4 Chromosomal Basis of Inheritance Flashcards
A geneticist wants to explain Mendel’s laws of segregation and independent assortment using modern genetic principles. How can the behavior of chromosomes during meiosis be used to explain these laws?
Mendel’s law of segregation states that allele pairs separate during gamete formation, which is explained by the separation of homologous chromosomes during meiosis I. His law of independent assortment states that genes for different traits are passed independently of each other, which is explained by the random alignment and separation of non-homologous chromosomes during meiosis I. This results in gametes with different combinations of chromosomes.
Describe Morgan’s experiments and explain how they provided evidence that genes are located on chromosomes.
Thomas Hunt Morgan used fruit flies (Drosophila melanogaster) to study inheritance patterns. He observed that certain traits (e.g., eye color) were inherited together more often than would be expected if genes were on separate chromosomes. This led to the conclusion that genes are located on chromosomes and are linked. His experiments with sex-linked traits, such as the white-eye mutation, further supported the chromosomal theory of inheritance.
How is the chromosomal basis of sex determined in humans, and what is sex-linkage?
In humans, sex is determined by the presence of sex chromosomes: females have two X chromosomes (XX), and males have one X and one Y chromosome (XY). Sex-linkage refers to genes located on the sex chromosomes, especially the X chromosome. Traits linked to the X chromosome show different inheritance patterns in males and females because males have only one X chromosome.
Explain X inactivation in female mammals and how it leads to genetic mosaicism.
X inactivation is a process in female mammals where one of the two X chromosomes in each cell is randomly inactivated to equalize gene dosage between males and females. This leads to genetic mosaicism, where different cells have different X chromosomes active, resulting in a mixture of phenotypes depending on which X chromosome is inactivated in each cell.
What is the difference between linkage and sex linkage?
Linkage refers to genes located close to each other on the same chromosome, which tend to be inherited together. Sex linkage specifically refers to genes located on the sex chromosomes, particularly the X chromosome, which show different inheritance patterns between males and females. While all sex-linked genes are linked (on the X chromosome), not all linked genes are sex-linked.
Define centimorgan and explain its relationship to recombinant frequency.
A centimorgan (cM) is a unit of genetic distance used in linkage maps. It represents a 1% chance of recombination between two genes during meiosis. The recombinant frequency is the percentage of offspring with recombinant phenotypes, which can be used to estimate the distance between genes on a chromosome in centimorgans.
How can linkage between genes be detected in a genetic cross?
Linkage can be detected by comparing the observed frequencies of different phenotypic combinations in offspring to the expected frequencies if genes were independently assorting. A higher frequency of parental types and a lower frequency of recombinant types than expected indicates linkage between the genes.
Given a genetic cross that results in 80 parental-type and 20 recombinant-type offspring, calculate the genetic distance between the linked genes.
The recombinant frequency is 20 / (80 + 20) = 20 / 100 = 0.20 or 20%. The genetic distance between the genes is 20 centimorgans (cM), as recombinant frequency is directly expressed as centimorgans.
Describe the process of constructing a linkage map using recombinant frequencies.
To construct a linkage map, you need to determine the recombinant frequencies between pairs of genes from genetic crosses. These frequencies are used to estimate the distances between genes in centimorgans. Genes are then arranged on a map based on these distances, with genes that are closer together having smaller distances and genes further apart having larger distances.
Describe the types of structural changes that can occur in chromosomes due to damage and their potential effects.
Structural changes in chromosomes can include:
Deletions: Loss of a chromosome segment, leading to loss of genes.
Duplications: Repetition of a chromosome segment, which can cause gene dosage imbalances.
Inversions: Reversal of a chromosome segment, which can disrupt gene function if breakpoints are within genes.
Translocations: Exchange of segments between non-homologous chromosomes, potentially leading to gene fusion or disruption.
Rings: Formation of a ring chromosome from a chromosome that has lost its ends and rejoined.
These changes can result in genetic disorders, cancers, and developmental abnormalities.
Describe two aneuploid human conditions and their karyotypes.
Down Syndrome (Trisomy 21): Characterized by an extra chromosome 21, resulting in a karyotype of 47 chromosomes with an additional 21st chromosome (47,XY,+21 or 47,XX,+21).
Turner Syndrome: Characterized by the absence of one X chromosome in females, resulting in a karyotype of 45 chromosomes with only one X chromosome (45,X).
Describe Cri du Chat syndrome and the chromosomal aberration involved.
Cri du Chat syndrome is caused by a deletion of a portion of the short arm of chromosome 5. The characteristic symptoms include a high-pitched cry resembling a cat’s meow, developmental delays, and distinct facial features. The karyotype for this syndrome includes a deletion in chromosome 5p (5p-).
Describe the Philadelphia chromosome and the disease associated with it.
The Philadelphia chromosome is a genetic abnormality caused by a translocation between chromosomes 9 and 22, specifically t(9;22)(q34;q11). This results in the fusion of the BCR and ABL genes, producing an oncogenic protein that promotes leukemia. The disease associated with this chromosome is chronic myeloid leukemia (CML).
