Chapter 4 vocab Flashcards
Sex chromosomes
A pair of chromosomes (e.g., X and Y in mammals) that determines sex in a species.
Heterogametic sex
In species with two types of sex chromosomes, the heterogametic sex produces two types of gametes. For example, in mammals, the male is the heterogametic sex, because a sperm can contain either an X or a Y chromosome.
Homogametic sex
In species with two types of sex chromosomes, the homogametic sex produces only one type of gamete. For example, in mammals, the female is the homogametic sex, because an egg can only contain an X chromosome.
autosomes
Chromosomes that are not sex chromosomes,humans have 22 of these
Haplodiploid system
A mechanism of sex determination found in some species, such as bees, in which one sex is haploid (i.e., male) and the other sex is diploid (i.e., female).
dioecious
Refers to a species of plants in which some individuals produce only male gametophytes and others produce only female gametophytes.
Dosage compensation
the mechanism that keeps levels of expression of X chromosome genes similar in both sexes
Example: Drosophila
XY(male)- increase X expression, increased expression in heterogametic
XX-(female)-lower X expression, decreased expression in homogametic
X-chromosome inactivation (XCI)
A process in which mammals equalize the expression of X-linked genes by randomly turning off one X chromosome in the somatic cells of females.
- In early embryonic development, one of the two X chromosomes is randomly inactivated in each somatic cell and becomes a Barr body
- Most of the genes on that X chromosomes cannot be expressed
- Each embryonic cell divides and the pattern of XCI is maintained in subsequent cell divisions
Barr body
A structure in the interphase nuclei of somatic cells of FEMALE mammals that is a highly condensed X chromosome.
-an inactivated X!!!
- only exists in females, XX must be present
- evidence for the Lyon hypothesis
Lyon hypothesis
A hypothesis to explain the pattern of X-chromosome inactivation seen in mammals. Initially, both X chromosomes are active. However, at an early stage of embryonic development, one of the two X chromosomes is randomly inactivated in each somatic cell of a female, then maintained.
-female mammals often exhibit variegation (calico kitty)
X- Inactivation center
A site on the X chromosome that appears to play a critical role in X-chromosome inactivation.
Sex-linked gene
A gene that is located on one of the sex chromosomes.
(NOT BOTH)
X-linked genes (alleles)
Genes (or alleles of genes) that are physically located on the X chromosome.
X-linked inheritance
An inheritance pattern in certain species that involves genes that are located only on the X chromosome.
Hemizygous
Describes the single copy of an X-linked gene in a male. A male mammal is said to be hemizygous for X-linked genes.
testcross
An experimental cross between a recessive individual and an individual whose genotype the experimenter wishes to determine.
X-linked recessive inheritance pattern
An inheritance pattern for an allele or trait in which the gene is found on the X chromosome and the allele is recessive relative to a corresponding dominant allele.
Y-linked
genes on the Y chromosome
What significant gene is in the Y chromosome?
- a gene, sry, which encodes sequence information to produce a
transcription factor protein (SRY) that causes the embryo to become a male.
If an an individual with an XY karyotype possesses a LOF mutation in the sry genomic locus, what is the phenotype and why?
Describe the mechanism of Dosage Compensation Among Placental mammals
One of the X chromosomes in the somatic cells of females is inactivated. In certain species, the paternal X chromosome is inactivated, and in other species, such as humans, either the maternal or paternal X chromosome is randomly inactivated throughout the somatic cells of females(mosaic-like).
Describe the process of X-inactivation
-Initiation – X-chromosome is selected for inactivation
-Spreading- Xist expressed on chromosome to be inactivated; Xist transcripts coat chromosome; Proteins recruited to chromosome to condense it
- Maintenance: through mitosis and beyond
X-inactivation center and Xist
Mechanism of counting relies on a region of the X-chromosome called the X-inactivation center (Xic):
- Contains the X-inactive specific transcript (Xist) gene
- Xist is active on the condensed chromosome in Barr body
- Xist gene product is an RNA molecule; coats the X chromosome and inactivates it
- After coating, proteins associate with Xist and promote chromosomal compaction
