Colour blind mother (X^cb X^cb) × Normal father (X^N Y) → sons: X^cb Y (all colour blind, 50% of children) + daughters: X^N X^cb (all carriers, not colour blind, 50% of children). So 50% of all children are colour blind (all sons).

Blood group O negative individuals are the true universal donors — their RBCs lack A, B, and Rh antigens. Blood group O individuals lack A and B antigens and can donate to all ABO groups. AB individuals are universal recipients.

Male grasshoppers have 23 chromosomes — 22 autosomes plus one X chromosome (XO system). Female grasshoppers have 24 chromosomes (22 autosomes + XX). The XO system was the first sex determination mechanism to be described.

Colour blind father (X^cb Y) × Normal mother (X^N X^N) → daughters are X^N X^cb (carriers, not colour blind) and sons are X^N Y (normal). No colour blind children in this cross — but the question says 50% which would apply in different scenarios.

Epistasis is the interaction between non-allelic genes where one gene (epistatic gene) masks or suppresses the expression of another gene (hypostatic gene) at a different locus. It produces modified Mendelian ratios in F2.

Crossing over is the physical exchange of chromosome segments between non-sister chromatids of homologous chromosomes during pachytene of prophase I of meiosis. The points of exchange are called chiasmata (singular: chiasma).

The Rh blood group system is encoded by three closely linked genes (RHD, RHCE, and RHAG) on chromosome 1. The most clinically important is the D antigen encoded by the RHD gene.

Tt is the heterozygous genotype for plant height in Mendel's peas. Since T (tall) is dominant over t (dwarf), the Tt plant expresses the tall phenotype. Only tt (homozygous recessive) plants are dwarf.

Mr. Shahbaz (A = IA i or IA IA) × Wife (AB = IA IB): If IA i × IA IB → children can be IA IA (A), IA IB (AB), i IA (A), or i IB (B). So A, B, and AB are all possible. If IA IA × IA IB → children are IA IA (A) or IA IB (AB).

The IB allele encodes a glycosyltransferase enzyme that adds N-acetylgalactosamine to the H antigen, creating antigen B on red blood cells. IA adds galactose to create antigen A. The i allele produces a non-functional enzyme.

Mendel designated the original true-breeding parents as the P (parental) generation — the first parental generation. Their offspring are F1 (first filial), and F1 × F1 produces F2 (second filial generation).

Blood group AB individuals have both antigen A and antigen B on their red blood cells. Their genotype is IA IB (codominant). They have neither anti-A nor anti-B antibodies and are called universal recipients.

Multiple alleles are three or more alternative forms of a gene that can exist at the same locus in a population. The ABO blood group system has three alleles (IA, IB, i). Any individual has only two of these three alleles.

Phenotype is the observable, physical expression of genes — what we can see or measure. Rounded vs wrinkled seeds and tall vs dwarf plants are classic phenotypic differences that Mendel used to discover laws of inheritance.

The genotype is the genetic constitution (the alleles present) of an organism for a trait or for all traits. The phenotype is the observable expression. Same phenotype can have different genotypes (e.g., TT and Tt are both tall phenotype).

Hypophosphataemic rickets (X-linked hypophosphataemia) is an X-linked dominant disorder — even one copy of the mutant PHEX gene causes disease. Unlike most X-linked conditions, it affects females as well as males.

Blood pressure is a multifactorial (complex) trait influenced by many genes and environmental factors (diet, salt intake, stress, exercise). It shows continuous variation and familial clustering without simple Mendelian inheritance.

Diabetes mellitus (especially type 2) is a multifactorial trait — influenced by multiple genes AND environmental factors (diet, lifestyle, obesity). It shows familial clustering but does not follow simple Mendelian inheritance patterns.

Testicular feminization syndrome (androgen insensitivity syndrome, AIS) is a rare X-linked recessive condition where XY individuals are phenotypically female because their cells cannot respond to androgens. Vitamin D-resistant rickets is X-linked dominant.

Deuteranopia is green colour blindness — inability to distinguish green from red due to absence or dysfunction of the M-cone (green opsin) photoreceptors. It is an X-linked recessive condition more common in males.

Males are more frequently affected by X-linked recessive conditions like haemophilia because they are hemizygous (only one X chromosome). A female needs two copies of the recessive allele to be affected — much less likely statistically.

Haemophilia A and B (clotting factor deficiencies) and colour blindness (opsin gene mutations) are both X-linked recessive traits. Males (XY) are more frequently affected because they only need one copy of the recessive allele.

Protanopia is red colour blindness — inability to perceive red light due to absence or deficiency of the L-cone (red opsin) photoreceptors. Deuteranopia is green blindness. Both are X-linked recessive conditions.

The genes encoding red opsin (L-opsin, OPN1LW) and green opsin (M-opsin, OPN1MW) are located on the X chromosome (Xq28). Mutations in these genes cause red-green colour blindness — an X-linked recessive condition.

Haemophilia passes from an affected grandfather to grandson through a carrier daughter (the grandfather's daughter who carries the haemophilia allele on one X chromosome but does not express it).

Haemophilia A is caused by a deficiency or abnormality of clotting Factor VIII. It is the most common form of haemophilia (80% of cases), X-linked recessive, affecting about 1 in 5,000 male births.

Y-linked (holandric) traits are passed exclusively from father to all sons — since sons inherit their Y chromosome from their father. Y-linked genes cannot be transmitted to daughters (who get X from father, not Y).

Hemizygous males (XY) carry only one copy of X-linked genes — they cannot be homozygous or heterozygous for X-linked alleles. Any allele on their single X chromosome is expressed, whether dominant or recessive.

In Drosophila, maleness is determined by autosomes — specifically by the X:Autosome ratio (X:A = 0.5 → male). Unlike in humans, the Y chromosome in Drosophila does not determine maleness (Y-less flies are females).

Maleness in humans is determined by the SRY gene on the Y chromosome — it is Y-linked. The presence of a Y chromosome (specifically the SRY gene) initiates testis development and male sex determination.

Morgan and his colleagues at Columbia University studied 85 different genetic traits in Drosophila melanogaster. They established that these traits formed 4 linkage groups corresponding to Drosophila's 4 pairs of chromosomes.

In Drosophila, sex is determined by the X:Autosome ratio. If X:A = 1.0 or greater (2X:2A = 1.0), the fly develops as female. If X:A = 0.5 (1X:2A), it develops as male. The Y chromosome does not determine sex in Drosophila.

The probability of a son or daughter in human births is 50:50 (1:1). This is because fathers produce equal numbers of X-bearing and Y-bearing sperm. If X sperm fertilises the egg (XX) → girl; Y sperm (XY) → boy.

A nullo gamete lacks a sex chromosome — it results from non-disjunction of sex chromosomes during meiosis. When a nullo gamete fuses with a normal gamete, it produces individuals with abnormal sex chromosome numbers (like Turner's 45,X).

In Drosophila, the Y chromosome is genetically almost inert — it carries very few functional genes. Sex determination in Drosophila depends on the X:autosome ratio, not on the presence of a Y chromosome as in humans.

The ZW-ZZ sex determination system (where females are ZW and males are ZZ) is found in birds, butterflies, moths, some fish, and some reptiles. It is the reverse of the XY system where males are heterogametic.

The SRY gene (Sex-determining Region of the Y chromosome) is located on the short arm of the Y chromosome. It encodes the testis-determining factor (TDF) that initiates male sex determination by activating a cascade of gene expression.

Linkage is the tendency of genes located on the same chromosome to be inherited together. Thomas Hunt Morgan discovered linkage in Drosophila. Linked genes violate Mendel's law of independent assortment.

Both colour blindness (opsin genes) and haemophilia (factor VIII and IX genes) are located on the X chromosome — they are X-linked traits forming a linkage group. Males with one affected X chromosome express these recessive conditions.

Crossing over is the physical exchange of corresponding segments between non-sister chromatids of homologous chromosomes during pachytene of meiosis I. It produces recombinant chromosomes and is the molecular basis of genetic recombination.

Sex chromosomes (allosomes) are a special pair of chromosomes that carry genes determining the sex of an organism. In humans, females are XX and males are XY. The Y chromosome carries the SRY male-determining gene.

All genes located on the same chromosome form a linkage group and tend to be inherited together. The number of linkage groups equals the haploid number of chromosomes. Crossing over can separate linked genes.

Tongue rolling is a genetic trait — individuals who can roll their tongue into a tube shape are called rollers. It is often cited as a dominant trait, though the genetics is more complex. Non-rollers cannot form this shape.

The Bombay phenotype (h/h) occurs when the H gene is homozygous recessive. The H antigen (precursor for A and B antigens) cannot be made, so even people with IA or IB alleles appear to be blood group O phenotypically.

Frequency histograms (bar graphs) illustrate the distribution of variations in a population. When showing continuous variation of polygenic traits (height, skin colour), they approach a normal distribution (bell curve).

Pleiotropy is when a single gene influences multiple phenotypic traits. Example: the gene for PKU affects both intelligence and skin/hair pigmentation. Sickle cell haemoglobin gene affects blood, heart, kidneys, and spleen.

Epistasis is gene interaction where one gene masks or modifies the expression of another gene at a different locus. It differs from dominance (interaction between alleles at the same locus). Epistasis can produce modified Mendelian ratios.

The Rh blood group is controlled by the D gene. DD (homozygous dominant) and Dd (heterozygous) individuals are Rh-positive — they have the D antigen on their red blood cells. Only dd individuals are Rh-negative.

A blood group B man (IB i or IB IB) × O woman (ii): If IB i × ii → offspring are IB i (B) and ii (O). If IB IB × ii → offspring are all IB i (B). In neither case can blood type A offspring appear.

Antiserum is blood serum (plasma minus clotting factors) that contains specific antibodies. It is obtained from immunised individuals and used for passive immunisation, blood typing, and diagnostic tests.

If Mr. Haris is IA i and Mrs. Haris is IA IB, possible offspring genotypes are: IA IA (A), IA IB (AB), i IB (B), i IA (A). Type AB (IA IB) is possible. If Mr. Haris is IA IA, all children with IA IB mother will be either IA IA (A) or IA IB (AB).

Blood group O individuals are called universal donors because their red blood cells lack both A and B antigens, so they can donate blood to people of any ABO blood group without triggering agglutination reactions.

Multiple alleles are three or more alternate forms of a gene that can occupy the same locus. In a population, any number of alleles may exist, but each diploid individual carries only two. ABO blood group has 3 alleles (IA, IB, i).

Although there may be many alleles for a gene in a population (multiple alleles), a diploid individual can only have a maximum of two alleles for any gene (one on each homologous chromosome).

The genotype ii (homozygous for the recessive i allele) produces blood group O — no antigen A or B on red blood cells. The i allele produces a non-functional enzyme that cannot add antigenic sugars to the H antigen.

The ABO blood group system has four phenotypes: A (IA IA or IA i), B (IB IB or IB i), AB (IA IB — codominant), and O (ii). These are determined by three alleles (IA, IB, i) at the ABO locus.

The product rule of probability states that the probability of two independent events both occurring simultaneously equals the product of their individual probabilities. It is used to calculate expected ratios in dihybrid and multihybrid crosses.

Codominance occurs when both alleles are fully expressed in the heterozygote and the phenotype shows both parental traits simultaneously. ABO blood group (IA and IB are codominant) is the classic example in humans.

Carl Correns studied incomplete dominance in Mirabilis jalapa (four o'clock flower). When red-flowered (RR) × white-flowered (WW) plants are crossed, F1 are pink (RW) — neither allele completely dominates the other.

Complete dominance occurs when the dominant allele completely masks the expression of the recessive allele in heterozygotes. The heterozygote's phenotype is identical to the dominant homozygote. This is what Mendel observed in peas.

Homozygous individuals have two identical alleles for a gene — either both dominant (TT, pure dominant) or both recessive (tt, pure recessive). They breed true and produce only one type of gamete for that gene.

A test cross involves crossing an individual of unknown genotype (dominant phenotype) with a homozygous recessive individual. The ratio of offspring phenotypes reveals whether the unknown is homozygous (TT) or heterozygous (Tt).

Heterozygous (hybrid) individuals have two different alleles for a gene (e.g., Tt). Homozygous individuals have identical alleles (TT or tt). Heterozygotes express the dominant phenotype but carry the recessive allele.

The dominant trait is the one that appears in the F1 generation when true-breeding parents with contrasting traits are crossed. The recessive trait is suppressed in F1 but reappears in F2 in 25% of offspring.

A monohybrid cross involves two individuals differing in only one character (e.g., tall × dwarf). Dihybrid crosses involve two characters; polyhybrid crosses involve multiple characters simultaneously.

Mendel performed his famous experiments on garden pea (Pisum sativum) in the monastery garden in Brno (now Czech Republic). He chose 7 pairs of contrasting traits and grew over 28,000 plants.

The genotype is the genetic constitution of an individual for a particular trait or for all traits — the specific alleles present at a locus (e.g., TT, Tt, or tt). Phenotype is the observable expression of the genotype.

The gene pool is the total collection of all genes and their alleles present in all individuals of a breeding population. It is the reservoir of genetic variation available to the population for evolution.

The gene is the basic unit of biological information — it is the hereditary unit that encodes information for a specific function (usually a protein or regulatory RNA). Genes are segments of DNA at specific loci on chromosomes.

Alleles are the alternative forms of a gene that occupy corresponding loci on homologous chromosomes. An individual typically has two alleles for each gene (one from each parent) — they can be the same (homozygous) or different (heterozygous).

In Morgan's cross of red-eyed male (X^R Y) × white-eyed female (X^r X^r), F1 gives red-eyed females (X^R X^r) and white-eyed males (X^r Y). F2 gives half red-eyed and half white-eyed males.

Abnormal chromosome numbers (aneuploidy — trisomy, monosomy) result from non-disjunction errors during meiosis, producing gametes with extra or missing chromosomes. When these fertilise normal gametes, the offspring has abnormal chromosome number.

Antonie van Leeuwenhoek first observed spermatozoa (sperm cells) in 1677 using his hand-crafted microscopes. He described them as tiny 'animalcules' swimming in semen from various animals including humans.

Parental plants (P generation) are the original plants used in a cross. They are usually true-breeding (homozygous) for the traits being studied. Their offspring are the F1 (first filial) generation.

Gregor Mendel used the term 'Elemente' (factors or unit factors) to describe the hereditary determinants of traits. These unit factors are what we now call genes or alleles.

The classic 9:3:3:1 ratio is the expected phenotypic ratio in F2 of a dihybrid cross (YyRr × YyRr). It gives: 9 yellow round : 3 yellow wrinkled : 3 green round : 1 green wrinkled in 16 offspring.

Female grasshoppers carry two X chromosomes (XX, 24 chromosomes) while males have only one X (XO, 23 chromosomes). In the XO system, the extra X chromosome in females determines female sex.

Blood group O individuals have neither antigen A nor antigen B on their red blood cells. They have both anti-A and anti-B antibodies in their plasma. Their genotype is ii (homozygous for the recessive i allele).

Walter Sutton (1902) and Theodor Boveri independently formulated the chromosome theory of heredity — proposing that chromosomes are the carriers of Mendel's hereditary factors (genes). This is the Sutton-Boveri theory.

Wilhelm Johannsen coined the term 'gene' in 1909 to replace Mendel's 'factor'. He also introduced the terms 'genotype' (genetic constitution) and 'phenotype' (observable characteristics).

Males produce two types of gametes (heterogametes) — in mammals, X-bearing sperm and Y-bearing sperm. Females produce homogametes (all eggs carry X in mammals). Male heterogamety determines offspring sex.

In Morgan's classic Drosophila experiment studying body colour and wing size genes, approximately 17% of offspring were recombinants (parental combination broken by crossing over). This 17% recombination frequency indicates the genes are 17 cM apart on the chromosome.

In birds, moths, and some fish, males are homogametic (ZZ) and females are heterogametic (ZW). In these species, females determine the sex of offspring since they produce two types of eggs.

In Drosophila, the X chromosome is rod-shaped (straight). The Y chromosome is hook-shaped (J-shaped). This morphological difference allows sex determination by karyotype analysis.

In birds, females are ZW (heterogametic) and males are ZZ (homogametic). This is the reverse of mammals. Female birds produce Z-bearing and W-bearing eggs; male birds produce only Z-bearing sperm.

Sex chromosomes in mammals are identified by their shape/size. The X chromosome is large and rod-shaped; the Y chromosome is much smaller. They can be distinguished cytologically by their morphology during cell division.

In mammals, females (XX) are homogametic — they produce only one type of gamete (all carrying X chromosome). Males (XY) are heterogametic — they produce two types of gametes (X-bearing and Y-bearing sperm).

Human males are XY — they have one X chromosome (inherited from mother) and one Y chromosome (inherited from father). Females are XX. The Y chromosome carries the SRY gene which determines male sex.

Female grasshoppers have 24 chromosomes (12 pairs including XX sex chromosomes). Male grasshoppers have 23 chromosomes (11 autosomes + 1 X chromosome, no Y) — this XO system was the first sex determination system discovered.

Colour blindness is caused by recessive alleles on the X chromosome affecting the opsin genes (for red or green colour vision). Males (XY) need only one copy to be affected; females need two copies.

When an Rh-ve mother carries an Rh+ve fetus, fetal RBCs can enter her blood (especially at delivery). Her immune system recognises the Rh antigen as foreign and produces anti-Rh antibodies (sensitisation).

Genes are located on chromosomes in the cell nucleus (nuclear DNA). Mitochondria also have a small amount of DNA (mitochondrial genes), but the vast majority of genes are in the nuclear chromosomes.

T.H. Morgan discovered the role of sex chromosomes (X and Y) in sex determination through his work on Drosophila melanogaster. He also discovered sex-linked inheritance, linkage, and crossing over.

In birds, males are homogametic (ZZ — produce only Z-bearing sperm) while females are heterogametic (ZW — produce Z- or W-bearing eggs). This is the reverse of the mammalian XX/XY system.

Female grasshoppers have 24 chromosomes (12 pairs, XX sex chromosomes), while males have only 23 chromosomes (11 pairs + 1 X chromosome, no Y chromosome). This is the XO sex determination system.

Blood group A individuals have antigen A on their red blood cells and anti-B antibodies in their plasma. They can be either homozygous (IAIA) or heterozygous (IAi) for the A allele.

Approximately 85% of the human population possess the Rh antigen (D antigen) on their red blood cells and are Rh positive (Rh+ve). The remaining 15% lack this antigen and are Rh negative (Rh-ve).

A dihybrid cross involves two pairs of contrasting traits simultaneously. When yellow round (YYRR) × green wrinkled (yyrr), all F1 offspring are YyRr (heterozygous yellow round) due to dominance of yellow and round alleles.

Normal (coloured) eye colour is produced by the dominant allele that promotes melanin production in the iris. The recessive allele produces blue eyes (less melanin) or in some conditions, albinism (no melanin).

Polygenic traits are controlled by multiple genes (polygenes) each contributing additively to the phenotype. Skin colour, height, and intelligence in humans are polygenic — they show continuous variation in populations.

When homozygous tall (TT) is crossed with dwarf (tt), all F1 offspring are Tt (heterozygous tall) because T (tall) is dominant over t (dwarf). All plants express the tall phenotype.

Mendel's Law of Independent Assortment states that alleles of different gene pairs assort independently during gamete formation — the alleles segregate independently and combine randomly in offspring.

An organism is heterozygous when it has two different alleles for a gene (one on each homologous chromosome), e.g., Tt. Homozygous means both alleles are identical (TT or tt).

In 1900, Hugo de Vries, Carl Correns, and Erich von Tschermak independently rediscovered Mendel's laws through their own experiments. This rediscovery marks the birth of genetics as a formal science.

The wild type is the most common phenotype (observable characteristics) found in a natural population. It is considered the standard or normal phenotype against which mutations are compared.

A recessive allele is unable to express itself phenotypically when in heterozygous condition (paired with a dominant allele). It is only expressed when homozygous (two copies of the recessive allele are present).

A locus (plural: loci) is the specific position or location of an allele (or gene) on a chromosome. Homologous chromosomes carry alleles of the same gene at corresponding loci.

Gregor Johann Mendel (1822-1884) is called the Father of Genetics. He discovered the laws of segregation and independent assortment through meticulous experiments on garden peas, published in 1866 but recognised only in 1900.

The scientific name of garden pea is Pisum sativum. Mendel chose it because it is easy to grow, has clearly distinguishable traits, can self-fertilise, and has a short generation time — making it ideal for genetic experiments.

Mendel's work was based on hybridization experiments — he crossed different varieties of garden pea with contrasting traits and analysed the offspring to discover the fundamental laws of inheritance.