Table of contents

1. Introduction to Genetics51m
2. Mendel's Laws of Inheritance3h 37m
3. Extensions to Mendelian Inheritance2h 41m
4. Genetic Mapping and Linkage2h 28m
5. Genetics of Bacteria and Viruses1h 21m
6. Chromosomal Variation1h 48m
7. DNA and Chromosome Structure56m
8. DNA Replication1h 10m
9. Mitosis and Meiosis1h 34m
10. Transcription1h 0m
11. Translation58m
12. Gene Regulation in Prokaryotes1h 19m
13. Gene Regulation in Eukaryotes44m
14. Genetic Control of Development44m
15. Genomes and Genomics1h 50m
16. Transposable Elements47m
17. Mutation, Repair, and Recombination1h 6m
18. Molecular Genetic Tools19m
- Genetic Cloning
  9m
- Methods for Analyzing DNA
  9m
19. Cancer Genetics29m
- Overview of Cancer
  21m
- Cancer Mutations
  7m
20. Quantitative Genetics1h 26m
21. Population Genetics50m
- Hardy Weinberg
  19m
- Allelic Frequency Changes
  31m
22. Evolutionary Genetics29m

3. Extensions to Mendelian Inheritance

Sex Chromosome

Genetics

3. Extensions to Mendelian Inheritance

Sex Chromosome

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Multiple Choice

Which of the follow sex chromosomes can be describes as homogametic?

XX/XY

ZZ/ZW

XX/XO

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Understand the concept of homogametic and heterogametic sex determination systems. In homogametic systems, the individual has two of the same type of sex chromosome, while in heterogametic systems, the individual has two different types of sex chromosomes.

Identify the sex determination system in humans and many other mammals, which is the XX/XY system. In this system, females are XX (homogametic) and males are XY (heterogametic).

Examine the ZZ/ZW system, which is found in birds, some reptiles, and some fish. In this system, males are ZZ (homogametic) and females are ZW (heterogametic).

Consider the XX/XO system, which is found in some insects like grasshoppers. In this system, females are XX (homogametic) and males are XO (heterogametic, where 'O' represents the absence of a second sex chromosome).

Conclude that in the given options, XX in the XX/XY system and XX in the XX/XO system are both examples of homogametic sex chromosomes.

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Open Question

In C. elegans there are two sexes: hermaphrodite and male. Sex is determined by the ratio of X chromosomes to haploid sets of autosomes (X/A). An X/A ratio of 1.0 produces a hermaphrodite (XX), and an X/A ratio of 0.5 results in a male (XO). In the 1970s, Jonathan Hodgkin and Sydney Brenner carried out genetic screens to identify mutations in three genes that result in either XX males (tra-1, tra-2) or XO hermaphrodites (her-1). Double-mutant strains were constructed to assess for epistatic interactions between the genes (see table). Propose a genetic model of how the her and tra genes control sex determination.

Genotypeᵃ XX Phenotype XO Phenotype
Wild-type Hermaphrodite Male
tra-1ʳᵉᶜ Male Male
tra-2ʳᵉᶜ Male Male
her-1ʳᵉᶜ Hermaphrodite Hermaphrodite
tra-1ᵈᵒᵐ/+ Hermaphrodite Hermaphrodite
tra-1ʳᵉᶜ tra-2ʳᵉᶜ Male Male
tra-1ʳᵉᶜ her-1ʳᵉᶜ Male Male
tra-2ʳᵉᶜ her-1ʳᵉᶜ Male Male
tra-2ʳᵉᶜ tra-1ᵈᵒᵐ/+ Hermaphrodite Hermaphrodite
ᵃrec = recessive mutation; dom = dominant mutation.

Open Question

A wild-type Drosophila male and female are crossed, producing 324 female progeny and 161 male progeny. All their progeny are wild type.

Propose a genetic hypothesis to explain these data.

Open Question

A wild-type Drosophila male and female are crossed, producing 324 female progeny and 161 male progeny. All their progeny are wild type.

Design an experiment that will test your hypothesis, using the wild-type progeny identified above. Describe the results you expect if your hypothesis is true.

Open Question

In chickens, a key gene involved in sex determination has recently been identified. Called DMRT1, it is located on the Z chromosome and is absent on the W chromosome. Like SRY in humans, it is male determining. Unlike SRY in humans, however, female chickens (ZW) have a single copy while males (ZZ) have two copies of the gene. Nevertheless, it is transcribed only in the developing testis. Working in the laboratory of Andrew Sinclair (a co-discoverer of the human SRY gene), Craig Smith and colleagues were able to 'knock down' expression of DMRT1 in ZZ embryos using RNA interference techniques (see Chapter 18). In such cases, the developing gonads look more like ovaries than testes [Nature 461: 267 (2009)]. What conclusions can you draw about the role that the DMRT1 gene plays in chickens in contrast to the role the SRY gene plays in humans?

Multiple Choice

In the ZZ/ZW sex determination system, the male is…?

Multiple Choice

In humans, which region on the Y chromosome determines maleness?

Multiple Choice

During meiosis the XY sex chromosomes segregate independently. Which of the following represents the gametes chromosomes after meiosis?

Open Question

How do we know that X chromosomal inactivation of either the paternal or maternal homolog is a random event during early development in mammalian females?

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