10.2 Inheritance Flashcards
What is it necessary to know in order to test whether inheritance of a trait follows a particular pattern?
It is necessary to know what types and proportions of offspring are predicted for that pattern.
What are Punnet squares?
One tool that can be used to make predictions about the genotypes and phenotypes of offspring from a cross between a particular set of parents.
What are monohybrid and dihybrid crosses?
- Monohybrid crosses, which examine one trait at a time, use Punnett squares to check for dominance and sex-linkage in a trait.
- Using the same principles, Punnett squares can be used when examining two or more traits at once.
- A dihybrid cross follows the inheritance of two traits, and a dihybrid Punnett square would give the expected ratio of phenotypes in the offspring if the traits are independently assorted.
Diagram showing a dihybrid cross
Homozygous yellow, round peas (genotype YYRR) were crossed with homozygous green, wrinkled peas (genotype yyrr).
The allele for yellow seeds, Y, is dominant to the allele for green seeds, y; the allele for round seeds, R, is dominant to the allele for wrinkled seeds, r.
How are dihybrid Punnet squares set up?
- Given the parental genotypes, the possible alleles found in the egg are shown along an outside edge of the grid, and the alleles possible in the sperm are shown along an adjacent edge.
- In a dihybrid cross, two traits are being examined, so there will be two different letters representing the potential gametes.
- The internal boxes show the allele combinations that can be produced by the fusion of the gametes. They represent the genotypes that could be found in the offspring.
-In a dihybrid cross, there will be two pairs of alleles, so two copies each of two different letters.
Why does the dihybrid Punnett square in this diagram has only one box?
- This is because, in the P generation, both parents are homozygous for both traits.
- There is only one possible genotype and one possible phenotype for the offspring.
Diaram of a dihybrid cross with two parents heterozygous for both traits
The individuals in the F 1 generation are heterozygous for both traits (genotype YyRr).
Therefore, they can produce four possible combinations of alleles (YR, Yr, yR, and yr) in their gametes.
When these allele combinations are placed along the top and left sides of the Punnett square, a 4 × 4 Punnett square with 16 boxes is created.
Explanation for how to complete a Punnet square
- To complete the Punnett square, fill each box with the alleles from the appropriate gametes on the outside of the square (above and to the left).
- For a dihybrid cross, write the two alleles for a trait next to each other, for example, YyRR not YRyR.
- This makes it easier to determine the genotype and phenotype of each offspring.
- If the genes show independent assortment, each box in the Punnett square is equally probable, so it can be used to calculate predicted genotypic and phenotypic outcomes.
- For example, out of the 16 possibilities, there are 3 allelic combinations that produce a phenotype of ‘round and green’; therefore 3/16, or 18.75%, of peas are expected to be round and green.
Rules to keep in mind when completing dihybrid Punnett squares
- The top and left sides of the Punnett square show each possible haploid gamete which each contains one copy of each gene in question, and, therefore, one of each letter.
- Each box in the Punnett square represents a possible genotype in the diploid offspring. Therefore, each box should have two copies of each gene, and therefore two of each letter.
- Punnett squares are not always square. Do not use 16 boxes unless they are required because it will take unnecessary time and increase the chance of error.
- Punnett squares are one tool for finding genotypic and phenotypic ratios, but other methods can be used, such as multiplying probabilities.
Diagram of Mendel’s recorded results of a dihybrid cross
When Mendel performed these types of crosses, he carefully recorded the results.
This diagram shows an example of the phenotypic results Mendel collected for one dihybrid cross as well as the modern explanation in terms of genotype.
What is the F2 phenotypic ratio for a dihybrid cross between two heterozygous parents if the two genes involved are unlinked and show independent assortment?
9:3:3:1
Calculating the predicted genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes for the exam
You should be able to calculate the predicted genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes. Sometimes shortcuts exist to answer a question more quickly.
- For example, what is the chance of a pea offspring being round and green if the parents have the genotypes RrYy and RRyy? (R=round, r=wrinkled, Y=yellow, y=green)
- Create a dihybrid Punnett square with four boxes, or
- See that all offspring will be round (the parent with RR always gives an R), and complete a monohybrid cross for pea color.
If a parent has the genotype Aabb, what should be shown along the edge of the Punnett square?
Ab and ab
The gametes shown at the top of the Punnett square must include one copy of each gene that is being investigated, and therefore one letter from each pair.
Two homozygous pea plants are crossed.
One parent has red, axial (side) flowers and the other has white, terminal (top of shoot) flowers; all F1 individuals have red, axial flowers.
The genes are unlinked.
Among the F2 offspring, what is the probability of plants with white, axial flowers?
3/16
Two homozygous pea plants are crossed. One parent has red, axial flowers and the other has white, terminal flowers; all F1 individuals have red, axial flowers. That means that red and axial are dominant traits. The genotype of P: RRAA × rraa that means that the F1 genotype is: RrAa. The Punnett square looks as follows:
Gametes RA Ra rA ra RA RRAA RRAa RrAA RrAa Ra RRAa RRaa RrAa Rraa rA RrAA RrAa rrAA rrAa ra RrAa Rraa rrAa rraa
That yields the following ratio of genotypes:
9 R-A- : 3 R-aa : 3 rrA- : 1 rraa (with ‘-‘ indicating the presence of either allele)
and phenotypes:
9 red axial : 3 red terminal : 3 white axial : 1 white terminal.
Out of 16 total possibilities 9/16 are red axial, 3/16 are red terminal, 3/16 are white axial, and 1/16 are white terminal.
Therefore, the correct answer is #3. Not #1: The 9/16 ratio represents two dominant phenotypes (red axial). Not #2: 1/16 represents two recessive phenotypes (white terminal). Not #4: 1/8 is not a ratio that occurs in this problem.
Two homozygous pea plants are crossed.
One parent has red, axial flowers and the other has white, terminal flowers; all F1 individuals have red, axial flowers.
The genes are unlinked.
If 1000 F2 offspring resulted from the cross, approximately how many of them would you expect to have red, terminal flowers?
187
In tomato plants, the allele for tall plants (T) is dominant to the allele for short plants (t), and the allele for hairy stems (H) is dominant to the allele for smooth stems (h).
The two genes are not linked. If a tall smooth plant (Tthh) is crossed with a short hairy plant (ttHh), which are possible genotypes of the offspring?
What is independent assortment?
- Genes found on different chromosomes will show independent assortment.
- There is no physical link between them and, due to the random orientation of homologous chromosomes in meiosis I, the allele inherited for one gene has no influence on the allele inherited for another.
What are linked genes?
- Genes found on the same chromosome
- They are physically connected since they occur along the same DNA molecule.
What is the result of linked genes?
- Many linked genes give the same results as unlinked genes in genetic crosses.
- In other words, even though they are on the same chromosome, the allele inherited for one gene does not affect which allele will be inherited for the other gene.
- This is because crossing over occurs with enough frequency to randomize the inheritance of the two traits.
When can crossing over occur and how does this affect loci?
- Crossing over may occur once between two loci, separating alleles, or twice between the loci, joining them back together.
- Crossing over can occur three or more times between loci, or not at all.
- Thus, when loci are far apart on the same chromosome they are inherited as though they are not connected at all.
- However, some linked genes have loci so close together that inheriting one allele for a gene increases the chance of inheriting whichever version of the other gene is found on the same chromosome.
Diagram of linked genes are found on the same chromosome
What will the ratio of observed offspring be when dihybrid crosses are performed using closely-linked genes?
- The ratio of observed offspring will not match the predictions of the Punnett square, which assumes independent assortment.
- The combination of alleles that were together on the parental chromosomes will always be more common in the offspring.
What are recombinant combinations?
New allele combinations, created by crossing over, are called recombinant and are always less common than parental combinations.
What are linked genes?
- Genes found on the same chromosome.
- The closer the loci of two linked genes, the less frequently they are separated by crossing over.
What is a linkage group?
Several genes (two or more) on the same chromosome that do not show independent assortment are called a linkage group.
What organism did Thomas Hunt Morgan use in his research?
He worked with fruit flies (Drosophila melanogaster) to demonstrate that genes are found on chromosomes and that they do not always follow Mendelian ratios.
Why are fruit flies a useful model organism for studying genetics?
- Because they are small and easy to care for, they go through an entire life cycle in about ten days, and they produce hundreds of offspring.
- The females and males have visible differences, especially in the abdomen.
- Fruit flies have three pairs of autosomes and one pair of sex chromosomes.
What methods did Morgan and his students use and what did they find?
- They used a variety of methods, including radiation, to cause heritable mutations with visible effects on phenotypes that could be studied.
- The first clear exception to Mendelian inheritance found in Drosophila was the inheritance of white eyes.
- Fruit flies usually have red eyes.
Drawing of a female (left) and male (right) fruit fly (Drosophila melanogaster)
What did Morgan observe and later discover?
- Morgan observed a male fruit fly with white eyes (instead of red). This unusual mutant male was mated with ‘wild type’ (normal, red-eyed) females and produced offspring that all had red eyes, demonstrating that red eye colour is dominant.
- However, the males and females of the F 1 generation produced offspring in F 2 (small 2) with a ratio of 2 red-eyed females: 1 red-eyed male: 1 white-eyed male.
- Although overall the ratio of red to white eyes was 3:1, the white-eyed flies were all males.
- If eye colour had followed Mendelian predictions, an equal number of males and females should have white eyes.
- Thus, Morgan established sex-linked traits and suggested that some genes, including eye colour in Drosophila, are located on the sex chromosomes.
Diagram of the mating of wild type with white-eyed mutants (the Y chromosome in males is not shown because it has no corresponding gene locus for this trait)
What system did Morgan use for the notation of alleles? (+ info for exam)
- Morgan used a different system of notation for alleles than the standard uppercase and lowercase letters.
- Morgan’s system is still frequently used for Drosophila.
- The most common allele of any trait is known as wild-type and is symbolized by ‘+’. The mutant alleles are represented by one or a few letters.
- For example, ‘w’ for white eyes (recessive), and ‘Cy’ for curly wings (dominant). A fly heterozygous for wing shape might be written ‘Cy/+’ or ‘Cy+ Cy’.
- It is unlikely but possible that a similar notation would be used for Drosophila on the exam.
- It is not necessary to know all the possibilities because you will be able to deduce the meaning from the information given.
- However, being aware that this notation exists may help prevent confusion during an exam.
What did Morgan do once he determined that some genes are located on the sex chromosome?
- He began experimenting to determine the location of other genes using dihybrid crosses.
- Many pairs of genes followed expected Mendelian ratios, which assume independent assortment.
- However, some gene pairs seemed to ‘stick together’.
- Most offspring had the same combination of alleles as the parents, while far fewer than expected had recombinant traits.
- These genes show autosomal linkage, meaning they are found on the same autosomal chromosome.
Explain how, when genes are linked, a special notation is used.
- To show dominance, a capital letter (or letters, if the organism is Drosophila) is still used.
- To show that the genes are linked, a schematic representation of chromosomes is used as shown below.
Schematic representation of chromosomes to show that the genes are linked
- The horizontal lines represent the homologous chromosomes, with one allele for each gene.
- The alleles for a gene are shown as a vertical pair, indicating the locus of the gene.
Diagram of crossing over and segregation of the alleles of linked genes
What are the two distinct possibilities for the genotype of a heterozygous parent in a linkage problem?
Digram showing what would be expected from a dihybrid test cross if there was no linkage between the genes
Recombinants in Drosophila , caused by crossing over between linked genes
Diagram showing Recombinants in Drosophila , caused by crossing over between linked genes
Explain this diagram
- The F 2 (little 2) offspring show the four expected phenotypes but the ratios are very different from the 1:1:1:1 that would be expected if the genes were not linked.
- The two most common phenotypes reveal the original allele combinations in the heterozygous parent: B G (underlined) and b g (underlined).
- The percentage of crossing over (recombinant frequency) is calculated by dividing the number of recombinant offspring by the total number of offspring and multiplying by 100.
- In this case, the calculation would be ((206+185)/(965+944+206+185))×100=17%.
What would it mean if the two genes (for body color and wing size) were not linked in the fruit flies
They would follow Mendel’s Law of Independent Assortment and a phenotypic ratio of approximately 1:1:1:1 would have been seen for the example in Figure 3
A parent is heterozygous for two linked genes. How would the parent’s genotype be represented using proper notation?
Graph showing an example of the distribution of height in a human population
Maps showing distribution of height increase in females and males across the world
Picture of mice (environmental differences in the fur color of cloned mice)
- These three mice are genetically identical; however, they developed in surrogate mothers that were fed different diets.
- In this case, fur color shows an environmental component.
