Simple Mendelian Genetics Question

Simple Mendelian Genetics Question

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here is my question:

In certain plants, tall is dominant to short. If a heterozygous plant is crossed with a homozygous tall plant, what is the probability that the offspring will be short?

My Solution:

Let T be the allele for the tall phenotype and t be the allele for the short phenotype.

Then the parental cross would be Tt x TT = 100% tall phenotype (1/2 Tt, 1/2 TT).

Hence the probability of having short offspring should be $fbox{0}$.

However, the answer key for this question says the answer is $fbox{$frac{1}{2}$}$. How can this be??? It really bothers me when the answer key is potentially incorrect because it makes me very unsure, and I end up wasting time trying to figure out if the key is wrong or if I am. Can somebody please let me know if what I did was correct/incorrect??


Assuming you have stated the question correctly, the answer key is incorrect for exactly the reasons you have given and your reasoning and consequent answer are correct.

I would double-check you have read the question correctly and then conclude the answer key is incorrect. It happens.

This is a test cross - a cross with a recessive pair of alleles and either a heterozygous or homozygous pair of dominant alleles( eg- Tt X tt or TT x tt respectively). These crosses are carried out to check the if genes are homozygous or not. In this the ratio is always 1:1. So don't worry. :) Be confident about your answer, because it is obvious that the answer cannot be half if you work it out using the Punnett square.

Mendelian Genetics Worksheet Answer Key

Use b or b for your genotypes. Click on pop out icon or print icon to worksheet to print or download.

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Show/hide words to know

Gene: a region of DNA that instructs the cell on how to build protein(s). As a human, you usually get a set of instructions from your mom and another set from your dad. more

Inheritance: genetic information passed down from a parent.

Phenotype: the appearance of an individual that results from the interaction between their genetic makeup and the environment. Phenotypic trait. more

Trait: a characteristic of an organism that can be the result of genes and/or influenced by the environment. Traits can be physical like hair color or the shape and size of a plant leaf. Traits can also be behaviors such as nest building behavior in birds.

**Some of the content on this page is out of date, please pardon us while we update it for accuracy.**

Below is a list of phenotypes easily identified in humans that follow the pattern of Mendelian inheritance. Look at yourself in the mirror to see if you carry the dominant or recessive alleles for these traits.

Mendelian Genetics

The laws proposed to explain the inheritance of traits (characters) from one generation to another are known as Mendel's Laws of inheritance.

Mendel's laws are as follows:

Law of Dominance: When two alternative alleles of a trait (e.g.height of plant) are present together, either of the two only is able to express itself and is called dominant allele or factor and the other allele, though present, is not able to express itself is called recessive allele or factor.

In Pisum sativum (edible pea) tallness is dominant and is written as (T) and dwarfness is recessive and is written as (t).

Recessive allele is represented by small letter of the same alphabet that represents the dominant allele.

A tall pea plant is either pure tall (TT) or impure tall (Tt). The allele of dwarfness (t) is not able to express itself when present along with dominant allele of tallness (T).

Recessive allele can express only when both the alleles are (tt).

Thus a tall plant can be pure (homozygous) or impure (heterozygous), but dwarf plant is always pure (homozygous).

Law of Segregation: According to law of segregation two factors or alleles representing same expression or alternate expression of a trait segregate or separate at the time of gamete formation so that gametes contain only one allele or factor and are thus always pure.

Law of Independent Assortment: According to this law segregation of the factors or alleles of two traits occurs independently of each other at the time of gamete formation.

In other words the segregation of the factors of one trait does not affect the segregation of the factors of the other trait.

11.3 Dihybrid cross (Experiment 2)

In the second experiment, we will study the result obtained from a dihybrid cross. A dihybrid cross is a cross between two different lines (varieties, strains) that differ in two observed traits. In the name “Dihybrid cross”, the “di” indicates that there are two traits involved (in our example designated R and Su), the “hybrid” means that each trait has two different alleles (in our example R and r, or Su and su), and “cross” means that there are two individuals who are combining or “crossing” their genetic information. In our example, a pure strain of corn producing purple-starchy kernels (RR SuSu) is crossed with a pure strain producing yellow-sweet (rr susu). The starchy seeds are smooth, the sweet seeds are wrinkled. The resulting F1 ears all bear purple-starchy (smooth) kernels. Plants that are heterozygous for two traits are called dihybrids. When the F1 is self-pollinated, the resulting F2 generation contains various combinations (Figure 11.4).

Figure 11.4: Dihybrid cross

The rules of meiosis, as they apply to the dihybrid, are codified in Mendel’s first law and Mendel’s second law, which are also called the Law of Segregation and the Law of Independent Assortment, respectively (Table 11.1). For genes on separate chromosomes, each allele pair showed independent segregation. If the first filial generation (F1 generation) produces four identical offspring, the second filial generation, which occurs by crossing the members of the first filial generation, shows a phenotypic (appearance) ratio of 9:3:3:1, where:

  • the 9 represents the proportion of individuals displaying both dominant traits
  • the first 3 represents the individuals displaying the first dominant trait and the second recessive trait
  • the second 3 represents those displaying the first recessive trait and second dominant trait
  • the 1 represents the homozygous, displaying both recessive traits.

11.3.1 Experimental procedures

  1. Carefully count the number of kernels of each phenotype appearing on a row of F2 ear. Tabulate the results and determine the totals and total ratios in Table 11.3.

Recombinant DNA

  • What does recombinant DNA technology allow scientists to do that they were not able to do before?
  • Recombinant DNA is a set of tools. What are those tools used for?
  • Where did these tools come from?

What does it mean to clone a DNA fragment?

What are restriction enzymes? Why are they important?

How do you find a piece of DNA or RNA in a library?

How can we determine the sequence of a specific segment of DNA?

What important feature of gene regulation needs to be adjusted when cloning a gene for expression in a new species?

How is the process of homologous recombination used in the recombinant DNA toolbox?

Why is PCR a powerful technique in the recombinant DNA toolbox?

What is DNA fingerprinting?

Mendel’s Experiments

Figure 2: Johann Gregor Mendel set the framework for the study of genetics.

Johann Gregor Mendel (1822–1884) (Figure 2) was a lifelong learner, teacher, scientist, and man of faith. As a young adult, he joined the Augustinian Abbey of St. Thomas in Brno in what is now the Czech Republic. Supported by the monastery, he taught physics, botany, and natural science courses at the secondary and university levels. In 1856, he began a decade-long research pursuit involving inheritance patterns in honeybees and plants, ultimately settling on pea plants as his primary model system (a system with convenient characteristics that is used to study a specific biological phenomenon to gain understanding to be applied to other systems). In 1865, Mendel presented the results of his experiments with nearly 30,000 pea plants to the local natural history society. He demonstrated that traits are transmitted faithfully from parents to offspring in specific patterns. In 1866, he published his work, Experiments in Plant Hybridization, in the proceedings of the Natural History Society of Brünn.

Mendel’s work went virtually unnoticed by the scientific community, which incorrectly believed that the process of inheritance involved a blending of parental traits that produced an intermediate physical appearance in offspring. This hypothetical process appeared to be correct because of what we know now as continuous variation. Continuous variation is the range of small differences we see among individuals in a characteristic like human height. It does appear that offspring are a “blend” of their parents’ traits when we look at characteristics that exhibit continuous variation. Mendel worked instead with traits that show discontinuous variation. Discontinuous variation is the variation seen among individuals when each individual shows one of two—or a very few—easily distinguishable traits, such as violet or white flowers. Mendel’s choice of these kinds of traits allowed him to see experimentally that the traits were not blended in the offspring as would have been expected at the time, but that they were inherited as distinct traits. In 1868, Mendel became abbot of the monastery and exchanged his scientific pursuits for his pastoral duties. He was not recognized for his extraordinary scientific contributions during his lifetime in fact, it was not until 1900 that his work was rediscovered, reproduced, and revitalized by scientists on the brink of discovering the chromosomal basis of heredity.

Simple Mendelian Genetics Question - Biology

Heredity is when certain traits are passed from the parents to the children. Traits are characteristics such as eye color, height, and athletic ability. Heredity is passed through genes in the DNA molecule. In biology the study of heredity is called genetics.

Scientist Gregor Mendel (1822 - 1884) is considered the father of the science of genetics. Through experimentation he found that certain traits were inherited following specific patterns.

Gregor studied inheritance by experimenting with peas in his garden. Peas work as an excellent test subject as they can self-pollinate, cross fertilize, and have several traits that only have two forms. This enabled Mendel to easily control his experiments and reduced the possibility of the outcomes to something he could record and manage.

Gregor studied seven traits of the pea plant: seed color, seed shape, flower position, flower color, pod shape, pod color, and the stem length. There were three major steps to Mendel's experiments:

1. First he produced a parent generation of true-breeding plants. He made these by self-fertilizing the plants until he knew they bred true to the seven traits. For example, the purple flowering plants always produced seeds that made purple flowers. He called these plants the P generation (for parent).
2. Next, he produced a second generation of plants (F1) by breeding two different true-breeding P plants.
3. He then produced a third generation of plants (F2) by self-pollinating two F1 generation plants that had the same traits.

Mendel found some incredible results from his experiments.

Mendel found that the F1 generation all produced the same trait. Even though the two parents had different traits, the offspring always had the same trait. For example, if he bred a P plant with a purple flower with a P plant with a white flower, all of the offspring (F1) plants would have purple flowers. This is because the purple flower is the dominate trait.

These results can be shown in a diagram called a Punnett square. The dominate gene is shown with a capital letter and the recessive gene with a lower case letter. Here the purple is the dominant gene shown with a "P" and the white is the recessive gene shown with a "w."

You can go to this page to learn more about inheritance patterns and Punnett Squares.

In the F2 generation he found that 75% of the flowers were purple and 25% were white. Even though both parents had purple flowers, 25% of the offspring had white flowers. This turned out to be because of a recessive gene or trait was present in both parents.

Here is the Punnett square showing that 25% of the offspring had two "w" genes causing them to have white flowers:

Homozygous and Heterozygous

When two of the genes are the same (like with "PP" or "ww" above) they are called homozygous. When they are different (like with "Pw") they are called Heterozygous.

Genetic Characteristics that cannot be Explained by Simple Mendelian Genetics

Mendel is the father of modern genetics, but there are some genetic characteristics that cannot be explained by simple Mendelian genetics. Such is the case with the human blood types in which there are 3 alleles for the same gene, A B, and o. A parent can pass allele A, B, or o to the offspring based on the parent's genotype.

From these 3 alleles, there are 4 blood types (phenotypes): A, B, AB, and O, and there are six genotypes: AA, Ao, BB, Bo, AB, or oo. This is an example of codominance in which both A and B alleles are codominant to each other.

Blood types can be used in forensics to determine if blood is from the victim or criminal. Blood types can be used to determine parental source in situation where the father is unknown however, blood types can only eliminate certain blood types. DNA fingerprinting is a better method that is used often in criminal and parental determination cases.

Punnett squares such as the one shown above are used to determine the probabilities (percentages) for genotypes of offspring given specific genotypes for the parents.

A) In the example above, the Punnett Square represents a cross (mating) between a male (on the left side) with blood type AB, and a female, (top of square), with blood type A, genotype Ao.

Answer the following for the cross represented above.

1) What are the possible blood types for the offspring?
2) What are the ratios or percentages for each possible blood type from this cross?
3) What blood type is not possible from this cross?

B) Fill out two Punnett squares for a cross between a male with blood type B and a female with blood type AB. (Note that we do not know if the father is genotype BB or Bo from the information given. Thus there are two solutions to the possible cross.)

Set up two Punnett squares and answer the following questions about them.

1) What are the possible blood types for the cross between the type B (BB or Bo?) male and AB female?
2) What are the percentages (%) or probabilities for each blood type in the offspring?
3) What blood type(s) would not be possible in a cross between these two parents?

Hint: There are two answers for questions 1 & 2 above and only one for 3.

Part 2: Cell division, mutations and genetic variability.

Eukaryotic cells can divide by mitosis or meiosis. In humans, mitosis produces new cells for growth and repair. And, meiosis produces sex cells (gametes), called sperm and eggs. Changes or mutations in genes in sex cells can be inherited by human offspring. Genetic variation in a population of organisms is good however, sometimes mutations can be harmful or cause genetic disorders.

Briefly, answer the following questions:

How do meiosis and sexual reproduction (fertilization) produce offspring that differ genetically from the parents?

© BrainMass Inc. March 4, 2021, 8:36 pm ad1c9bdddf


Solution Summary

In the first part, this solution creates Punnett squares to answer questions relating to blood type of offsprings and ratios for these percentages. The second part of the answer discusses how offspring differ genetically from parents through meiosis and fertilization.

Cooties Activity

Cootie Genetics is a hands-on, inquiry-based activity that enables students to learn the Mendelian laws of inheritance by conducting experiments similar to those famously pioneered by Gregor Mendel. Using Hasbro-brand Cooties toys, students simulate Mendel’s experiments in order to gain a practical and enduring understanding of foundational genetics principles and terminology. The activity begins with two true-breeding Cooties of the same species that exhibit five observable trait differences between the two populations. Students “breed” members of the two populations and observe the retention or loss of traits among first-generation heterozygotes. Using the scientific method, students then hypothesize what happened to these traits and design an experiment to test their hypotheses by mating the first-generation Cooties. With the production of 40 second-generation offspring, students begin to observe Mendel’s principles of segregation and the independent assortment of alleles, manifested through the 3:1 phenotypic ratio and 1:2:1 genotypic ratio. Students will obtain a practical comprehension of Mendelian genetics, learn to readily identify dominant and recessive traits, and absorb a durable and functional understanding of genetic terminology. This foundation, built through hands-on experimentation and a firsthand, participatory introduction to the scientific method, will provide students the tools they need to conquer more complicated tasks like constructing Punnett squares and, ultimately, acquiring the STEM skills they will need to compete in a knowledge-based economy.

Watch the video: How to solve simple Mendelian genetics problems (February 2023).