Fig. 2.5. Process of meiosis

Fig. 2.6. Process of meiosis (continuation)

two sex cells unite, the new organism has the normal number of chromosomes for that species and the especial individual make-up.

Significance of meiosis: prior to the mixing of one individual’s genotype with that of another at fertilization, meiosis provides the opportunity for new combinations of the existing alleles of genes to arise as follows.

Crossing over takes place when breaks occur in chromatids at the beginning of meiosis, when the chromosomes are paired, and the broken end of each chromatid joins with the chromatid of an homologous chromosome. By this means, alleles of linked genes can become separated. This can result in the formation of new combinations of alleles and give four genetically different chromatids each of which ends up in a different gamete. This may lead to the formation of new phenotypes in the next generation. Alleles are exchanged between chromosomes.

Sex-Linked Traits

The human Y chromosome is unusual in that it carries very few genes. The X chromosome, however, carries many genes. A trait controlled by genes on the sex chromosomes is called a sex-linked trait.

One sex-linked gene determines red-green color-blindness and has trouble telling red colors from green colors.

Females are seldom color-blind. Each of their two X chromosomes has one gene that influences the trait of color vision. The gene for normal color vision is dominant over the gene for color-blindness.

More males than females tend to be color-blind. The Y chromosome has no genes for color vision. If the male’s only X chromosome has the gene for color-blindness, then here will be color-blind. This is because his Y chromosome has no color vision to mask the color-blind gene.

Another sex-linked gene causes the disease hemophilia. A person with hemophilia has blood that does not clot. Hemophilia is caused by a recessive gene on the X chromosome, so it occurs generally in males. Females, however, may be carriers. These females have one normal gene that dominates the recessive gene for the trait. Carriers do not have hemophilia, but they can pass the recessive gene to their offspring.

Mendelian inheritance in man

A number of man’s many inherited characteristics have been shown to follow the simple patterns of inheritance first observed by Mendel. One such characteristic is brachydactylism; this and other characteristics involving fingers and toes, including possession of extra ones, seem to involve simple dominants. Another simple dominant trait is tongue rolling. Can you roll your tongue? Can your parents? What is your genotype for tongue rolling? If you cannot roll your tongue, does this mean that neither of your parents can? This may seem to be a very trivial sort of characteristic, one that neither natural nor society would favor, but oddly enough, very small differences such as this are often the reflection of more fundamental differences which may have considerable importance over an evolutionary time span.

Of more immediate consequence are a number of congenital diseases which are the result of the coming together of recessive genes. One such disease is sickle cell anemia. In persons homozygous for the stickling gene, a large proportion of the red blood cells “sickle”- that is, form a sickle shape-and then clog the small capillaries, causing blood clots and depriving vital organs of their full supply of blood. This produces continuous, painful illness and, usually, death at an early age. About 4 percents of the population in certain tropical regions in Africa are born with sickle cell anemia, and almost half of the members of some African tribes are known to carry the recessive gene. In this country, it is found almost exclusively among blacks.

Mental deficient in infants, is also the result of a “double dose” of a recessive gene; so is Tay-Sachs disease, which appears almost exclusively among Jews of Central European ancestry.

Human Genetics Disorders

As in Drosophila, the Y chromosome of a man carries much less genetic information than the chromosome. Genes for color vision, for example, are carried on the X chromosome in humans but not on the Y chromosome. Color-blindness is produced by a recessive allele of the normal gene. The normal allele is dominant; a woman with the normal allele and one X chromosome with the allele for color blindness will have normal color vision. If she transmits the X chromosome with the recessive allele to a daughter, the daughter also will have color vision if she receives a normal X chromosome from her father (that is, if he is not color-blind). If, however, the X chromosome with the recessive allele is transmitted to a son, he will be color-blind since, lacking a second X chromosome, he has only the recessive allele.

Blood Groups

Probably the most familiar characteristic in human beings that is determined by a single group of alleles is the ABO blood series. The existence of blood groups was discovered in 1900 by Karl Landsteiner. Mixing samples of blood taken from members of his laboratory staff, Landsteiner found that sometimes the red blood cells would clump together, or agglutinate, & sometimes they would not. He worked out four major groups of blood: A, B, AB, & O. Before long, it was established that these blood types are inherited according to Mendelian laws.

If your blood type is A, this means that on the surface of your red blood cells is a specific polysaccharide, A, that is not found on the surface of blood cells of persons with type O or B. Persons with type B have polysaccharide B on their red blood cells; persons with type AB have two types of polysaccharides; & persons with type O have neither A nor B polysaccharides. People of blood type A have in their blood antibodies to B. Similarly, type B has antibodies to A. Type O individuals have antibodies to both A & B, while type AB ones have neither. As a consequence, if you – still hypothetically blood type A- are given a transfusion of blood type B or blood type AB, your body’s antibodies against the B or AB cells will agglutinate the donor B-type cells in their bloodstream. This reaction can be so violent that it is sometimes fatal. You can receive O cells safely, however, since they contain no polysaccharide that your body will recognize as foreign.

The agglutination phenomenon is caused by antibodies, globular proteins that react against foreign substances in the blood.