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MHT-CET : Biology Entrance Exam

MHT - CET : Biology - Sex Determination and Sex Linkage Know More

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Types of Sex Determination:

1. XX-XY Type:

  • Here, the females are homogametic having XX pair of sex chromosomes and males are heterogametic having XY pair of sex chromosomes.
  • Males are, therefore, responsible for determining the sex of the child as it depends on the type of sperm that fertilises the egg.
  • For example: drosophila and man.

2. XX-XO Type:

  • Here, the females are homogametic having XX pair of sex chromosomes and males have only one sex chromosome (X).
  • They, therefore, produce two types of sperms.
  • One type has an X chromosome, whereas the other is without any chromosome (O).
  • Sex of the offspring, therefore, depends upon the type of sperm that fertilises the egg. For example, grasshopper and bugs.

3. ZZ-ZO Type:

  • Here, the males are homogametic and produce only one type of sperms, i.e. (Z).
  • The females are heterogametic, producing two types of eggs - one containing Z chromosome and the other 'O'.
  • Here, the female is responsible for determining the sex of the offspring, which depends on the type of egg that fuses with the sperm. Example: butterflies and moths.

4. ZZ-ZW Type:

  • Here, the males are homogametic having ZZ pair of sex chromosomes, while the females are heterogametic having ZW pair.
  • Females are, therefore, responsible for determination of sex.
  • Example: birds and fishes.

Linkage

  • The main observations about the phenomenon of linkage by Morgan and Castle are as follows:

1.      The genes that show linkage are located on the same chromosome.

2.      A linkage group is formed by all the linked genes in a chromosome.

3.      The strength of linkage between two genes is directly proportional to the distance between them.

Types of Linkage
1. Complete Linkage:

  • When two or more genes tend to remain together on the same chromosome and are inherited together for many generations, the phenomenon is known as complete linkage.
  • The genes that are present on the non-homologous part of the sex chromosomes exhibit the phenomenon of complete linkage.

2. Incomplete Linkage:

  • When the linked genes tend to separate on some occasions during the course of inheritance, the phenomenon is known as incomplete linkage.
  • This occurs in the process of crossing over during gametogenesis.
  • The genes that are present on the homologous part of sex chromosomes exhibit the phenomenon of incomplete linkage.
  • The linkage may be autosomal linkage, if the linked genes are present on the autosomes or it may be sex linkage, if the linked genes are present on the sex chromosomes.

Sex-limited Genes:

  • These genes are present only in one sex and are located on the autosomes. For example, development of beard in males and development of breast in females.

Sex-influenced Genes:

  • These genes are also located on the autosomes.
  • Sex-influenced genes are dominant in case of males and recessive in case of females.
  • In males, a single gene can act in the presence of a male hormone, whereas females need two genes to show the effect.
  • Baldness is a sex-influenced character.

Sex-linked Inheritance:

Thomas Hunt Morgan was awarded the Nobel Prize for medicine in the year 1933 for his research work in genetics.

  • He, along with his other associates, studied the inheritance pattern for the eye colour in Drosophila melanogaster and found that the gene for eye colour is present on the X chromosome.
  • The Y chromosome does not carry any gene locus for eye colour.
  • Eye colour in drosophila is thus an X-linked character.
  • Like other X-linked diseases in man such as colour-blindness and haemophilia, it follows the criss-cross pattern of inheritance, which means that the male transmits the X-linked traits to his grand children through his daughters.


Colour-blindness:

  • Some people are totally colour-blind. They are known as monochromats.
  • Some people are partially colour-blind. They are known as dichromats. They generally suffer from the red-green colour vision deficiency.

Rh Factor Incompatibility:

  • Rhesus factor or Rh factor is an antigen, which was first discovered in the blood cells of Rhesus monkey by Landsteiner and Weiner.
  • Rh factor is due to the presence of a dominant gene called 'R' gene.
  • The recessive gene 'r' neither produces the antigen nor its antibodies.
  • Rh positive (Rh+ve) individuals may be RR (homozygous dominant) or Rr (heterozygous). They have the Rh+ ve antigen on their RBCs.
  • On the other hand, (Rh-ve) individuals are RR (homozygous recessive). They do not bear the Rh antigen.
  • Rh-ve blood responds to the Rh+ve blood by making antibodies against the Rh factor.
  • If a person with Rh+ve blood group donates blood to a person with Rh-ve blood group, the recipient will produce antibodies against the Rh+ve antigen of the donor.
  • The antibodies may not be sufficient to cause harm during the first transfusion, but during the second transfusion these antibodies may cause agglutination of the donor's blood and may prove to be fatal.
  • Similarly, if an Rh-ve mother bears an Rh+ve child, the first pregnancy may be safe. But if the second or third child is also Rh+ve, the situation is risky. In such a situation, the antibodies from the mother's blood enter the foetal blood and may cause agglutination of the foetal blood cells. Such a condition is called Erythroblastosis foetalis.
  • But now with proper medical treatment, the foetus can develop normally.

Down's Syndrome:

  • It is an autosomal abnormality.
  • It is due to presence of an extra chromosome on chromosome number 21. So the total number of chromosomes are 47.
  • Sometimes it is called as Trisomy 21.
  • Such individuals show severe mental retardation.
  • They also suffer from abnormalities in facial structure such as flattened nasal bridge and protruding lower lip.
  • They have Mongoloid face hence this syndrome is also referred to as Mongolism.

Kline Felter's Syndrome:

  • It is a sex chromosome abnormality.
  • The genotype of the individual in this case is XXY or XXXY.
  • These are underdeveloped males.
  • They also show some female characters like enlarged breasts.
  • They are mentally retarded.

Turner's Syndrome:

  • It is also a sex chromosome abnormality.
  • The genotype of such individuals is XO.
  • These are underdeveloped female individuals.
  • They are often sterile.
  • They have short stature and a characteristic wide-webbed neck.

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