The study of inheritance pattern in human beings is difficult, for you cannot make any individual cross with any other as can be done in plants and animals. Data can be collected by studying family history.
This is known as Pedigree studies, which records the data of all individuals in a family for as many generations as possible and the probability of the character in question appearing in future generations can be predicted. In the following paragraphs, we shall discuss briefly the inheritance pattern of some of the characters in man.
This is a trait in which the pigments in skin, eye, and hair are completely or near totally absent. The trait follows the Mendelian inheritance. Albinism is a recessive trait in comparison with the coloured character.
Albinos occur in all human races, but of quite frequent occurrence in Caucasoid race of Europe. The skin is pale, so also the iris and the hair is .like straw. Albinism is due to a recessive allele a present in the homozygous condition.
The wild gene for this is A (colored) and it is dominant. We shall study one or two examples to know the inheritance pattern. If two normal human beings (male and female) produce an albino child, the genotype should be as follows.
The two parents are heterozygous for the albino gene, but phenotypically they are normal as the dominant gene suppresses the albino gene. Among the progenies however phenotypically there are two kinds – 3/4 are normal (1, 2 and 3) and 1 /4 albino.
Genotypically however there are 3 categories viz. homozygous normal, heterozygous normal and homozygous albino in the ratio of 1:2:1. This is the typical ratio in the monohybird cross. The situation here is comparable to what happens when it is salted in the monohybrid cross.
Among the offspring who are normal 1/3 is completely normal (albino gene in absent); remaining 2/3 are normal but carriers; they carry the gene for albinism in heterozygous state.
Taste blindness to PTC:
Phenylthiocarbamide or PTC is a bitter compound, but curiously, certain percentage of human population cannot find out the taste: obviously because their taste buds cannot recognise PTC. Thus we have two groups, tasters and non tasters.
In the American population from where the data is available about 70% are tasters and 30% non tasters. The pedigree analysis of several families indicates that the ability or inability to taste PTC is a Mendelian character.
Tasters have one or two dominant gene T and taste blindness is due to recessive allele t. obviously the non tasters have a genotype tt. While the tasters are of two types – Homozygous TT or Heterozygous Tt. Marriage between heterozygous tasters would produce 3:1 ratio as in albinism.
Brown and blue eye color:
The color of the iris in human beings is either black (usually brown) or blue. Pedigree studies have once again shown that the difference in eye color is due to an allelic difference in a gene pair. Black eye color is governed by a dominant gene B; while blue eye color is decided by recessive gene b. Blue eyed individuals always has a genotype bb, while black eyed individuals may be homozygous BB or heterozygous Bb.
M – N blood groups:
Landsteiner and Levine have discovered that in all human beings the blood group may be divided into M, N and MN (Note this is different from A, B, AB and 0 blood group which is a multiple allelic character) and they follow the typical Mendelian inheritance.
These blood groups however do not cause any agglutination in human blood, so that there can be blood transfusion between these unlike in A, AB, AB and 0 groups. Then how are these blood groups M, N and MN can be identified?
They are identified by the agglutination they cause in the RBC of rabbits. The red blood cells of M people carry an antigen M and these can be agglutinated by the anti M sera of the rabbit (A rabbit produces anti M sera when injected with M blood).
The red cells of N people carry N antigen and it is agglutinated by N anti sera. MN people have both M and N antigens and can be agglutinated by both M and N antisera. The genes that control these blood groups function as follows M people will have Mm gene and N people have M” gene.
These two do not show any dominance hence the heterozygote show MN blood groups and are genetically M”M”. This can occur when M and N individuals marry among themselves. The following table gives the frequency of occurrence of blood groups in various marriages.
Deviations from Mendelism:
Mendel believed that the characters in individuals exist either as dominant or as recessive alleles only. He did not visualize any intermediate behavior. This is a deviation from Mendelian postulate and is called incomplete dominance.
The phenomenon of incomplete dominance has been seen in snapdragons (Antirrhinum majus), four o clock plants (Mirabilisjalapa), Andalusian fowls etc.
We shall study the example of 4 O’clock plants in 4° clock plants also just as in pea plants, the flower color is controlled by a pair of alleles. Red flower color is RR and white flower colour is rr genotypically.
When a red flowered plant is crossed with a white flowered one (both are pure breeding) the? Heterozygote progeny will be pink in color, (it is red in the case of a Mendelian cross) It appears as if the red and white are blended to produce the pink color. When the F2 is raised by selfing the F1 not only red and pink flower colors appear but the white color also appears in the ratio of 1:2:1 of red, pink and white respectively.