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.