5 Genetics and Evolution
5.4 Sex Determination
Sex determination
Dr V Malathi
Sex Chromosomes
In humans and in many other animal species the sex of the organism is determined by specific chromosomes.
• The chromosomes that carry those genes related to sexual characteristics are called the sex chromosomes.
• Experiments conducted by German biologist Hermann Henking in 1891 showed that sex chromosomes were unique from all other
chromosomes.
Henking was studying sperm formation in wasps. Henking saw that some wasp sperm cells had 12 chromosomes, while others had only 11.
• While observing the stages of meiosis that formed these sperm cells, Henking noticed that the twelfth chromosome looked different from all the others.
• He thus named this chromosome the “X element,” to represent its unknown nature.
• Henking hypothesized that this extra chromosome must play some role in sex determination in insects.
Humans and most other mammals have two sex chromosomes, the X and the Y.
• Females have two X chromosomes in their cells, while males have both X and a Y chromosomes in their cells.
These are able to produce two types of gametes.
• While members of the homogametic sex can only produce one type of gamete.
• In humans (as well as many other animals), males are heterogametic, while females are homogametic .
• This system is reversed in butterflies and moths (order Lepidoptera): females are the heterogametic sex, while males are
homogametic
Sex Determination in Mammals
• In placental mammals, the presence of a Y chromosome determines sex.
• Normally, cells from females contain two X chromosomes, and cells from males contain an X and a Y chromosome.
• Occasionally, individuals are born with sex chromosome aneuploidies, and the sex of
these individuals is always determined by the absence or presence of a Y chromosome
The Y Chromosome
• The Y chromosome of the males is the smallest chromosome .
• It’s a kind of hemizygous- single chromosome.
• It is inherited from fathers to their male child.
• It has very few genes present on its single-arm, in comparison with other chromosomes.
• The Y chromosome evolved before 300,000 years ago from autosomes.
• The Y chromosome is the smallest acrocentric chromosome
• The size of it is 57,227,415bp.=57.2 Mb
• Genes on Y chromosome: around 200 in which the structure and function of only 50 to 60 genes are known to us.
• The centromere is located at the position of 10.4Mb.
• Only a single Y chromosome is present in the male individuals of the mammalian population
Note:
• kilo base pairs = 1,000bp
• 1 Mb (Mbp) = mega base pairs = 1,000,000bp
“Human male Karyotype after G-banding. Y chromosome highlighted” by National Human Genome Research Institute, via Wikimedia Commons is in the Public Domain, CC0
Structure:
• Structurally, the Y chromosome is a smaller- acrocentric chromosome having a short p arm and a long q arm.
• The p arm mostly contains genes and DNA sequences that are necessary for maleness, for example, the TDF a