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What are sex-linked genes?
genes located on the sex chromosomes (X and Y), meaning their inheritance patterns differ between males and females due to the different combinations of sex chromosomes they carry
How did T.H. Morgan discovered sex-linked genes?
discovered sex-linked genes by observing a mutant male fruit fly with white eyes, which he then bred with normal red-eyed females; he noticed that all the female offspring had red eyes, while only the male offspring inherited the white-eye trait, leading him to conclude that the gene for eye color was located on the X chromosome
Use proper notation for wild-type and mutants in Drosophila (fruit flies)
in genetic notation, you can use a superscript plus sign (+) over the mutant letter code
For example, if the mutant "ebony" has a darker body than the wild-type fly, the notation for the wild-type fly would be "e+"
Explain the importance of the X chromosome in humans
one of the two sex chromosomes, determining an individual's gender, and also carries numerous genes responsible for various biological functions beyond sex development, including brain function, coagulation, and color vision, making it vital for overall health
About how many genes are located on the X-chromosome?
approximately 900 genes
What are some genes found on the X-chromosome?
genes related to color vision (causing color blindness), the DMD gene responsible for muscular dystrophy, genes associated with cognitive function like JARID1C and PHF8, and genes involved in immune response like TLR7
Why are males more susceptible to color blindness and hemophilia?
because the genes responsible for these conditions are located on the X chromosome, and males only have one X chromosome, meaning if they inherit the defective gene, they will express the condition, while females, with two X chromosomes, can carry the gene without showing symptoms unless they inherit a defective copy on both X chromosomes.
Solve genetics problems involving X linked genes, and be sure to use the proper nomenclature
Identify the Genes: Determine which traits are X-linked (e.g., color blindness, hemophilia).
Use Proper Nomenclature:
For males (XY), use the format: X^A (dominant) or X^a (recessive).
For females (XX), use: X^A X^A (homozygous dominant), X^A X^a (heterozygous), or X^a X^a (homozygous recessive).
Punnett Squares: Create Punnett squares to predict offspring genotypes and phenotypes.
Calculate Ratios: Analyze the results to determine the expected ratios of traits in offspring.
Example: If a color-blind male (X^aY) mates with a carrier female (X^AX^a), the Punnett square will show the potential offspring genotypes.
bout how many genes are located on the Y chromosome?
70 to 200
Explain the importance of the Y chromosome in humans
it primarily determines male sex development by carrying the SRY gene, which triggers the development of male reproductive organs, essentially making the presence of a Y chromosome the key factor in defining a biological male; beyond sex determination, the Y chromosome also contains genes important for male fertility and may play a role in certain aspects of male health
Solve genetics problems involving color blindness, hemophilia, and Duschenne muscular dystrophy
Color Blindness:
X-linked recessive trait.
If a carrier female (X^C X^c) has a child with a normal male (X^C Y), the probability of a color-blind son (X^c Y) is 50%.
Hemophilia:
Also X-linked recessive.
A carrier female (X^H X^h) and a normal male (X^H Y) have a 25% chance of having a hemophiliac son (X^h Y).
Duchenne Muscular Dystrophy:
X-linked recessive.
A carrier female (X^D X^d) and a normal male (X^D Y) have a 25% chance of having an affected son (X^d Y).
Color Blindness: 50% chance for sons.
Hemophilia: 25% chance for sons.
Duchenne: 25% chance for sons.
Explain X-inactivation and how it leads to Calico color in cats.
X-inactivation is a process where one of the two X chromosomes in female mammals is randomly deactivated in each cell during early embryonic development, resulting in a mosaic of cells expressing either one of the X chromosome's alleles; in calico cats, this means that the gene for fur color, located on the X chromosome, is expressed differently in different patches of fur, leading to the distinctive orange and black patches on a white background, almost exclusively seen in female cats because they have two X chromosomes to inactivate
What is a Barr body?
a condensed, inactivated X chromosome found in female mammals
Explain T.H. Morgan’s use of test cross to discover gene linkage between black body and vestigial wing
by breeding a heterozygous fly (carrying both the dominant and recessive alleles for both traits) with a homozygous recessive fly, observing that the offspring did not show the expected Mendelian ratio of phenotypic variation, indicating that the genes for these traits were located close together on the same chromosome and were therefore linked, not assorting independently
Are crossovers more likely to occur between two loci that are close together or two loci that are far apart?
are more likely to occur between two loci that are far apart on a chromosome
Nondisjunction
the failure of chromosomes or sister chromatids to separate properly during cell division (mitosis or meiosis), resulting in daughter cells with an abnormal number of chromosomes
Aneuploidy
a condition where a cell or organism has an abnormal number of chromosomes, either missing or extra
Monosomic
having one chromosome instead of the usual pair in a cell
Trisomic
having an extra copy of a specific chromosome in a cell, resulting in three copies of that chromosome instead of the usual two
Polyploidy
the condition where an organism possesses more than two complete sets of chromosomes in its cells
Triploid
a cell or organism that has three complete sets of chromosomes
Tetraploid
an organism or cell that has four sets of chromosomes
Octaploid
an organism has eight sets of chromosomes, or eight times the basic haploid chromosome number
Explain how nondisjunction can occur during meosis
when homologous chromosomes fail to separate properly during anaphase I, or when sister chromatids fail to separate during anaphase II, resulting in daughter cells with an abnormal number of chromosomes (aneuploidy) due to one cell receiving an extra chromosome and the other lacking one; this can occur due to issues with the spindle fibers that pull chromosomes apart, improper attachment of the spindle fibers to the centromeres, or problems with the cohesion proteins holding sister chromatids together
Describe some examples of aneuploidy in humans
in humans refers to an abnormal number of chromosomes in a cell, and common examples include Down syndrome (trisomy 21), where an individual has an extra copy of chromosome 21, Turner syndrome (monosomy X) with a missing X chromosome, Klinefelter syndrome (47, XXY) with an extra X chromosome, and conditions like trisomy 18 (Edwards syndrome) and trisomy 13 (Patau syndrome) which involve extra copies of chromosomes 18 and 13 respectively
Deletion
a genetic mutation that occurs when a segment of DNA is missing
Duplication
a genetic mutation where a segment of DNA is copied, resulting in the presence of multiple identical copies of that DNA sequence within a chromosome, essentially creating extra genetic material
Inversion
a chromosomal defect where a segment of a chromosome breaks off and reattaches in the opposite direction
Translocation
the process where a piece of a chromosome breaks off and attaches to a different chromosome
In gene imprinting, what chemical modification occurs and how does gene imprinting affect expression?
where methyl groups are added to specific DNA sequences, which can silence the gene depending on whether the methylated allele is inherited from the mother or father, resulting in only one parental copy of the gene being expressed
About how many genes are imprinted in humans and what are some examples?
approximately 150 imprinted genes, with some examples including IGF2 (insulin-like growth factor 2), H19, and genes involved in disorders like Prader-Willi syndrome and Angelman syndrome; these genes are expressed differently depending on whether they are inherited from the mother or father.
Does it usually occur on sex chromosomes or autosomes
typically occurs on autosomes (non-sex chromosomes) rather than sex chromosomes, although there is some evidence that imprinting can happen on the X chromosome in certain cases; the majority of identified imprinted genes are found on autosomes
Would both alleles for a particular
gene be imprinted?
No, in gene imprinting, only one allele of a particular gene will be imprinted, meaning only one copy (either from the mother or father) will be expressed while the other copy is silenced; both alleles are not typically imprinted for a single gene
What happens to these chemical modifications for gamete formation in the subsequent generation?
During gamete formation in the subsequent generation, the chemical modifications established by gene imprinting are "erased" and then "reset" based on the sex of the developing gamete, meaning the parental imprints are removed and new ones are established specific to the new offspring's sex, ensuring proper allele-specific gene expression in the next generation.
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