Each body cell contains two copies of the genome in diploid organisms. This means that each somatic cell contains two copies of each chromosome and two copies of each gene. Sex chromosomes are the only exception to this rule. Sex chromosomes determine an organism's sex. Humans have 46 chromosomes, 22 pairs are autosomal and 1 pair is the sex chromosome. Alleles are different forms of genes found at a specific locus of a chromosome. Diploid organisms have two alleles for every autosomal gene. One is from the mother and one is from the father. Organisms that have two copies of an allele are homozygous for that allele and organisms that have different copies of alleles are heterozygous for that allele. Observed inheritance patterns depend on whether the allele is on an autosomal chromosome or a se chromosome and whether or not the allele is dominant or recessive. An allele is autosomal dominant when the phenotype associated with it is seen when the organism has only one copy. The dominant phenotype will be seen if the organism is heterozygous (only has one copy of that allele) or if it is homozygous (has two copies of the allele).  An allele is autosomal recessive when the phenotype associated with it is seen when the organism has two copies. The phenotype is seen only when the organism is homozygous for the allele. An organism that has one copy of the autosomal recessive allele will be able to pass it down though. So an organism that is heterozygous for an autosomal recessive allele is called a carrier. For humans and other mammals the XY system is used to determine sex. Genes on the X chromosome show a pattern of inheritance called sex linkage. Females have two copies of the genes on the X chromosome, so they can be either homozygous or heterozygous for an allele. Males only have one copy of genes expressed on the X chromosome, so it doesn't matter if they are dominant or recessive. Humans and plants have mitochondria and chloroplasts respectively. Mitochondria and chloroplasts have their own DNA.  Mitochondria get scattered throughout the cytoplasm of plant and animal cells. As a part of mitochondrial division, Mitochondrial DNA is replicated. Newly formed embryos get their mitochondria from their mothers. A few human genes are influenced by whether or not they are received from the mother or the father. This process is called genomic imprinting. It means that the organism only shows one of its genes, not both.  |
Class Notes > Genetics >