Question

1) Before DNA sequencing, how were scientists able to determine the identity and number of chromosomes a person had?

2) What is scientifically accurate about the assumption that two X chromosomes establishes the female sex? Under what genetic or developmental conditions might two X chromosomes not result in femaleness? Under what genetic or developmental conditions might a phenotypically female contain a Y chromosome?

3) Assume a zygote started with a (1) XY (only) genotype, or (2) an XXY genotype. What mitotic events might have occurred during early embryonic development that could cause the person to have both XX and XXY cells as an adult?

Answer 1

Question 1:

Chromosomes:

When scientists discovered the cell and nucleus, further studies led to the discovery of chromosomes. It was first observed in plant cells. Basophilic aniline dye was used to stain the chromatin material in the nucleus. Later Gregor mendel discovered the inheritance of chromosomes.

Theophilus painter in the year 1923 using microscope counted 24pairs of chromosomes, which means 48chromosomes. Later in the year 1956 scientist joe Hin Tjio discovered 22 pairs of autosomes and 1 pair of sex chromosome, which means 46 chromosomes.

DNA sequencing is the recent technique by which number of nucleotides in the genome (Entire gene content) of an organism is identified by sequencing techniques.

Question 2:

Sex determination is done by the presence of XX to be female and presence of XY to be male.

• X chromosome has number of genes about 1000 genes and Y has only few genes.
• Majority of these genes are not involved in the sex determination.
• The Y chromosome has sry gene. Expression of sry gene early in embryonic development initiates male specific development. Absence of this gene expression results in the development of ovaries.

XX male syndrome:

This is the condition in which the X chromosome of the father undergoes crossing over with the Y gene resulting in the presence of sry gene in the X chromosome. When this X chromosome joins with the mothers X chromosome results in XX male syndrome.

XY swyer syndrome:

This condition has phenotipically female and genetically male Y chromosome is present. Uterus and fallopian tubes are present but ovary or testis is absent. Swyer syndrome is also called complete gonadal dysgenesis.

Question 3:

1. XY zygote:

X chromosome from mother and Y chromosome from father results in the formation of zygote with XY chromosome which will grow as male.

2. XXY zygote:

• These types result when during gamete formation during meiosis, XY chromosome of male fail to separate and result in the same gamete. When this is fertilized with normal egg with X chromosome results in XXY zygote.
• This also happens when XX of female fail to separate during gamete formation and result in the egg, when fertilized with the normal Y sperm, results in XXY zygote.
• This disease condition is known as klinefelter syndrome.