Chapter 4: The Chromosome Theory of Inheritance Learning Objectives

Question 1

o Describe the key chromosome behaviors during mitosis

Answer 1

 Cell division that preserves chromosome number

 During, sister chromatids separate and two daughter nuclei form

Question 2

o Describe the key events of meiosis that explain Mendel’s first and second laws

Answer 2

 Law of Segregation: two alleles from two parents will be separated from each other during meiosis; in meiosis II, two copies of each chromosome will be separated from each other causing two distinct alleles located on those chromosomes to segregate from one another
 Law of Independent Assortment: the way allele pairs gets segregated into two daughter cells during meiosis II has no effect on how any other allele pair gets segregated/traits inherited through one gene will be inherited independently of traits inherited through another gene because genes reside on different chromosomes

Question 3

o Distinguish between the sex chromosome complements of human female and male germ-line cells at different stages of gametogenesis

Answer 3

 Human females are born with oocytes arrested in prophase I of meiosis I; meiosis resumes at ovulation but is not completed until fertilization
 Human males, spermatogenesis begins at puberty and continues through lifetime of human males

Question 4

o Predict the sex of humans with different complements of X and Y chromosomes

Answer 4

 In humans, males sex determination is triggered by a Y linked (SYR) female sex determination occurs in XX embryos by default

Question 5

o Compare chromosome behaviors during mitosis and meiosis

Answer 5

 Mitosis:
• Occurs in somatic cells
• Haploid and diploid cells can undergo mitosis
• One round of division
• Mitosis is preceded by S phase (chromosome duplication)
• Homologous chromosomes don’t pair
• Genetic exchange between homologous chromosome is very rare
• Sister chromatids attach to spindle fibers from opposite poles during metaphase
• Centromere splits at beginning of anaphase
• Produces two daughter cells
 Meiosis
• Occurs in germ cells as part of the sexual cycle
• Two rounds of division, meiosis I and II
• Only diploid cells undergo meiosis
• Chromosomes duplicate prior to meiosis I but not before meiosis II
• During prophase of meiosis I, homologous chromosomes pair (synapse) along their length
• Crossing-over occurs between homologous chromosomes during prophase of meiosis I
• Homologous chromosomes (not sister chromatids) attach to spindle fibers from opposite poles during metaphase I
• Centromere don’t split during meiosis I
• Sister chromatids attach to spindle fibers from opposite poles during metaphase II
• Centromeres split at beginning of anaphase II
• Produces 4 haploid cells

Question 6

o Differentiate among somatic cells, gametes, and zygotes with regard to the number and origin of their chromosomes

Answer 6

 Gametes: haploid/single set of chromosome present in egg and sperm cells of animals (n)
 Zygotes: diploid/two matching sets of chromosomes (2n); most body cells are diploid

Question 7

o Distinguish between homologous and nonhomologous chromosomes

Answer 7

 Homologous chromosome/homologs: chromosomes that match in size, shape, and bandings/containing same linear gene sequence (each derived from one parent)
 Nonhomologous: carry unrelated sets of genetic info (appear in different colors)

Question 8

o Describe the basis of sex reversal in humans

Answer 8

 Phenomenon where males are XX or females are XY

 Males have part of Y included in SRY gene on one of their X chromosomes; females lack SRY in their Y chromosome

Question 9

o Describe the key chromosome behaviors during meiosis that lead to haploid gametes

Answer 9

 Chromosomes replicate once, nuclei divide twice
 Meiosis I: parent nucleus divides to form two daughter nuclei; previously replicated homologous chromosomes segregate to different daughter cells
• Prophase I: homologs condense and pair and crossing-over occurs
• Metaphase I: paired homologs attach to spindle fibers from opposite poles
• Anaphase I: homologs move to opposite spindle poles
• Telophase 1: nuclear envelope reforms
 Meiosis II: both daughter nuclei resulting from meiosis I divide to produce 4 nuclei/sister chromatids separate from each other so that gametes only have one copy of each chromosome (because chromosome don’t duplicate at start of meiosis II, these 4 nuclei are haploid)
• Prophase II: chromosomes condense
• Metaphase II: chromosomes align at metaphase plate
• Anaphase II: sister chromatids move to opposite spindle poles
• Telophase II: nuclear membrane reform and cytokinesis follows

Question 10

o Explain how the independent alignment of homologs, and also crossing-over during the first meiotic division, each contribute to the genetic diversity of gametes

Answer 10

 During metaphase of meiosis I, homologous chromosomes connect to opposite spindle poles; independent alignment of each pair of homologs ensure independent assortment of genes carried on different chromosomes

Question 11

o Diagram the forces and structures that dictate chromosomal movement during mitosis

Answer 11

 Prophase: chromosomes condense and become visible, centromeres move apart towards opposite poles and generate new microtubules
 Prometaphase: nuclear envelope breaks down, microtubules from centromeres invade nucleus, sister chromatids attach to microtubules from centrosomes
 Metaphase: chromosomes align on metaphase plate with sister chromatids facing opposite poles
 Anaphase: centromeres divide, now separated sister chromatids move to opposite poles
 Telophase: nuclear membrane and nucleoli re-form, spindle fibers disappear, chromosomes uncoil and become a tangle of chromatin
 Cytokinesis: cytoplasm divide/splitting elongated parent cell into two daughter cells

Question 12

o Compare the processes of oogenesis and spermatogenesis in humans

Answer 12

 Oogenesis: formation of female gametes (eggs); human females are born with oocytes arrested in prophase of meiosis I; meiosis resumes at ovulation but is not completed until fertilization
 Spermatogenesis: production of sperm; begins at puberty and continues through lifetime of human males

Question 13

o Predict phenotypes associated with nondisjunction of sex chromosomes

Answer 13

 Too many chromosomes in a nucleus or not enough

 Down syndrome

Question 14

o Distinguish between sister chromatids and nonsister chromatids

Answer 14

 Sister chromatids are two identical copies of a chromatid having the same genes and alleles whereas in case of non-sister chromatid, one strand is inherited from its mother while the other one is inherited from its father. Because of this, sister chromatids are called identical whereas non-sister chromatids are called non-identical
 Sister chromatids are produced during S phase of the interphase period whereas non-sister chromatids are produced during metaphase period of meiotic cellular divisions
 Sister chromatids are used in cell division like cell replacement whereas non-sister chromatids are used in reproductive divisions with the production of gametes

Question 15

o Compare the means of sex determination in different organisms

Answer 15

 Mechanisms of sex determination vary; in some sex is determined by environmental factors rather than by specific chromosomes

Question 16

o Infer from the results of crosses whether or not a trait is sex-linked

Answer 16

 X-Linked Recessive Trait:
• Trait appears in more males than females (females must receive two copies of rare allele to display phenotype; males only need one copy of allele)
• Mutation will never pass from father to son (sons receive only Y chromosome from fathers)
• Affected male passes X linked mutation to all his daughters (who are carriers)
• Trait often skips generation as mutation passes from grandfather through carrier daughter to grandson
• Trait can appear in successive generations when a sister of an affected male is carrier
• With rare affected (homozygous) female, all her sons will be affected and all her daughter will be carriers
 X-Linked Dominant Trait
• More females that males show trait
• Trait is seen in every generation
• All daughters but none of the sons of affected male will be affected
• One-half the sons and one-half the daughters of affected female will be affected
• For incompletely dominant X-linked traits, carrier females may show trait in less extreme form than males with defected allele
 Y-Linked Trait
• Trait is seen only in males
• All male descendants of affected man will exhibit trait
• Not only do females no exhibit trait, can’t transmit it

Question 17

o Explain how human cells compensate for the X-linked gene dosage difference in XX and XY nuclei

Answer 17

 Random inactivation of either the maternal or paternal X chromosome in XX cells ensures that male and female mammalian cells express equivalent amounts of the proteins encoded by most X-linked genes