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.
-In an isolated population of 10,000 rabbits, 900 are homozygous for a recessive mutant Sp1 allele. Given this, what is the percentage of rabbits in the population who carry the mutant Sp1 allele but do not express the mutant phenotype? (Note: Assume the population is in Hardy-Weinberg equilibrium.)

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.
-In an isolated population of 10,000 rabbits, 900 are homozygous for a recessive mutant Sp1 allele. Given this, what is the percentage of rabbits in the population who carry the mutant Sp1 allele but do not express the mutant phenotype? (Note: Assume the population is in Hardy-Weinberg equilibrium.)

Quizplus · Accepted Answer

11ecba2d_1008_ce8e_a3bf_2368c4038b59_MD0008_00 For populations in Hardy-Weinberg equilibrium, the allele frequencies can be used to predict the genotype frequencies using the following two equations: p + q = 1: Applies to a gene with only two alleles (S and s), where p represents the frequency of one allele (S) and q represents the frequency of the other (s). p² + 2pq + q² = 1: Can be used to calculate frequencies of possible genotypes (SS, Ss, and ss) from the allele frequencies. Here p² and q² represent the frequencies of the two homozygous genotypes SS and ss, respectively. The frequency of the heterozygous genotype (Ss) is 2pq. In this case, heterozygous rabbits (Ss) have one dominant and one recessive Sp1 mutant allele, which means they carry the recessive allele but do not express the associated mutant phenotype. Therefore, the fraction of rabbits who meet this criterion is equal to the frequency of the heterozygous genotype (2pq). To determine 2pq, the following calculations must be performed. Solve for q. Let p represent the frequency of the WT Sp1 allele (S) and q represent the frequency of the recessive mutant allele (s). Here, 900 out of 10,000 rabbits are homozygous recessive (ss) for the mutant allele. Given that q² is equal to the frequency of the homozygous recessive genotype (ss), the following solves for q: 11ecba2d_1008_f59f_a3bf_abd60fca39c7_MD0008_00 q2=90010,000=0.09 11ecba2d_1008_f5a0_a3bf_29b16399b805_MD0008_00 q2=0.09 11ecba2d_1008_f5a1_a3bf_e189aaedaf44_MD0008_00 q=0.3 Use p + q = 1 to calculate p: 11ecba2d_1008_f5a2_a3bf_6ded37f0622f_MD0008_00 p+q=1 11ecba2d_1008_f5a3_a3bf_5d673864a7d2_MD0008_00 p+0.3=1 11ecba2d_1008_f5a4_a3bf_27ba4af1a70e_MD0008_00 p=1-0.3=0.7 Calculate the frequency of the heterozygous genotype (Ss) using 2pq: 11ecba2d_1009_1cb5_a3bf_dbf7ed553662_MD0008_00 2pq=20.70.3=0.42 The percentage of rabbits that carry the mutant Sp1 allele but do not express its associated phenotype is 42%. (Choice A) 9% is the frequency of the homozygous recessive genotype (q²). (Choice B) 21% is half the frequency of heterozygotes in the population. (Choice D) 70% is the frequency of the WT (dominant) SP1 allele (p). Educational objective: The Hardy-Weinberg equations can be used to relate allele frequencies to genotype frequencies. Allele frequencies are used to calculate the expected genotype frequencies of populations in Hardy-Weinberg equilibrium.

Passage The Sex Chromosomes X and Y Are Involved in Mammalian