Book - Aids to Embryology (1948) 14
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Baxter JS. Aids to Embryology. (1948) 4th Edition, Bailliere, Tindall And Cox, London.
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Chapter XIV The Transmission of Hereditary Characters
Every potential individual, that is, every fertilised ovum, commences development with a complement of hereditary factors (genes) derived from both parents at the time of union of the germ cells in fertilisation. These inherited factors operate during development, both before and after birth, to produce an individual resembling the parents. The environment, both prenatal and post-natal, may influence some of these hereditary characters and modify them, but there are certain characters which cannot be affected by the environment and hence are said to be determined at fertilisation. Examples of these are the blood group to which the person belongs and the colour of the eyes. The study of these hereditary factors, and the laws which govern their transmission from parents to offspring, is known as the science of genetics.
Genetic laws are fundamentally the same for plants and animals, and since many generations of plants and lower animals can be investigated in a relatively short period of time, the study of genetics has largely been based on them. In addition, certain forms possess chromosomes particularly favourable to the genetic analysis of experimental procedures. But in all cases where genetic laws have been tested in lower forms they have been found applicable to human hereditary.
Parent ^ Tfc\l (T) "H Dw&rf (d)
fj. AU hybrid Tidls (Td)
1 ^ l
Td
Td
Hybrid Tidls
dd
Pure Dvj&rfs
1 1 1 r
Td Td dd TT
1
i
d Td dd
k dd All Pure Ow^ris
Fig. 42 . - Schematic Table showing Mendelian Inheritance AS APPLIED TO TALL AND DWARF PEAS.
T = tall ; d = dwarf.
The fundamental genetic laws are based on the experiments of Mendel (1866) on garden peas. Mendel studied the inheritance of a number of characters of this form, of which tallness and dwarfness may be taken as a typical example. If a tall pea and a dwarf pea were cross-fertilised and the resultant peas planted, all of the plants that grew from them were tall. These he called the first filial generation (Fj). These tall cross-bred plants were allowed to produce peas by self-fertilisation, and when such were sown the second filial generation (F a ) showed some plants that were tall and some that were dwarf, in the proportion of three tall to one dwarf. Peas derived from self-fertilisation of each of the dwarf plants were then sown, and they produced dwarf plants only ; seed from some of the self -fertilised tall plants produced both tall and dwarf offspring, in the proportion of three tall to one dwarf ; the remainder of the seed from the tall plants produced nothing but tall offspring in the third (F s ) generation (see Fig. 42 for details).
From these experiments it is clear that an attempt was being made by nature to separate the original pure characters of tallness and dwarfness in the parent stock from the hybrids. This is known as the law of segregation. The character of tallness, which was found in the F x generation, is known as a dominant character since it overshadows the recessive character of dwarfness. These characters are known to be caused by certain elements on the maternal and paternal chromosomes called genes.
Parents
— Y
+ dd
4
P&rent G&mefes
T
»
d
j 1
F 1
.Td
Fj. Q&metes
_T.
d. T. d.
F t T ~ v7~ 1 ' >
F * TT. Td. Td. dd.
Fig. 43. — Schematic Table to show Segregation of the Tall and Dwarf Genes in the Gametes of Peas.
T = tall ; d = dwarf.
Consideration of Fig. 43 will indicate how these dominant and recessive factors become separated out during breeding. The parent stock possessed either the character of tallness or dwarfness in pure form, that is, when self -fertilised they always bred true. The first filial generation (Fj) were all tall since the gene for dwarfness was present but masked by the dominant tall (T) gene. The gametes (sex cells) of the Fj generation contained the genes for tallness and dwarfness in equal numbers so that when such plants were self-fertilised the dominant T gene would be present in three out of four of the offspring and they would all be tall plants. In one out of the four, two d (dwarfness) genes would be present and hence these plants would be dwarfs, and on further selffertilisation would continue to breed truly dwarf. One of the three tall plants would contain only genes for tallness and so on self-fertilisation it would continue to breed true for the character of tallness. The remaining two tall plants contained Td genes, and further inbreeding of them would result in the formation of dominant and recessive forms in the ratio of three dominant to one recessive.
In human genetics there are certain clear-cut cases of inheritance due to dominant factors. Examples of these are congenital brachydactyly, syndactyly, and congenital night blindness. Also black hair colour is dominant over brown hair colour, and so is brown iris colour over blue.
Mendelian laws have been applied to animals and they have been found to hold equally good for them. For example, the colour of the ordinary brownishgrey wild mouse is due to bands of pigment in its hair ; black at the base and yellow at the tip. This is known as “ agouti †colouration. But in fancy mice there is a variety in which the hair appears black, due to the absence of the yellow tip to the hair. If a pure agouti mouse is crossed with a black or non-agouti, the offspring at Fj are all agouti in colour — that is, the agouti factor is dominant over the non-agouti, which is the recessive factor. If however these hybrids are inbred, the F 2 generation comes out as 25 per cent, pure agouti, 50 per cent. hybrid agouti, and 25 per cent, pure non-agouti. This shows the Mendelian ratio of 1 : 2 : 1.
In both the agouti (A) and the non-agouti (b) mice there is a variety with straight hair (S) and a variety with wavy hair (w). If a non-agouti wavy mouse (bw) be crossed with an agouti straight mouse (AS), the F! animals will be agouti straight (AbSw) since A is dominant over b, and S over w. Then if these F x hybrids are inbred the genes will be segregated out as follows :
Agouti straight (AS) Agouti wavy (Aw) Non-agouti straight (bS) Non-agouti wavy (bw) .
9
3
3
1
In these experiments two new types have been produced — the agouti wavy and the non-agouti straight. The combinations of dominant and recessive characters which have produced these adult forms are shown in Fig. 44. It will be seen that the AbSw hybrids gave rise to four gametes in equal numbers :
AS : Aw : bS : bw.
These combinations take place in the ova as well as in the spermatozoa and since every kind of ovum is likely to be fertilised by any kind of spermatozoon, there are 4 s = 16 possible combinations yielding 9 agouti straight, 3 non-agouti straight, 3 agouti wavy and 1 non-agouti wavy. The several genes of the cross are being segregated out as is shown in Fig. 44. This rule holds good for other possible combinations.
The blood groups A, B, AB and O are transmitted from parents to offspring in accordance with Mendelian laws. The principles involved may be briefly stated as follows :
When red blood corpuscles of one animal species are mixed with the serum of another species they become clumped together or agglutinated. There is in the serum a substance (an agglutinin) which attaches itself to an agglutinable substance (agglutinogen) in the red cells and clumping occurs. In man, blood cannot be transfused from one person to another unless the two bloods are compatible. The important factor is the agglutinogen in the red cells of the donor ; if that is incompatible with the agglutinin in the recipient’s serum, serious consequences will result in transfusion. From the standpoint of heredity the A and the B agglutinogen factors are dominant over the O factor. If a child’s blood belongs to either group A or B one or other of its parents must have blood belonging to such a group.
AS
Aw
bS
bw
Sperms
" J t
A5
A A 55
Pure Agouti.
^ure
Straight.
AASw Pure Agouti. Hybrid Straight.
AbS5
Hybrid Agouti.
Pure
Straight.
AbSw Hybrid Agouti, | Hybrid
Straight. I
Aw
A A5w
Pure Agouti. Hybrid
Straight.
AAww
Pure Agouti. Pure Wave.
AbSw
Hybrid Agouti. Hybrid Straight.
Abww 1 Hybrid j
Agouti.
Pure Wave. #
bS
AbSS
Hybrid Agouti .
Pure
Straight.
AbSw
4
Hybrid
Agouti.
Hybrid
Straight.
bbSS
Pure
Mon Agouti . Pure
Straight.
bbSw
Pure
Mon Agouti . Hybrid Str&igW t .
bw
Ab Sw
Hybrid Agouti . Hybrid Straight.
Abww
Hybrid Agout i .
Pure
Wave
bbSw
Pure
Non Agouti . Hybrid Straight.
bbw w
Pure
Non Agouti.
Pure
Wave.
Fig . 44. - Schematic Table to show the possible Combinations of the genes for straight hair, wavy hair, Agouti Colour and Non-Agouti Colour in Mice. A = agouti ; S = straight ; b = non-agouti ; w - wavy.
Genes do not always act in the relatively simple manner just described. There may be incomplete
Parents
XhY
[H&emopluVic
Daughter free from disease but CAN TRANSMIT.
XbX?
Marries
Son free from ® AY disease but CANNOT TRANSMIT, normd male.XY
XkY <?
T
XwX?
~~\ —
XX?
“1
XY o* *
fUemophi'ic Son. Daughter free Normal Daughter. Normal Son.
from disease but CA N
TRANSMIT.
Fig. 45. — Schematic Table to show the Transmission of haemophiliac genes through three generations.
X, female chromosome with non-haemophiliac gene ; Xh, female chromosome with haemophiliac gene ; Y, normal male chromosome.
action of a gene or there may be a lethal gene present. An example of the latter is the disease in man called Huntingdon’s chorea. Here the disease appears first about 35 years of age and the patient rarely lives longer than ten to twelve years after that time. The carrier is eventually killed by his or her predisposition to the disease but only after he or she has passed on the lethal gene to the offspring.
Sex-linked characters are important in certain rare diseases such as haemophilia. The factor here is transmitted on an X chromosome. If a haemophiliac male marries a normal unrelated female all their daughters must necessarily receive one haemophiliac X chromosome from the father and a normal X chromosome from the mother. The sons all receive a normal Y chromosome from the father and a normal X chromosome from the mother. They therefore do not suffer from the disease, nor can they transmit it to their offspring. But the daughters all have a haemophilac X chromosome, and they will necessarily transmit the disease to 50 per cent, of their sons after marriage with a normal male.
Appendix
Ossification Times
The ossification periods have been widely studied, but the work of Hess (i9 2 3)> although differing in some respects from that usually given in textbooks on osteology, is now recognized as being the most accurate series of observations at present available. These observations may be summarized as follows :
7th week : Mandible, diaphysis of clavicle.
8th week : Diaphysis of humerus, radius, and ulna. 9th week : The terminal phalanges, the 2nd and 3rd basal phalanges, and the 2nd and 3rd metacarpal bones of the hand ; the ilium ; the 2nd and 3rd metatarsals, and the terminal phalanges of the foot bones.
10th week : 1st rib, and the 4th and 1st basal phalanges of the hand.
10th to 12th week : 4th, 5th, and 1st metatarsals.
11th to 1 2th week : The basal phalanx of the 5th digit, and the middle phalanges of the 2nd, 3rd, and 4th digits of the hand.
13th to 14th week : All the remaining metatarsals and the phalanges of the foot except the last phalanx of the 5th digit.
13th to 1 6th week : The middle phalanx of the 5th finger.
1 6th to 17th week : Descending ramus of ischium.
17th to 20th week : Odontoid process of axis.
2 1st to 24th week : Sternum.
2 1st to 28th week : Descending ramus of pubis.
2 1st to 29th week : Calcaneus (os calcis).
24th to 32nd week : Talus (astragalus).
33rd to 36th week : Last phalanx of the 5th digit of foot.
35th to 48th week : Distal epiphysis of femur, and occasionally the proximal epiphysis of tibia.
The wide variation in these figures might be accounted for by the statement of Pryor (1927), that ossification begins at an earlier date in female foetuses than in the male.
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Cite this page: Hill, M.A. (2024, April 16) Embryology Book - Aids to Embryology (1948) 14. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Aids_to_Embryology_(1948)_14
- © Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G
