Genetics for Biology: Dihybrid, Codominant, and Intermediate Dominant Traits

Need help with understanding basic punnett squares, including monohybrid crosses. Start with this site;

For a basic review: http://users.adelphia.net/~lubehawk/BioHELP!/mendel.htm

Dihybrid Cross Problems:

In the following 3 problems, assume that in guinea pigs black and short-haired are dominant over both brown and long-haired pigs (B-black; b – brown, S- short; s- long)

1.  George is a pig that is homozygous for black and short-hair. He is crossed with Linda, who is brown and longhaired. State the genotypes of both parents.

2.  If pigs from the F-1 generation of the parent pigs in question 1 are crossed with each other, what are the phenotypes of the F-2 generation, expressed as a ratio.

3.  Give the genotype and phenotypic ratios of the offspring resulting from a cross of the F-1 in te above problem with its brown, long-haired parent.

4. In man, assume that brown eyes are dominant over blue eyes, and right-handedness is dominant over left-handedness. What are the possible genotypes and phenotypes of the F-1 offspring when both parents are hybrids for both of the above traits (hybrid means heterozygous).

Answers for the following problems at:

http://www.ksu.edu/biology/pob/genetics/dihy.htm

1. About 70% of Americans perceive a bitter taste from the chemical phenylthiocarbamide (PTC). The ability to taste this chemical results from a dominant allele (T) and not being able to taste PTC is the result of having two recessive alleles (t). Albinism is also a single locus trait with normal pigment being dominant (A) and the lack of pigment being recessive (a). A normally pigmented woman who cannot taste PTC has a father who is an albino taster. She marries a homozygous, normally pigmented man who is a taster but who has a mother that does not taste PTC. What are the genotypes of the possible children (choose all that apply)?

2. Wolves are sometimes observed to have black coats and blue eyes. Assume that these traits are controlled by single locus genes and are located on different chromosomes. Assume further that normal coat color (N) is dominant to black (n) and brown eyes (B) are dominant to blue (b). Suppose the alpha male and alpha female of a pack (these are the dominant individuals who do most of the breeding) are black with blue eyes and normal colored with brown eyes, respectively. The female is also heterozygous for both traits. How many of the offspring (assume 16) living in the pack will have each of the following genotypes?

3. In the breeding season, male Anole lizards court females by bobbing their heads up and down while displaying a colorful throat patch. Assume for this question that both males and females bob their heads and have throat patches. Assume also, that both traits are controlled by single locus genes on separate chromosomes. Now, suppose that anoles prefer to mate with lizards who bob their heads fast (F) and have red throat patches (R) and that these two alleles are dominant to their counterparts, slow bobbing and yellow throats. A male lizard heterozygous for head bobbing and homozygous dominant for the red throat patch mates with a female that is also heterozygous for head bobbing but is homozygous recessive for yellow throat patches.

A. How many of the F1 offspring have the preferred fast bobbing / red throat phenotype (assume 16 young)?

B. What percentage of the offspring will lack mates because they have both slow head bobbing and yellow throats?

C. What percentage of the offspring will have trouble finding mates because because they lack one of the dominant traits?

Intermediate inheritance (also called incomplete dominance)

  1. In cats, long tail displays intermediate with no tail. (TLTN is the heterozygote which has a short tail).
  2. Felix, long tailed cat, is crossed with Alice, a cat with no tail. What are the expected and phenotypic ratios of the first litters.
  3. Alex and Samantha are bred several times. The resulting litters yield a total of 30 cats. Of this total, 16 have short tails. What are the most likely genotypes and phenotypes of the Alex and Samantha.

The following problems are from a KSU Internet site. Go to this site to find the answers, and to find hints for working out the problems:

http://www.ksu.edu/biology/pob/genetics/incom.htm

  1. In northeast Kansas there is a creature know as a wildcat. It comes in three colors, blue, red, and purple. This trait is controlled by a single locus gene with incomplete dominance. A homozygous (BB) individual is blue, a homozygous (bb) individual is red, and a heterozygous (Bb) individual is purple. What would be the genotypes and phenotypes of the offspring if a blue wildcat were crossed with a red one?
  2. The lubber grasshopper is a very large grasshopper, and is black with red and yellow stripes. Assume that red stripes are expressed from the homozygous RR genotype, yellow stripes from the homozygous rr genotype, and both from the heterozygous genotype. What will be the phenotypic ratio of the F1 generation resulting from a cross of two grasshoppers, both with red and yellow stripes (red : both : yellow)?
  3. Suppose you have two rose plants, both with pink flowers. You cross the two plants and are surprised to find that, while most of the offspring are pink, some are red and some are white. You decide that you like the red flowers and would like to make more. What cross would you perform to produce the most red flowered plants?
  4. Incomplete dominance and a dihybrid cross:

Racoons have rings around their tails and a habit of washing their food in water before eating it. Suppose that both of these traits are controlled via incomplete dominance so that wide bands on the tail are BB, medium sized bands are Bb, and narrow bands are bb and that washing all their food is WW, washing some of their food is Ww, and washing no food is ww. How many of each genotype will be in the F1 generation resulting from a cross of two racoons, both with medium sized tail bands and that wash some of their food (assume 16)?

Codominance and Multiple Alleles – blood types

For the answers to the following problems go to:

http://users.adelphia.net/~lubehawk/BioHELP!/multalle.htm

1. A woman with Type O blood and a man who is Type AB have are expecting a child. What are the possible blood types of the kid?

2. What are the possible blood types of a child who's parents are both heterozygous for "B" blood type?

3. What are the chances of a woman with Type AB and a man with Type A having a child with Type O?

4. Determine the possible genotypes & phenotypes with respect to blood type for a couple who's blood types are homozygous A & heterozygous B.

5. Jill is blood Type O. She has two older brothers (who tease her like crazy) with blood types A & B. What are the genotypes of her parents with respect to this trait?

6. A test was done to determine the biological father of a child. The child's blood Type is A and the mother's is B. Dude #1 has a blood type of O, & dude #2 has blood type AB. Which dude is the biological father?

Codominance – Some additional problems:

http://www.ksu.edu/biology/pob/genetics/incom.htm

1 The lubber grasshopper is a very large grasshopper, and is black with red and yellow stripes. Assume that red stripes are expressed from the homozygous RR genotype, yellow stripes from the homozygous rr genotype, and both from the heterozygous genotype. What will be the phenotypic ratio of the F1 generation resulting from a cross of two grasshoppers, both with red and yellow stripes (red : both : yellow)?

Go to this site for a great tutorial and the answers to the following questions:

http://users.adelphia.net/~lubehawk/BioHELP!/inccodom.htm

A very common phenotype used in questions about codominance is roan fur in cattle. Cattle can be red (RR = all red hairs), white (WW = all white hairs), or roan (RW = red & white hairs together). A good example of codominance.


1. Predict the phenotypic ratios of offspring when a homozygous white cow is crossed with a roan bull.


2. What should the genotypes & phenotypes for parent cattle be if a farmer wanted only cattle with red fur?

Another example of codominance is the black, tan, and tabby colors of cats:


3. A cross between a black cat & a tan cat produces a tabby pattern (black & tan fur together).
a) What pattern of inheritance does this illustrate?
b) What percent of kittens would have tan fur if a tabby cat is crossed with a black cat?