How are homozygous and heterozygous genotypes for a particular trait in plants different?
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Recommended textbook solutionsIB Biology Student Workbook2nd EditionRichard Allan, Tracey Greenwood 1,112 solutions Introduction to Anatomy and Physiology1st EditionMichelle Provost-Craig, Susan J. Hall, William C. Rose 1,678 solutions Miller and Levine Biology1st EditionJoseph S. Levine, Kenneth R. Miller 1,773 solutions Lifetime Health1st EditionRinehart, Winston and Holt 1,294 solutions updated: October 26, 2022 DefinitionHeterozygous, as related to genetics, refers to having inherited different versions (alleles) of a genomic marker from each biological parent. Thus, an individual who is heterozygous for a genomic marker has two different versions of that marker. By contrast, an individual who is homozygous for a marker has identical versions of that marker.
NarrationHeterozygous. In diploid species, there are two alleles for each trait of genes in each pair of chromosomes, one coming from the father and one from the mother. An allele is one of two or more alternative forms of a gene, and they are found at the same place, or locus, on the chromosome. Heterozygous refers to having different alleles for a particular trait. If the two versions are different, you have a heterozygous genotype for that gene. The relationship between the two alleles affects which traits are expressed. SearchMENDEL'S GENETIC LAWS Mendel's Laws are as follows: 1. the Law of Dominance Now, notice in that very brief description of his work that the words "chromosomes" or "genes" are nowhere to be found. That is because the role of these things in relation to inheritance & heredity had not been discovered yet. What makes Mendel's contributions so impressive is that he described the basic patterns of inheritance before the mechanism for inheritance (namely genes) was even discovered. There are a few importantvocabulary terms we should iron-out before diving into Mendel's Laws. � GENOTYPE = the genes present in the DNA of an organism. We will use a pair of letters (ex: Tt or YY or ss, etc.) to represent genotypes for one particular trait. There are always two letters in the genotype because (as a result of sexual reproduction) one code for the trait comes from mama organism & the other comes from papa organism, so every offspring gets two codes (two letters). Now, turns out there are three possible GENOTYPES - two big letters (like "TT"), one of each ("Tt"), or two lowercase letters ("tt"). Since WE LOVE VOCABULARY, each possible combo has a term for it. When we have two capital or two lowercase letters in the GENOTYPE (ex: TT or tt) it's called HOMOZYGOUS("homo" means "the same"). Sometimes the term "PURE" is used instead of homozygous. When the GENOTYPE is made up of one capital letter & one lowercase letter (ex: Tt) it's called HETEROZYGOUS ("hetero" means "other"). Just to confuse you, a heterozygous genotype can also be referred to as HYBRID. OK? Let's Summarize:
� PHENOTYPE = how the trait physically shows-up in the organism. Wanna know the simplest way to determine an organism's phenotype ? Look at it. Examples of phenotypes: blue eyes, brown fur, striped fruit, yellow flowers. � ALLELES = (WARNING - THIS WORD CONFUSES PEOPLE; READ SLOW)
alternative forms of the same gene. Alleles for a trait are located at corresponding positions on homologous chromosomes. For example, there is a gene for hair texture (whether hair is curly or straight). One form of the hair texture gene codes for curly hair. A different code for of the same gene makes hair straight. So the gene for hair texture exists as two alleles --- one curly code, and one straight code. Let's try & illustrate with a diagram. Getting back to our abbreviations, we could use a "C" for the curly allele, and a "c" for the straight allele. A person's genotype with respect to hair texture has three possiblilties: CC, Cc, or cc. So to review some vocab, homozygous means having two of the same allele in the genotype (2 big or 2 little letters --- CC or cc). Heterozygous means one of each allele in the genotype (ex: Cc). Now I could tell you which genotypes create curls & which do not, but then I'd be stealing some of Mr. Mendel's thunder. More on that in a minute ........
1. Which of the following is a possible abbreviation for a genotype? A.
BC 2. What is the best way to determine the phenotype of the feathers on a bird? A. analyze the bird's DNA (genes) 3. Which of the following pairs is not correct? A. kk = hybrid 4. The genes present in an organism represent the organism's __________. A. genotype 5. Which choice represents a possible pair of alleles? A. k & t 6. How many alleles for one trait are normally found in the genotype of an organism? A. 1 7. Which statement is not true? A. genotype determines
phenotype The Law of Dominance While Mendel was crossing (reproducing) his pea plants (over & over & over again), he noticed something interesting. When he crossed pure tall plants with pure short plants, all the new pea plants (referred to as the F1 generation) were tall. Similarly, crossing pure yellow seeded pea plants and pure green seeded pea plants produced an F1 generation of all yellow seeded pea plants. The same was true for other pea traits:
So, what he noticed was that when the parent plants had contrasting forms of a trait (tall vs short, green vs yellow, etc.) the phenotypes of the offspring resembled only one of the parent plants with respect to that trait. So, he said to himself, "Greg, there is a factor that makes pea plants tall, and another factor that makes pea plants short. Furthermore Greg ol' boy, when the factors are mixed, the tall factor seems to DOMINATE the short factor". Now, in our modern wisdom, we use "allele" or "gene" instead of what Mendel called "factors". There is a gene in the DNA of pea plants that controls plant height (makes them either tall or short). One form of the gene (allele) codes for tall, and the other allele for plant height codes for short. For abbreviations, we use the capital "T" for the dominant tall allele, and the lowercase "t" for the recessive short allele. Let's revisit the three possible genotypes for pea plant height & add some MORE VOCABULARY.
Note: the only way the recessive trait shows-up in the phenotype is if the geneotype has 2 lowercase letters (i.e. is homozygous recessive). The PUNNETT SQUARE (P-Square for short) OK, now is as good of time as any to introduce you to a new friend, the Punnett Square. This little thing helps us illustrate the crosses Mendel did, and will assist you in figuring out a multitude of genetics problems. We will start by using a P-Square to illustrate Mendels Law of Dominance. Recall that he "discovered" this law by crossing a pure tall pea plant & a pure short pea plant. In symbols, that cross looks like this: Parents (P): TT x tt where T = the dominant allele for tall stems The P-Square for such a cross looks like this: To "fill in the boxes" of the Punnett Square, say to yourself "letter from the left & letter from the top". The "t" from the left is partnered with the "T" from the top to complete each of the four squares. A summary of this cross would be:
Now, a helpful thing to recognize is this: ANY TIME TWO PARENT ORGANISMS LOOK DIFFERENT FOR A TRAIT, All the offspring are heterozygous for the trait, one parent is homozygous dominant, and the other is homozygous recessive. Does setting up & using the Punnett Square confuse you? Would you like to see a step-by-step "how to" about the good ol' p-square? For some practice Punnett Square problems visit my very own: "P-Square Practice Page". Don't forget to come back & learn more about Mendel! The Law of Segregation The way I figure it, Mendel probably got really bored crossing pure dominant trait pea plants with pure recessive trait pea plants (over & over & over again) & getting nothing but pea plants with the dominant trait as a result. Except for gaining more & more evidence for his Law of Dominance, this probably grew tiresome. So, at one point he takes the offspring of a previous cross & crosses them. Ooooooooh ............ Recall that his original cross for the tall & short pea plants was:
So, he takes two of the "F1" generation (which are tall) & crosses them. I would think that he is figuring that he's gonna get all tall again (since tall is dominant). But no! Low & behold he gets some short plants from this cross! His new batch of pea plants (the "F2" generation) is about 3/4 tall & 1/4 short. So he says to himself, "Greg ol' boy, the parent plants for this cross each have one tall factor that dominates the short factor & causes them to grow tall. To get short plants from these parents, the tall & short factors must separate, otherwise a plant with just short factors couldn't be produced. The factors must SEGREGATE themselves somewhere between the production of sex cells & fertilization." I think it's easier to picture this law by using a p-square. Our cross is two hybrid parents, Tt x Tt. You can see from the p-square that any time you cross two hybrids, 3 of the 4 boxes will produce an organism with the dominant trait (in this example "TT", "Tt", & "Tt"), and 1 of the 4 boxes ends up homozygous recessive, producing an organism with the recessive phenotype ("tt" in this example). Our summary:
A helpful thing to recognize: Any time two parents have the same phenotype for a trait The Law of Independent Assortment OK. So far we've been dealing with one trait at a time. For example, height (tall or short), seed shape (round or wrinkled), pod color (green or yellow), etc. Mendel noticed during all his work that the height of the plant and the shape of the seeds and the color of the pods had no impact on one another. In other words, being tall didn't automatically mean the plants had to have green pods, nor did green pods have to be filled only with wrinkled seeds, the different traits seem to be inherited INDEPENDENTLY. Please note my emphasis on the word "different". Nine times out of ten, in a question involving two different traits, your answer will be "independent assortment". There is a big ugly punnet square that illustrates this law so I guess we should take a look at it. It involves what's known as a "dihybrid cross", meaning that the parents are hybrid for two different traits. The genotypes of our parent pea plants will be: RrGg x RrGg where Notice that we are dealing with two different traits: (1) seed texture (round or wrinkled) & (2) pod color (green or yellow). Notice also that each parent is hybrid for each trait (one dominant & one recessive allele for each trait). We need to "split" the genotype letters & come up with the possible gametes for each parent. Keep in mind that a gamete (sex cell) should get half as many total letters (alleles) as the parent and only one of each letter. So each gamete should have one "are" and one "gee" for a total of two letters. There are four possible letter combinations: RG, Rg, rG, and rg. These gametes are going "outside" the p-square, above 4 columns & in front of 4 rows. We fill things in just like before --- "letters from the left, letters from the top". When we finish each box gets four letters total (two "are's" & two "gees"). This is what it looks like:
The results from a dihybrid cross are always the same: So, as you can see from the results, a green pod can have round or wrinkled seeds, and the same is true of a yellow pod. The different traits do not influence the inheritance of each other. They are inherited INDEPENDENTLY. Interesting to note is that if you consider one trait at a time, we get "the usual" 3:1 ratio of a single hybrid cross (like we did for the LAw of Segregation). For
example, just compare the color trait in the offspring; 12 green & 4 yellow (3:1 dominant:recessive). Same deal with the seed texture; 12 round & 4 wrinkled (3:1 ratio). The traits are inherited INDEPENDENTLY of eachother --- Mendel's 3rd Law. I would like to summarize Mendel's Laws by listing the cross that illustrates each.
There you have them, Mendel's huge contributions to the world of science. A very smart cookie. His work has stood the test of time, even as the discovery & understanding of chromosomes & genes has developed in the 140 years after he published his findings. New discoveries have found "exceptions" to Mendel's basic laws, but none of Mendel's things have been proven to be flat-out wrong. Review Questions 1. Which cross would best illustrate Mendel's Law of Segregation? A. TT x tt 2. In the cross Yy x Yy, what percent of offspring would have the same phenotype as the parents? A. 25% 3. In a certain plant, purple flowers are dominant to red flowers. If the cross of two purple-flowered plants produces some some purple-flowered and some red-flowered plants, what is the genotype of the parent plants? A. PP x Pp Base questions #4-8 on the following information: A white-flowered plant is crossed with a pink-flowered plant. All of the F1 offspring from the cross are white. 4. Which phenotype is dominant? 9. Crossing two dihybrid organisms results in which phenotypic ratio? A. 1:2:1 10. The outward appearance (gene expression) of a trait in an organism is referred to as: A.
genotype
A. cD
A. analyzing its genes 13. Mendel formulated his Law of Segregation after he had: A. studied F1
offspring 14. Which cross would produce phenotypic ratios that would illustrate the Law of Dominance? A. TT x tt 15. The mating of two curly-haired brown guinea pigs results in some offspring with brown curly hair, some with brown straight hair, some with white curly hair, and even some with white straight hair. This mating illustrates which of Mendel's Laws? A. Dominance
Vocabulary Term Review Questions- CORRECT ANSWERS ARE UNDERLINED 1. Which of the following is a possible abbreviation for a genotype? A.
BC 2. What is the best way to determine the phenotype of the feathers on a bird? A. analyze the bird's DNA (genes) 3. Which of the following pairs is not correct? A. kk = hybrid - Kk would be hybrid (one capital, one lowercase of the same letter) 4. The genes present in an organism represent the organism's __________. A. genotype 5. Which choice represents a possible pair of alleles? A. k & t 6. How many alleles for one trait are normally found in the genotype of an organism? A. 1 7. Which statement is not true? A. genotype determines phenotype - (note that the environment does play a role
in influencing phenotype too) Review Questions - ANSWERED & EXPLAINED 1. Which cross would best illustrate Mendel's Law of Segregation? A. TT x
tt 2. In the cross Yy x Yy, what percent of offspring would have the same phenotype as the parents? A. 25% 3. In a certain plant, purple flowers are dominant to red flowers. If the cross of two purple-flowered plants produces some some purple-flowered and some red-flowered plants, what is the genotype of the parent plants? A. PP x Pp Base questions #4-8 on the following information: A white-flowered plant is crossed with a pink-flowered plant. All of the F1 offspring from the cross are white. 4.
Which phenotype is dominant? white 50% heterozygous white & 50% homozygous recessive pink. The cross for this question would be "Ww (white F1) x ww (pink)". 8. Which of Mendel's Laws is/are illustrated in this question? Dominance is illustrated by the original cross (WW x ww). 9. Crossing two dihybrid organisms results in which phenotypic ratio? A. 1:2:1 - genotype
ratio of a hybrid cross, ex: Tt x Tt 10. The outward appearance (gene expression) of a trait in an organism is referred to as: A. genotype
A. cD
A. analyzing its genes 13. Mendel formulated his Law of Segregation after he had: A. studied F1 offspring - 14. Which cross would produce phenotypic ratios that would illustrate the Law of Dominance? A. TT x tt - one parent tall, the other short, all offspring would be tall 15. The mating of two curly-haired brown guinea pigs results in some offspring with brown curly hair, some with brown straight hair, some with white curly hair, and even some with white straight hair. This mating illustrates which of Mendel's Laws? A. Dominance How are homozygous and heterozygous allele combinations for a particular trait in plants different give an example of an allele combination for each type of plant?Give an example of an allele combination for each type of plant. Homozygous plants have two identical alleles for a particular trait, TT or tt. In contrast, heterozygous plants have two different alleles for a particular trait, Tt.
What is homozygous and heterozygous in plants?In a homozygous plant, a gene pair (alleles) contains two identical genes controlling similar trait of a character. In heterozygous plant, a gene pair (alleles) contains two different genes controlling different traits of a character in an organism.
How could you tell if your plant is homozygous or heterozygous for the trait?Heterozygous means that an organism has two different alleles of a gene. For example, pea plants can have red flowers and either be homozygous dominant (red-red), or heterozygous (red-white). If they have white flowers, then they are homozygous recessive (white-white). Carriers are always heterozygous.
What is the difference between homozygous and heterozygous alleles quizlet?Homozygous is having two identical alleles for a particular gene. Heterozygous is having two different alleles for a particular gene.
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