Haemophilia is an X-linked recessive disorder. Males (XY) express the disease if their single X carries the recessive allele; females (XX) need two copies. Hence greater frequency in males.
ABO blood group is controlled by three alleles (IA, IB, Io) at the same locus (multiple alleles). IA and IB are codominant; Io is recessive.
Parents IAIo (type A) × IBIo (type B) produce gametes IA/Io and IB/Io giving offspring IAIB (AB), IAIo (A), IOIB (B) and IOIO (O). Presence of A, AB and B is consistent with these genotypes.
Multiple alleles are different forms of a gene at the same locus; they do not map to different loci. A diploid individual carries only two alleles (one per homologous chromosome).
If parents are phenotypically A (IAIo) and B (IBIo), crossing IA/Io × IB/Io can produce IAIB (AB), IAIo (A), IOIB (B) and IOIO (O). Thus all four phenotypes are possible.
Gametes: IA or IO from IAIO; IA or IB from IAIB. Possible offspring: IAIA (A), IAIB (AB), IOIA (A), IOIB (B). IOIO (O) cannot be formed, so O phenotype is not possible.
Dd × Dd gives genotypes 1 DD : 2 Dd : 1 dd. DD and Dd are Rh positive (3/4); dd is Rh negative (1/4). Thus about one fourth will be Rh negative.
Parents IAIB × IAIB produce gametes IA and IB. Offspring genotypes: IAIA (A), IAIB (AB), IBIB (B). IO (O) cannot occur because neither parent carries Io.
For a child with blood group O (io/io), each parent must provide an io allele. Thus both parents must be heterozygous carriers: IAio (type A) and IBio (type B).
In both XO and XY systems males produce two different kinds of gametes regarding sex chromosome (X and O or X and Y) and are therefore the heterogametic sex (male heterogamety).
Type O negative blood lacks A, B and Rh antigens and is the universal donor for red cells; it minimizes risk when blood type is unknown.
- Site of fertilization: External – outside the body (in water); Internal – inside the female reproductive tract.
- Gamete release: External – usually large numbers of gametes released into environment; Internal – fewer gametes, direct delivery (e.g., via copulation).
- Parental care: External – usually little or no parental care; Internal – often greater parental investment and protection of developing embryo.
- Fertilization chances: External – lower probability per gamete (compensated by quantity); Internal – higher probability of successful fertilization.
- Examples: External – most fishes, many amphibians; Internal – reptiles, birds, mammals.
- Extent: Lizard – limited regeneration (e.g., tail regrowth but usually not whole body); Planaria – extensive regeneration (a small fragment can regenerate an entire worm).
- Cellular mechanism: Lizard – regeneration involves dedifferentiation and proliferation of cells at wound to form a blastema that replaces some structures; Planaria – regeneration is driven by pluripotent stem cells (neoblasts) that proliferate and differentiate to replace all tissues.
- Complexity of structures restored: Lizard – regenerates mainly tail tissues (cartilage, muscle, skin) but regenerated tail is structurally simpler; Planaria – can regenerate complete head, brain and other organs with restoration of proper polarity.
- Examples: Lizard – autotomized tail regrowth; Planaria – regeneration after transverse or longitudinal cutting.
If the normal woman is homozygous normal (XA XA) and the man is colourblind (Xc Y), all daughters receive Xc from father and XA from mother → carriers (XA Xc); all sons receive Y from father and XA from mother → normal (XA Y).
Down's syndrome is caused by trisomy of chromosome 21 (presence of an extra copy of chromosome 21).
Klinefelter's syndrome results from the presence of an extra X chromosome in males, producing the karyotype 47,XXY; it leads to hypogonadism, gynecomastia and infertility.
Turner's syndrome (45,XO) females typically have short stature, rudimentary (streak) ovaries, underdeveloped secondary sexual characteristics (breasts) and often a small or immature uterus.
Patau's syndrome is trisomy 13 (an extra chromosome 13) and is associated with severe developmental defects and low survival.
Type O (especially O negative) is the universal donor for red cells (no A/B/Rh antigens). Type AB is the universal recipient (has both A and B antigens and does not form anti-A or anti-B antibodies).
The ZW–ZZ system is characteristic of birds: females are heterogametic (ZW) and males are homogametic (ZZ). Some other groups may show variations, but birds are the classical example.
Blood group AB results from codominance of IA and IB alleles; both A and B antigens are expressed equally on red cells.
In the ZW-ZZ system females are heterogametic (ZW) and males are homogametic (ZZ). Thus statement b is incorrect. Statements a, c and d are correct: ZW-ZZ occurs in birds and some reptiles; males (ZZ) produce only one type of sex chromosome-bearing gamete (Z); gypsy moth exhibits ZW females and ZZ males.
Haplodiploidy is a sex-determination system in which males are haploid (develop from unfertilized eggs) and females are diploid (develop from fertilized eggs). Common in Hymenoptera (honeybees, ants, wasps). Key terms: haploid, diploid, arrhenotoky.
In haplodiploidy, the sex of the offspring is determined by the number of sets of chromosomes it receives. Fertilized eggs develop into females (Queen or Worker) and unfertilized eggs develop into males (drones) by parthenogenesis. It means that the males have half the number of chromosomes (haploid) and the females have double the number (diploid).
Heterogametic sex: individuals produce two types of sex chromosomes in gametes (e.g., human males XY produce X- and Y-bearing sperm; birds males are ZZ so not heterogametic). Homogametic sex: individuals produce only one type of sex chromosome in gametes (e.g., human females XX produce only X-bearing eggs; bird males ZZ produce only Z-bearing sperm). Example: male heterogamety = XY system (humans); female heterogamety = ZW system (birds).
Heterogametic Sex: * Organisms producing two different types of gametes. * Example: Human male. Sperm with X chromosome Sperm with Y chromosome Homogametic Sex: * Organisms producing only one type of gametes. * Example: Human female. Every egg produced contain X chromosomes.
Lyonisation (X‑chromosome inactivation) is the random inactivation of one X chromosome in each somatic cell of female mammals early in embryogenesis, producing a Barr body. It provides dosage compensation between XX females and XY males and leads to mosaic expression of X‑linked genes.
Lyonisation is the process of inactivation or silencing of one of the two X chromosomes in female mammals. In female cells, one X chromosome is randomly selected and becomes highly condensed into a structure called a Barr body, rendering most of its genes transcriptionally inactive. This process occurs early in female development and is random, meaning that in some cells the maternal X chromosome is inactivated while in other cells the paternal X chromosome is inactivated. Lyonisation ensures dosage compensation, equalizing X-linked gene expression between males (who have one X chromosome) and females (who have two X chromosomes). This prevents females from having a double dose of X-linked gene products. The inactivated X chromosome remains condensed and transcriptionally silent throughout the life of the cell and is passed to daughter cells during cell division, maintaining the same pattern of X inactivation in clonal cell populations.
Criss-cross inheritance describes the pattern in X‑linked traits where an affected male transmits the allele to all daughters (who become carriers) and those carrier daughters can transmit the trait to their sons. Thus the trait appears to 'cross' sexes each generation (father → daughter → grandson).
Criss-cross inheritance is the pattern of inheritance of X-linked genes where the trait appears to skip generations or pass from one sex to the other in an alternating pattern. In this inheritance pattern, a gene is transmitted from an affected male parent to his carrier daughter (who is phenotypically normal if the allele is recessive) and then to an affected grandson through the daughter. Alternatively, a carrier female can pass the recessive allele to her son who expresses the trait, and then the trait can reappear in the granddaughter if she inherits the allele from her affected father. The characteristic criss-cross pattern occurs because males have only one X chromosome, so they express X-linked recessive traits, while heterozygous females typically do not express recessive traits but can transmit them to their offspring. Classic examples of criss-cross inheritance include X-linked traits such as colour blindness and haemophilia, where affected males have unaffected carrier daughters whose sons may be affected.
Males are hemizygous for X chromosome (only one X). A single recessive allele on the X is sufficient to express the trait. Females have two Xs and must have two copies of the recessive allele to express the trait, so expression is rarer in females.
Sex-linked recessive characters are more common in male human beings because males have only one X chromosome (XY), whereas females have two X chromosomes (XX). In males, a single recessive allele on the X chromosome is expressed phenotypically because there is no corresponding allele on the Y chromosome to mask it. Males are therefore hemizygous for X-linked genes, meaning they have only one copy of each X-linked gene. In contrast, females require two copies of a recessive allele (one on each X chromosome) to express the recessive phenotype. If a female has only one recessive allele, she is a carrier and typically expresses the dominant phenotype. This fundamental difference in chromosome composition makes males much more likely to display X-linked recessive traits such as colour blindness and haemophilia, even though the allele frequency may be the same in both sexes.
Holandric genes are genes located on the Y chromosome and therefore transmitted strictly from father to son. They govern Y‑linked traits (e.g., some determinants of maleness, Y‑specific markers).
Holandric genes are genes present in the differential region of the Y chromosome that have no corresponding alleles on the X chromosome. These genes are found only on the Y chromosome and are therefore exclusive to males. Holandric genes are also called Y-linked genes. Since males have only one Y chromosome and females have none, holandric genes are transmitted directly from father to son and appear in every male descendant of an affected male. These genes do not show the typical criss-cross inheritance pattern seen with X-linked genes. Examples of holandric genes include those controlling male sex determination and spermatogenesis. The inheritance of holandric genes is strictly patrilineal, with the trait appearing in all male offspring of an affected father but never in female offspring or in any descendants through females.
Symptoms: intellectual disability/mental retardation, seizures, microcephaly, pale skin and hair (hypopigmentation), musty or 'mousy' body odor, hyperphenylalaninemia due to deficiency of phenylalanine hydroxylase. Early dietary management prevents severe outcomes.
Phenylketonuria (PKU) is an autosomal recessive metabolic disorder caused by deficiency of the enzyme phenylalanine hydroxylase, which normally converts the amino acid phenylalanine to tyrosine. The primary symptoms include severe mental retardation or intellectual disability if the condition is not detected and treated early in life. Affected individuals typically have light pigmentation of the skin and hair due to reduced melanin synthesis, as tyrosine is a precursor for melanin. A characteristic musty or mousy odour is often present in the body odour and urine of affected individuals. Phenylpyruvic acid, a metabolite of phenylalanine, accumulates and is excreted in the urine. Early diagnosis through newborn screening and implementation of a phenylalanine-restricted diet can prevent the neurological damage and intellectual disability associated with this condition, making early detection and dietary management crucial for affected individuals.
Symptoms: intellectual disability, characteristic facies (flat facial profile, epicanthic folds, upward slanting palpebral fissures), hypotonia, short stature, single transverse palmar crease, clinodactyly, congenital heart defects, increased risk of leukemia; karyotype 47, +21 (trisomy 21) usually due to nondisjunction.
Down's syndrome, also called trisomy 21, is a chromosomal disorder caused by the presence of three copies of chromosome 21 instead of the normal two copies. The characteristic symptoms include severe mental retardation or intellectual disability of varying degrees. Affected individuals show defective development of the central nervous system, resulting in developmental delays and learning difficulties. Distinctive facial features include increased separation between the eyes (hypertelorism), a flattened nose with a low nasal bridge, and malformed ears that may be small or positioned lower than normal. The mouth is characteristically held open and the tongue protrudes, a condition called macroglossia. Additional features often include short stature, poor muscle tone (hypotonia), congenital heart defects, and increased susceptibility to infections. Individuals with Down's syndrome may also have hearing and vision problems. Despite these challenges, many individuals with Down's syndrome can learn to perform daily activities and some can be employed with appropriate support and education.
ABO blood group is controlled by a single gene (I) with three common alleles: I^A, I^B and i. I^A and I^B are codominant — heterozygote I^A I^B expresses both A and B antigens — while i is recessive (no A or B antigen = O). The gene encodes glycosyltransferases that modify the H‑antigen on red cell membrane to produce A or B antigens. Genotype–phenotype examples: I^A I^A or I^A i → blood group A; I^B I^B or I^B i → B; I^A I^B → AB; ii → O. ABO locus is on chromosome 9. Applications: blood transfusion compatibility, paternity testing.
The ABO blood grouping system in humans is controlled by a single gene with three alleles designated as I^A, I^B, and i. These alleles show codominance and complete dominance relationships. The I^A and I^B alleles are codominant to each other, meaning both are expressed when present together. Both I^A and I^B alleles are completely dominant over the recessive i allele. The four possible blood groups result from different combinations of these three alleles. Blood group A individuals have genotype I^A I^A or I^A i, possessing A antigens on red blood cells. Blood group B individuals have genotype I^B I^B or I^B i, possessing B antigens on red blood cells. Blood group AB individuals have genotype I^A I^B, possessing both A and B antigens on red blood cells due to codominance of the two alleles. Blood group O individuals have genotype ii, lacking both A and B antigens. The plasma of each blood group contains naturally occurring antibodies against the antigens that are absent from their own red blood cells. Blood group A individuals have anti-B antibodies, blood group B individuals have anti-A antibodies, blood group AB individuals have no anti-A or anti-B antibodies, and blood group O individuals have both anti-A and anti-B antibodies. This genetic basis explains the inheritance patterns of blood groups and the compatibility requirements for blood transfusions.
Human sex is chromosomally determined: females are XX, males are XY. Sperm (produced by male) carry either X or Y; eggs always carry X. Fertilization by X‑sperm → XX (female); by Y‑sperm → XY (male). The Y chromosome has SRY (sex‑determining region) which initiates male development.
Sex determination in humans follows an XY chromosomal system where males are heterogametic and females are homogametic. Females have two X chromosomes (XX) and produce eggs that all contain a single X chromosome. Males have one X chromosome and one Y chromosome (XY) and produce two types of sperm, approximately half carrying an X chromosome and half carrying a Y chromosome. During fertilization, if an X-bearing sperm fertilizes an egg, the resulting zygote has an XX chromosome combination and develops as a female. If a Y-bearing sperm fertilizes an egg, the resulting zygote has an XY chromosome combination and develops as a male. Therefore, the sex of the offspring is determined by the male parent, as the sperm contributes either an X or Y chromosome. The Y chromosome carries the sex-determining region (SRY gene) which triggers the development of male characteristics. In the absence of the Y chromosome and SRY gene, the default developmental pathway results in female characteristics. This chromosomal mechanism of sex determination is consistent and reliable, producing approximately equal numbers of male and female offspring in populations.
Male heterogamety is a sex-determination system in which males produce two different types of gametes with respect to sex chromosomes (X and Y), while females produce only one type (X). Example: human XY system — males are heterogametic, females homogametic (XX).
Male heterogamety refers to the condition where males produce two different types of gametes (sperm cells) during meiosis. In mammals including humans, males have XY sex chromosomes, so they produce two types of sperm: some carrying the X chromosome and some carrying the Y chromosome. When a sperm carrying the X chromosome fertilizes an egg (which always carries an X chromosome), the resulting offspring is female (XX). When a sperm carrying the Y chromosome fertilizes an egg, the resulting offspring is male (XY). Thus, males are the heterogametic sex because they determine the sex of the offspring through their gamete contribution.
Female heterogamety is a system where females are heterogametic (produce two types of sex chromosome-bearing eggs) and males are homogametic. Example: birds and some reptiles — females ZW (produce Z and W eggs), males ZZ (produce only Z sperm).
Female heterogamety (ZO females) refers to the condition where female produces two types of egg cells. Some with Z chromosome and some without Z chromosome. This pattern occurs in birds, butterflies, and some other organisms where females have ZO sex chromosomes (one Z and no second sex chromosome) while males have ZZ chromosomes. During meiosis in females, the Z chromosome segregates into some eggs while other eggs receive no sex chromosome (O). When a Z-bearing egg is fertilized by a Z-bearing sperm from the male, the offspring is female (ZO). When an O-bearing egg is fertilized by a Z-bearing sperm, the offspring is male (ZZ). Thus, in this system, females are heterogametic and determine the sex of offspring.
The Rh system is primarily determined by the presence or absence of the D antigen (RhD). The D (dominant) and d (absence) alleles determine Rh status: genotypes DD or Dd → Rh positive (Rh+); dd → Rh negative (Rh−). Rh incompatibility: an Rh− mother carrying an Rh+ fetus can become sensitized to RhD antigen and produce anti‑D IgG antibodies that cross the placenta in subsequent pregnancies and cause hemolytic disease of the newborn (erythroblastosis fetalis). Prevention: anti‑D (Rh immunoglobulin) given to Rh− mothers to prevent sensitization. The Rh locus is complex with several linked antigens (C, c, E, e) but D antigen is clinically most important.
Genetic control of Rh factor Fisher and Race hypothesis: Rh factor involves three different pairs of alleles located on three different closely linked loci on the chromosome pair. This system is more commonly in use today, and uses the ‘Cde’ nomenclature. In the given figure, three pairs of Rh alleles (Cc, Dd and Ee) occur at 3 different loci on homologous chromosome pair-1. The possible genotypes will be one C or c, one D or d, one E or e from each chromosome. For e.g. CDE/cde; CdE/cDe; cde/cde; CDe/CdE etc. All genotypes carrying a dominant ‘D’ allele will produce Rh+positive phenotype and double recessive genotype ‘dd’ will give rise to Rh negative phenotype. Wiener Hypothesis Wiener proposed the existence of eight alleles (R 1, R 2, R 0, R z, r, r 1, r 11, r y ) at a single Rh locus. All genotypes carrying a dominant ‘R allele’ (R 1, R 2,R 0,R z ) will produce ‘Rh-positive’ ^phenotype and double recessive genotypes (rr, rr 1, rr 11, rr y ) will give rise to Rh-negative phenotype.
Honeybees use haplodiploidy (arrhenotoky): fertilized eggs develop into diploid females (workers or queens); unfertilized eggs develop into haploid males (drones). Sex is determined by ploidy and in some species by the complementary sex determiner (csd) gene — heterozygosity at csd → female, hemizygous or homozygous → male (diploid males are usually inviable).
In hymenopteran insects such as honeybees, ants and wasps, a mechanism of sex determination called haplodiploidy mechanism of sex determination is common. In this system, the sex of the offspring is determined by the number of sets of chromosomes it receives. Fertilized eggs develop into females (Queen or Worker) and unfertilized eggs develop into males (drones) by parthenogenesis. It means that the males have half the number of chromosomes (haploid) and the females have double the number (diploid), hence the name haplodiploid for this system of sex determination. This mode of sex determination facilitates the evolution of sociality in which only one diploid female becomes a queen and lays the eggs for the colony. All other females which are diploid having developed from fertilized eggs help to raise the queen’s eggs and so contribute to the queen’s reproductive success and indirectly to their own, a phenomenon known as Kin Selection. The queen constructs their social environment by releasing a hormone that suppresses fertility of the workers.
Applications: diagnosis of chromosomal abnormalities (Down syndrome 47,+21; Turner syndrome XO; Klinefelter XXY), prenatal diagnosis (amniocentesis, CVS), fertility investigations, cancer cytogenetics (chromosomal translocations), species identification and evolutionary studies, and detection of structural rearrangements (deletions, duplications, translocations).
* Karyotyping helps in gender identification. * It is used to detect the chromosomal aberrations like deletion, duplication, translocation, non-disjunction of chromosomes. * It helps to identify the abnormalities of chromosomes like aneuploidy. * It is also used in predicting the evolutionary relationships between species. * Genetic diseases in human beings can be detected by this technique.
X‑linked recessive: Trait is carried on X chromosome. Males (XY) are hemizygous so a single recessive allele causes expression (e.g., haemophilia, red‑green colour blindness). Affected males pass the mutant X to all daughters (who become carriers) but to no sons; carrier mothers have a 50% chance of passing the allele to sons (affected) and 50% to daughters (carriers). This produces criss‑cross inheritance. X‑linked dominant: a single mutant X allele causes disease in both sexes; affected fathers transmit the trait to all daughters but no sons; affected mothers transmit to 50% of children of each sex. Y‑linked (holandric) inheritance: genes on Y chromosome transmitted father→son only. Concepts: hemizygous, carrier, criss‑cross inheritance, dosage compensation (X inactivation) influence expression in females.
Sex-linked characters in humans are traits controlled by genes located on the sex chromosomes, primarily the X chromosome. Since males have only one X chromosome (XY), they express both dominant and recessive alleles on the X chromosome, making them hemizygous. Females have two X chromosomes (XX), so they can be homozygous or heterozygous for X-linked traits. For X-linked recessive traits like color blindness and hemophilia, affected males have the genotype X^a Y where X^a represents the recessive allele. Carrier females have the genotype X^A X^a (one normal and one mutant allele) and typically show the dominant phenotype but can pass the recessive allele to offspring. When a carrier female (X^A X^a) mates with a normal male (X^A Y), the offspring show a 1:1 ratio in males (half normal X^A Y and half affected X^a Y) and all females are normal phenotypically (half X^A X^A and half X^A X^a carriers). When an affected female (X^a X^a) mates with a normal male (X^A Y), all daughters are carriers (X^A X^a) and all sons are affected (X^a Y). This criss-cross inheritance pattern explains why X-linked recessive traits appear more frequently in males than females. The inheritance of X-linked dominant traits differs because affected heterozygous females (X^A X^a) pass the dominant allele to half their offspring of both sexes, while affected males (X^A Y) pass the dominant allele to all daughters but no sons.