Sex chromosomes are referred to as allosomes. They determine the sex of an individual. The sex determination also happens in most animals and many plants.
Humans have only 2 sex chromosomes in their genome which are labeled as X chromosome and Y chromosome. A female individual is determined by XX and a male individual is determined by XY. A female contains the same two copies of sex determining genes arranged in the same order in both X chromosomes homomorphic. Therefore the sex chromosomes in a female are homologous to each other. During Meiosis , female gametes are made of a single X chromosome plus 22 autosomal chromosomes.
Male gametes are made either of an X or Y chromosome plus 22 autosomal chromosomes. The joining of two gametes containing both X chromosomes produces a female offspring. On the contrary, the joining of two gametes, containing either an X or Y chromosomes produce a male offspring.
The fertilization of two gametes, each containing a haploid set of chromosomes makes human genome diploid. Some unfertilized eggs of ants and bees develop into haploid males while fertilization makes females. Sex-linked genetic disorders such as hemophilia and Duchenne muscular dystrophy occur due to the defective second copy of the same gene. Figure 2: X-linked recessive inheritance. Autosomes: Autosomes determine the trait.
Males and females contain the same copy of autosomes. Sex Chromosomes: Sex chromosomes determine the gender. They are different in males and females by their size, form, and behavior. Autosomes: Autosomes are labeled with numbers, from 1 to Autosomes: Most of the chromosomes within a genome are autosomes. Sex Chromosomes: A few of the chromosomes within a genome are sex chromosomes.
In contrast, gene mutations as we shall see in Chapter 24 owe their level of incidence to a complex interplay of mutation rates and environmental selection that spans many human generations. The fate of a million implanted human zygotes. Robertsonian translocations are due to fusion or dissociation of centromeres. From K. Sankaranarayanan, Mutation Research 61, When the frequencies of various chromosome mutations in live births are compared with the corresponding frequencies found in spontaneous abortions Table , it becomes clear that the chromosome mutations that we know about as clinical abnormalities are just the tip of an iceberg of chromosome mutations.
First, we see that many more types of abnormalities are produced than survive to birth; for example, trisomies of chromosome 2, 16, and 22 are relatively common in abortuses but never survive to birth. Second, the specific aberrations that survive are part of a much larger number that do not survive; for example, Down syndrome trisomy 21 is produced at almost 20 times the frequency in live births.
The comparison is even more striking for Turner syndrome XO. An estimated minimum of 10 percent of conceptions have a major chromosome abnormality; our reproductive success depends on the natural weeding-out process that eliminates most of these abnormalities before birth.
Incidentally, no evidence suggests that these aberrations are produced by environmental insult to our reproductive systems or that the frequency of the aberrations is increasing. Trisomics show the deleterious effects of genome inbalance and produce chromosome -specific modified phenotypic ratios.
A disomic is an aberration of a haploid organism. In fungi, they can result from meiotic nondisjunction. These diagrams correspond exactly to the outcomes of the chromosomal events shown in Figure The abortion patterns themselves are diagnostic for the presence of disomics in the asci. Another way of detecting disomics in fungi is to cross two strains with homologous chromosomes bearing multiple auxotrophic mutations; for example:.
From such a cross , large populations of ascospores are plated onto minimal medium. Most of these colonies are found to be disomics and not multiple crossover types. Disomics in fungi can be selected from asci showing special spore abortion patterns or as meiotic progeny that must contain homologous chromosomes from both parents.
Aneuploid cells can arise spontaneously in somatic tissue or in cell culture. In such cases, the initial result is a genetic mosaic of cell types. Human sexual mosaics—individuals whose bodies are a mixture of male and female tissue—are good examples.
One type of sexual mosaic , XO XYY , can be explained by postulating an XY zygote in which the Y chromatids fail to disjoin at an early mitotic division, so both go to one pole:. The phenotypic sex of such individuals depends on where the male and female sectors end up in the body. In the type of nondisjunction being considered, nondisjunction at a later mitotic division would produce a three-way mosaic XY XO XYY , which contains a clone of normal male cells.
In general, sexual mosaics are called gynandromorphs. Origin of a human sexual mosaic XY XO by Y chromosome loss at the first mitotic division of the zygote. After C. Stern, Principles of Human Genetics, more Geneticists working with many species of experimental animals occasionally find gynandromorphs among their stocks. A classic example is the Drosophila gynandromorph shown in Figure Loss of the wild-type allele —bearing X chromosome at the first mitotic division resulted in the two cell lines and ultimately in a fly differing from one side to the other in sex, eye color, and size of wing.
A similar gynandromorph in the Io moth is shown in Figure A bilateral gynandromorph of Drosophila. A bilateral gynandromorph in the Io moth, Automeris io io. Mitotic nondisjunction and other types of aberrant mitotic chromosome behavior can give rise to mosaics consisting of two or more chromosomally distinct cell types, including aneuploids.
Somatic aneuploidy and its resulting mosaics are often observed in association with cancer. People suffering from chronic myeloid leukemia CML , a cancer of the white blood cells, frequently harbor cells containing the so-called Philadelphia chromosome. This chromosome was once thought to represent an aneuploid condition, but it is now known to be a translocation product in which part of the long arm of chromosome 22 attaches to the long arm of chromosome 9.
However, CML patients often show aneuploidy in addition to the Philadelphia chromosome. In one study of 67 people with CML, 33 proved to have an extra Philadelphia chromosome and the remainder had various aneuploidies; the most common aneuploidy was trisomy for the long arm of chromosome 17, which was detected in 28 people. Of 58 people with acute myeloid leukemia, 21 were shown to have aneuploidy for chromosome 8; 16 for chromosome 9; and 10 for chromosome In another study of 15 patients with intestinal tumors, 12 had cells with abnormal chromosomes, at least some with trisomy for chromosome 8, 13, 15, 17, or Such studies merely established correlations, and it is not clear whether the abnormalities are best thought of as a cause or as an effect of cancer.
Aneuploids are produced by nondisjunction or some other type of chromosome misdivision at either meiosis or mitosis. By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.
Turn recording back on. National Center for Biotechnology Information , U. Freeman ; Search term. Figure Ears of the nullisomics of wheat. Figure Meiosis in which the chromosome of interest is monosomic. MESSAGE Monosomics show the deleterious effects of genome inbalance, as well as unexpected expression of recessive alleles carried on the monosomic chromosome. Figure Characteristics of Down syndrome trisomy Figure Maternal age and the production of Down-syndrome offspring. Figure The fate of a million implanted human zygotes.
MESSAGE Disomics in fungi can be selected from asci showing special spore abortion patterns or as meiotic progeny that must contain homologous chromosomes from both parents. This two-step division process produces four haploid daughter cells. Haploid cells contain only one set of chromosomes. When the haploid male and female gametes unite in a process called fertilization, they form what is called a zygote. The zygote is diploid and contains two sets of chromosomes.
Fertilization occurs when male and female gametes fuse. In animal organisms, the union of sperm and egg occurs in the fallopian tubes of the female reproductive tract. Millions of sperm are released during sexual intercourse and these travel from the vagina to the fallopian tubes.
Sperm are specially equipped with burrowing catalysts and mechanisms for fertilizing an egg. The head region contains a cap-like covering called an acrosome that contains enzymes that help the sperm cell penetrate the zona pellucida, the outer covering of an egg cell membrane.
When a sperm reaches the egg cell membrane , its head fuses with the egg. This triggers the release of substances that modify the zona pellucida to prevent any other sperm from fertilizing the egg. This process is crucial as fertilization by multiple sperm cells, or polyspermy, produces a zygote with extra chromosomes. Polyspermy is lethal to a zygote.
Upon fertilization, two haploid gametes become one diploid zygote. A human zygote has 23 pairs of homologous chromosomes and 46 chromosomes total—half from the mother and half from the father. The zygote continues to divide by mitosis until a fully functional individual is formed. The biological sex of this human is decided by the sex chromosomes it inherits. A sperm cell may either have an X or Y sex chromosome, but an egg cell can only have an X chromosome.
A sperm cell with a Y sex chromosome results in a male XY and a sperm cell with an X sex chromosome results in a female XX. The type of sexual reproduction of an organism is largely dependent on the size and shape of its gametes. Some male and female gametes are of similar size and shape, while others are vastly different.
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