Download Through the selective cross-breeding of common pea plants (Pisum

Mendelian Genetics (pgs. 277-282)
Through the selective cross-breeding of common pea plants (Pisum sativum) over many generations, Mendel discovered that certain
traits show up in offspring without any blending of parent characteristics. For instance, the pea flowers are either purple or white-intermediate colors do not appear in the offspring of cross-pollinated pea plants. Mendel observed seven traits that are easily
recognized and apparently only occur in one of two forms. One of the traits he studied was the color of the peas.
In cross-pollinating plants that either produce yellow or green pea seeds exclusively (purebred or true breeding), Mendel found that
the first offspring generation (F1) always has yellow seeds. However, the following generation (F2) consistently has a 3:1 ratio of
yellow to green.
He came to three important conclusions from these experimental results:
1.
2.
3.
that the inheritance of each trait is determined by "units" or "factors" that are passed on to descendents unchanged (these units are
now called genes)
that an individual inherits one such unit from each parent for each trait
that a trait may not show up in an individual but can still be passed on to the next generation.
It is important to realize that, in this experiment, the starting parent
plants were homozygous for pea seed color. That is to say, they
each had two identical forms (or alleles) of the gene for this trait--2
yellows or 2 greens. The plants in the F1 generation were all
heterozygous. In other words, they each had inherited two
different alleles--one from each parent plant. It becomes clearer
when we look at the actual genetic makeup, or genotype, of the pea
plants instead of only the phenotype, or observable physical
characteristics.
Note that each of the F1 generation plants (shown left) inherited a Y
(yellow) allele from one parent and a y (green) allele from the
other. When the F1 plants breed, each has an equal chance of
passing on either Y or y alleles to each offspring.
With all of the seven pea plant traits that Mendel examined, one
form appeared dominant over the other; it masked the presence of
the other allele. For example, when the genotype for pea seed color
is Yy (heterozygous), the phenotype is yellow. However, the
dominant yellow allele does not alter the recessive green one in any
way. Both alleles can be passed on to the next generation.
A phenotypic ratio is a mathematical way of showing what fraction of the total offspring produced would be expected to have a certain
phenotype. A genotypic ration is a mathematical way of showing what fraction of the total offspring would be expected to have a
certain genotype.
Consider the following example: Yellow color is dominant over green color.
A heterozygous plant for yellow seeds is bred with a heterozygous plant with yellow seeds. What would be the phenotypic and
genotypic ratios for their offspring?
Heterozygous with yellow seeds = Yy
Heterozygous with yellow seeds = Yy
YY
Yy
Now plug these genotypes into a Punnett Square.
Fill in each box of the Punnett square by
transferring the letter above and in front of each
box into each appropriate box. As a general rule,
the capital letter goes first and a lowercase
letter follows.
Yy
yy
Phenotypic ratio:
3/4 yellow seeded plants : 1/4 green
seeded plants
Genotypic ratio:
1/4 YY : 2/4 Yy : 1/4 yy
1. flower color is purple (dominant, P) or white (recessive, p)
5. seed color is yellow (dominant, Y) or green (recessive, y)
2. flower position is axial (dominant, A) or terminal (recessive, a)
6. pod shape is inflated (dominant, I) or constricted (recessive, i)
3. stem length is long (dominant, L) or short (recessive, l)
7. pod color is green (dominant, G) or yellow (recessive, g)
4. seed shape is round (dominant, R) or wrinkled (recessive, r)
Use the following table above to figure out the phenotypic and genotypic ratios of the offspring. Show your work by filling out the
Punnett Square for each.
1. Cross an Aa axial plant with an Aa axial plant.
5. Cross a heterozygous round seed plant with a homozygous
recessive wrinkled seed plant.
Phenotypic ratio
Phenotypic ratio
Genotypic ratio
Genotypic ratio
2. Cross an II inflated plant with an Ii inflated plant.
6. Cross heterozygous purple flowered plant with another
heterozygote purple flowered plant.
Phenotypic ratio
Phenotypic ratio
Genotypic ratio
Genotypic ratio
3. Cross Gg green pod plant with gg yellow pod plant.
7. Cross a homozygous constricted pod plant with a
heterozygous plant for pod shape.
Phenotypic ratio
Phenotypic ratio
Genotypic ratio
Genotypic ratio
4. Cross a homozygous dominant long plant with a
homozygous recessive short plant.
8. Cross a heterozygous round seeded plant with another
heterozygous round seeded plant
Phenotypic ratio
Phenotypic ratio
Genotypic ratio
Genotypic ratio
9. In pea plants, spherical seeds (S) are dominant to dented
seeds (s). In a genetic cross of two plants that are
heterozygous for the seed shape trait, what fraction of the
offspring should have spherical seeds?
Answer:
13. Saguaro cacti are very tall cylindrical plants that usually
have two L-shaped arms, one on each side. Suppose you lived
in southern Arizona where the Saguaro cactus is common and
you happen to have one growing in your yard. Your Saguaro
has two arms but one is longer than the other. Now, assume
that arm length in these cacti are controlled by a single gene
with arms of the same length (A) being dominant to arms of
different lengths. What is the genotype of your cactus?
Genotype of your cactus = ___________________
10. A genetic cross between two F1-hybrid pea plants having
yellow seeds will yield what percent green-seeded plants in
the F2 generation? Yellow seeds are dominant to green.
14. Suppose you cross your cactus with that of your neighbor
which has arms of the same length. Your great grandchildren
(it takes a Saguaro cactus a long time to mature) find that half
of the resulting offspring have arms the same length and half
have arms of different lengths. What was the genotype of your
neighbor's cactus?
Genotype of neighbor’s cactus = ___________________
Answer:
11. In dogs, wire hair (S) is dominant to smooth (s). In a cross
of a homozygous wire-haired dog with a smooth-haired dog,
what will be the phenotype and genotype of the F1 generation?
15. The common grackle is a species of robin-sized
blackbirds that are fairly common (hence the name) over most
of the United States. Suppose that long tails (L) were
dominant to short tails in these birds. A female short-tailed
grackle mates with a male long-tailed grackle who had one
parent with a long tail and one parent with a short tail. What is
the male's genotype?
Answer:
Phenotype
Genotype
12. Woodrats are medium sized rodents with lots of
interesting behaviors. You may know of them as packrats.
Let's assume that the trait of bringing home shiny objects (H)
is controlled by a single locus gene and is dominant to the trait
of carrying home only dull objects (h). Suppose two
heterozygous individuals are crossed. How many of each
genotype would be expected if only 4 offspring were
produced?
# of offspring
with each
genotype
16. The ability to curl your tongue up on the sides (T, tongue
rolling) is dominant to not being able to roll your tongue. A
woman who can roll her tongue marries a man who cannot.
Their first child has his father's phenotype. What are the
genotypes of the mother, father, and child?
Mother’s genotype
Father’s genotype
Child’s genotype
Dihybrid Cross
Punnett squares are used to predict the probability of certain genotypes and phenotypes. We have been looking at monohybrid crosses
in which we are dealing with one trait, for example, height in pea plant (tall or short). What happens in a Punnett square when two
traits are considered?
Dihybrid crosses deal with two traits at the same time, for example, height and seed color (tall or short and yellow or green).
Use the following information to complete problems #1 and #2. Tall (T) is dominant over short (t) for height and yellow pea seed
color (Y) is dominant over green seed color (y). Remember it takes two alleles for every trait.
1. Cross a homozygous tall and yellow seeded pea plant (TTYY) with a homozygous short and green seeded pea plant (ttyy).
What is the genotypic ratio?
What is the phenotypic ratio?
2. Cross a heterozygous tall and yellow seeded plant (TtYy) with a heterozygous tall and yellow seed plant (TtYy)
What is the genotypic ratio?
What is the phenotypic ratio?