WORKSHEET 6.4-6.6 Section 6.4 – Traits, Genes and Alleles 1

WORKSHEET
6.4-6.6
Section 6.4 – Traits, Genes and Alleles
1. What is the relationship between a gene and a protein?
Genes code for proteins
2. What is an allele?
Any of the alternative forms of a gene that may occur at a specific locus
3. What term describes a pair of alleles that are the same? What about when they are different?
Homozygous; heterozygous
4. Write a definition of homologous chromosomes using the terms “gene” and “allele.”
Homologous chromosomes are two chromosomes, one from the mother and one from the father, that have the same
length, overall appearance, and genes, although the alleles may differ.
In the space below, draw a pair of homologous chromosomes. Label the chromosomes with two sets of genes, one with
homozygous alleles (Gene A, Gene A) and one with heterozygous alleles (Gene B, Gene b).
Genes influence the development of traits.
5. Write an analogy to show the difference between genotype and phenotype.
Genotype is the underlying genetics of an organism, which could be compared to someone’s thoughts that you can’t
read. Phenotype is the observable traits, which could be compared to someone’s words that tell you what they’re
thinking about.
6. How are alleles represented on paper?
as letters, uppercase for dominant alleles and lowercase for recessive alleles
7. Fill in the table below with the missing genotype, phenotype (dominant or recessive), or alleles (TT, Tt, tt).
Genotype
Phenotype
Alleles
homozygous dominant
dominant
TT
homozygous recessive
recessive
tt
heterozygous
dominant
Tt
8. If an organism has a recessive trait, can you determine its genotype for that trait?
Yes, it has to be homozygous recessive.
9. What factors besides alleles affect phenotype?
Environment, such as nutrients and sunshine
10. What type of alleles are present in an organism with a QQ genotype?
Homozygous dominant
11. What is an alternative form of a gene?
Allele
12. What is the opposite of the term” homozygous”? What about the term “dominant”?
Heterozygous; recessive
13. Alleles may be represented using letters. Uppercase letters represent dominant alleles.
Lowercase letters represent recessive alleles. Use the letters B and b to represent the following genotypes:
heterozygous, homozygous recessive, homozygous dominant.
Bb; bb; BB
Section 6.5 – Traits and Probability
Punnett squares illustrate genetic crosses.
Identify what each of the numbered parts represents in the Punnett square below. Then draw lines from each of the
parents’ alleles to the corresponding alleles in the offspring.
4. Why does each parent contribute only one allele to the offspring?
Because the alleles segregated during gamete formation (meiosis) when the homologous chromosomes separated.
5. You know a ratio is a comparison that tells how two or more things relate. What is a genotypic ratio? a phenotypic
ratio?
A comparison that tells the proportion of offspring that have a particular genotype; a comparison that tells the
proportion of offspring that have a particular phenotype.
6. What is the genotypic ratio of the offspring in Figure 6.15 on pg 184 of the textbook?
1:2:1
7. What is the phenotypic ratio of the offspring in Figure 6.15 pg 184 of the textbook?
3:1
8. What is a dihybrid cross?
A cross that examines the inheritance of two different traits
9. Look at figure 6.17 on pg 186 of the textbook and explain why each parent organism in the F1 generation has four
alleles listed.
Figure 6.17 represents a dihybrid cross. Each parent organism has two alleles for both traits, which makes a total of
four alleles.
10. Suppose an organism had the genotype AABb. What two types of gametes could result from this allele combination?
AB, Ab
11. What is the phenotypic ratio that results from a dihybrid cross between two organisms that are heterozygous for
both traits? See Figure 6.17 on pg 186 of the textbook for help.
9:3:3:1
12. Probability predicts the __________________ number of occurrences, not the ________________ number of
occurrences.
Average, exact
13. To calculate the probability that two independent events will happen together, ______________ the probability of
each individual event.
multiply
14. In Figure 6.18, the probability of getting one coin that is heads up and one coin that is tails up is
____________________.
One-fourth (1/4)
15. What is a testcross?
A testcross is a cross between an organism with the recessive phenotype and an organism with an unknown
genotype.
16. What is independent in the law of independent assortment?
Allele pairs are independent. They separate independently of each other during gamete formation (meiosis).
17. Use the Punnett square below and fill out the data for the cross between organisms that have the genotypes “Aa”
and “aa” to answer the following questions.
18. Is this a monohybrid cross or a dihybrid cross?
Monohybrid
19. What is the genotypic ratio of the offspring?
1:1 for heterozygous: homozygous recessive
20. What is the phenotypic ratio of the offspring?
1:1 for dominant: recessive
Section 6.6 – Meiosis and Genetic Variation
1. What factors contribute to genetic diversity?
Independent assortment of chromosomes, random fertilization, crossing over
2. What is crossing over?
The exchange of chromosome segments between homologous chromosomes
3. If two genes are located close together on the same chromosome, are they likely to follow Mendel’s law of
independent assortment? Explain.
No. The two genes are unlikely to be separated by crossing over, so they will be inherited together.
4. Which does sexual reproduction create; new alleles or new combinations of alleles?
New combinations of alleles
5. How is the production of unique genetic combinations an advantage to organisms and species?
Unique genetic combinations result in organisms with unique phenotypes, which increases the likelihood that some,
will survive under changing conditions.
6. Are chromosomes in a duplicated or an unduplicated state when crossing over occurs?
Duplicated
Use sketches to illustrate how crossing over contributes to genetic diversity. Use Figure 6.20 on pg 190 of the
textbook for reference.
Refer to your cell sketch in the last box on the previous page. Also refer to Figure 6.5 on pg 174-175 of the textbook if
necessary. 1. In the first box below, show what your cell would look like at the end of meiosis I. Remember, the result
will be two cells that have one duplicated chromosome from each homologous pair. 2. In the second box, show what
your cell would look like at the end of meiosis II. Remember, the result will be four cells that have one (unduplicated)
chromosome from each homologous pair.
7. If genes A and B are located on separate, nonhomologous chromosomes, will they follow Mendel’s law of
independent assortment? Explain.
Yes. The chromosomes carrying those genes will line up randomly and separate randomly during meiosis.
8. If genes A and B are located at opposite ends on the same chromosome, are they likely to follow Mendel’s law of
independent assortment? Explain.
Yes. The genes will be far enough from each other that crossing over is very likely to occur between them.
No. The genes are likely to be linked and to travel together during meiosis.
9. The exchange of chromosome segments between homologous chromosomes is called
Crossing over
10. The tendency for two genes that are located close together on a chromosome to be inherited together is called.
Genetic linkeage