Sex chromosomes produce very unique patterns of inheritance in living organisms. In humans, there is an X chromosome and a Y chromosome that determines the sex of the offspring. A male has an X and Y chromosome, whereas a female has two X chromosomes. The sex of an offspring is determined by whether the sperm cell that fertilizes the egg is made up of an X or a Y chromosome; ultimately the male is the determining factor of the sex of the offspring. Even though the Y chromosome determines if the offspring will be male, the X chromosome has genes that control certain human characters; these genes are called sex-linked genes because they are linked to a certain sex. There are approximately 1,098 sex-linked genes that code unusual inheritance patterns other than the female sex trait. Some of these sex-linked genes carry unusual inheritance such as color blindness, Duchenne muscular dystrophy and hemophilia. (O’Neil, 2012). In the case of the recessive mutation that causes hemophilia, a female has to be the carrier of the mutated allele, since she has the X chromosomes only. If the female carrier of the disease mates with a male who has hemophilia (XhY), there would be a one-quarter chance that they would produce a female offspring that has hemophilia (XhXh). Males though are more susceptible to inheriting the hemophilia trait if the mother is a carrier, because the dominant gene (XH) will prevent the disease. (Simon, 2015). Since males can only have an X and a Y chromosome, there is a 50/50 chance, in this case that the hemophilia trait would be passed on. Every living thing has chromosomes and on these chromosomes are genes, which are “individual units of inheritance that comprise stretches of DNA“(Simon, 2015). Alleles, which are variant forms of these genes, are always found on the X chromosome. If two alleles are identical on the gene for that particular trait, then the individual is homozygous for that trait. An example of this would be hair color; a female has the blonde hair trait and mates with a male who also has the blonde hair trait will produce a homozygous recessive individual with the recessive blonde hair trait. Both parents carry the recessive genotype for blonde hair trait, therefore will pass the two copies of the recessive allele to their offspring and that would be that individual’s phenotype, or physical trait. If two alleles are different for that particular trait, then the individual will be heterozygous for that trait. An example is a male with the Bb genotype for brown eyes mates with a female genotype bb for blue eyes. If they had four children, two would have brown eyes and two would have blue eyes; if they only had one child there would be a one-quarter chance they would have either eye color. The reason any of the children would have blue eyes is because the male was heterozygous for that particular trait. Cell division plays an important role in the life cycle of sexual reproduction. (Simon, 2015). Meiosis is a stage in cell division that produces unique haploid gametes and is only involved in sexual reproduction. In males, meiosis starts at puberty and is indefinite, whereas in females, meiosis starts even before birth and ends between puberty and menopause. (Simon, 2015). During cell division in the interphase stage, chromosomes are duplicated in that cell. During meiosis I, the cell splits into two cells further dividing the chromosomes equally in each new cell. During the meiosis II phase, the two cells split in to four equal cells producing four unique haploid (n) gamete cells.

The splitting and production of gamete cells during meiosis is important because this helps increase the unique genetic possibilities of traits of that specie; between the sperm and egg, of living organisms, there is roughly 64 trillion possible combinations of chromosomes that could be passed on to the offspring (Simon, 2015). The process of meiosis also allows for the endless combinations of genes passed on from parent to offspring. (“Genetics Basics”, 2016).
Errors during meiosis can lead to abnormal numbers of chromosomes that can have either a mild to severe effect on the offspring. Turner syndrome, which only affects 1:5000 females, is a condition where the individual one normal X chromosome and one missing or altered chromosome. About half of affected females may have a monosomy X, which is having only 1 copy of the X chromosome instead of the usual two XX chromosomes. Other females can have a partially missing or rearranged chromosome, which results in the same abnormalities of that individual. The most common visual feature of females with Turner’s syndrome is short stature but isn’t evident until the age of 5. Another non visible, but common feature is loss of ovarian function and many do not undergo puberty without the aid of hormone injections. 30% of affected females have webbed neck, puffiness of the hands and feet and kidney problems and about 1/3 are born with heart defects which could be life-threatening. (“Turners syndrome”, 2016). There are many cross-bred dog breeds around the world that are bred to inherit a specific coat color, size, behavior, etc. Genetic cross is a cross-breeding experiment where two P generation (parents) are bred to produce an F1 generation or a first-generation offspring. A monohybrid cross is the genetic breeding that focuses on only one characteristic of the parents, such as in the case of a dogs coat color.

This type of cross-breeding can be explained using the Punnett Square, which shows the probability of each outcome of the crossing of the genotype and phenotype of the parent expressed as a ratio. (Simon, 2015). For example, a homozygous recessive brown coat female Labrador retriever (bb) is bred with a male heterozygous black coated Labrador retriever (Bb. With the female dog all the eggs will express the b allele. With the male dog half the sperm will have the B allele and the other half will have the b allele. The ratio of phenotypes of brown and black coated offspring will be 2:2 or a 50% chance; two brown coated Labradors (bb) and two black coated Labradors (Bb). (Simon, 2015).

References
Genetics Basics Lesson 1: The basics of DNA and genetic disease, including phenotype and genotype. (n.d.). Retrieved March 31, 2016, from http://hihg.med.miami.edu/code/http/modules/education/Design/Print.asp?CourseNum=1
O'Neil, D. (n.d.). Biological Basis of Heredity: Sex Linked Genes. Retrieved March 31, 2016, from http://anthro.palomar.edu/biobasis/bio_4.htm
Simon, E.J. (2015). Biology: The core. Glenview, IL: Pearson Education
Turner syndrome. (2016, March 28). Retrieved March 31, 2016, from https://ghr.nlm.nil.gov/condition/turner-syndrome