X-chromosomes and Y-chromosomes {sex chromosome}| determine sexual characteristics.
male
Male cells have one X-chromosome and one Y-chromosome.
female
Female cells have two X-chromosomes.
genes
Sex-chromosome genes determine sex-linked traits and diseases, such as hemophilia and color blindness. Sex hormones influence some autosomal genes, such as baldness gene and horn gene.
crossing over
X-chromosome and Y-chromosome stopped crossing over in birds and mammals.
meiosis
Tips of Y recombine with tips of X, to allow meiosis.
evolution
Reptiles that led to mammals had two X-chromosomes. SRY gene arose 350,000,000 years ago. Between 320,000,000 to 240,000,000 years ago, ancient X-chromosome larger half, containing SRY, inverted or failed to recombine, making monotreme Y-chromosome. Between 170,000,000 and 130,000,000 years ago, a region on other centromere side inverted or failed to recombine, resulting in marsupial Y-chromosome. From -130,000,000 to -80,000,000 years ago, a large region on other centromere side inverted or failed to recombine, resulting in Eutheria Y-chromosome. From -50,000,000 to -30,000,000, a large region on other centromere side inverted or failed to recombine, resulting in human Y-chromosome. During this succession, SRY gene moved to other arm.
evolution: human
Y-chromosome variations track human migrations. M91 is only in south-Africa San people. Yap, M60, M2, MT68, M89M96, M35, M172, and M304 are in Africa. M170, M343, and LLY22 are in Europe. M9, M201, M17, M173, and M69 are in Near East. M20 and M45 are in India. M175, M174, and M122 are in southeast Asia. M130, M4, and M130 are in Pacific islands. M242 and M130 are in northeast Asia and Americas.
Biological Sciences>Genetics>Chromosome>Kinds>Sex
4-Genetics-Chromosome-Kinds-Sex
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Date Modified: 2022.0224