Genes, Chromosomes, and Human Genetics
Summary Questions for Chapter 10 – Genes, Chromosomes, and Human Genetics
1. What are genes?
Definition: Genes are segments of DNA that code for proteins or functional RNA.
Structure: Composed of nucleotide sequences that determine hereditary traits.
2. How are they arranged in chromosomes?
Chromosome Structure: Genes are located on chromosomes, which are long strands of DNA wrapped around proteins called histones.
Arrangement: Genes are organized linearly along the chromosomes, and each chromosome contains many genes.
3. What is meant by linked genes?
Definition of Linked Genes: Linked genes are genes that are located close together on the same chromosome and tend to be inherited together during meiosis.
4. Are linked genes always inherited together? If not, why?
Not Always Inherited Together: Linked genes may not always be inherited together due to genetic recombination during meiosis.
Recombination Process: Crossing over can occur between homologous chromosomes, resulting in new combinations of alleles.
5. What are the symbols for genes in Drosophila?
Gene Symbols: In Drosophila melanogaster (fruit fly), genes are typically represented by a combination of one or more letters, with dominant alleles written in uppercase and recessive alleles in lowercase (e.g., "w" for white eyes, "W" for red eyes).
6. Which chromosomes participate in genetic recombination and when does this happen?
Chromosomes Involved: Genetic recombination primarily occurs between homologous chromosomes during prophase I of meiosis.
Timing: This process happens when chromosomes align and exchange segments of genetic material.
7. Describe Morgan's experiment with eye color and wing shape that provided evidence for gene linkage.
Morgan's Experiment: Thomas Hunt Morgan studied Drosophila and observed that certain traits, such as eye color and wing shape, were inherited together more frequently than expected if they assort independently.
Conclusion: This led to the conclusion that these traits are linked, residing on the same chromosome.
8. What is the expected phenotype ratio if genes are not linked?
Expected Ratio: If genes assort independently, the expected phenotypic ratio in offspring of a dihybrid cross is 9:3:3:1.
9. What is recombination frequency? How does distance between two genes on a chromosome affect this?
Recombination Frequency: The recombination frequency is the likelihood that two alleles will be separated during meiosis due to crossing over.
Distance Effect: The further apart two genes are on a chromosome, the higher the recombination frequency, as there is a greater chance of crossover occurring between them.
10. What does the recombination frequency mean when trying to map genes on a chromosome?
Gene Mapping: Recombination frequency can be used to create genetic maps, where distance in centimorgans (cM) indicates the likelihood of recombination occurring between specific genes i.e., 1% recombination frequency equals 1 cM.
11. Why do some genes on the same chromosome appear to assort independently?
Apparent Independent Assortment: This phenomenon can occur due to mechanisms like epistasis, where the expression of one gene masks the effect of another, or because of sufficient distance that increases recombination likelihood.
12. What are the sex chromosomes in human males and females?
Sex Chromosomes: Males have one X and one Y chromosome (XY), while females have two X chromosomes (XX).
13. What are autosomes?
Definition: Autosomes are chromosomes that are not involved in determining the sex of an organism; humans have 22 pairs of autosomes.
14. How are X and Y chromosomes paired during meiosis?
Pairing Mechanism: During meiosis, the X and Y chromosomes pair along homologous regions allowing recombination, despite being different in size and gene content.
15. Compare the X and Y chromosomes.
X Chromosome: Larger and carries more genes, involved in many functions including those unrelated to sex; around 1,100 genes identified.
Y Chromosome: Smaller and primarily contains genes related to male sex determination and spermatogenesis; contains about 50-200 functional genes.
16. Where is the SRY gene located and what is its function?
Location: The SRY (Sex-determining Region Y) gene is located on the Y chromosome.
Function: It triggers the development of male gonads and promotes male sex differentiation in early embryonic development.
17. Compare sex-linkage in males and females.
Males: Have only one X chromosome; thus, any allele on the X chromosome is fully expressed (hemizygous).
Females: Have two X chromosomes and can be carriers of recessive alleles; if one X has a dominant allele, that trait is expressed.
18. What is a pedigree and how can it be used?
Definition: A pedigree is a diagram that shows the occurrence of a genetic trait in several generations of a family.
Usage: It helps track heritable traits, understand inheritance patterns, identify carriers, and assess risks of genetic disorders.
19. How do we compensate for the increase in X-chromosome gene expression in females as compared to males?
Dosage Compensation: In females, one of the two X chromosomes becomes inactivated (Barr body), equalizing the gene dosage with males who have one X chromosome.
20. How does an X-chromosome become inactivated? What is it called when it is inactivated?
Process: An X chromosome undergoes a process called X-inactivation where one X is randomly selected to be transcriptionally silenced in early embryonic development.
Name: This phenomenon is referred to as Lyonization.
21. Which of the two X-chromosomes becomes inactive?
Random Inactivation: Either the paternal or maternal X chromosome can be inactivated, and this selection is random in each cell.
22. Explain how X-inactivation affects fur color in calico cats.
Calico Cats: X-inactivation leads to a mosaic pattern of fur coloration in calico cats, as different cells express different color traits (from separate X chromosomes containing differing alleles for fur color).
23. Describe the four ways in which chromosomes can alter their structure.
Alteration Mechanisms:
Deletion: Loss of a chromosome segment.
Duplication: A segment is copied, resulting in extra genetic material.
Inversion: A segment is reversed within the chromosome.
Translocation: A segment of one chromosome breaks off and attaches to another chromosome.
24. Explain the reason and mechanism behind chronic myelogenous leukemia.
Cause: Chronic myelogenous leukemia (CML) is often caused by a genetic translocation between chromosomes 9 and 22, creating the Philadelphia chromosome.
Mechanism: This results in the fusion of the BCR and ABL genes, leading to deregulated cell division and increased proliferation of myeloid cells.
25. Explain the results of nondisjunction during meiosis I versus meiosis II.
Meiosis I Nondisjunction: Results in gametes with an abnormal number of chromosomes (n+1 or n-1), leading to all resulting gametes being affected.
Meiosis II Nondisjunction: Results in two normal and two abnormal gametes (half normal and half aneuploidy).
26. What is the difference between euploids, aneuploids, and polyploids?
Euploids: Organisms with a complete set of chromosomes (normal chromosome number).
Aneuploid: Organisms with an abnormal number of chromosomes (not an exact multiple of the haploid number).
Polyploids: Organisms with more than two complete sets of chromosomes (e.g., triploid, tetraploid).
27. What is Down Syndrome?
Definition: Down Syndrome, also known as Trisomy 21, is a genetic disorder caused by the presence of an extra copy of chromosome 21.
Characteristics: Features include intellectual disability, characteristic facial features, and increased risk of congenital anomalies.
28. What is the difference between Turner’s syndrome, Klienfelter’s syndrome, and Triple X Syndrome?
Turner’s Syndrome: A condition where females have only one X chromosome (45,X), leading to underdeveloped female traits and infertility.
Klinefelter’s Syndrome: Males have an extra X chromosome (47,XXY), causing symptoms like reduced testosterone and infertility.
Triple X Syndrome: Females have three X chromosomes (47,XXX), may have taller stature and can lead to learning difficulties.
29. What are the differences between the three modes of inheritance in humans?
Autosomal Dominant: Only one copy of the dominant allele is needed for the phenotype to be expressed.
Autosomal Recessive: Two copies of the recessive allele are required for the phenotype to be expressed.
X-Linked Inheritance: Traits associated with genes on the X chromosome, with unique inheritance patterns in males and females.
30. What does it mean if someone is a carrier?
Carrier Definition: A carrier is an individual who has one copy of a recessive allele for a trait and does not exhibit the trait but can pass the allele to offspring.
31. What are the two main ways you can do genetic testing in an embryo or fetus?
Amniocentesis: A procedure where amniotic fluid is sampled under ultrasound guidance to analyze fetal cells for genetic abnormalities.
Chorionic Villus Sampling (CVS): A procedure that involves taking a sample of chorionic tissue from the placenta to analyze fetal chromosomes for genetic disorders.
32. Describe cytoplasmic inheritance and how it does not follow traditional Mendelian inheritance.
Cytoplasmic Inheritance: The transmission of genetic material (often mtDNA) from the cytoplasm of the egg rather than the nuclear DNA; traits are inherited only from the mother.
Non-Mendelian Nature: Traits do not follow Mendelian ratios due to maternal inheritance.
33. What is genomic imprinting?
Definition: Genomic imprinting is a genetic phenomenon by which certain genes are expressed in a manner dependent on whether they are inherited from the mother or father; specific alleles are silenced depending on their parent of origin.
34. What happens when genes are methylated?
Methylation Effect: Methylation of genes typically leads to gene silencing; the addition of methyl groups to DNA can reduce gene expression and ultimately impact development and environmental response.