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.
Haplodiploidy
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 vs Homogametic
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
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
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 recessives more common in males
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
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 symptoms
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 symptoms
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.
Genetic basis of ABO blood group
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
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
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
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
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).
Sex determination in honeybees
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).
Applications of karyotyping
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.
Inheritance of sex‑linked characters in humans