Gene And Inheritance , Single-factor inheritance



FROM its parents an individual inherits the characteristics of the species, e.g. man inherits highly developed cerebral hemispheres, vocal cords and the nervous co-ordination necessary for speech, a characteristic arrangement of the teeth and the ability to stand upright with all its attendant skeletal features. In addition, he inherits certain characteristics peculiar to his parents and not common to the species as a whole, e hair and eye colour, blood group and facial appearance. The study of the method of inheritance of these "characters s called genetics.

in sexual reproduction a new individual is derived only from the gametes of its parents. The hereditary information must therefore be contained in the gametes. For many reasons, this information is thought to be present in the nucleus of the gamete and located on the chromosomes

Genes and inheritance

The term gene was originally applied to purely theoretical units or particles in the nucleus. These particles, in conjunction with the environment, were thought to determine the presence or absence of a particular characteristic. It was suggested that the genes may correspond to regions on the chromosomes and may consist of a large group of chemicals linked in a particular sequence in the chromosome.

In some cases, the presence of a single gene may determine the appearance of one characteristic, as in the eye colour of the fruit fly, but most human characteristics are controlled by more than one gene. This multifactorial inheritance and the impossibility with humans of breeding experiments, make it difficult to collect and present simple, clear-cut genetical information about man. In order to provide some clear ideas about heredity, simple cases amongst other animals will first be considered.

Single-factor inheritance. If a pure-breeding, ie. homorygous (see below) black mouse is mated with a pure-breeding brown mouse, the offspring will not be intermediate in colour, ie. dark brown or some combination of brown and black, but will all be black. The gene for black fur is said to be dominane to that for brown fur because, although each of the baby mice, being the product of fusion of sperm and egg. must carry genes for both blackness and brownness, only that for blackness is expressed in the visible characteristics of the animal. The gene foe brown fur is said to be recessive, The black babies are called the first filial or E, generation. If, when they are mature, these black mice are mated amongst themselves, their of speing the F generation will include both black and brown mice and if the total number for all the F, families are added up the eate of black to brown babies will be approximately 3 to1 must net be assurmed, however that if two black E, mice have 4 babies 3 will be black and one brown ln a mating which prodced soy 3 babies, it would not be at all unasual to find all black, or 5 biack to 3 brown ete. The ratio 3 :1 appears only when large numbers of individuals are considered.

The appearance of brown fur in the second generation is proof of the fact that the F black mice carried the recessive gene for brown fur even though it did not find expression in their observable features.
In explanation, it will be assumed that a pure-breeding black mouse carries, on homologous chromosomes. a pair of genes controlling the production of black pigment. The genes are represented in subsequent diagrams by the letters BB, the capital letters signifying dominance.
In the same position on the corresponding chromosomes in brown mice are carried the genes bb for brownness. The genes B and b are called allelomorphic genes or alleles. During the formation of gametes the process of meiosis will separate the homologous chromosomes, so that the gametes will contain only one gene from each pair. All the sperms from the pure-breeding black parent will carry the factor B and all the eggs from the brown parent will carry the factor b. When the gametes fuse, the zygotes will contain both factors B and b but since B is dominant to b, only the former gene is expressed, i.e.
the offspring will all be black.
When, later on, these black F, mice produce gametes, the process of meiosis will separate the chromosomes carrying the
B and b factors so that half the sperms of the male parent will carry B and half will carry b. Similarly, half the ova from the female will contain B and half b. At fertilization there are equal chances that a B-carrying sperm will fuse with either an egg carrying the B gene or an egg with the b gene so producing either a BB or a Bb zygote. Similarly there are equal chances of a b-carrying sperm fusing with either a B- or a b- carrying ovum to give bB or bb zygotes. This results in the theoretical expectation of finding, in every four F2 offspring.
one pure-breeding black mouse BB, one pure-breeding brown mouse bb, and two "impure" black mice Bb.
The separation at meiosis of the alleles B and b into different gametes is called segregation. The pure-breeding black (B and brown (bb) mice are called homozygous for coat colour and the "impure" black mice (Bb) are called heterozygous. The heterozygous mice will not breed true, ie. if mated with each other their litters are likely to include some brown mice. The homozygous BB mice mated together can produce only black otfspring and the bb homozygotes only brown oflspring Genotype and phenotype. The BB mice and Bb mice will be indistinguishable in their appearance, ie. they will both Ba black fur and they are thus said to be the same phenotypes, in other words they are identical in appearance for a partica Characteristic, in this case blackness. Their genetic conshons, of genotypes, however, are different, namely BB and Bb In short. the black phenotypes have different genotype distinguish between the black phenotypes, it further breeding experiments by mating them brown, homozygous, recessive individual. 

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