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human inheritance

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Unit 22: Human Inheritance P1: Explain the process of meiosis in relation to the production of sperm and ova. Meiosis is the process of cell division which results in a cell where the 23 pairs of chromosomes divide into two, each with 23 chromosomes. These cells are the female ova (eggs) and the male sperm. Each of the resulting reproductive cells, or gamates (sperm and egg), has only a single set of 22 autosomes plus a single sex chromosome, either an X or a Y. A cell with a single chromosome set is called a haploid cell. Diploid is the term refers to a cell or an organism consisting of two sets of chromosomes: generally, one set from the mother and another set from the father. Meiosis involves a reduction in the amount of genetic material. Meiosis comprises two successive nuclear divisions with only one round of DNA reproduction. Chiasma is describes as the point where two chromatids are intertwined (interwoven) in a cell. A chromatid is one of two identical copies of DNA making up a chromosome, which are joined at their centromeres, for the process of cell division (mitosis or meiosis). Gametogenesis is the production of haploid sex cells (in humans, ovum and spermatozoa) that each carry one-half the genetic compliment of the parents from the germ cell line of each parent. The production of ovum is termed oogenesis and the production of spermatozoa is called spermatogenesis. Both oogenesis and spermatogenesis provide a mechanism through which genetic information may be passed to offspring. The fusion of spermatozoa and ova during fertilization results in a zygote with a fully restored diploid genome. Four stages can be described for each nuclear division. www.accessexcellence.org/AB/GG/meiosis.html Interphase: Before meiosis begins, genetic material is duplicated. First division of meiosis Meiosis I: this is the separation series that split the sets of homologous chromosomes to create two daughter cells that have half the normal number of chromosomes. ...read more.


Mendel showed that the inheritance of traits follows particular laws, which were later named after him. The significance of Mendel's work was not recognized until the turn of the 20th century. Its rediscovery prompted the foundation of the discipline of genetics. Independent assortment: The Law of Independent Assortment this is also known as "Inheritance Law" or Mendel's Second Law, this states that the inheritance pattern of one trait will not affect the inheritance pattern of another. While his experiments with mixing one trait always resulted in a 3:1 ratio between dominant and recessive phenotypes, his experiments with mixing two traits (dihybrid cross) showed 9:3:3:1 ratios. But the 9:3:3:1 table shows that each of the two genes is independently inherited with a 3:1 ratio. Mendel concluded that different traits are inherited independently of each other, so that there is no relation, for example, between a cat's color and tail length. This is actually only true for genes that are not linked to each other. Independent assortment happens during meiosis I in eukaryotic organisms, specifically anaphase I of meiosis, to produce a gamete with a mixture of the organism's maternal and paternal chromosomes. Along with chromosomal crossover, this process aids in rising genetic diversity by producing novel genetic combinations. Of the 46 chromosomes in a normal diploid human cell, half are maternally-derived (from the mother's egg) and half are paternally-derived (from the father's sperm). This occurs as sexual reproduction involves the fusion of two haploid gametes (the egg and sperm) to produce a new organism having the full complement of chromosomes. During gametogenesis that is the production of new gametes by an adult - the normal complement of 46 chromosomes needs to be halved to 23 to ensure that the resulting haploid gamete can join with another gamete to produce a diploid organism. An error in the number of chromosomes, such as those caused by a diploid gamete joining with a haploid gamete, is termed aneuploidy. ...read more.


Everyone has two copies of the gene for haemoglobin; one from their mother and one from their father. If one of these genes carries the instructions to make sickle haemoglobin (HbS) and the other carries the instructions to make normal haemoglobin (HbA) then the person has Sickle Cell Trait and is a carrier of the sickle haemoglobin gene. This means that this person has enough normal haemoglobin in their red blood cells to keep the cells flexible and they don't have the symptoms of the sickle cell disorders. If both copies of the haemoglobin gene carry instructions to make sickle haemoglobin then this will be the only type of haemoglobin they can make and sickled cells can occur. These people have Sickle Cell Anaemia and can suffer from anaemia and severe pain. These severe attacks are known as Crises. Over time Sickle Cell sufferers can experience damage to organs such as liver, kidney, lungs, heart and spleen. Death can be a result. Another problem is that red blood cells containing sickle haemoglobin do not live as long as the normal 120 days and this results in a chronic state of anaemia. Symptoms Blockage of a blood vessel causes an attack known as a crisis. This is more likely to happen when the person is stressed by another illness, exhaustion, cold, dehydration and other problems. Organs such as the liver, kidney, lungs, heart and spleen become damaged, causing severe pain, especially in the bones. The red blood cells also break up easily, leading to anaemia Inheritance Pattern for Sickle Cell Trait The illustration shows how sickle cell genes are inherited. A person inherits two copies of the hemoglobin gene-one from each parent. A normal gene will produce normal hemoglobin (A). An abnormal (sickle cell) gene will produce abnormal hemoglobin (S). When each parent has a normal gene and an abnormal gene, each child has: a 25 percent chance of inheriting two normal genes; a 50 percent chance of inheriting one normal gene and one abnormal gene; and a 25 percent chance of inheriting two abnormal genes ...read more.

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