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Mendelian inheritance patterns
Within a population, there may be a number of alleles for a given gene. Individuals that have two copies of the same allele are referred to as homozygous for that allele; individuals that have copies of different alleles are known as heterozygous for that allele. The inheritance patterns observed will depend on whether the allele is found on an autosomal chromosome or a sex chromosome, and on whether the allele is dominant or recessive. Autosomal dominant If the phenotype associated with a given version of a gene is observed when an individual has only one copy, the allele is said to be autosomal dominant. The phenotype will be observed whether the individual has one copy of the allele (is heterozygous) or has two copies of the allele (is homozygous).
Autosomal recessive
If the phenotype associated with a given version of a gene is observed only when an individual has two copies, the allele is said to be autosomal recessive. The phenotype will be observed only when the individual is homozygous for the allele concerned. An individual with only one copy of the allele will not show the phenotype, but will be able to pass the allele on to subsequent generations. As a result, an individual heterozygous for an autosomal recessive allele is known as a carrier.
Sex-linked or X-linked inheritance
In many organisms, the determination of sex involves a pair of chromosomes that differ in length and genetic content - for example, the XY system used in human beings and other mammals. The X chromosome carries hundreds of genes, and many of these are not connected with the determination of sex. The smaller Y chromosome contains a number of genes responsible for the initiation and maintenance of maleness, but it lacks copies of most of the genes that are found on the X chromosome. As a result, the genes located on the X chromosome display a characteristic pattern of inheritance referred to as sex-linkage or X-linkage. Females (XX) have two copies of each gene on the X chromosome, so they can be heterozygous or homozygous for a given allele. However, males (XY) will express all the alleles present on the single X chromosome that they receive from their mother, and concepts such as 'dominant' or 'recessive' are irrelevant. A number of medical conditions in humans are associated with genes on the X chromosome, including haemophilia, muscular dystrophy and some forms of colour blindness.
Non-Mendelian inheritance patterns
Genomic imprinting
The expression of a small number of human genes is influenced by whether the gene has been inherited from the mother or father. This process - called genomic (or parental) imprinting - usually means that the organism expresses one of its alleles but not both. In many cases the non-expressed allele is inactivated - for example, by DNA methylation. (High levels of DNA methylation are known to inhibit gene activity.) Imprinting involves three stages: The inactivation of an allele in the ovaries or testes before or during the formation of egg cells or sperm. The maintenance of that inactivation in the somatic cells of the offspring organism. The removal, then re-establishment, of the inactivation during the formation of egg cells or sperm in the offspring organism. The pattern of imprinting is maintained in the somatic cells of the organism but can alter from generation to generation.
Structure of Genomes and Chromosomes Chromosome size, shape and organization vary widely Size: The size of chromosome is normally measured at mitotic metaphase and may be as short as 0.25μm in fungi and birds to as long as 30 μm in some plants such as Trillium. However, most mitotic chromosome falls in the range of 3μm in Drosophila to 5μm in man and 8-12μm in maize. The monocots contain large sized chromosomes as compared to dicots. Organisms with less number of chromosomes contain comparatively large sized chromosomes. The chromosomes in set vary in size. Shape: The shape of the chromosome changes from phase to phase in the continuous process of cell growth and cell division. During the resting/interphase stage of the cell, the chromosomes occur in the form of thin, coiled, elastic and contractile, thread like stainable structures, the chromatin threads. In the metaphase and the anaphase, the chromosome becomes thick and filamentous. Each chromosome contains a clear zone, known as centromere or kinetochore, along their length. The centromere divides the chromosome into two parts and each part is called chromosome arm. The position of centromere varies from chromosome to chromosome providing it a different shape. They could be telocentric (centromere on the proximal end of the chromosome), acrocentric (centromere at one end giving
more diversity or are not conserved. They may also differ between different tissues of same organism. Roger Kornberg in 1974 described the basic structural unit of chromatin which is called the nucleosome (is the fundamental subunit of chromatin. Each nucleosome is composed of a little less than two turns of DNA wrapped around a set of eight proteins called histones, which are known as a histone octamer. Each histone octamer is composed of two copies each of the histone proteins H2A, H2B, H3, and H4). Euchromatin: The lightly-stained regions in chromosome when stained with basic dyes are called euchromatin and contain single-copy of genetically-active DNA. The extent of chromatin condensation varies during the life cycle of the cell and plays an important role in regulating gene expression. In the interphase of cell cycle the chromatin are decondensed and known as euchromatin leading to gene transcription and DNA replication. Heterochromatin: The word heterochromatin was coined by Emil Heitz based on cytological observations. They are highly condensed and ordered areas in nucleosomal arrays. About 10% of interphase chromatin is called heterochromatin and is in a very highly condensed state that resembles the chromatin of cells undergoing mitosis. They contain a high density of repetitive DNA found at centromeres and telomeres form heterochromatin. Centromeres: Centromeres are those condensed regions within the chromosome that are responsible for the accurate segregation of the replicated chromosome during mitosis and meiosis. When chromosomes are stained they typically show a dark-stained region that is the centromere. The actual location where the attachments of spindle fibers occur is called the kinetochore and is composed of both DNA and protein. The DNA sequence within these regions is called CEN DNA. Because CEN DNA can be moved from one chromosome to another and still provide the chromosome with the ability to segregate, these sequences must not provide any other function. Typically CEN DNA is about 120 base pairs long and consists of several sub-domains, CDE-I, CDE-II and CDE-III.
Sex Determination and Changes in Chromosome Number
Sex Determination In humans, it is the presence or absence of the Y that makes the difference; a person carrying a Y will look like a male; a person with one X and lacking a Y (XO) will look female. Drosophila also use an XX female, XY male strategy of sex determination, but it is the ratio of X chromosomes to autosomes (non sex chromosomes) that makes the difference. Flies with X:A =1 are female, flies with X:A = 0.5 are male, and flies with X:A ratios between 0.5-1 are "intersex". Genes must be equalized in the two sexes. Although humans and fruit flies both use a XX female/ XYmale strategy for sex determination, they use two different mechanisms for dosage compensation. In human females, one of the X-chromosomes is inactivated in each somatic cell. In fruit flies, transcription of the X chromosome is hyperactivated in males versus females. X-Chromosome Inactivation in mammals aneuploidy for the X chromosome (either extra or missing sex chromosomes) is much less severe than aneuploidy of autosomal genes. In 1961, Mary Lyon proposed the X chromosome inactivation hypothesis to explain dosage compensation in mammals. Her observations of X-linked coat color genes in mice led her to propose that the amount of products derived from X-linked genes were equalized in the sexes by inactivating one of the X chromosomes in females. Inactive X chromosomes can be seen in interphase nuclei as darkly staining heterochromatic masses. These masses are called Barr bodies after the cytologist who discovered them. XX females have one Barr body per cell, XXX females have 2 Barr bodies per cell, XXY Klinefelter males have one Barr body per cell (Barr bodies are not observed in XY males). This is why X chromosome aneuploidy can be tolerated; all but one of the extra X chromosomes is unactivated. discuss, not all of the genes on the X are inactivated. A possible explanation is that in
Changes in Chromosome Number
Incomplete segregation during meiosis, called nondisjunction, can result in abnormal chromosome number. Experimental manipulation and treatment with certain chemicals during mitosis or meiosis can also modify chromosome number. o euploidy is change in multiples of the entire set of chromosomes monoploid - one set (n) as in male wasps, bees and ants: different from haploid, where all individuals are n. monoploid plants can be cultivated from pollen grains cultured on agar plants supplemented with hormones; they are commonly use in agriculture polyploid - multiple sets of chromosomes (3n, 4n, etc) autopolyploid - chromosome sets from same species (most common polyploidy). autotetraploids are new species, because when they are crossed with the stock from which they arose, the hybrids are triploid, which are sterile. allopolyploid - chromosome sets from different species usually involves production of a sterile monoploid hybrid and chromosome doubling to restore fertility. o aneuploidy is change in number for a single chromosome aneuploids are produced by nondisjunction during meiosis, usually anaphase I. Pairing and segregation more sensitive than during meiosis II and dependent on crossing over. If this does not occur normally, nondisjunction can result. monosomy = one copy of a specific chromosome. X chromosome monosomics are the only monosomics that can survive in humans trisomy = three copies of a specific chromosome sex chromosome aneuploidy in humans results in
XO - Turner syndrome - only human monosomic - sterile females, 8/1360 occurrences survive XXX - normal females XXY - Kleinfelter syndrome - sterile males w/ mild mental impairment XYY - slightly taller, delayed speech, language and motor skills abnormality of XO, XXY and XYY is due substantially to a region of homology (pseudoautosomal region) between X and Y, which is not inactivated and exists in two copies in both females and males. XXY, XYY males have 1.5 times as much of this as normal males, even though one of their Xs is inactivated, XXX females have 1.5 times as much as normal females autosomal monosomics are lethal, most trisomics. 21, 13 and 18 are very small chromosomes, so imbalance is minimal, most 21, 13 and 18 trisomics die before birth, all other autosomal trisomics do. autosomal aneuploidy in humans: trisomy 21 - Down syndrome - highly variable phenotypic expression
