Males: the production of sperm takes place in the testes. Cells are called spermatogonia, line the tubules of the testes and divide by mitosis from puberty until death, producing daughter cells called spermatocytes. These undergo meiosis and the four haploid cells that result are called spermatids. Each spermatid develops into mature sperm.
Females: the production of gametes is called oogenesis and takes place in the ovaries. Cells in the ovary known as oogonia begin mitosis early in embryonic development and finish a few weeks later. Females are born with all the primary oocytes they will ever have. All of the primary oocytes begin meiosis during embryonic development and then stop. They remain in meiosis 1 until a female undergoes puberty. After puberty, one primary oocyte per month completes the first meiotic division, and the secondary oocyte (the large cell produced by the first meiotic division) is released from the ovary and moves into the oviduct. Cytokinesis follows mitosis 1. One cell, destined to be female gamete, receives about 95% of the cytoplasm and is called secondary oocyte. The larger cell becomes the functional gamete (the ovum) If the secondary oocyte is fertilized, meiosis 2 is completed.
Differences between males and females: The timing of meiosis and gamete formation is different for males and females.
Also, only one gamete is formed for each round of meiosis in females, while the other 3 cells become polar bodies. Males produce four gametes from each spermatogonia that goes through meiosis
Males produce gametes continuously from puberty on
Females born with exact number of possible gametes, maturing once a month
Meiosis produces new combinations of genes in two ways.
Random Assortment- When pairs of homologous chromosomes enter metaphase 1, the maternal and paternal members of each pair line up at random with respect to all the other pairs. (Arrangement can be maternal:paternal, or paternal:maternal.) As a result, cells produced in meiosis 1 are much more likely to receive a combination of maternal and paternal chromosomes than they are to receive a complete set of maternal chromosomes and a complete set of paternal chromosomes. The number of combinations produced by random assortment during meiosis is equal to 2n, where 2 represents the number of chromosomes in each pair and n represents the number of chromosomes in the haploid set. Humans have 23 chromosomes in their haploid set, and therefore 2 to the 23rd power-or 8,388,608 different combinations of maternal and paternal chromosomes are possible in cells produced by meiosis 1. **isn't this called independent assortment? same thing
Crossing over- This process involves the physical exchange of parts between non-sister chromatids. This adds to the genetic variation produced by random alignment of chromosome pairs in metaphase 1. Members of a chromosome pair carry identical genes, but may carry different versions of a gene. These different versions are called alleles.For example, a chromosome may carry a gene for eye color. One copy of the chromosome may carry the allele for blue eyes, and the other copy of the chromosome may carry the allele for brown eyes. The exchange of chromosome parts during crossing over creates new combinations of alleles inherited from each parent.
In mitosis, a diploid parental cell undergoes chromosome replication. When the cell enters prophase, the chromosome becomes visible as replicated structures with sister chromatids held together by a common centromere. Unpaired chromosomes line up at the center of the cell during metaphase. In anaphase the centromeres separate, converting the sister chromatids into chromosomes. The result is two daughter cells, each of which is genetically identical to the parental cell.
In Meiosis, the parental diploid cell undergoes chromosome replication. When the cell enters prophase 1, the chromosomes become visible as replicated structures with sister chromatids held together by a common centromere. Members of a chromosome pair line up at the equator of the cell during metaphase 1. Members of a chromosome pair separate in anaphase 1 and move to opposite poles of the cell. In meiosis 2, the unpaired chromosomes in each cell line up on the equator of the cell. During anaphase 2 the centromeres split, converting the sister chromatids into chromosomes, which are distributed to daughter cells. The result is four haploid daughter cells, each with one copy of the chromosomes.
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
Females: In oogenesis ,cells in the ovaries (called oogonia) produce primary oocytes by mitosis. Later these cells begin meiosis 1 during embryonic development and then stop. They remain in meiosis 1 until the female undergoes puberty.Usually one oocyte per menstrual cycle completes the first meiotic division, is released from the ovary, and moves into the oviduct. Fertilized eggs complete meiosis 2 producing a diploid zygote. All together, a woman produces and releases about 450 oocytes during reproductive phase of her life. In females, meiosis takes years to complete. Beginning with prophase 1 while an embryo, and continues until completion of meiosis 2 after fertilization. (can take anywhere from 12-50 years)
oogonium 2-3 months after conception---->primary oocyte---->secondary oocyte---->forms at 2-3 months after gestation. Remains in meiosis 1 until ovulation 12-50 years after formation. ------> Ootid, less than 1 day when fertilization occurs. Mature egg zygote total time=12-50 years
Males:The process of sperm production are called spermatogenesis. In the seminiferous tubules, cells called spermatocytes divide by meiosis to produce four haploid spermatids, which in turn differentiate to form mature sperm. Spermatogenesis begins at puberty and continues throughout life. Each day, several hundred million sperm are in various stages of maturation.
primary spermatocyte=16 days --------> Secondary spermatocyte=16 days-------> spermatid=16 days-------->Mature sperm (48 days total time)
Autosomal dominant inheritance:
In autosomal dominant disorders, heterozygotes have an abnormal phenotype. Unaffected individuals carry two recessive alleles and have a normal phenotype. Dominant traits usually have a distinctive pattern of inheritance and usually have affected family members in each generation.
>Every affected individual has at least one affected parent. Exceptions occur when the gene has a high mutation rate.
>Most affected individuals are heterozygotes with a homozygous recessive (unaffected) spouse, so each child has a 50% chance of being affected.
>because the trait is autosomal, the numbers of affected males and females are roughly equal.
>Two affected individuals may have unaffected children because most affected individuals are heterozygous.
>The phenotype in homozygous dominant individuals is often more severe than the heterozygous phenotype.
Marfan syndrome is an example of an inherited autosomal dominant trait affecting the skeletal system, eyes ,and cardiovascular system.
Autosomal Recessive Traits:
>For rare or relatively rare traits, affected individuals have unaffected parents.
>All the children of two affected (homozygous) individuals are affected
>The risk of an affected child from a mating of two heterozygous is 25%
>Because the trait is autosomal, it is expressed in both males and females who are affected in roughly equal matters. Either the male or the female parent can transmit the trait.
>In pedigrees involving rare traits, the unaffected (heterozygous) parents of an affected (homozygous) individual may be related to each other.
Cystic Fibrosis is an example of an autosomal recessive trait which affects the glands that produce mucus, digestive enzymes, and sweat.