Download Name - cloudfront.net

Name: _____________________
Mendelian Genetics
Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned
experiments. Read the chapter on Mendel and the Gene Idea and complete the chapter outline.
1. Explain why peas in nature can’t be used to study genetics. What is the likely reason Mendel chose to
work with pea plants and what did he do to combat the problem presented in the answer to part one of
this question?
2. Explain how you would determine if a plant is true breeding.
4. Describe the results of the experiment on
the left. Use the following words in your
description: Crossing, hybridization, P
generation, F1 generation.
5. Predict what results would be obtained from
mating two F1 plants from the experiment.
6. Mendel postulated that alternative versions
of genes account for variations in inherited
characters. With our current knowledge this
postulate can be even more specific. Rewrite Mendel’s concept using the words,
gene, locus, DNA, etc.
7. Describe the law of segregation in your own
words.
8. What is the difference between genotype
and phenotype? Do you think natural
selection would act on genotype of
phenotype? Explain why.
Review how to do Punnett squares (use your
textbook or find an online tutorial). You will
need to know what heterozygous and
homozygous mean. Complete the following problems. Show your work:
9. One cat carries heterozygous, long-haired traits (Ss), and its mate carries homozygous short-haired
traits (ss). Use a Punnett square to determine the probability of one of their offspring having long hair.
10. One cat carries heterozygous, long-haired traits (Ss), and its mate carries homozygous short-haired
traits (ss). Use a Punnett square to determine the probability of one of their offspring having short
hair.
11. Explain how a test cross is done and what it’s function is.
12. Define the following: Monohybrid cross and dihybrid cross.
13. Describe the law of independent assortment.
14. Pea plants heterozygous for flower position and stem length (AaTt) are allowed to self-pollinate, and
400 of the resulting seeds are planted. Draw a Punnett square for this cross. How many offspring
would be predicted to have terminal flowers (A - axial; a - terminal) and be dwarf(T - tall; t recessive)?
15. List all gametes that could be made by crossing two pea plants heterozygous for seed color, seed
shape, and pod shape (YyRrIi). Yellow is dominant to green, round is dominant to wrinkly, and inflated
is dominant to constricted. How large a Punnett square would you need to draw to predict the
offspring of a self-pollination of this “trihybrid”?
16. Explain the multiplication rule and addition rules applied to monohybrid crosses and explain when you
would use each.
17. Three characters (flower color, seed color, and pod shape) are considered in a cross between two pea
plants (PpYyIi X ppYyii). What fraction of offspring are predicted to be homozygous recessive for
at least two of the three traits? Show your work.
The inheritance of characters determined by a single gene deviates from simple Mendelian patterns
when alleles are not completely dominant or recessive, when a particular gene has more than two alleles,
or when a single gene produces multiple phenotypes.
18. Contrast complete dominance, incomplete dominance, and co-dominance.
The Relationship Between Dominance and Phenotype
We’ve now seen that the relative effects of two alleles range from complete dominance of one allele,
through incomplete dominance of either allele, to codominance of both alleles. It is important to
understand that an allele is called dominant because it is seen in the phenotype, not because it somehow
subdues a recessive allele. Alleles are simply variations in a gene’s nucleotide sequence. When a
dominant allele coexists with a recessive allele in a heterozygote, they do not actually interact at all. it
is in the pathway from genotype to phenotype that dominance and recessiveness come into play.
To illustrate the relationship between dominance and phenotype, we can use one of the characters
Mendel studied - round versus wrinkled pea seed shape. The dominant allele (round) codes for an enzyme
that helps convert an unbranched form of starch into a branched form in the seed. The recessive allele
(wrinkled) codes for a defective form of this enzyme, leading to an accumulation of unbranched starch,
which causes excess water to enter the seed by osmosis. Later, when the seed dries, it wrinkles. If a
dominant allele is present, no excess water enters the seed and it does not wrinkle when it dries. One
dominant allele results in enough of the enzyme to synthesize adequate amounts of branched starch,
which means that dominant homozygotes and heterozygotes have the same phenotype: round seeds.
A closer look at the relationship between dominance and phenotype reveals an intriguing fact: For any
character, the observed dominant/recessive relationship of alleles depends on the level at which we
examine the phenotype. Tay-Sachs disease, an inherited disorder in humans, provides an example. The
brain cells of a child with Tay-Sachs disease cannot metabolize certain lipids because a crucial enzyme
does not work properly. As these lipids accumulate in brain cells, the child begins to suffer seizures,
blindness, and degeneration of motor and mental performance and dies within a few years.
Only children who inherit two copies of the Tay-Sachs allele have the disease. Thus, at the organismal
level, the Tay-Sachs allele qualifies as recessive. However, the activity level of the lipid metabolizing
enzyme in heterozygotes is intermediate between that in individuals homozygous for the normal allele
and that in individuals with Tay-Sachs disease. The intermediate phenotype observed at the biochemical
level is characteristic of incomplete dominance of either allele. Fortunately, the heterozygote condition
does not lead to disease symptoms, apparently because half the normal enzyme activity is sufficient to
prevent lipid accumulation in the brain. Extending our analysis to yet another level, we find that
heterozygous individuals produce equal numbers of normal and dysfunctional enzyme molecules. thus, at
the molecular level, the normal allele and the Tay-Sachs allele are codominant. As you can see, whether
alleles appear to be completely dominant, incompletely dominant, or codominant depends on the level at
which the phenotype is analyzed.
19. When we learned about dominant and recessive genes in regular biology we greatly simplified what it
means for a gene to be dominant. You probably learned that a dominant gene masks or hides a recessive
gene. From the reading above you see that this is not really true. Succinctly explain how dominant and
recessive genes work using the pea seed shape and Tay-Sachs allele to support your explanation.
20. Explain multiple alleles using the example of blood type above.
21. If a man with type AB blood marries a woman with type O blood, what blood types would you expect
in theeir children? What fraction would you expect of each type. Show your work.
22. Define pleiotropy.
23. Explain how epistasis governs gene expression. Use the Labrador retriever example in your
explanation.
24. Define polygenic inheritance.
24. What does it mean when geneticists say that characters are multifactorial?
25. A rooster with gray feathers and a hen of the same phenotype produce 15 gray, 6 black, and 8 white
chicks. What is the simplest explanation for these colors in chickens? What phenotypes would you
expect in the offspring of a cross between a gray rooster and a black hen?
26. Albinism is inherited as an autosomal recessive. In the figure below, assuming that persons from the
general population are not heterozygous for albinism (unless they need to be for the gene to be passed
on), what are the genotypes of all persons whose genotypes are known? (i.e., indicate the genotypes on
the figure for all known AA, Aa, and aa individuals)
27. What does it mean to be a carrier?
28. Would individuals with sickle cell anemia be more affected if they lived at high or low altitudes?
Explain. Why is the allele frequency for the trait so high among African Americans? In other words,
what advantage is conferred by having the gene that would cause it to remain in the population and not
get weeded out by natural selection.
29. Are you homozygous dominant, heterozygous, or homozygous recessive for achnodroplasia? How do
you know for sure?
Mendelian genes have specific loci (positions) along chromosomes, and it is the chromosomes that
undergo segregation and independent assortment. You will read about this in the Chromosomal Basis of
Inheritance chapter of your textbook. Answer the following reading guide questions as you proceed.
1. Contrast wild type and mutant phenotypes.
2. Examine the figure above. Propose an explanation for why no females in the F2 generation have white
eyes.
3. Describe the patterns of inheritance in sex-linked traits.
4. Why aren't there many y-liked genes?
5. Neither Tim nor Rhonda has Duchenne muscular dystrophy (an X-linked trait), but their firstborn son
does have it. What is the probability that a second child of this couple will have the disease? What is
the probability if the second child is a boy? A girl?
6. Do males, females, or both have Barr bodies? Explain what a Barr body is.
7. Explain what a linked gene is (not the same as a sex-linked).
8. Look at the figure below. Find a similar one in your textbook. Why are the predicted ratios
different for genes on the same and different chromosomes. Look at the actual results. What
conclusion can be drawn about the location of the gene?
9. What is a recombinant?
10. What process of genetic variation accounts for the recombination of linked genes?
11. Describe a linkage map.
12. What do the following terms mean? Nondisjunction, aneuploiidy, monosomic, trisomic, polyploidy,
inversion, and translocation.