XIST stands for X-inactivation specific transcript, which acts in the inactivation process in females since only one X will be active. The nuclei of cells in females contain the barr body. Female heterozygotes are mosaics. Calico cats are almost always female.

Since the father has only dominant alleles for the brown phenotype, all of the litter will be brown. However, half the litter should carry the recessive allele. The punnet square below shows the dad's genotype on top and the mom's genotype on the left. The outcome is that half the progeny will have the genotype BB and half will have the genotype Bb. Therefore, all offspring will have the brown phenotype.

B B

B BB BB

b Bb Bb

Since the disease is found on the X-chromosome, we need to find the scenario in which both sons and daughters have an equal 50% probability of getting the disease. Regardless of gender, mothers will always donate one X-chromosome to the offspring. If the mother is heterozygous for the disease, she has a 50% chance of giving an offspring the diseased allele. As a result, a heterozygous mother will have children that display the disease in the observed ratio.

Parents: XXR x XY

Offspring: XX, XXR, XY, XRY

Note that this ratio of expression is only possible when the allele for the disorder is dominant; otherwise the heterozygous female would be a carrier, and not express the disorder.

X-linked disorders are inherited when a parent passes on his or her X-chromosome. Since females have two X-chromosomes, they are less likely to exhibit symptoms of a recessive disorder than males, who have only one. Females are capable of carrying a recessive X-linked trait without expressing it, while males are not. A male must inherit his Y-chromosome from the father and an X-chromosome from the mother, while a female must inherit X-chromosomes from both parents.

If a genotypically healthy mother and a colorblind father have a son, then this child must inherit an X-chromosome from the mother and a Y-chromosome from the father. The mother's chromosome are both genotypically normal, and do not possess the colorblind allele. This means that the son cannot possibly inherit a colorblind allele if the mother is genotypically normal.

All other presented answer represent scenarios that are possible.

Genomic imprinting is when certain genes are expressed in a parent-of-origin inheritance pattern. In the example, angleman syndrome goes to the offspring if the mother has a deletion in chromosome 15. Mitochondrial inheritance is only from the mother and refers to the number of defective mitochondria that may have been inherited. Trinucleotide repeat is the expansion of 3 nucleotide repeats usually associated with Huntington's. Multifactorial inheritance has to do with polygenic (genes & environment), co inheritance, risk factors, and environmental influence.

Mitochondrial inheritance only involves mother-to-offspring (son or daughter) transmission. Hence, both those choices can be eliminated. Both mitochondrial and X-linked inheritance do not include father-to-son transmission. Hence, the only differentiating transmission is father-to-daughter (not seen in mitochondrial).

Humans with the genotype XX are female, as females get two X chromosomes (one from the mother, one from the father). On the other hand, humans with the genotype XY are male, as males get a Y chromosome from the father and an X chromosome from the mother.

When you do a punnet square you will see the portion of genotypes being 25% AA, 50% Aa and 25% aa. The AA allele is not viable, however, so only the Aa and aa genotypes would make it to term. Since 1 out of 3 children would have aa then the percentage would be 33%.

This problem asks you to use concepts of inheritance and Mendelian genetics. The best approach to this problem is to rule out possiblities rather than to find the actual mode of inheritance, as the latter can be a much more difficult and time-consuming process. First off, we know that Y-linked inheritance could not explain this pattern because we see that in generation 1 (G1), the male is affected. If he is affected, all of his sons (who inherit his Y chromosome) would also be affected. There is one son in G2 who is not. Similarly, dominant X-linked inheritance could not explain this pattern; recall that the daughters inherit two copies of the X chromosome, and one is always inactivated. Were the trait X-linked dominant, then the girls of generation 3 (G3) would be affected, having received a copy of the affected gene from their father. Revisiting all other options, we see that any of the remaining inheritance patterns could possibly explain what we see.

A Barr body is an inactivated X-chromosome found in females. Females have two X chromosomes (XX) while males have one X-chromosome and one Y-chromosome (XY). Only one X-chromosome is expressed in females, and the other is "unexpressed". The inactivated X-chromosome in females is termed a Barr body.