Chromosomes and Inheritance- Genes are units of information about heritable traits. The genes of eukaryotic cells are distributed among a number of chromosomes. Each gene has its own location - a gene locus - in one type of chromosome.
- Any cell with a diploid chromosome number has inherited pairs of homologous chromosomes. All but one pair are identical in length, shape, and gene sequence. The single exception is a pairing of nonidentical sex chromosomes, such as X with Y. The two members of a pair of homologous chromosomes interact and segregate from each other during meiosis.
- A gene at one locus may have the same form or a slightly different one compared to its partner gene on the homologous chromosome. When considering a population as a whole, which forms are inherited usually varies from one individual to the next.
- All the different molecular forms of a gene that are possible at a given locus are called alleles. New alleles arise only through mutation.
- A wild-type allele is the most common form of a gene, either in a natural population or in a standard, laboratory-bred strain of species. Any one of the less common forms of a gene is a mutant allele.
- Genes on the same chromosome are physically linked together. The farther apart two linked genes are, the more vulnerable they are to crossing over. By this event, homologous chromosomes exchange corresponding segments.
- Crossing over results in genetic recombination. The term refers to nonparental combinations of alleles in gametes, then in offspring.
- Independent assortment refers to the random alignment of each pair of homologous chromosomes at metaphase 1 of meiosis. It results in nonparental combination of alleles in gametes and offspring.
- On rare occasions, the structure of chromosomes changes abnormally during mitosis or meiosis. So does the parental chromosome number.
Remember:
Diploid cells have pairs of genes, on pairs of homologous chromosomes. At each gene locus, the alleles may be identical or non identical.
Crossing over and other events during meiosis give offspring new combination of alleles and parental chromosomes.
Abnormal events during meiosis or mitosis can change the structure and number of chromosomes.
Autosomes are the pairs of chromosomes that are the same in males and females of a species. One other pair, the sex chromosomes govern the sex of a new individual.
[All information taken from Biology Concepts and Applications by Cecie Starr Fifth Edition]Labels: genetics
