Examining genetics
I’m looking into how an XYS genotype results in a female mouse once the S gene is lost, leading to the production of viable female gametes.
Assessing genetic patterns
I’m pinpointing the female's genotype, with suggestions pointing to XY. This female, due to the lack of the S gene, is phenotypically female and typically can't produce male offspring.
Mapping out the genes
Tracing the female's XY genotype and unseen male genotype to spot male-to-female ratios, considering factors like the S gene on the X chromosome.
Assessing the father's genotype
OK, let's think it through: the father likely has at least one S gene. This leads to considering possibilities like XY^S, X^S X, or X^S Y^S, while ruling out certain combinations.
Mapping out male patterns
I’m thinking through the possibilities for male genotypes: XY^S, X^S X, and X^S Y^S, steering clear of combinations like Y^S Y^S.
Focusing on sex chromosomes
I'm pulling together the father’s genotype, considering combinations like XY^S and X^S X, while mindful of 2 Y chromosomes being unviable.
Weighing possibilities
I'm collecting info on genotypes and the father: XYS and X^S Y look promising. X^S Y^S is a non-starter due to its lethal implications.
Debunking the theory
OK, let's see. The X^S Y^S genotype works since only one Y chromosome is present, making fatal complications unlikely. Other conditions like Y^S Y^S and X^S X^S are avoided.
Taking a closer look
I'm starting to see that X^S X^S might be plausible if the two X chromosomes don't cause inviability. Other genotypes like X^S Y and Y^S X are also potentially viable.
Piecing together
I’m thinking the mother’s genotype is X Y, with the S gene lost. Normally, XY is male with S, so this female development is intriguing.
Examining the Y chromosome
I’m curious about the mother’s Y chromosome without the S gene, which still lets her produce viable female gametes, as viable gametes emerge as X or Y.
Piecing it together
Shifting my focus to the mother’s ability to produce both X and Y gametes, which is unusual yet follows the problem's logic. The father must carry at least one S chromosome.
Mapping the options
I’m listing each crossing possibility between the mother and father, aiming to pinpoint the correct male to female ratio among the F2 mice. It’s interesting to see where this leads.
Analyzing fertilization
I’m thinking through how the mother’s and father’s gametes combine, considering viability by removing any potential deformations or inviable combinations, and then examining the resulting F1 and F2 generations.
Tracking gene impact
I’m suggesting a simpler method: analyzing the S gene distribution in the population and examining how the mother’s Y gametes and the father’s Y^S gametes influence the F2 sex ratio.
Assessing genotype outcomes
I’m mapping out the distribution of genotypes and sex determination for F1 and F2 generations, using a standard XY^S father and a mother with X Y genotypes.
Analyzing gamete combinations
I’m examining how the mother’s X and Y gametes combine with the father’s X and Y^S gametes, resulting in viable male and female F1 zygotes.
Ratios among viable F1
I’m thinking through the survival rate of F1 zygotes, excluding the lethal combination, and finding the genotypic ratio among the viable ones to be 1:1:1.
Examining gender ratio
I’m noting the gender ratio of F1 progeny from a XY^S father: 2/3 female and 1/3 male, considering the viable genotypes XX, X Y^S, and X Y.
Analyzing F1 mating
Considering female F1 genotypes (XX and XY) and male F1 genotype (XY^S). Female F1 split equally into XX and XY, each 1/3.
Assessing mating dynamics
Females (XX and XY) mate with males (XY^S). With a 2:1 female to male ratio, each male pairs with 2 females on average.
Assessing F2 outcomes
First, half of F2 zygotes are female (XX), the other half male (XY^S). This trend might continue, but let me see further.
Evaluating gamete possibilities
Crossing XY females with XY^S males reveals three viable F2 zygote outcomes: XX (female), XY^S (male), and XY (female), each with a 1/4 chance.
Tracking outcomes
OK, focusing on the critical 3/4 probability for survival, observing a 2:1 female to male ratio among the viable offspring. This makes me think about selection impacts.
Analyzing mating dynamics
I’m weighing the likelihood of XX and XY^S males for Cross 1, then XY females and XY^S males for Cross 2. Random mating ratios could tilt the scales.
Mapping mating dynamics
OK, let’s look at N F1 individuals where 1/3 are XX females, 1/3 are XY females, and 1/3 are XY^S males. Considering random pairing within these groups.
Calculating mating odds
Interestingly enough, with a 1/2 chance of picking XX or XY females, each cross is equally likely. The sex ratio from Cross 1 remains 1:1.
Balancing ratios
Taking a closer look at Cross 1 and Cross 2, each contributing 2k offspring, the combined sex ratio averages out to a balanced 1:1. Progressing through the calculations reveals more clarity.
Breaking down ratios
OK, I’m thinking through the ratio of 2:1 in Cross 2. This leads me to conclude that among 2k offspring, 2/3 are female and 1/3 are male.
Weighing the odds
I'm thinking through the fractions of XX and XY females from a 1/3 ratio, comparing this with the total weighting, and noting the absence of 7:5 in the answer choices.
Mapping out mating odds
OK, let me see. I’m calculating the probability of different pairs within the population and aligning the resulting sex ratios with the provided options to check for congruence.
Refining details
Let's refine the sex ratio, considering cross 2's 1/4 lethal chance, leading to a 3/4 survival and revealing the struggle to match a finite ratio from discrete outcomes.
Mapping out offspring
I'm calculating the total number of litters and their composition by considering the ratio of XX and XY females to XY^S males, keeping in mind the likelihood of each offspring type.
Mapping out the pairing process
I'm detailing the pairing of XX and XY females with XY^S males, ensuring equal mating capacity and averaging offspring per mating to simplify the calculation.
Evaluating potential outcomes
I'm thinking through the expected number of surviving offspring per cross, defined as 1 for simplicity, and exploring the probability of each sex ratio outcome from the offspring.
Piecing it together
I’m examining expected surviving offspring and their sex ratios for both cross types. Cross 1 has a 1:1 ratio, while Cross 2 has a 2:1 ratio with 3/4 surviving children.
Calculating total offspring
First, I’m figuring out the N/3 type 1 matings producing N/6 females and N/6 males. Then, adding type 2 matings, I get N/6 females and N/12 males from N/4 offspring.
Summing up offspring
OK, let me see. I’m balancing the male-to-female ratio, which is 4:3, aligning with the provided answer choices for the scenario. It's interesting to see the ratio match.
Reasoned for 10m 19s
