6.genetics 🥂

• Pleiotropy: The ability of a gene to have multiple phenotypic effects
• Example: Phenylketonuria (PKU)
• Antigens causing 'ABO blood grouping: Antigen A and Antigen B
• These antigens are present on the plasma membrane of RBC
• Antibodies of 'ABO blood grouping:

  • Isoagglutinin A (anti A)
  • Isoagglutinin B (anti B)
• These antibodies are present in blood plasma
• Multiple alleles: If a gene has more than two alleles, they are multiple alleles
• Example: In humans, ABO blood groups are the best example for multiple allelism
• Erythroblastosis foetalis:

  • An alloimmune condition that develops in an Rh positive foetus whose father is Rh positive and mother is Rh negative
  • Antibodies developed against the Rh antigen in the mother cross the placenta and destroy the RBC cells of the Rh +ve foetus during the second pregnancy
• Genotypes:

  • Child's blood group 'O' genotype: IOIO
  • Father's blood group 'A' genotype: IAIO
  • Mother's blood group 'B' genotype: IBIO
  • Possible genotypes of other offspring: IAIB, IAIO, IBIO, IOIO
• Genetic basis of blood types in ABO system in man:

  • Three alleles IA, IB, IO of gene I are responsible for ABO blood grouping
  • Genotypes for blood groups:
  • IAIA/IAIO for A blood group
  • IBIB/IBIO for B blood group
  • IAIB for AB blood group
  • IOIO for O blood group
• Polygenic inheritance is a cumulative effect of two or more genes on a single phenotypic character
• Example: Skin colour in humans
• In human beings, XX chromosomes refer to female and XY chromosomes refer to male. The Y chromosome determines the sex of the child
• In Drosophila, the Y chromosome lacks male determining factors but contains only genetic information essential to male fertility
• Heterogametic sex determination systems involve the formation of two different types of gametes. Example: XY in males
• Homogametic individuals form only one type of similar gametes. Example: XX in females
• Haplo-diploidy is a mechanism of sex determination where the sex of the offspring is determined by the number of sets of chromosomes. Example: Honeybees
• In honeybees, fertilized eggs develop into females and unfertilized eggs develop into males

  • Males have half the number of chromosomes (haploid)
  • Females have double the number of chromosomes (diploid), hence the name haplo-diploidy
• Barr bodies are the inactive X-chromosome in a female somatic cell
• The extra X-chromosome undergoes heterochromatinization and becomes inactive during early embryonic development
• Klinefelter syndrome is a genetic disorder caused by trisomy in the 23rd pair, with a genotype of 47XXY
• Klinefelter males have an extra X chromosome and Barr body is present

  • Symptoms include hypogonadism, reduced fertility, slight enlargement of breasts
• Turner's syndrome is a genetic disorder caused by monosomy in the 23rd pair, with a karyotype of 45, X
• Barr bodies are absent

  • Symptoms include short stature, gonadal dysgenesis, webbed neck, flat chest with widely spaced nipples
• Down's syndrome is a genetic disorder that causes delay in physical and intellectual development
• It is due to the presence of an additional copy of the chromosome numbered 21
• Lyonisation is a process by which one of two copies of X-chromosome present in the body cells of female mammals is inactivated
• The inactive X-chromosome is transcriptionally inactive and called heterochromatic body
• Sex-linked inheritance is the inheritance of a character that is influenced by a gene located on one of the sex chromosomes
• Examples include Colour blindness and Haemophilia
• Hemizygous condition is when the genes are present on non-homologous portion of either X-chromosome or Y-chromosomes
• For these genes, related alleles are absent on corresponding paired chromosomes
• Criss-Cross inheritance is X linked inheritance where the X linked character present in father is inherited to the daughter, and later to her son
• The character is passed from father to grandson through the daughter, and from mother to granddaughter through son
• Sex-linked recessive characters are more common in male human beings because these genes are located in the X-chromosome
• Males need only one copy of the mutant allele to express the phenotype as they possess only one X-chromosome
• Sex-linked dominant characters are more common in female human beings as females have 2X-chromosomes, giving them double chance to inherit the character
• Sex limited characters are autosomal characters that are limited to only one sex
• They are due to internal hormonal environment