# Determining the mode of inheritance of genotypes

In pings, coat color may be sandy, red, or white. A geneticist spent several years mating true-breeding pigs of all different color combinations, even going so far as to obtain true-breeding lines from different parts of the country. For crosses 1 and 4 below, she encountered a major problem: her computer crashed and she lost the F2 data. She nevertheless persevered, and using the limited data shown here, she was able to predict the mode of inheritance, the number of genes involved, and assign genotypes to each coat color. Based on the available data generated from the crosses shown, attempt to duplicate her analysis.

Cross P1 F1 F2

1 sandy X sandy all red data lost

2 red X sandy all red 3/4 red: 1/4 sandy

3 sandy X white all sandy 3/4 sandy: 1/4 white

4 white X red all red data lost

Once you have formulated a hypothesis to explain the mode of inheritance and assigned genotype to the respective coat colors, predict the outcomes of the F2 generations where the data were lost.

After considering the above problem, concentrate on the following one.

Labrador retrievers may be black, brown, or golden in color. While each color may breed true, many different outcomes occur if numerous litters are examined from a variety of matings, where the parents are not necessarily true-breeding. Shown here are just some of the many possibilities. Propose a mode of inheritance that is consistent with these data, and indicated the corresponding genotypes of the parents in each mating. Indicated as well the genotypes of dongs that breed true for each color.

A. black X brown ---> all black

B. black X brown ---> 1/2 black; 1/2 brown

C. Black X Brown ---> 3/4 black; 1/4 golden

D. Black X Golden ---> all black

E. Black X golden ---> 4/8 golden; 3/8 black; 1/8 brown

F. Black X golden ---> 2/4 golden ; 1/4 black; 1/4 brown

G Brown X Brown ---> 3/4 brown; 1/4 golden

H. Black X Black ---> 9/16 black; 4/16 golden; 3/16 brown

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Question 1

To start we must first establish the number of alleles and genes.

- 3 traits could mean one gene with 2 co-dominant alleles or 2 genes in a more complex relationships.

Co-dominance cannot explain the test crosses available so we will make a model that has two genes.

gene 1 (R,r) encodes for red such that

RR or Rr= red

rr= no color

gene 2 (S,s) encodes for sandy such that

Ss or Ss= sandy

ss= no color

Now we must look at the relationship between these two genes that contributes coat color.

-Since sandyx sandy= red and redx sandy or white = red ...

Problem Set

(See attached file for full problem description)

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1. In the following pedigree, assume no outsiders marrying in carry a disease allele.

A. What is the mode of inheritance of this disease? Circle one. (1 point)

Autosomal dominant; Autosomal recessive; X-linked dominant;

Y-linked; mitochondrial inheritance; X-linked recessive

B. Explain your choice in A. (Give two lines of reasoning.) [2 points]

C. Write the genotypes of the following individuals. (If more than one genotype is possible, write down all the possibilities.) [5 points]

#1_________ #2_________ #3_________ #4_________ # 5_________

D. What is the probability that the asterisked individual will be affected with the disease (4 points)

if male?______________ if female?______________

2. Imagine you are a genetic counselor, and a couple planning to start a family came to you for information. Charles, who is phenotypically normal, was married once before, and he and his first wife had a child who has cystic fibrosis. The brother of his current wife Elaine died of cystic fibrosis. What is the probability that Charles and Elaine will have a baby with cystic fibrosis? (Neither Charles nor Elaine has the disease) [2 points]

3. A man with achondroplasia, which is an autosomal dominant condition, marries a phenotypically normal woman. The man's mother did not suffer from achondroplasia.

A. What is the probability that none of their children will be affected with this disorder? (2 points)

B. What is the probability that all of them will be affected? (2 points)

C. What is the probability that their first child will be an affected female? (2 points)

D. What is the probability that their second child will be either an affected male or a normal female? (2 points)

E. If this couple have five children what is the probability that 4 will be normal and 3 will have achondroplasia? (3 points)

4. Two true breeding white-flowered lines of sweet peas (Lathyrus odoratus) when crossed produced purple-flowered F1's which when crossed produced 100 purple- and 72 white-flowered F2's. Show statistically that this could not be considered a random deviation from a 1:1 ratio. Use the chi-square table in your textbook. (5 points)

5. Phenylketonuria (PKU) is an inborn error of metabolism of the amino acid phenylalanine. The following pedigree is of an affected family:

A. What is the mode of inheritance of PKU? Explain. (4 points)

B. Which individuals in this pedigree are heterozygous for PKU? (5 points)

C. What is the probability that III-1 is heterozygous? (2 points)

D. If III-3 and III-4 marry:

(1) what is the probability that their first child will have PKU? (2 points)

(2) what is the probability that their second child with be a normal male? (2 points)

6. In Drosophila melanogaster the alleles Dp and dp determine long versus short wings, and E and e determine gray versus ebony body. Flies heterozygous at both loci were crossed and the following progeny were obtained:

Long wings, gray body 462

Long wings, ebony body 167

Short wings, gray body 127

Short wings, ebony body 44

A. Assume the two pairs of alleles undergo independent assortment and state the null hypothesis (Ho) you intend to test. (2 points)

B. Test your null hypothesis using chi-square analysis. (10 points)

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