One or more pairs of chromosomes are different in males and females. Genes located on sex chromosomes are called SEX-LINKED GENES and are inherited differently in males and females. Note that sex-linked genes means the gene is ONLY on the sex chromosome.
TWO TYPES OF CHROMOSOMES
- Autosomes : All other chromosomes other than sex chromosomes. Genes on these chromosomes have same patterns of inheritance in both sexes which involve chromosome 1-22.
- Sex chromosomes: Chromosome 23 which is different in males and females
- Female : 2 copies of chromosome X , forming a homologous pair as XX pair
- Male: One copy of chromosome X and another copy of chromosome Y, producing heterozygous pair XY
- Note: Y chromosome is a short region of homology which allows X chromosome to pair during meiosis
- A sperm cell that carries an X chromosome fertilizes an X-bearing egg cell, individual = XX female
- A sperm cell that carries Y chromosome fertilizes an X-bearing egg cell, individual = XY male
A gene carried on Y chromosome, SRY ( sex-determining region of Y) appears to be the master switch that directs the development towards maleness during early stages of embryonic development
HOW DOES IT WORK?
For first month, rudimentary structures that give rise to reproductive organs and tissues are the same. By 6-8 weeks, SRY gene becomes active in XY embryos, producing a protein that regulates expression of other genes, stimulating the production of testes. Due to the stimulation by hormones secreted by developing testes, tissues degenerate that would develop into female reproductive organs. Remaining structures develop to male reproductive organs. Note: In XX embryos, they don't contain the SRY gene and proceeds to development of female organs. The rudimentary male structures degenerate in XX embryos due to the non presence of hormones for the development of testes.
IMPORTANT TO CONSIDER:
- Sex chromosomes do little part in sex determination, rather it is involved in activities required for both sexes
- Genes involved for one sex is usually coded by autosomes
- Males contain the gene for uterine development but is not expressed and instead passed on to offspring ( If daughter = gene expressed)
- Females contain the gene for penis development but is not expressed and instead passed on to offspring ( If son=gene expressed)
Genes carried on sex chromosomes can be inherited by Non-Mendelian pattern called SEX-LINKAGE. Sex linkage arises from 2 differences in males and females:
- Males have one X chromosome and thus one allele for each gene on this chromosome. Females have 2 copies of X chromosome and thus have 2 alleles for all genes on X chromosome
- Males have one copy of the Y chromosome. Females have no Y chromosome and thus no Y alleles
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