticists are a lazy lot and use as little notation as possible: Since eukaryotic species carry many tens of thousands of genes we can not possible list them all every time we are dealing with a few traits. If a genotype is known, but not required for the problem at hand, it is just left out. For example, consider the F2 data from Mendel's Expt.1 with round vs wrinkled seeds. If all the seed, in every generation was yellow, our previous discussion leads us to expect all seeds to be of genotype ? Nevertheless, as we are interested only in seed shape we would ignore the GG notation. Similarly seed shape segregating in a green ? genotype would continue to use the same notation - that is, only "W" / "w" notation.

Allele __ notation: A short hand notation is often used when a phenotype may be either homozygous dominant or heterozygous. For example,consider "565 plants which were raised from round seeds of the first generation, 193 yielded round seeds only, and remained therefore constant in this character; 372, however, gave both round and wrinkled seeds, in the proportion of 3:1." As we don't know which of the 565 F2 plants are homozygous or heterozygous, they would be indicated as W_, where _ may represent either W or w.

A Mendrill to get you started Genetics makes extensive use of abbreviated notation to indicate genotypes. The implied relationship between genotype and phenotype - that the phenotype results from the genotype, requires the agile flipping back and forth between the two terms. Furthermore the geneticist must assign genotypes that sensibly recall the phenotype and be consistent in the use of the designated terminology. The first question is a small drill on genotype assignment / utilization and the Law of Segregation.

1. A red flowered pea plant was crossed to a white flowered pea plant and the F1 progeny observed was 1/2 red and 1/2 white.

(i). the dominant phenotype is  red ? ; white ? ;  [ red or white] ?

(ii).Fill in the four suggested notations for this cross

1. white is dominant, use lower case first letter for the recessive trait red.

P1 red	P2 white	F1 red	F1 white
?	?	?	?

2. white is dominant, use upper case first letter for the dominant trait

P1 red	P2 white	F1 red	F1 white
?	?	?	?

3. red is dominant, use upper case first letter for the dominant trait

P1 red	P2 white	F1 red	F1 white
?	?	?	?

4. red is dominant, use lower case first letter for the recessive trait

P1 red	P2 white	F1 red	F1 white
?	?	?	?

 

(iii) Assume red is dominant; red F1 or white F1 does not segregate in the F2 and using notation 4 above, the genotype of white is ?

(iv) Assume white is dominant; red F1 or white F1 does not segregate in the F2.

(v) Assume red is dominant; red F1 or white F1 segregates in the F2 in a ratio of ? white : ? red

(vi) Assume white is dominant; red F1 or white F1 segregates in the F2 in a ratio of ? white : ? red ; assuming the notation in 1 above, the genotype(s) of white is ? ; assuming the notation in 2 above, the genotype(s) of white is ? .

(vii) Enough already! Design another drill based on (vi) and try it out on another student.

2.

Hornless (polled) cattle are H_ ,which is dominant to horned hh. Farmer Brown wants to establish a pure-breeding herd of hornless Texas longhorns and at great cost purchased a rare polled Texas longhorn bull. Tex, the Texan who sold him the bull, said at least half the progeny from breeding horned cows would be polled. In the first year farmer Brown's polled bull breed four cows ( <-- you judge the best cow); three horned and one polled calves were born. Farmer Brown loaded the bull in the truck, drove to Texas, and demanded his money back. Did Tex cheat farmer Brown?

answer:

---

3 .

pigeons fly when reloaded

                                      Pigeons may have a checkered or plain feather pattern. The following results were obtained.

Cross				F1 Progeny
				checkered	plain
(i) checkered X checkered				37	0
(ii) checkered X plain				43	0
(iii) plain X plain				0	38

Use the lower case of the first three letters of the recessive phenotype to indicate the recessive allele and three UPPERCASE letters for the dominant trait. The recessive genotype is ? .  For cross (ii), the genotype of the F1 checkered phenotype is: ? which would result in an F2 genotypic ratio of 3 ? : 1 ? . The expected F2 phenotypic ratio from cross (ii) is: ?

 

4. Consider the following four generation pedigree.

In the pedigree IV-2 and IV-4 are first ? . III-3 is the ? of IV-9. I-1 is the ? of III-2. Which of the following is a consanguineous mating? IV-1 and IV-9 or III-1 and III-8 or both matings

If great grandmother was heterozygous for a recessive non-lethal such as albinism (OCA1OCA1⁺), what would be the probability of passing an OCA1 allele to II-4 ? . If both great grandparents were OCA1OCA1⁺ what fraction of the progeny would be albino? ? If both great grandparents were OCA1OCA1⁺ what is the probability that II-1 is an adult heterozygous carrier? ?

If great grandmother was heterozygous for a recessive childhood lethal lethal such as Tay Sachs Disease (TSDTSD⁺), what would be the probability of passing an TSD allele to II-4 ? If both great grandparents wereTSDTSD⁺ what fraction of the progeny would be Tay Sachs? ? If both great grandparents were TSDTSD⁺ what is the probability that II-1 is an adult heterozygous carrier ?

(i) The abnormal phenoype indicated in black is for the condition of dwarfism in humans.

Velazquez " The Dwarf Sebastian de Morra"

Achondroplasia is a dominant form of dwarfism in humans.The allele ACH allele is dominant to wild type ACH+. In the pedigree the genotype of affected individuals is ? Individual ? shows lack of penetrance ! of ACH.

(ii) The abnormal phenoype indicated in black is for the condition of obesity in mice.

Obesity is inherited as a recessive allele( Ob). The wild-type allele (Ob+) controls the hormone leptin which regulates the desire to eat. In the pedigree the genotype of: II-2 is ? , II-4 ? , III-5 ? .

 

 

5. Apparently normal 5-6 month old infants with Tay Sachs disease develop paralysis, mental retardation and usually die by age 6. TSDTSD recessive homozygotes have a deficiency in the enzyme hexosaminidase A which leads to a deteriorated nervous system.  Jews of Eastern European descent had TSD at a frequency of ca 1/3500 - ca 10 times higher than the general population. However, this disease has been virtually eliminated in those populations that used genetic technology , in combination with genetic counseling, to detect TSDTSD⁺ carriers. For example, in some segments of the Jewish population the Rabbi "matchmaker" was informed of carrier status and counseled against marriages between two TSDTSD⁺ carriers.

You have just put up your shingle as a qualified genetic councellor and a childless, penotypically normal couple has come to your office. The man has a paternal female first cousin who died from TSD and the woman had a maternal uncle who died from TSD.

(i) draw a pedigree showing the relevant individuals and their putative genotypes ?

(ii) calculate the probability that; they are both carriers , that only one of them is a carrier , that neither is a carrier

(iii) what is the probability that they will have a TSDTSD child ?

(iv) what is your best advise to them ?

additional problems -do problems 1 - 8

---

F1 Hybrids in Which Several Different Traits are Associated

The seven monohybrid analyses all followed the inheritance of only one trait at a time. What might we expect if the inheritance of two traits are followed simultaneously? Perhaps the normal dominance relationships would fail, new characteristics may appear amongst the progeny, or simple reproducible ratios may no longer be discernable in different generations. Mendel sought answers to these questions by following the inheritance of two traits at the same time.

Dominance in the F1 dihybrid

Mendel again took advantage of the traits for seed shape and color. P1 was round yellow and P2 was wrinkled green. The dominance relationships followed the two related monohybrid crosses and thus the F1 phenotype was ?

The first generation from the F1 dihybrid - the F2 generation ?

Mendel described the results: "The plants raised therefrom yielded seeds of four sorts, which frequently presented themselves in one pod. In all, 556 seeds were yielded by 15 plants, and of these there were: 315 round and yellow, 101 wrinkled and yellow, 108 round and green, 32 wrinkled and green."

The dihybrid F2 phenotypic ratio is 9/16 yellow round : 3/16 yellow wrinkled : 3/16 green round : 1/16 green wrinkled . The following table summarizes the observations in this generation.

	seed color phenotype	seed shape phenotype	number	F2 fraction	genotype
	yellow(dominant)	round ( Choose One dominant recessive ) ?	315	?	G_W_
	yellow ( Choose One dominant recessive ) ?	wrinkled ( Choose One dominant recessive ) ?	101	?	?
	green ( Choose One dominant recessive ) ?	round ( Choose One dominant recessive ) ?	108	?	?
	green ( Choose One dominant recessive ) ?	wrinkled ( Choose One dominant recessive ) ?	32	?	?

Recall that the dominant phenotype of the monohybrid F2 was found to be 1/3 homozygous and 2/3 heterozygous. In the monohybrid, the recognition of 1 /3 true-breeding and 2/3 hybrid F2 genotypes was only known by the absence or presence of segregation in the F3 generation.

Similarly we do not know if the dihybrid dominant F2 genotypes are homozygous or heterozygous until we analyse for segregation in the F3. For example, the F2 phenotype may be one of four genotypes GGWW, GG ? , WW ? ,or ? What is the frequency of these genotypes amongst the 9/16 yellow round F2 seeds? What frequency of genotypes are found in the other three , , F2 phenotypes?

The second generation from the F1 dihybrid - the F3 generation ?

Only 301 of the315 yellow round F2 seeds planted bore F3 seed.

F3 progeny classes from F2 seed	Number of F2 seeds	F2 genotype	F2 frequency
yellow round	38	?	1/16
yellow round & green round in a 3:1 ratio	65	?	?
yellow round & yellow wrinkled in a 3:1 ratio	60	?	?
yellow round , green round , yellow wrinkled , green wrinkled in a 9:3:3:1 ratio	138	?	?
	301

96 of the 101 yellow wrinkled F2 seeds planted bore F3 seed.

F3 progeny classes from F2 seed	Number of F2 seeds	F2 genotype	F2 frequency
yellow wrinkled	28	?	?
yellow wrinkled & green wrinkled in a 3:1 ratio	68	?	?
	96

102 of the 108 green round F2 seeds planted bore F3 progeny.

F3 progeny classes from F2 seed	Number of F2 seeds	F2 genotype	F2 frequency
green round	35	?	?
green round & green wrinkled in a 3:1 ratio	67	?	?
	102

30 of the 32 green wrinkled F2 seed planted bore F3 progeny.

F3 progeny classes from F2 seed	Number of F2 seeds	F2 genotype	F2 frequency
green wrinkled	30	?	?
	30

The four phenotypes of the the classical 9:3:3:1 F2 ratio were seen to consist of ? different genotypes in frequencies of 4/16, 2/16 or 1/16. Fill in F2 frequencies in the above tables.

F2 genotypes are recognized as homozygous when there is no segregation for the recessive phenotype in the F3. Conversely, segregation of recessive genotypes in the F3 indicates heterozygosity in the F2 parent. Fill in the nine F2 genotypes in the above tables.

---

Modern interpretation - The Law of Independent Assortment

It is assumed that both the seed shape and seed color phenotypes are controlled by two alleles of two different genes.Thus the genotype of is GGWW and the genotype of is ? As both parents are homozygous they produce only one type of gamete, union of these gametes results in a uniform dihybrid GgWw. This is the case whether the cross is in coupling (dominant alleles all in one parent) or repulsion (each parent contains a dominant and a recessive allele).

The Law of Independent Assortment states that, during the formation of gametes, the alleles of different genes segregate independently of each other. Gametes then combine at random to form progeny.

In the dihybrid GgWw, the two alleles Gg would segregate to give 1/2 G and 1/2 g gametes and the two alleles Ww would segregate to give 1/2 W and 1/2 w gametes. The independent assortment of these two pairs of alleles results in four gametes with equal frequency.

Independent assortment of GgWw alleles

alleles	1/2 G	1/2 g
1/2 W	1/4 WG	1/4 Wg
1/2 w	1/4 wG	1/4 wg

These four gametes then combine at random to form diploid F2 progeny.

 

The 9:3:3:1 phenotypic ratio breaks down into a 1:2:2:4:1:2:1:2:1 genotypic ratio.

Problems

1. The secretor phenotype is a dominant trait ( FUT2__ ) with A and B blood group antigens in saliva and body fluids. FUTFUT lacks I^A or I^B antigens. In the offspring of I^AI^BFUT²FUT by I^AI^BFUT²FUT what is the apparent blood group types if the phenotype was determined by saliva tests? 6/16 AB : 3/16 A ; 3/16 B : 4/16 O. ?

 

2. Radishes may be red (RR), purple (Rr) or white (rr) and long (LL), oval (Ll) or round (ll). If you selfed purple oval what progeny phenotypes would you expect ?