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Chapter 2 Part 2
Genetics- Exam 2- Fall 2026
| Question | Answer |
|---|---|
| What is the chromosome theory of inheritance? | The theory that chromosome transmission accounts for Mendelian patterns of inheritance. |
| Who independently proposed the chromosome theory of inheritance? | Theodor Boveri and Walter Sutton, in 1902–1903. |
| What is one principle of the chromosome theory? | Chromosomes contain the genetic material. |
| How are chromosomes transmitted across generations? | They are replicated and passed from parent to offspring. |
| What chromosome arrangement is found in nuclei of most eukaryotic cells? | Chromosomes are found in homologous pairs. |
| What happens to homologous chromosomes during meiosis? | Each homologue segregates into one of the two daughter nuclei. |
| What happens to different nonhomologous chromosome types during gamete formation? | They segregate independently. |
| How many chromosome sets does each parent contribute to offspring? | One set; the parental sets are functionally equivalent and each carries a full complement of genes. |
| How does meiosis explain Mendel's law of segregation? | Homologous chromosomes pair and then segregate, separating alleles into different gametes. |
| How does meiosis explain Mendel's law of independent assortment? | Different nonhomologous chromosomes align and segregate independently during meiosis. |
| What is the XY sex-determination system? | A system found in mammals, other animals, and some plants in which females are XX and males are XY. |
| What does homogametic mean in the XY system? | The sex has two of the same sex chromosomes and produces one sex-chromosome type of gamete; females are XX. |
| What does heterogametic mean in the XY system? | The sex has two different sex chromosomes; males are XY. |
| What is the ZW sex-determination system? | A system found in birds, butterflies, moths, and some fishes; females are ZW and males ZZ. |
| Which sex is heterogametic in the ZW system? | Females (ZW). |
| What is the XO sex-determination system? | A system found in many insects in which females are XX and males have a single X chromosome (XO). |
| What determines sex in Drosophila's X:A balance system? | The ratio of the number of X chromosomes to the number of sets of autosomes. |
| What role does the Y chromosome play in Drosophila, according to the slides? | It is essential for male fertility, not male development. |
| What does an X:A ratio of 1.00 produce in Drosophila? | A female. |
| What does an X:A ratio of 0.50 produce in Drosophila? | A male. |
| What does an X:A ratio between 0.50 and 1.00 produce in Drosophila? | An intersex fly. |
| What does an X:A ratio greater than 1.00 produce in Drosophila? | A metafemale. |
| What does an X:A ratio below 0.50 produce in Drosophila? | A metamale. |
| What is the haplo-diploid system in bees? | Males (drones) are haploid and develop from unfertilized eggs; females (workers and queens) are diploid and develop from fertilized eggs. |
| Who confirmed the chromosome theory through Drosophila experiments? | Thomas Hunt Morgan. |
| What mutation did Morgan observe in Drosophila around 1912? | A male with white eyes instead of the usual red eyes. |
| What did Morgan hypothesize about the white-eye trait? | It arose from a mutation converting a red-eye allele to a white-eye allele. |
| What did Morgan's F2 cross show about white-eyed females? | No white-eyed females appeared, supporting an X-linked location for eye-color alleles. |
| What were the F1 genotypes in Morgan's white-eye cross as given in the slides? | F1 male: Xw+Y; F1 female: Xw+Xw. |
| Why did the F2 red:white ratio differ from the predicted 3:1 ratio? | The slides suggest decreased survival of white-eyed flies. |
| What did Morgan's testcross produce? | Red-eyed females and males and white-eyed females and males in approximately equal numbers, consistent with X-linked inheritance. |
| What were the observed testcross counts shown? | 129 red-eyed females, 132 red-eyed males, 88 white-eyed females, and 86 white-eyed males. |
| What is a reciprocal cross? | Crosses between different strains in which the sexes are reversed. |
| Why are reciprocal crosses useful? | They help reveal whether a trait is carried on a sex chromosome or an autosome. |
| Why do X-linked traits differ in reciprocal crosses? | Males transmit X-linked genes only to daughters, while females transmit X-linked genes to all children. |
| What are sex-linked genes? | Genes found on one type of sex chromosome but not on both. |
| What are X-linked genes? | Genes located on the X chromosome. |
| What does hemizygous mean in the context of X-linked genes? | A male has only one copy of most X-linked genes, so dominance/recessiveness comparisons mainly apply in females. |
| What are holandric genes? | Genes found on the Y chromosome. |
| What are pseudoautosomal regions (PARs)? | Short homologous regions found at one end of the X and Y chromosomes. |
| What is the role of PARs in meiosis? | They promote the necessary pairing of X and Y chromosomes during meiosis I of spermatogenesis. |
| How are pseudoautosomal genes inherited? | They follow an autosome-like pattern of inheritance. |
| What is the primary sex ratio? | The proportion of males to females conceived in a population. |
| What is the secondary sex ratio? | The proportion of males to females born in a population. |
| Why is a 1:1 primary sex ratio expected in humans? | Males produce equal numbers of X- and Y-bearing sperm, which are described as equally viable and motile, and the egg surface is equally receptive. |
| What is dosage compensation? | A genetic mechanism that addresses differences in X-linked gene dosage between sexes with different numbers of X chromosomes. |
| What is a Barr body? | A darkly staining body, also called sex chromatin, observed in interphase cells and associated with an inactive X chromosome. |
| Who observed the Barr body in 1949? | Murray Barr, in interphase nerve cells of female cats but not male cats. |
| What did Mary Lyon propose in 1961? | In female mammalian somatic cells, one of the two X chromosomes becomes condensed and inactive early in embryonic life. |
| Is X inactivation random? | Yes. It occurs independently and randomly in each cell. |
| What happens to X inactivation through mitotic divisions? | Once chosen, the same X remains inactive through subsequent mitotic divisions within that cell lineage. |
| Are both X chromosomes active in normal female germline cells? | Yes, both X chromosomes are active in normal female germline cells. |
| How is the number of Barr bodies calculated? | Number of Barr bodies = number of X chromosomes − 1. |
| How can random X inactivation affect external phenotype? | It can create patches of different phenotypes, such as the calico coat pattern in cats. |
| Why can a heterozygous female mouse have a variegated coat? | Different cell lineages inactivate different X chromosomes carrying black- or white-coat alleles. |
| What happens if the X chromosome with the black allele is inactive in a cell lineage? | That lineage produces white fur. |
| What happens if the X chromosome with the white allele is inactive in a cell lineage? | That lineage produces black fur. |
| How does Caenorhabditis elegans achieve dosage compensation? | In XX hermaphrodites, genes on the X chromosomes are down-regulated by 50%. |
| How does Drosophila achieve dosage compensation? | In males, genes on the X chromosome are up-regulated two-fold. |
| What do mammalian cells do when more than one X chromosome is present? | They maintain one active X chromosome in somatic cells and convert additional X chromosomes into Barr bodies. |
| Which gene and region are identified in the slides as involved in mammalian X inactivation? | The Xic region and the Xist gene. |
Created by:
KristenR2025