Unit 18: Inheritance — Short Questions

The main exceptions to Mendel's laws include: incomplete dominance and co-dominance (exceptions to complete dominance), multiple alleles (e.g. ABO blood groups), polygenic inheritance and epistasis (deviations from simple single-gene ratios), gene linkage and crossing over (which counter independent assortment), pleiotropy (one gene affecting multiple traits), environmental effects on phenotype, and sex-linkage (unequal inheritance pattern in males and females).

Probability is a measure of how often a particular outcome occurs; it is calculated by dividing the number of times an event occurs by the total number of possible outcomes. In genetics, the transfer of genes from parents to offspring is random, so probability rules (the product rule for independent events and the sum rule for mutually exclusive events) are used to predict the likely genotype and phenotype ratios of offspring, especially when crosses involve many genes and Punnett squares become too complex.

A multiple allelic system has more than two alternative forms of a gene in the population, even though any individual carries only two of them. For example, the ABO gene has three alleles (IA, IB, i); combinations of these three alleles produce six possible genotypes but, because IA and IB are codominant over each other and both are dominant over i, these combinations result in four distinct phenotypes (blood groups A, B, AB, O) — more phenotypic variety than a simple two-allele gene, which can give at most three genotypes and two phenotypes.

The major human blood group systems are the ABO system and the Rh system. Other blood group systems that do not usually cause transfusion-matching problems include the MNS system, Lutheran system, Kell system, Lewis system, and Duffy system.

A multiple allelic system occurs when a single gene has more than two alternative forms (alleles) present in a population, although each individual carries only two of them, creating more variation in the trait. The ABO blood group system is controlled by the I gene, which has three alleles: IA (produces antigen-A), IB (produces antigen-B), and i (produces no antigen).

Co-dominance is a type of inheritance in which both alleles in a gene pair are fully expressed in a heterozygous individual, without blending or either allele masking the other, so the products of both alleles appear together in the phenotype. Example: the human MN blood group, where alleles LM and LN are codominant — a heterozygous individual (LMLN) expresses both the M and N antigens on the RBCs, giving the MN blood group.

In the ABO system, the antigens are antigen-A and antigen-B, found on the surface of RBCs. Blood group A has antigen-A and anti-B antibodies in plasma; blood group B has antigen-B and anti-A antibodies; blood group AB has both antigen-A and antigen-B and neither antibody; and blood group O has neither antigen but has both anti-A and anti-B antibodies in plasma (Landsteiner's rule).

O-negative blood has no antigen-A, no antigen-B, and no Rh-antigen on its RBCs, so it cannot be attacked by the recipient's anti-A, anti-B, or anti-Rh antibodies, meaning it can be transfused to a person of any ABO and Rh blood group — hence O-negative is the universal donor. AB-positive blood has antigen-A, antigen-B and Rh-antigen but its plasma has no anti-A, anti-B, or anti-Rh antibodies, so it can safely receive blood from any ABO and Rh group — hence AB-positive is the universal recipient.

A human male has XY sex chromosomes, so during meiosis he produces two different types of sperm in equal numbers — half carrying an X chromosome and half carrying a Y chromosome. Producing two types of gametes makes the male heterogametic, in contrast to the female (XX), who produces only one type of egg (all carrying X), making her homogametic.

Gene linkage is the phenomenon in which genes located on the same chromosome, especially those close together, tend to be inherited together rather than assorting independently during gamete formation.

Linked genes can separate from each other through chromosomal crossing over during meiosis-I, in which homologous chromatids exchange segments of DNA. This recombination shuffles the alleles between homologous chromosomes, producing new (recombinant) gamete combinations, so the linked genes can effectively show some degree of independent assortment, especially if they are located relatively far apart on the chromosome.

Y-linked (holandric) genes are located only on the non-homologous region of the Y chromosome and have no corresponding alleles on the X chromosome. Because only males have a Y chromosome, Y-linked traits appear only in males and are passed directly from father to son (never through daughters). Examples in humans include hypertrichosis (hair growth on the rim of the pinna), porcupine man (straight body hair), and webbing of toes.

Sex linkage is the phenomenon in which traits are controlled by alleles present on the sex chromosomes (mainly the X chromosome, since the Y chromosome carries almost no functional genes), so such traits show a different inheritance ratio in males and females. Example: eye colour in Drosophila — the white-eye allele is recessive and X-linked, so it is inherited differently by male and female flies, as shown in Morgan's experiments.

A gene is a sequence of nucleotides on DNA that codes for the formation of a polypeptide and controls a particular character, occupying a specific location (locus) on a chromosome. An allele is an alternative form of that same gene — different versions of the gene that can occupy the same locus and produce alternative traits (e.g., the alleles R and r for seed shape).

Genotype is the combination of alleles present in an individual for a given gene/trait — the genetic 'blueprint'. Phenotype is the observable character or trait that results from the genotype — the visible outcome, which may also be influenced by environmental factors.

A homozygous individual has two identical alleles for a gene pair (e.g., RR or rr), formed when two gametes carrying the same allele fuse. A heterozygous individual has two different alleles for a gene pair (e.g., Rr), formed when gametes with different alleles fuse; in such an individual, only the dominant allele is expressed while the recessive allele remains present but unexpressed.

A monohybrid cross studies the inheritance of a single character/gene pair (e.g., seed shape: Rr × Rr), typically producing a 3:1 phenotypic ratio in F2. A dihybrid cross studies the inheritance of two characters/gene pairs together (e.g., seed shape and seed colour: RrYy × RrYy), typically producing a 9:3:3:1 phenotypic ratio in F2.

In incomplete dominance, both alleles in a heterozygote blend their phenotypic effects, producing a single intermediate phenotype between the two parental types (e.g., pink flowers in four-o'clock plants from red × white). In co-dominance, both alleles are fully expressed without blending, and the heterozygote shows both parental phenotypes together at the same time (e.g., human MN blood group, where both M and N antigens appear together).

In polygenic inheritance, many genes act together in an additive way, each contributing a small effect to produce a single trait, resulting in continuous variation (e.g., skin colour, height). In epistasis, one gene's allele (epistatic) masks or modifies the effect of another non-allelic gene's alleles (hypostatic), which usually produces discontinuous variation with distinct phenotype categories (e.g., coat colour in Labrador dogs) rather than a continuous range.

In the XX-XY system (found in Drosophila, mammals and humans), females are homogametic (XX, producing only X-carrying eggs) and males are heterogametic (XY, producing both X- and Y-carrying sperm); the sperm determines the offspring's sex. In the ZZ-ZW system (found in birds), the sexes are reversed — males are homogametic (ZZ, all sperm carry Z) and females are heterogametic (ZW, producing Z-carrying and W-carrying eggs); the egg determines the offspring's sex.

In the XX-XY system (mammals, Drosophila), males have an XY sex chromosome pair (a distinct Y chromosome is present) and females are XX; males produce X-bearing and Y-bearing sperm. In the XX-XO system (found in some insects like grasshoppers), there is no Y chromosome at all — males have only a single X chromosome (XO) and produce X-bearing sperm and 'nullo' sperm (lacking any sex chromosome), while females are XX; an egg fertilized by a nullo sperm produces an XO male, and one fertilized by an X-bearing sperm produces an XX female.

Sex-linked traits are controlled by genes located on the sex chromosomes (mainly the X chromosome), so they show different inheritance ratios in males and females (e.g., colour blindness, haemophilia). Sex-limited traits are controlled by genes present on autosomes in both sexes, but due to anatomical or hormonal differences, these genes are expressed in only one sex (e.g., milk production genes expressed only in females, horn-development genes expressed only in male sheep).

Sex-limited traits are genes present in both sexes but expressed (switched on) in only one sex due to anatomical/hormonal differences (e.g., milk production in female mammals, beard growth in men). Sex-influenced traits are genes present and expressed in both sexes, but the same allele behaves as dominant in one sex and recessive in the other due to hormonal differences (e.g., the baldness allele is dominant in men but recessive in women).

In a cross between two blood group O (ii) parents, all offspring will have blood group O (ii). In a cross between heterozygous IAi × IAi parents, 25% of offspring will be blood group O (ii); similarly, in a cross between heterozygous IBi × IBi, 25% will be O (ii); and in a cross between heterozygous IAi × IBi, 25% will also be O (ii). Because the recessive allele i is carried silently (unexpressed) in heterozygous A and B individuals (IAi and IBi) in addition to being expressed in all O individuals (ii), it keeps getting passed on and reappearing in the population at a high rate across many types of crosses — this is why the allele i is more frequent in the population than IA or IB.

Blood cannot currently be manufactured artificially in sufficient quantity, so patients needing transfusions — due to severe blood loss from accidents or surgery, blood disorders, cancer treatments, or childbirth complications — depend entirely on donated blood to survive. A single blood donation can be separated into RBCs, plasma, and platelets, potentially helping multiple patients at once. Since matching ABO and Rh blood groups is essential for safe transfusion (as incompatible transfusions cause dangerous agglutination reactions), a wide base of regular voluntary donors across all blood groups, including rare types, is needed to ensure blood is available when and where it is needed. Donating blood is therefore a direct, low-risk way for a healthy individual to save the life of someone in need, making it a valuable service to humanity.

Knowledge from this chapter can save lives in several ways: knowing one's own ABO and Rh blood group helps ensure safe, compatible blood transfusions during emergencies or surgery and avoids dangerous agglutination reactions; understanding Rh incompatibility allows Rh-negative mothers to receive preventive anti-Rh antiserum injections during pregnancy to avoid Erythroblastosis foetalis in the baby; understanding the inheritance pattern of X-linked disorders like haemophilia and colour blindness helps families assess the risk of passing these conditions to children, assisted by genetic counselling; and embryonic screening techniques such as amniocentesis and chorionic villus sampling allow early detection of chromosomal or genetic abnormalities in a developing foetus, enabling timely medical decisions and care. Regular voluntary blood donation, informed by understanding of universal donors (O-negative) and universal recipients (AB-positive), also directly helps meet transfusion needs in the community.