Some characteristics are conrolled by a single gene. Each gene may have different forms called alleles. An allele which controls the development of a characteristic when it is present on only one of the chromosomes is a dominant allele is not present is a recessive allele.
Chromosomes are made up of large molecules of DNA. A gene is a small section of DNA.
In human cells, one of the 23 pairs of chromosomes carries the genes which determine sex. In females, the sex chromosomes are the same (**) in males the sex chromosomes are different (XY).
Some disorders are inherited. Huntingtons disease is caused by a dominant allele of a gene. Cystic fibrosis is caused by a recessive allele and must be inherited from both parents. Embryos can be screened for the alleles that cause these other genetic disorders.
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