The sex chromosomes X and Y are involved in mammalian sex determination.  Organisms with two wild-type (WT) X chromosomes typically develop as female (XX), but those with a combination of WT X and Y chromosomes typically develop as male (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 Female and Male Rabbits (Note: The Y chromosome has fewer alleles than the X)Specifically, the SRY gene, normally located on the mammalian Y chromosome, is crucial for male sex determination and induction of testis development.  Studies in mammals have indicated that, regardless of an organism's combination of sex chromosomes, fetuses 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 many functions, including regulatory control of SRY gene expression.  XY individuals with mutations affecting the Sp1 binding site on the SRY gene have been shown to develop female sex characteristics.  Unlike SRY, the Sp1 gene 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.Phenotypically female 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 with WT,XY males.WT,XY male rabbits had significantly higher testosterone levels when compared with 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. Question 19Based on the information given in the passage, which of the following observations best supports the conclusion that the second X chromosome that WT,XX females inherit affects their fertility?A.Sp1 KO,XY rabbits have lower testosterone levels compared with WT,XY males.B.Sp1 KO,XY rabbits and WT,XX rabbits exhibit no differences in uterus and cervix structure.C.Sp1 KO,XY rabbits have smaller ovaries and fewer follicles than WT,XX females.D.Sp1 KO,XY rabbits show reduced SRY protein expression compared to WT,XY males.