10.2 Inheritance

Question 1

What is it necessary to know in order to test whether inheritance of a trait follows a particular pattern?

Answer 1

It is necessary to know what types and proportions of offspring are predicted for that pattern.

Question 2

What are Punnet squares?

Answer 2

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.

Question 3

What are monohybrid and dihybrid crosses?

Answer 3

• 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.

Question 4

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.

Answer 4

Question 5

How are dihybrid Punnet squares set up?

Answer 5

• 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.

Question 6

Why does the dihybrid Punnett square in this diagram has only one box?

Answer 6

• 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.

Question 7

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.

Answer 7

Question 8

Explanation for how to complete a Punnet square

Answer 8

• 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.

Question 9

Rules to keep in mind when completing dihybrid Punnett squares

Answer 9

• 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.

Question 10

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.

Answer 10

Question 11

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?

Answer 11

9:3:3:1

Question 12

Calculating the predicted genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes for the exam

Answer 12

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.

Question 13

If a parent has the genotype Aabb, what should be shown along the edge of the Punnett square?

Answer 13

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.

Question 14

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?

Answer 14

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.

Question 15

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?

Answer 15

187

Question 16

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?

Answer 16

Question 17

What is independent assortment?

Answer 17

• 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.

Question 18

What are linked genes?

Answer 18

• Genes found on the same chromosome
• They are physically connected since they occur along the same DNA molecule.

Question 19

What is the result of linked genes?

Answer 19

• 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.

Question 20

When can crossing over occur and how does this affect loci?

Answer 20

• 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.

Question 21

Diagram of linked genes are found on the same chromosome

Answer 21

Question 22

What will the ratio of observed offspring be when dihybrid crosses are performed using closely-linked genes?

Answer 22

• 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.

Question 23

What are recombinant combinations?

Answer 23

New allele combinations, created by crossing over, are called recombinant and are always less common than parental combinations.

Question 24

What are linked genes?

Answer 24

• Genes found on the same chromosome.
• The closer the loci of two linked genes, the less frequently they are separated by crossing over.

Question 25

What is a linkage group?

Answer 25

Several genes (two or more) on the same chromosome that do not show independent assortment are called a linkage group.

Question 26

What organism did Thomas Hunt Morgan use in his research?

Answer 26

He worked with fruit flies (Drosophila melanogaster) to demonstrate that genes are found on chromosomes and that they do not always follow Mendelian ratios.

Question 27

Why are fruit flies a useful model organism for studying genetics?

Answer 27

- 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.

Question 28

What methods did Morgan and his students use and what did they find?

Answer 28

- 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.

Question 29

Drawing of a female (left) and male (right) fruit fly (Drosophila melanogaster)

Answer 29

Question 30

What did Morgan observe and later discover?

Answer 30

- 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.

Question 31

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)

Answer 31

Question 32

What system did Morgan use for the notation of alleles? (+ info for exam)

Answer 32

- 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.

Question 33

What did Morgan do once he determined that some genes are located on the sex chromosome?

Answer 33

- 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.

Question 34

Thomas Hunt Morgan discovered non-Mendelian ratios using fruit flies ( Drosophila melanogaster ). What are these ratios due to?

Answer 34

- Sex linkage (any gene found on the sex chromosomes) - Autosomal linkage (two genes found on the same autosomal chromosome)

Question 35

Thomas Hunt Morgan crossed two red-eyed F1 generation flies to each other. The F2 generation included both red- and white-eyed flies. However, all the white-eyed flies were male. What is the most likely explanation for this result?

Answer 35

The gene involved is on the X chromosome.

Question 36

Genes that are linked together on the same chromosome often do not show ___

Answer 36

Mendelian ratios, depending on the distance between the loci.

Question 37

Effect of distance on crossing over

Answer 37

- When two genes are very close together, it is very rare that a chiasma will form between them; thus, they are almost always inherited together. - The greater the distance between two genes, the more likely that crossing over will occur between them.

Question 38

When genes are linked, a special ___ is used.

Answer 38

Notation

Question 39

Explain how, when genes are linked, a special notation is used.

Answer 39

- 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.

Question 40

Schematic representation of chromosomes to show that the genes are linked

Answer 40

- 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.

Question 41

Format for drawing linked genes for exam

Answer 41

- The format for linked genes shown above will be expected in examination papers. - If you are asked to demonstrate or solve a linkage question, you are expected to use this format.

Question 42

Diagram of crossing over and segregation of the alleles of linked genes

Answer 42

Question 43

In a linkage problem, there are ___ distinct possibilities for the genotype of a heterozygous parent

Answer 43

Two

Question 44

What are the two distinct possibilities for the genotype of a heterozygous parent in a linkage problem?

Answer 44

Question 45

How does crossing over happen?

Answer 45

It is random and can occur multiple times between loci, so alleles can be separated and rejoined

Question 46

When can linked genes show independent assortment?

Answer 46

If the distance between them is great.

Question 47

If two alleles are inherited together more than 50% of the time, it is always because ___

Answer 47

They were connected in the parental chromosomes.

Question 48

Which combinations are the most common in offspring

Answer 48

The two parental combinations are always the most common .

Question 49

What is the easiest way to test for linkage between genes?

Answer 49

- To use a test cross, where a heterozygous parent is crossed with a homozygous recessive parent. - Crossing over events in the heterozygous parent are visible in the offspring because the second parent has only recessive alleles. - Thus, the combination of dominant and recessive phenotypes in the offspring is always determined by alleles inherited from the heterozygous parent.

Question 50

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

Answer 50

Question 51

Diagram showing Recombinants in Drosophila , caused by crossing over between linked genes

Answer 51

Question 52

Explain this diagram

Answer 52

- 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%.

Question 53

What would it mean if the two genes (for body color and wing size) were not linked in the fruit flies

Answer 53

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

Question 54

Whenever non-Mendelian ratios are obtained in a dihybrid cross, the genes are ___

Answer 54

Likely to be linked

Question 55

The lower the recombinant frequency, the closer the ___

Answer 55

Linked genes are on the chromosome.

Question 56

Define recombinant

Answer 56

An organism that contains a different combination of alleles than either of its parents. (also called non-parental)

Question 57

How would you explain a test cross involving dihybrid flies in which parental-type offspring and small numbers of new combinations of parental phenotypes are produced?

Answer 57

The two genes are linked on the same chromosome and crossing over has occurred.

Question 58

A parent is heterozygous for two linked genes. How would the parent's genotype be represented using proper notation?

Answer 58

Question 59

Following a genetic cross involving two linked genes, 1902 offspring with parental combinations and 208 recombinants are obtained. Calculate the percentage of offspring which are recombinants and give your answer to three significant figures.

Answer 59

9.86 Percentage of recombinants = the number of recombinants divided by the total number of offspring, multiplied by 100 =(208/(208+1902))×100 =(208/2110)×100 =9.857819905 Answer (3 significant figures)=9.86

Question 60

What can genetic variation lead directly to?

Answer 60

Specific phenotypes

Question 61

Genetic variation can lead directly to specific phenotypes, as Mendel demonstrated using ___

Answer 61

Traits in pea plants

Question 62

Traits are often determined in more complex ways. The range of variation seen in many organisms cannot be fully explained by ___

Answer 62

Classical genetics

Question 63

What factors influence phenotype?

Answer 63

- More than one gene may affect a single trait ( polygenic traits ) - A single gene may affect multiple traits ( pleiotropic genes) - A gene may impact the expression of another gene ( epistatic genes) - A gene may impact the expression of another gene ( epistatic genes) - Some alleles are expressed with varying intensity ( expressivity of the allele) - Some genetically influenced traits also have important environmental influences

Question 64

Explain how more than one gene may affect a single trait (polygenic traits)

Answer 64

For example, human eye color is polygenic because it is determined largely by two genes with some additional influence from genes on at least a dozen loci

Question 65

Explain how a single gene may affect multiple traits (pleiotropic genes)

Answer 65

- For example, the CFTR gene codes for a transmembrane protein with effects in multiple organs. - When the cystic fibrosis allele is homozygous, the functions of the lungs, intestines, and pancreas are all affected.

Question 66

Explain how a gene may impact the expression of another gene (epistatic genes)

Answer 66

- For example, the TYR gene produces a polypeptide that is a necessary first step in creating the melanin that gives skin, hair, and eyes their many possible colors. - When the TYR gene is not functional, the individual will exhibit albinism (lack of pigment) no matter which alleles are present in the genes that normally determine the level of pigmentation.

Question 67

Explain how some alleles are not expressed in all individuals (penetrance of the allele)

Answer 67

For example, BRCA1 alleles have incomplete penetrance because even though they greatly increase the risk of breast cancer, some women with the allele never develop cancer.

Question 68

Explain how some alleles are expressed with varying intensity (expressivity of the allele)

Answer 68

For example, the polydactyly allele in cats (which causes extra toes) has varied expressivity because some cats with the allele have only one or two extra toes, while others have four extra toes per paw.

Question 69

Explain how some genetically influenced traits also have important environmental influences

Answer 69

- For example, some alleles of the MC1R gene cause freckling when the skin is exposed to the sun. - The development of freckles has both genetic components and environmental aspects related to the amount and intensity of sunlight the individual experiences.

Question 70

What is discrete variation?

Answer 70

- A trait with phenotypes that can be grouped into distinct categories shows discrete variation (or discontinuous variation) and is usually coded for by a single gene. - An example is the ABO blood group in humans.

Question 71

What is continuous variation?

Answer 71

- Traits that show continuous variation display an unbroken range of phenotypes in the population. - In other words, for any two phenotypes, there can be an intermediate between them. - Traits with continuous variation are often polygenic traits, with the many variations created by combining the effects of various alleles of multiple genes.

Question 72

The phenotypes of polygenic characteristics tend to show ___ variation.

Answer 72

Continuous

Question 73

Give some examples of continuous variation

Answer 73

- Some examples include height and skin color in humans, fur color in some mammals, and milk yield in cows. - For example, height is continuous because for people of any two different heights there could be an intermediate height between them. - All of these traits have been shown to be polygenic.

Question 74

Continuous variation is very ___.

Answer 74

Common

Question 75

Height in humans as an example of continuous variation

Answer 75

- Height is continuous because for people of any two different heights there could be an intermediate height between them. - All of these traits have been shown to be polygenic. - Hundreds of alleles at dozens of gene loci have been found that exert an influence on height in humans.

Question 76

Many traits with continuous variation have a ___ distribution when graphed.

Answer 76

Normal (bell-shaped)

Question 77

Graph showing an example of the distribution of height in a human population

Answer 77

Question 78

Analyzing graphs showing the distribution of characteristics in a population

Answer 78

- You may be shown these graphs and should be able to analyze them. - For example, data regarding human height from different regions could be analyzed in terms of mean height, the difference in mean heights, overlap between categories, and the range and standard deviation of height in these populations.

Question 79

Explain how height is influenced by the environment

Answer 79

- Height is also influenced by the environment, with some experts estimating that 60–80% of height variation is genetic and 20–40% is environmental. - One of the most important environmental influences is diet. - In the last century, an increase in the availability of both vitamins and overall calories has led to a marked increase in average height. - Another environmental factor that has increased average height is a reduction in childhood infections and disease. - Height has increased more in some regions than others.

Question 80

Maps showing distribution of height increase in females and males across the world

Answer 80

Question 81

Give an example of a polygenic trait where the influence of the environment has been confirmed

Answer 81

- Mouse fur color - In a recent publication in Nature, scientists were able to manipulate the inheritance of the fur color by changing the diet of pregnant mice.

Question 82

Picture of mice (environmental differences in the fur color of cloned mice)

Answer 82

- 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.

Question 83

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