Study Questions 10
Meiosis, Fertilization, and Mendelian Genetics1. What is the "goal" of meiosis?
The goal of meiosis is to produce daughter cells with one of each homologous pair of chromosomes: a reduction division changing a cell from 2N -> 1N.2. How are mitosis and meiosis similar? different?
Meiosis and mitosis are similar in that both go through the cell cycle, cell division, and cytokinesis. They are different in that mitosis results in cells that are exact copies of the parental cell (whether it is 1N or 2N) whereas meiosis must start with a diploid cell and results in haploid daughter cells.3. Be able to follow chromosomes through both mitosis and meiosis and label the stages.
4. If you mixed a substance that prevented proper functioning of Na+ channels into some sperm and eggs, what would the consequences on the fertilization reaction be? Would the egg get fertilized?
The consequence would likely be that the egg would get fertilized by more than one sperm. Na+ channels function in the fast block to polyspermy. Without these channels functioning, more than one sperm would likely enter the egg before the slow block was in place.
5. What would happen if you injected some Ca++ into an unfertilized sea urchin egg?
Injecting Ca++ ions into an unfertilized sea urchin egg would trigger the slow block to polypsermy: the formation of the fertilization membrane because Ca++ triggers the exocytosis of the cortical granules, which contain both solutes to draw in water and lift the vitelline membrane away from the egg and enzymes that harden the vitelline membrane into the fertilization membrane.
6. How do eggs floating around in the briny deep make sure that it is the sperm of the correct species that is fertilizing them?
Bindin receptors on the vitelline membrane must match the bindin protein contained in the acrosome of the sperm.
7. The cortical reaction functions directly in the (Circle all correct answers)
a. formation of a fertilization membraneb. fast block to polyspermy
c. release of hydrolytic enzymes from the sperm cell.
d. slow block to polyspermy
e. the fusion of the egg and sperm nuclei.
8. What do the following terms mean?
gene - a position on a chromosome that contains a DNA sequence responsible for producing a protein that performs a specific task.
allele - a nucleotide sequence that occupies the site of a gene.
genotype - the specific allele(s) present at a given gene for an individual
phenotype - the appearance of an individual
dominant - an allele whose expression is able to mask the effects of another allele
recessive - an allele whose expression is masked by the effects of another allele
homozygous - a gene or an individual that has two of the same kind of allele at a given gene
heterozygous - a gene or an individual that has two different alleles at a give gene
incomplete dominance - heterozygotes have a different phenotype from either homozygote; in this case, one allele produces a product and the other does not. The heterozygote ends up with half as much product as one homozygote, and this difference is phenotypically detectable.
co-dominance - heterozygotes have a different phenotype from either homozygote; in this case, both alleles produce products, but different products, and the phenotype is a combination of the two products.
multiple alleles - more than two possible alleles for a given gene. Any diploid individual can carry only two of these alleles at a time, but within a populations, many alleles can exist.
monohybrid cross - a cross that follows only one gene
dihybrid cross - a cross that follows two genes
linkage - the phenomenon that two genes can occur on the same chromosome, and therefore not assort independently
crossing over - the exchange of parts of chromosomes between homologs during prophase I of meiosis.
9. How is a gene different from an allele?
A gene is a position on a chromosome; an allele is the nucleotide sequence that occupies that position.10. What is an intercross? A testcross?
An intercross is a cross between two F1 offspring. A testcross is a cross between an individual of unknown genotype and a homozygous recessive. It is used to detect recessive alleles in the individual of unknown genotype.11. Be able to follow alleles through a monohybrid cross (from gamete generation in the parents to predicted genotype and phenotype ratios in the offspring).
12. How is a testcross valuable in genetic analysis?
A testcross can identify whether an individual with a dominant phenotype is a homozygote or a heterozygote.13. What is the expected phenotypic ratio from a cross of two monohybrids when the alleles show complete dominance? incomplete or codominance?
Aa x Aa --> ??
Complete dominance: 3 A- to 1 aa
Incomplete or codominance: 1 AA : 2 Aa : 1 aa14. What is the principle of independent assortment? Does it occur during mitosis or meiosis?
Independent assortment means that homologs are randomly assigned to daughter cells regardless of the assignment of any other chromosomes.
15. Assuming independent assortment and complete dominance, what is the expected phenotypic ratio from a cross of two dihybrids?
AaBb x AaBb --> 9 A-B- : 3 A-bb : 3 aaB- : 1 aabb (or 9/16 dominant for both traits, 3/16 each dominant for only one trait, and 1/16 recessive for both traits)
16. Assuming independent assortment and complete dominance, what is the expected phenotypic ratio from a testcross of a dihybrid?
AaBb x aabb --> 1 AaBb : 1 Aabb : 1 aaBb : 1 aabb (or 1/4 dominant for both traits, 1/4 each dominant for only one trait, and 1/4 recessive for both traits)
17. How would the phenotypic ratio in the previous question change if the two genes were completely linked?
If the two genes were completely linked, then the gametes would be all AB and ab or all Ab and aB. That would result in the offspring being of only two equal types: 1/2 dominant for both traits and 1/2 recessive for both OR 1/2 dominant for one and recessive for the other and 1/2 dominant for the other trait and recessive for the first.
18. Having dimples and having an extra finger are both dominant traits. If a dimpled woman (with five fingers) marries a six-fingered man (without dimples), and they have an undimpled, five-fingered daughter ...
a. What are the genotypes of the parents? Ddff x ddFf (D = dimpled; F = six-fingered)b. What proportion of their kids would be expected to have both dimples and six fingers?
To have both dimples and six fingers they would have to be D-F-. Since the only way they can be that way is to be DdFf, then (1/2 Dd)(1/2 Ff) = 1/4 of the kids
19. What do the following terms mean?
autosome - a chromosome not involved in the determination of gender.
X-linked gene - a gene carried on the X chromosome
Y-linked gene - a gene carried on the Y chromosome
20. What sex chromosomes are present in a human female? a human male?
A human female is XX; a human male is XY
21. Can X-linked traits show up in both males and females? In which sex are recessive X-linked traits more likely to show up? What about dominant X-linked traits?
Because both males and females have X chromosomes, both can show X-linked traits. Recessive X-linked traits are more likely to show up in males, however, because males have only one X and therefore, no chance for a dominant trait to mask the recessive. Dominant X-linked traits are equally likely to show up in males and females.
22. Can Y-linked traits show up in both males and females?
Y-linked traits only show up in males because only males have a Y chromosome. If a female were to have a Y chromosome, "she" would be male!