Passage
The sex chromosomes X and Y are involved in mammalian sex determination. Organisms with two wild-type (WT) X chromosomes develop as females (XX) , but those with a combination of WT X and Y chromosomes develop as males (XY) (Table 1) . Both sexes also inherit a set of autosomes (nonsex chromosomes) . Humans inherit 22 pairs of autosomes (44 total) . In contrast, rabbits inherit 21 pairs of autosomes (42 total) .Table 1 Chromosome Types Inherited by a Male and Female Rabbit (Note: The Y chromosome has fewer alleles than the X chromosome.)
Specifically, the SRY gene, located on the mammalian Y chromosome, is crucial for male sex determination and induction of testis development. Studies in mammals have shown that regardless of an organism's combination of sex chromosomes, fetuses will always develop as female in the absence of the SRY protein. In addition, one of the X chromosomes becomes inactivated in WT,XX females to prevent them from having double the amount of X-chromosome gene products as WT,XY males.Sp1 is a transcription factor with demonstrated regulatory control of SRY gene expression, and XY individuals with mutations affecting the Sp1 binding site on the SRY gene have been shown to develop female sex characteristics. Unlike SRY, the gene for Sp1 is located on an autosome.Researchers knocked out (KO) Sp1 binding sites on the SRY gene in rabbit embryos (Sp1 KO,XY rabbits) and observed their sexual development following implantation in surrogate mothers. The relationship between the SRY gene and sexual development can be explained by the following experimental observations: Most Sp1 KO,XY rabbits had female genitalia and displayed normal mating behaviors.
Sp1 KO,XY rabbits had significantly smaller ovaries and fewer follicles than WT,XX rabbits.
No significant differences were observed in the cervix and uterus between Sp1 KO,XY and WT,XX rabbits.
There was significantly reduced SRY expression in Sp1 KO,XY rabbits when compared to WT,XY males.
WT,XY male rabbits had significantly higher testosterone levels when compared to Sp1 KO,XY rabbits.
Sp1 KO,XY rabbits were infertile when mated with WT,XY males.
Sp1 KO,XY rabbits could become pregnant via embryo transfer.
Song Y et al. Mutation of the Sp1 binding site in the 5' flanking region of SRY causes sex reversal in rabbits. Oncotarget. 2017; 8(24) :38176-83.
-The number of each type of sex chromosome in a group of 5 male (WT,XY) and 5 female (WT,XX) rabbits is shown below. In the combined population of 15 X chromosomes and 5 Y chromosomes, which of the following alleles, if any, would be most susceptible to loss due to random chance alone?

Question

Passage
The sex chromosomes X and Y are involved in mammalian sex determination. Organisms with two wild-type (WT) X chromosomes develop as females (XX) , but those with a combination of WT X and Y chromosomes develop as males (XY) (Table 1) . Both sexes also inherit a set of autosomes (nonsex chromosomes) . Humans inherit 22 pairs of autosomes (44 total) . In contrast, rabbits inherit 21 pairs of autosomes (42 total) .Table 1 Chromosome Types Inherited by a Male and Female Rabbit (Note: The Y chromosome has fewer alleles than the X chromosome.)

Specifically, the SRY gene, located on the mammalian Y chromosome, is crucial for male sex determination and induction of testis development. Studies in mammals have shown that regardless of an organism's combination of sex chromosomes, fetuses will always develop as female in the absence of the SRY protein. In addition, one of the X chromosomes becomes inactivated in WT,XX females to prevent them from having double the amount of X-chromosome gene products as WT,XY males.Sp1 is a transcription factor with demonstrated regulatory control of SRY gene expression, and XY individuals with mutations affecting the Sp1 binding site on the SRY gene have been shown to develop female sex characteristics. Unlike SRY, the gene for Sp1 is located on an autosome.Researchers knocked out (KO) Sp1 binding sites on the SRY gene in rabbit embryos (Sp1 KO,XY rabbits) and observed their sexual development following implantation in surrogate mothers. The relationship between the SRY gene and sexual development can be explained by the following experimental observations: Most Sp1 KO,XY rabbits had female genitalia and displayed normal mating behaviors.
Sp1 KO,XY rabbits had significantly smaller ovaries and fewer follicles than WT,XX rabbits.
No significant differences were observed in the cervix and uterus between Sp1 KO,XY and WT,XX rabbits.
There was significantly reduced SRY expression in Sp1 KO,XY rabbits when compared to WT,XY males.
WT,XY male rabbits had significantly higher testosterone levels when compared to Sp1 KO,XY rabbits.
Sp1 KO,XY rabbits were infertile when mated with WT,XY males.
Sp1 KO,XY rabbits could become pregnant via embryo transfer.
Song Y et al. Mutation of the Sp1 binding site in the 5' flanking region of SRY causes sex reversal in rabbits. Oncotarget. 2017; 8(24) :38176-83.
-The number of each type of sex chromosome in a group of 5 male (WT,XY) and 5 female (WT,XX) rabbits is shown below.

In the combined population of 15 X chromosomes and 5 Y chromosomes, which of the following alleles, if any, would be most susceptible to loss due to random chance alone?

Quizplus · Accepted Answer

11ecba2d_1008_324d_a3bf_0dc1730b9740_MD0008_00 According to the passage, the sex of an organism is determined by the combination of sex chromosomes (XX or XY) that they inherit. Males receive an X from their mother and a Y from their father whereas females receive two X chromosomes, one from each parent. In this scenario, the population size of each chromosome was analyzed in a group of 5 male (XY) and 5 female (XX) rabbits. The combined sex chromosome population is 20, with 15 X chromosomes (2 per female and 1 per male) and 5 Y chromosomes (1 per male). Because the Y chromosome is present only in males, its population size is automatically smaller than that of the X chromosome. In addition, the passage states that the Y chromosome has fewer alleles, meaning that Y chromosome alleles will exist at lower frequencies than alleles on the X chromosome. Genetic drift is the fluctuation of allele frequencies within a population due to chance events. This can lead to random loss of alleles within a population. In general, low-frequency alleles have an increased probability of being eliminated by random events than do high-frequency alleles. Because alleles on the Y chromosome exist at the lowest frequencies, they are most susceptible to loss by random chance alone. (Choices B, C, and D) The probability of an allele being lost by random chance (ie, genetic drift) is influenced by that allele's frequency in the population. The passage states that both male and female rabbits inherit 42 autosomes (nonsex chromosomes), which usually have more alleles than either sex chromosome. Because alleles exist on all chromosomes, alleles from both sex and nonsex chromosomes would still be susceptible to loss by random chance alone. However, alleles on the Y chromosome would be most susceptible as they exist at the lowest frequencies. Educational objective: Genetic drift refers to random genetic changes in allele frequency that are due to chance events (not natural selection). Low-frequency alleles have an increased probability of being lost by genetic drift compared to those present at higher frequencies.

Passage The Sex Chromosomes X and Y Are Involved in Mammalian