11-1
1. Dominant: form of an allele whose trait always shows up if it is present; recessive: form of an allele whose trait shows up only when the dominant allele is not present
2. Separation of paired alleles; the alleles are separated during the formation of gametes, with the result that each gamete carries only a single allele from the original pair.
3. Factors that are passed from one generation to the next
4. Mendel cut away the male parts of one flower, and then dusted it with pollen from another flower.
5. Only one-fourth of the possible gamete combinations did not have a dominant allele.
6. True-breeding pea plants have two identical alleles for a gene, so in a genetic cross each parent contributed only one form of a gene, making inheritance patterns more detectable.
11-2
1. The way in which the alleles segregate is random, and probability allows the calculation of the likelihood that a particular allele combination will occur in offspring.
2. To predict and compare the genetic variations that will result from a cross
3. The likelihood that a particular event will occur
4. Genotype: actual alleles present for a trait, or genetic makeup; phenotype: visible expression of the alleles, or physical characteristics
5. 50 percent; Punnett square (using A instead of T):
1. During gamete formation, pairs of alleles for different traits segregate, or separate, independently of each other.
2. Answers include descriptions for any two:
incomplete dominance, codominance, multiple alleles, or polygenic traits.
3. In incomplete dominance, two alleles combine their effects to produce a single in-between phenotype, such as pink flowers from red and white parents. In codominance, each allele is expressed separately, as when erminette chickens have both black and white feathers.
4. They are small, easy to keep in the laboratory, and produce large numbers of offspring in a short period of time.
5. The offspring in the second cross will show greater variation because 100 percent of the offspring from the first cross (CC x Cc) will be full color.
11-4
1. Four haploid cells genetically different from one another and from the original cell
2. Mitosis produces two genetically identical diploid cells; meiosis produces four genetically different haploid cells.
3. Diploid: two sets of chromosomes; haploid: one set of chromosomes
4. Homologous chromosomes pair up and form tetrads, which may exchange portions of chromatids, resulting in the exchange of alleles between the homologous chromosomes.
5. Both sperm and egg cells have 23 chromosomes because they are gametes, which are haploid cells. A white blood cell has 46 chromosomes because it is a diploid body cell.
11-5
1. It is the chromosomes that assort independently, not individual genes.
2. A gene map shows the relative locations of genes on a chromosome. The frequency of crossing-over between genes is used to produce a map of distances between genes.
3. The farther apart two genes are, the more likely they are to be separated during a crossover in meiosis. Therefore, the frequency of crossing-over is equal to the distance between two genes.
4. The two genes are located very far apart from each other.
SA 11
Reviewing Content
1.c 5.c 9.d
2.a 6.d 10.b
3.a 7.d
4.c 8.d
Understanding Concepts
11. (1) The inheritance of biological characteristics is determined by genes. (2) Where there are two or more forms (alleles) of the gene for a single trait, some forms of the gene may be dominant and others recessive. (3) In most sexually reproducing organisms, each adult has two copies of each gene, one from each parent. These genes are segregated when gametes form. (4) The alleles for different genes (actually, the chromosomes) usually segregate independently.
12. Probability is the likelihood that an event will occur. This principle can be used to predict the outcomes of genetic crosses.
13.1 YY: 2 Yy: 1 yy
Yy
Y YY Yy
y Yy yy
14. A gene has multiple alleles if it has more than two alleles. Two or more genes control polygenic traits.
15. With two alleles for a trait, up to three phenotypes are possible. With three alleles, up to six phenotypes are possible.
16. No, genes provide a plan for development, but how the plan unfolds depends on the environment.
17. Four
18. Homozygous black coat: BB; heterozygous black coat: Bb
19. Melosis is a process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes.
20. DNA replicates during interphase so that during meiosis I, all of the chromosomes are doubled and consist of duplicate chromosomes (sister chromatids). At anaphase I, the homologous chromosomes separate, with the sister chromatids still together, as two haploid daughter cells form. During meiosis II, the sister chromatids separate to produce four haploid daughter cells.
21. It is the chromosomes that are separated during gamete formation. The genes are linked to the chromosomes.
Critical Thinking
22. By crossing the white ram to a number of black ewes; if any offspring are black, then the white ram
is heterozygous
23.
| | Number of cells produced | Type of cell | Chromosome number |
| Mitosis | 2 | Body | Diploid (2N) |
| Meiosis | 4 | gamete | Haploid (N) |
24. Both parents are heterozygous.
25. The predicted outcome of the cross is 50% rough and 50% smooth. However, since the result of each fertilization (joining of egg and sperm) is independent of any previous fertilization, it is possible for all offspring to have smooth coats.
26. The original genotypes and the crosses could have been Tt x tt or Tt x Tt. The genotype TT could not have been present; if it were, all the offspring would be tall.
27. There would be less genetic variation in the F2 generation between two closely linked genes because these genes will not be separated during the chromosomal movements of meiosis and the chances of crossing-over events separating the genes are slim.
28. Gene M
29. The allele for black color is dominant, and the allele for brown color is recessive. The black parent is homozygous, and the brown parent is heterozygous.
30. The color helps the ptarmigan hide from predators. In winter, its white coat color blends in with its snowy surroundings. In summer, its brown coat blends in with the bare ground and grass.